The 100 most-cited articles in Journal of Applied Physics, 2018

Terahertz time-domain spectroscopy (THz-TDS) is a powerful technique for material’s characterization and process control. It has been used for contact-free conductivity measurements of metals, semiconductors, 2D materials, and superconductors. Furthermore, THz-TDS has been used to identify chemical components such as amino acids, peptides, pharmaceuticals, and explosives, which makes it particularly valuable for fundamental science, security, and medical applications. This tutorial is intended for a reader completely new to the field of THz-TDS and presents a basic understanding of THz-TDS. Hundreds of articles and many books can be consulted after reading this tutorial. We explore the basic concepts of TDS and discuss the relationship between temporal and frequency domain information. We illustrate how THz radiation can be generated and detected, and we discuss common noise sources and limitations for THz-TDS. This tutorial concludes by discussing some common experimental scenarios and explains how THz-TDS measurements can be used to identify materials, determine complex refractive…

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics with capabilities beyond existing technologies due to its large bandgap, controllable doping, and the availability of large diameter, relatively inexpensive substrates. These applications include power conditioning systems, including pulsed power for avionics and electric ships, solid-state drivers for heavy electric motors, and advanced power management and control electronics. Wide bandgap (WBG) power devices offer potential savings in both energy and cost. However, converters powered by WBG devices require innovation at all levels, entailing changes to system design, circuit architecture, qualification metrics, and even market models. The performance of high voltage rectifiers and enhancement-mode metal-oxide field effect transistors benefits from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. Reverse breakdown voltages of over 2 kV for β-Ga2O3 have been reported, either with or without edge termination and over 3 kV for a lateral field-plated Ga2O3…

Within a decade, the field of magnetic skyrmionics has developed from a niche prediction to a huge and active research field. Not only do magnetic skyrmions—magnetic whirls with a unique topology—reveal fundamentally new physics, but they have also risen to prominence as up-and-coming candidates for next-generation high-density efficient information encoding. Within a few years, it has been possible to efficiently create, manipulate, and destroy nanometer-size skyrmions in device-compatible materials at room-temperature by all electrical means. Despite the incredibly rapid progress, several challenges still remain to obtain fully functional and competitive skyrmion devices, as discussed in this perspective article with a focus on recent results.

The efficient use of reversible thermal effects in magnetocaloric, electrocaloric, and elastocaloric materials is a promising avenue that can lead to a substantially increased efficiency of refrigeration and heat pumping devices, most importantly, those used in household and commercial cooling applications near ambient temperature. A proliferation in caloric material research has resulted in a wide array of materials where only the isothermal change in entropy in response to a handful of different field strengths over a limited range of temperatures has been evaluated and reported. Given the abundance of such data, there is a clear need for a simple and reliable figure of merit enabling fast screening and down-selection to justify further detailed characterization of those material systems that hold the greatest promise. Based on the analysis of several well-known materials that exhibit vastly different magnetocaloric effects, the Temperature averaged Entropy Change is introduced as a suitable early indicator of the material's utility for magnetocaloric cooling applications, and its…

Measuring thermal properties of materials is not only of fundamental importance in understanding the transport processes of energy carriers (electrons and phonons in solids) but also of practical interest in developing novel materials with desired thermal properties for applications in energy conversion and storage, electronics, and photonic systems. Over the past two decades, ultrafast laser-based time-domain thermoreflectance (TDTR) has emerged and evolved as a reliable, powerful, and versatile technique to measure the thermal properties of a wide range of bulk and thin film materials and their interfaces. This tutorial discusses the basics as well as the recent advances of the TDTR technique and its applications in the thermal characterization of a variety of materials. The tutorial begins with the fundamentals of the TDTR technique, serving as a guideline for understanding the basic principles of this technique. Several variations of the TDTR technique that function similarly as the standard TDTR but with their own unique features are introduced, followed by introducing…

There is a significant need to build efficient non-von Neumann computing systems for highly data-centric artificial intelligence related applications. Brain-inspired computing is one such approach that shows significant promise. Memory is expected to play a key role in this form of computing and, in particular, phase-change memory (PCM), arguably the most advanced emerging non-volatile memory technology. Given a lack of comprehensive understanding of the working principles of the brain, brain-inspired computing is likely to be realized in multiple levels of inspiration. In the first level of inspiration, the idea would be to build computing units where memory and processing co-exist in some form. Computational memory is an example where the physical attributes and the state dynamics of memory devices are exploited to perform certain computational tasks in the memory itself with very high areal and energy efficiency. In a second level of brain-inspired computing using PCM devices, one could design a co-processor comprising multiple cross-bar arrays of PCM devices to accelerate the…

This tutorial describes challenges and possible avenues for the implementation of the components of a solid-state system, which emulates a biological brain. The tutorial is devoted mostly to a charge-based (i.e. electric controlled) implementation using transition metal oxide materials, which exhibit unique properties that emulate key functionalities needed for this application. In Sec. I, we compare the main differences between a conventional computational machine, based on the Turing-von Neumann paradigm, and a neuromorphic machine, which tries to emulate important functionalities of a biological brain. We also describe the main electrical properties of biological systems, which would be useful to implement in a charge-based system. In Sec. II, we describe the main components of a possible solid-state implementation. In Sec. III, we describe a variety of Resistive Switching phenomena, which may serve as the functional basis for the implementation of key devices for neuromorphic computing. In Sec. IV, we describe why transition metal oxides are promising materials for future…

Neuromorphic computation is one of the axes of parallel distributed processing, and memristor-based synaptic weight is considered as a key component of this type of computation. However, the material properties of memristors, including material related physics, are not yet matured. In parallel with memristors, CMOS based Graphics Processing Unit, Field Programmable Gate Array, and Application Specific Integrated Circuit are also being developed as dedicated artificial intelligence (AI) chips for fast computation. Therefore, it is necessary to analyze the competitiveness of the memristor-based neuromorphic device in order to position the memristor in the appropriate position of the future AI ecosystem. In this article, the status of memristor-based neuromorphic computation was analyzed on the basis of papers and patents to identify the competitiveness of the memristor properties by reviewing industrial trends and academic pursuits. In addition, material issues and challenges are discussed for implementing the memristor-based neural processor.

Chemical versatility and compatibility with a vast array of processing techniques has led to the incorporation of organic semiconductors in various electronic and opto-electronic devices. One such device is the organic field-effect transistor (OFET). In this tutorial, we describe the structure, operation, and characterization of OFETs. Following a short historical perspective, we introduce the architectures possible for OFETs and then describe the device physics and the methods for extracting relevant device parameters. We then provide a brief overview of the myriad organic semiconductors and deposition methods that were adopted for OFETs in the past decades. Non-ideal device characteristics, including contact resistance, are then discussed along with their effects on electrical performance and on the accuracy of extracting device parameters. Finally, we highlight several measurements involving OFETs that allow access to fundamental properties of organic semiconductors and the mechanism of charge transport in these materials.

This paper presents a theoretical, numerical, and experimental investigation of a deep-subwavelength absorber based on the concept of coiled-up space. By adjusting a partition panel in the cavity to form an unequal-section channel, it is found that the resonance frequency of the absorber is easily tuned and near-total absorption is acquired under a fixed deep-subwavelength thickness. The absorption mechanism induced by nearly critical coupling is revealed by graphically analyzing the reflection coefficient in the complex plane. In contrast to conventional techniques, near-total absorption can be adjusted over a wider frequency range. To further enhance the absorption, we demonstrate a broadband absorber with a relative bandwidth up to 33.3%.

Phase-change memory (PCM) is an emerging non-volatile memory technology that is based on the reversible and rapid phase transition between the amorphous and crystalline phases of certain phase-change materials. The ability to alter the conductance levels in a controllable way makes PCM devices particularly well-suited for synaptic realizations in neuromorphic computing. A key attribute that enables this application is the progressive crystallization of the phase-change material and subsequent increase in device conductance by the successive application of appropriate electrical pulses. There is significant inter- and intra-device randomness associated with this cumulative conductance evolution, and it is essential to develop a statistical model to capture this. PCM also exhibits a temporal evolution of the conductance values (drift), which could also influence applications in neuromorphic computing. In this paper, we have developed a statistical model that describes both the cumulative conductance evolution and conductance drift. This model is based on extensive characterization…

Reservoir computing was achieved by constructing a network of virtual nodes multiplexed in time and sharing a single silicon beam exhibiting a classical Duffing non-linearity as the source of nonlinearity. The delay-coupled electromechanical system performed well on time series classification tasks, with error rates below 0.1% for the 1st, 2nd, and 3rd order parity benchmarks and an accuracy of (78±2)% for the TI-46 spoken word recognition benchmark. As a first demonstration of reservoir computing using a non-linear mass-spring system in MEMS, this result paves the way to the creation of a new class of compact devices combining the functions of sensing and computing.

Perovskite solar cells are notorious for exhibiting transient behavior not seen in conventional inorganic semiconductor devices. Significant inroads have been made into understanding this fact in terms of rapid ion migration, now a well-established property of the prototype photovoltaic perovskite MAPbI3 and strongly implicated in the newer mixed compositions. Here, we study the manifestations of ion migration in frequency-domain small-signal measurements, focusing on the popular technique of Electrical Impedance Spectroscopy (EIS). We provide new interpretations for a variety of previously puzzling features, including giant photoinduced low-frequency capacitance and negative capacitance in a variety of forms. We show that these apparently strange measurements can be rationalized by the splitting of AC current into two components, one associated with charge-storage and the other with the quasi-steady-state recombination current of electrons and holes. The latter contribution to the capacitance can take either a positive or a negative sign and is potentially very large when slow,…

A theoretical investigation of extremely high field transport in an emerging wide-bandgap material β-Ga2O3 is reported from first principles. The signature high-field effect explored here is impact ionization. The interaction between a valence-band electron and an excited electron is computed from the matrix elements of a screened Coulomb operator. Maximally localized Wannier functions are utilized in computing the impact ionization rates. A full-band Monte Carlo simulation is carried out incorporating the impact ionization rates and electron-phonon scattering rates. This work brings out valuable insights into the impact ionization coefficient (IIC) of electrons in β-Ga2O3. The isolation of the Γ point conduction band minimum by a significantly high energy from other satellite band pockets plays a vital role in determining ionization co-efficients. IICs are calculated for electric fields ranging up to 8 MV/cm for different crystal directions. A Chynoweth fitting of the computed IICs is done to calibrate ionization models in device simulators.

III-valent dopants have shown to be most effective in stabilizing the ferroelectric, crystalline phase in atomic layer deposited, polycrystalline HfO2 thin films. On the other hand, such dopants are commonly used for tetragonal and cubic phase stabilization in ceramic HfO2. This difference in the impact has not been elucidated so far. The prospect is a suitable doping to produce ferroelectric HfO2 ceramics with a technological impact. In this paper, we investigate the impact of Al, Y, and La doping, which have experimentally proven to stabilize the ferroelectric Pca21 phase in HfO2, in a comprehensive first-principles study. Density functional theory calculations reveal the structure, formation energy, and total energy of various defects in HfO2. Most relevant are substitutional electronically compensated defects without oxygen vacancy, substitutional mixed compensated defects paired with a vacancy, and ionically compensated defect complexes containing two substitutional dopants paired with a vacancy. The ferroelectric phase is strongly favored with La and Y in the substitutional…

Photonic delay systems have revolutionized the hardware implementation of Recurrent Neural Networks and Reservoir Computing in particular. The fundamental principles of Reservoir Computing strongly facilitate a realization in such complex analog systems. Especially delay systems, which potentially provide large numbers of degrees of freedom even in simple architectures, can efficiently be exploited for information processing. The numerous demonstrations of their performance led to a revival of photonic Artificial Neural Network. Today, an astonishing variety of physical substrates, implementation techniques as well as network architectures based on this approach have been successfully employed. Important fundamental aspects of analog hardware Artificial Neural Networks have been investigated, and multiple high-performance applications have been demonstrated. Here, we introduce and explain the most relevant aspects of Artificial Neural Networks and delay systems, the seminal experimental demonstrations of Reservoir Computing in photonic delay systems, plus the most recent and…

Reactive magnetron sputtering is a well-established physical vapor technique to deposit thin compound films on different substrates, ranging from insulating glass windows over wear-resistant car parts to high-responsive touch screens. In this way, the industrial and technological relevance drives the need to understand this process on a more profound level to make optimal use of it. Notwithstanding, the basic principles of the technique can be summarized on a single sheet of paper, and truly mastering and understanding the process behavior is not a simple task. One of the main reasons is the often strong non-linear response of the reactive system to changes in the operation parameters or to small system fluctuations. This aspect of reactive sputtering is embodied by the occurrence of a hysteresis in the system observables as a function of the operation parameters. It is the existence of the hysteresis that troubles optimal deposition and process control on the one hand and gives voice to the intertwined physical and chemical complexity on the other hand. The aim of this tutorial can…

The nonradiative lifetime (τNR) of the near-band-edge emission in various quality GaN samples is compared with the results of positron annihilation measurement, in order to identify the origin and to determine the capture-cross-section of the major intrinsic nonradiative recombination centers (NRCs). The room-temperature τNR of various n-type GaN samples increased with decreasing the concentration of divacancies composed of a Ga vacancy (VGa) and a N vacancy (VN), namely, VGaVN. The τNR value also increased with increasing the diffusion length of positrons, which is almost proportional to the inverse third root of the gross concentration of all point defects. The results indicate that major intrinsic NRC in n-type GaN is VGaVN. From the relationship between its concentration and τNR, its hole capture-cross-section is estimated to be about 7 × 10−14 cm2. Different from the case of 4H-SiC, the major NRCs in p-type and n-type GaN are different: the major NRCs in Mg-doped p-type GaN epilayers are assigned to multiple vacancies containing a VGa and two (or three) VNs, namely, VGa(VN)n (n…

In the field of nanomagnetism and spintronics, integral magnetometry is nowadays challenged by samples with low magnetic moments and/or low coercive fields. Commercial superconducting quantum interference device magnetometers are versatile experimental tools to magnetically characterize samples with ultimate sensitivity as well as with a high degree of automation. For realistic experimental conditions, the as-recorded magnetic signal contains several artifacts, especially if small signals are measured on top of a large magnetic background or low magnetic fields are required. In this Tutorial, we will briefly review the basic principles of magnetometry and present a representative discussion of artifacts which can occur in studying samples like soft magnetic materials as well as low moment samples. It turns out that special attention is needed to quantify and correct the residual fields of the superconducting magnet to derive useful information from integral magnetometry while pushing the limits of detection and to avoid erroneous conclusions.

The term junction spectroscopy embraces a wide range of techniques used to explore the properties of semiconductor materials and semiconductor devices. In this tutorial review, we describe the most widely used junction spectroscopy approaches for characterizing deep-level defects in semiconductors and present some of the early work on which the principles of today's methodology are based. We outline ab-initio calculations of defect properties and give examples of how density functional theory in conjunction with formation energy and marker methods can be used to guide the interpretation of experimental results. We review recombination, generation, and trapping of charge carriers associated with defects. We consider thermally driven emission and capture and describe the techniques of Deep Level Transient Spectroscopy (DLTS), high resolution Laplace DLTS, admittance spectroscopy, and scanning DLTS. For the study of minority carrier related processes and wide gap materials, we consider Minority Carrier Transient Spectroscopy (MCTS), Optical DLTS, and deep level optical transient…

Temperature-dependent cathodoluminescence spectra of (2¯01) Si-doped β-Ga2O3 single crystals and (010) N-doped epitaxial films were comprehensively shown to investigate their electronic structure and defect states. The decrease in the self-trapped exciton (STE) emission at low temperatures in heavily Si-doped crystals implied the Debye-Hückel screening of STEs with the critical charge density larger than 2 × 1018 cm−3. The analysis based on the rate equation model suggested a significant influence of the donor-acceptor-pair recombination involving a SiGa donor on the UV luminescence band in the Si-doped crystals. The blue luminescence band was suppressed for the heavily N-doped epitaxial films with a N concentration of 1 × 1018 cm−3, implying the decrease in the oxygen vacancy (VO) concentration by N doping. The increase in the NO acceptor concentration as well as the decrease in the VO concentration were found to contribute to the compensation of residual Si donors resulting in high resistivity of N-doped epitaxial films.

Manifold research has been done to understand the detailed mechanisms behind the performance instabilities of ferroelectric capacitors based on hafnia. The wake-up together with the imprint might be the most controversially discussed phenomena so far. Among crystallographic phase change contributions and oxygen vacancy diffusion, electron trapping as the origin has been discussed recently. In this publication, we provide evidence that the imprint is indeed caused by electron trapping into deep states at oxygen vacancies. This impedes the ferroelectric switching and causes a shift of the hysteresis. Moreover, we show that the wake-up mechanism can be caused by a local imprint of the domains in the pristine state by the very same root cause. The various domain orientations together with an electron trapping can cause a constriction of the hysteresis and an internal bias field in the pristine state. Additionally, we show that this local imprint can even cause almost anti-ferroelectric like behavior in ferroelectric films.

Deep level defects were characterized in Ge-doped (010) β-Ga2O3 layers grown by plasma-assisted molecular beam epitaxy (PAMBE) using deep level optical spectroscopy (DLOS) and deep level transient (thermal) spectroscopy (DLTS) applied to Ni/β-Ga2O3:Ge (010) Schottky diodes that displayed Schottky barrier heights of 1.50 eV. DLOS revealed states at EC − 2.00 eV, EC − 3.25 eV, and EC − 4.37 eV with concentrations on the order of 1016 cm−3, and a lower concentration level at EC − 1.27 eV. In contrast to these states within the middle and lower parts of the bandgap probed by DLOS, DLTS measurements revealed much lower concentrations of states within the upper bandgap region at EC − 0.1 – 0.2 eV and EC − 0.98 eV. There was no evidence of the commonly observed trap state at ∼EC − 0.82 eV that has been reported to dominate the DLTS spectrum in substrate materials synthesized by melt-based growth methods such as edge defined film fed growth (EFG) and Czochralski methods [Zhang et al., Appl. Phys. Lett. 108, 052105 (2016) and Irmscher et al., J. Appl. Phys. 110, 063720 (2011)]. This strong…

While InGaN/GaN blue and green light-emitting diodes (LEDs) are commercially available, the search for an efficient red LED based on GaN is ongoing. The realization of this LED is crucial for the monolithic integration of the three primary colors and the development of nitride-based full-color high-resolution displays. In this perspective, we will address the challenges of attaining red luminescence from GaN under current injection and the methods that have been developed to circumvent them. While several approaches will be mentioned, a large emphasis will be placed on the recent developments of doping GaN with Eu3+ to achieve an efficient red GaN-based LED. Finally, we will provide an outlook to the future of this material as a candidate for small scale displays such as mobile device screens or micro-LED displays.

The paper proposes a modified metamaterial beam for simultaneous vibration suppression and energy harvesting. Local resonators in the modified metamaterial beam are alternately coupled, and each resonator is associated with a piezoelectric element for converting vibrations into electrical energy. First, the mathematical model of the modified metamaterial beam based piezoelectric energy harvester (PEH) is developed. The vibration suppression and energy harvesting performances of this system are analysed and compared with those of a conventional metamaterial beam PEH. The analytical results predict that not only the energy harvesting performance can be massively reinforced in the low frequency range, but also the vibration suppression performance can be slightly enhanced due to the appearance of an additional band gap. Subsequently, two finite element models, Models A and B, are developed. Model A is expected to be equivalent to the analytical model for validation and the local oscillators represented by lumped parameters in the analytical model are modelled by using cantilevers with…

There is an increasing demand for thin films with tailored properties, which requires the use and control of adequate deposition techniques. Ion beam sputter deposition (IBSD) is a physical vapor deposition (PVD) technique that is capable of fulfilling the technological challenges. In contrast to other PVD techniques, IBSD offers a unique opportunity to tailor the properties of the film-forming particles (sputtered target and scattered primary particles) and, hence, thin film properties. This is related to the fact that the generation and acceleration of the primary particles from the ion beam source, the generation of film-forming particles at the target, and thin film growth on the substrate are spatially separated. Thus, by changing ion beam parameters (ion species and ion energy) and geometrical parameters (ion incidence angle and emission angle), the energy distributions of the film-forming particles are modified. Even though in use for several decades, IBSD was not investigated systematically until lately. Utilizing the full potential of IBSD requires a comprehensive…

Energy and water are deeply interconnected, and each sector is both central to society and under increasing stress. Innovations in materials will be a powerful tool in efforts to overcome these challenges by providing sustainable solutions to treating water and rendering it fit-for-purpose with minimal expenditure of energy and other resources. Interfaces between components of water systems and the water-based fluids themselves govern the performance of the vast majority of water treatment and conveyance processes. This perspective examines many of these interfaces, ranging from those in sorbents and sensors to membranes and catalysts, and surveys opportunities for scientists and engineers to reveal new insights into their function and, thereby, to design novel technologies for next-generation solutions to our collective energy-water challenges.

Two-dimensional (2D) materials have attracted growing interest in the past decade because of their extraordinary properties and great potential in a wide range of applications. Strain is regarded as a simple yet powerful tool to modulate the properties of 2D materials, as it directly affects lattice structures and thus alters electronic structures. In this tutorial, we first discuss the fundamentals of strain engineering in 2D materials from macro and atomic perspective and then review some approaches to strain engineering as well as their merits and drawbacks. After that, we examine in detail how strain modulates physical and chemical properties in various types of 2D materials. In the last section of this tutorial, the applications of strain engineering in functional 2D materials are exhibited.

We study extensional vibrations of a ZnO piezoelectric semiconductor fiber driven by a time-harmonic end force. A theoretical analysis is performed using a one-dimensional model based on the phenomenological theory of piezoelectric semiconductors consisting of the equations of piezoelectricity coupled to the continuity equation of electrons. A series of resonance frequencies are identified. It is found that these frequencies are lower than those of the corresponding piezoelectric dielectric fiber because the mobile charges in the semiconductor screen the effective polarization charges and thus weaken the piezoelectric stiffening effect. Various electromechanical fields at resonances are calculated and examined. Numerical results show that the effective surface polarization charge plays a more dominant role than the effective body polarization charge. The mobile charges can screen the body polarization charges greatly, but not the surface polarization charges.

Acoustic and elastic metamaterials are media with a subwavelength structure that behave as effective materials displaying atypical effective dynamic properties. These material systems are of interest because the design of their sub-wavelength structure allows for direct control of macroscopic wave dispersion. One major design limitation of most metamaterial structures is that the dynamic response cannot be altered once the microstructure is manufactured. However, the ability to modify wave propagation in the metamaterial with an external stimulus is highly desirable for numerous applications and therefore remains a significant challenge in elastic metamaterials research. In this work, a honeycomb structure composed of a doubly periodic array of curved beams, known as a negative stiffness honeycomb (NSH), is analyzed as a tunable elastic metamaterial. The nonlinear static elastic response that results from large deformations of the NSH unit cell leads to a large variation in linear elastic wave dispersion associated with infinitesimal motion superposed on the externally imposed…

We study the extension of a composite fiber of piezoelectric dielectrics and nonpiezoelectric semiconductors. A theoretical analysis is performed using a one-dimensional model. It is shown that mechanical loads cause redistribution of mobile charges in such a composite fiber. Thus, the composite fiber exhibits piezotronic couplings like a homogeneous piezoelectric semiconducting fiber. The couplings are a product property of the composite, enabling the design of piezotronics devices beyond materials with direct piezotronics couplings. The basic behavior of the composite fiber and the effects of various parameters on piezotronic couplings are calculated and examined. It is observed that piezotronic couplings in these fibers are sensitive to material and geometric parameters and can be optimized through design.

Metamaterials can be engineered to interact with waves in entirely new ways, finding application on the nanoscale in various fields such as optics and acoustics. In addition, acoustic metamaterials can be used in large-scale experiments for filtering and manipulating seismic waves (seismic metamaterials). Here, we propose seismic isolation based on a device that combines some properties of seismic metamaterials (e.g., periodic mass-in-mass systems) with that of a standard foundation positioned right below the building for isolation purposes. The concepts on which this solution is based are the local resonance and a dual-stiffness structure that preserves large (small) rigidity for compression (shear) effects. In other words, this paper introduces a different approach to seismic isolation by using certain principles of seismic metamaterials. The experimental demonstrator tested on the laboratory scale exhibits a spectral bandgap that begins at 4.5 Hz. Within the bandgap, it filters more than 50% of the seismic energy via an internal dissipation process. Our results open a path toward…

The accumulated charges on the insulator surface become a key factor to incur surface flashover. The charge accumulation process is closely related to the surface condition. This research investigates the effects of surface roughness on surface charge accumulation behavior and surface flashover performance of alumina-filled epoxy resin spacers in SF6/N2 mixtures under DC voltage. The insulator is prepared with only half surface subjected to rough treatment. The accumulated charges are distributed in two regions. Considerable homo-charges are located near the high voltage (HV) electrode and few hetero-charges are near the grounded (GND) electrode (region I). These charges are injected by electrodes. The bipolar charges between HV and GND electrodes (region II) originate from gas ionization. Surface rough treatment can suppress surface charge accumulation and improve surface insulation strength. When increasing surface roughness, surface charge declines first slowly (stage I) and then rapidly (stage II), which coincides with the two stages of surface flashover voltage increment.…

Naturally occurring water is a promising candidate for achieving broadband absorption. In this work, by virtue of the optically transparent character of the water, the water-based metamaterial absorbers (MAs) are proposed to achieve the broadband absorption at microwave frequencies and optical transparence simultaneously. For this purpose, the transparent indium tin oxide (ITO) and polymethyl methacrylate (PMMA) are chosen as the constitutive materials. The water is encapsulated between the ITO backed plate and PMMA, serving as the microwave loss as well as optically transparent material. Numerical simulations show that the broadband absorption with the efficiency over 90% in the frequency band of 6.4–30 GHz and highly optical transparency of about 85% in the visible region can be achieved and have been well demonstrated experimentally. Additionally, the proposed water-based MA displays a wide-angle absorption performance for both TE and TM waves and is also robust to the variations of the structure parameters, which is much desired in a practical application.

Epitaxial Ru(0001) layers are sputter deposited onto Al2O3(0001) substrates and their resistivity ρ measured both in situ and ex situ as a function of thickness d = 5–80 nm in order to quantify the resistivity scaling associated with electron-surface scattering. All layers have smooth surfaces with a root-mean-square roughness <0.4 nm, exhibit an epitaxial relationship with the substrate: Ru[0001]||Al2O3[0001] and Ru[101¯0]||Al2O3[112¯0], and show no resistance change upon air exposure, suggesting negligible resistivity contributions from geometric surface roughness and grain boundary scattering and negligible changes in the surface scattering specularity p upon oxygen exposure. The room temperature ρ vs d data are well described by the semiclassical Fuchs-Sondheimer (FS) model, indicating a bulk electron mean free path λ = 6.7 ± 0.3 nm. However, the measured ρo × λ product at 77 K is 43% lower than at 295 K, suggesting a breakdown of the FS model and/or a thickness-dependent electron-phonon coupling and/or a temperature- or environment-dependent p. Transport simulations employing…

In this paper, we describe and analytically substantiate an alternate explanation for the negative capacitance (NC) effect in ferroelectrics (FE). We claim that the NC effect previously demonstrated in resistance-ferroelectric (R-FE) networks does not necessarily validate the existence of “S” shaped relation between polarization and voltage (according to Landau theory). In fact, the NC effect can be explained without invoking the “S”-shaped behavior of FE. We employ an analytical model for FE (Miller model) in which the steady state polarization strictly increases with the voltage across the FE and show that despite the inherent positive FE capacitance, reduction in FE voltage with the increase in its charge is possible in a R-FE network as well as in a ferroelectric-dielectric (FE-DE) stack. This can be attributed to a large increase in FE capacitance near the coercive voltage coupled with the polarization lag with respect to the electric field. Under certain conditions, these two factors yield transient NC effect. We analytically derive conditions for NC effect in R-FE and FE-DE…

While digital integrated circuits with von Neumann architectures, having exponentially evolved for half a century, are an indispensable building block of today's information society, recently growing demand on executing more complex tasks like the human brain has allowed a revisit to the architecture of information processing. Brain-inspired hardware using artificial neural networks is expected to offer a complementary approach to deal with complex problems. Since the neuron and synapse are key components of brains, most of the mathematical models of artificial neural networks require artificial neurons and synapses. Consequently, much effort has been devoted to creating artificial neurons and synapses using various solid-state systems with ferroelectric materials, phase-change materials, oxide-based memristive materials, and so on. Here, we review an example of studies on an artificial synapse based on spintronics and its application to artificial neural networks. The spintronic synapse, having analog and nonvolatile memory functionality, consists of an antiferromagnet/ferromagnet…

The role of carbon impurities in p-type GaN layers grown by metalorganic vapor phase epitaxy (MOVPE) was investigated. The lightly Mg-doped (∼1017 cm−3) p-type GaN samples with different carbon concentration [C] were prepared by controlling growth temperature and pressure. Temperature-dependent Hall-effect analyses exhibited an increase in donor concentration with increasing [C]. The low-temperature mobility also decreased with increasing [C], as a result of mobility limitation due to ionized impurity scattering. These results show that carbon atoms in MOVPE-grown p-GaN layers act as ionized donors and cause carrier compensation. Deep-level transient spectroscopy (DLTS) using bias pulses detected the existence of Hd traps (EV +0.88 eV) arising from the 0/−1 charge state of carbon on nitrogen sites (CN). The concentrations of Hd traps closely corresponded to [C] values in p-type GaN layers. Employing low-frequency capacitance DLTS to avoid carrier freeze-out at low temperatures, we newly discovered the Ha trap (EV +0.29 eV) whose concentration was directly proportional to the [C]…

The injection-seeded terahertz (THz) parametric generator (is-TPG) is one of the most high-power single-longitudinal mode THz-wave sources. Our system is less influenced by scattering, refraction, and multiple reflections by samples because it is a narrow-linewidth source, and the detection area of the THz parametric detector is large. Thus, it is suitable for nondestructive inspection of practical samples in the real world. In 2003, we reported on the development of a mail inspection system that employed a THz parametric oscillator. However, with a dynamic range of less than four orders of magnitude, this system could only identify reagents through thin envelopes. Recently, we succeeded in developing a high-power, highly sensitive THz-wave spectroscopic imaging system with a dynamic range of 100 dB using the is-TPG and a THz parametric detector. Nondestructive inspection of reagents inside thick envelopes and three-dimensional computed tomography of plastics, which attenuate THz-waves by more than 60 dB, were conducted using this system. More recently, we have focused our efforts…

It is well known that physical phenomena may be of great help in computing some difficult problems efficiently. A typical example is prime factorization that may be solved in polynomial time by exploiting quantum entanglement on a quantum computer. There are, however, other types of (non-quantum) physical properties that one may leverage to compute efficiently a wide range of hard problems. In this perspective, we discuss how to employ one such property, memory (time non-locality), in a novel physics-based approach to computation: Memcomputing. In particular, we focus on digital memcomputing machines (DMMs) that are scalable. DMMs can be realized with non-linear dynamical systems with memory. The latter property allows the realization of a new type of Boolean logic, one that is self-organizing. Self-organizing logic gates are “terminal-agnostic,” namely, they do not distinguish between the input and output terminals. When appropriately assembled to represent a given combinatorial/optimization problem, the corresponding self-organizing circuit converges to the equilibrium points that…

We carried out electrical and impedance studies on solution derived Al:ZnO/ZnO/CdS/Cu2ZnSnS4/Mo/Glass multilayered solar cell structures to understand their impact on photovoltaic performance. The Cu2ZnSnS4 layer is synthesized on a molybdenum (Mo) coated soda lime glass substrate as an absorber and characterized intensively to optimize the absorber physical properties. The optimized Cu2ZnSnS4 is p-type with 5.8 × 1017 cm−3 hole carrier concentration. The depletion width of the junction is around 20.5 nm and the diffusion capacitance is ∼35.5 nF for these devices. We observed relatively large minority carrier life time ∼23 μs for these structures using open voltage decay analysis. The measured Cu2ZnSnS4/MoS2 and Cu2ZnSnS4/CdS interface resistances are 7.6 kΩ and 12.5 kΩ, respectively. The spatial inhomogeneities are considered and the corresponding resistance is ∼11.4 kΩ. The impedance measurements suggest that in conjunction with series resistance ∼350 Ω, the interface and spatial inhomogeneity resistances also give a significant contribution to the photovoltaic performance.

The severe plastic deformation strongly changes the microstructure and properties of titanium-based alloys. The structure and microhardness of four binary and ternary titanium-based alloys (Ti–4 wt. % V, Ti–4 wt. % V–6 wt. % Al, Ti–4 wt. % V–3 wt. % Al, and Ti–5 wt. % V–6 wt. % Al) have been studied after preliminary annealing and following high pressure torsion (HPT). After HPT, the Ti–4 wt. % V alloy contains much less (ωTi) phase than Ti–4 wt. % Fe and Ti–4 wt. % Co alloys. The addition of aluminum to the binary Ti–V alloys completely suppresses the formation of the high-pressure (ωTi)-phase. HPT leads to the partial decomposition of the annealed (αTi) solid solution and “purification” of α-phase similar to that in the Ti–Fe alloys. After HPT of the studied ternary alloys, the (βTi)-phase completely disappears and nanoparticles of Ti2Fe form instead. This fact explains why the addition of aluminum leads to the increase of microhardness of alloys after annealing between 600 °C and 950 °C and after HPT-treatment. The increase of the temperature of the preliminary annealing also…

Conventional sound shielding structures typically prevent fluid transport between the exterior and interior. A design of a two-dimensional acoustic metacage with subwavelength thickness which can shield acoustic waves from all directions while allowing steady fluid flow is presented in this paper. The structure is designed based on acoustic gradient-index metasurfaces composed of open channels and shunted Helmholtz resonators. In-plane sound at an arbitrary angle of incidence is reflected due to the strong parallel momentum on the metacage surface, which leads to low sound transmission through the metacage. The performance of the proposed metacage is verified by numerical simulations and measurements on a three-dimensional printed prototype. The acoustic metacage has potential applications in sound insulation where steady fluid flow is necessary or advantageous.

Elastic constants and their derived properties of various cubic Heusler compounds were calculated using the first-principles density functional theory. To begin with, Cu2MnAl is used as a case study to explain the interpretation of the basic quantities and compare them with experiments. The main part of the work focuses on Co2-based compounds that are Co2MnM with the main group elements M=Al, Ga, In, Si, Ge, Sn, Pb, Sb, Bi, and Co2TM with the main group elements Al or Si, and the 3d transition metals T=Sc, Ti, V, Cr, Mn, and Fe. It is found that many properties of Heusler compounds correlate to the mass or nuclear charge Z of the main group element. Additional representation and compact simplification of the elastic data is useful to investigate and compare their influence on crystal stability and physical properties. Here, Blackman’s and Every’s diagrams are used to compare the elastic properties of the materials, whereas Pugh’s and Poisson’s ratios are used to analyze the relationship between interatomic bonding and physical properties. It is found that Pugh’s criterion on…

The electrical properties of epitaxial β-Ga2O3 doped with Sn (1016–9 × 1018 cm−3) and grown by metalorganic chemical vapor deposition on semi-insulating β-Ga2O3 substrates are reported. Shallow donors attributable to Sn were observed only in a narrow region near the film/substrate interface and with a much lower concentration than the total Sn density. For heavily Sn doped films (Sn concentration, 9 × 1018 cm−3), the electrical properties in the top portion of the layer were determined by deep centers with a level at Ec-0.21 eV not described previously. In more lightly doped layers, the Ec-0.21 eV centers and deeper traps at Ec-0.8 eV were present, with the latter pinning the Fermi level. Low temperature photocapacitance and capacitance voltage measurements of illuminated samples indicated the presence of high densities (1017–1018 cm−3) of deep acceptors with an optical ionization threshold of 2.3 eV. Optical deep level transient spectroscopy (ODLTS) and photoinduced current transient spectroscopy (PICTS) detected electron traps at Ec-0.8 eV and Ec-1.1 eV. For lightly doped layers,…

The microwave dielectric behavior of sandwich-like Ti3C2 MXene nanosheets with efficient microwave absorption was investigated by a combination of experiments and simulations. The obvious frequency dispersion effect and the double-peaked feature of dielectric spectra in Ti3C2 MXene nanosheets could be observed over the frequency range of 2–18 GHz, giving rise to superior microwave attenuation capability. Furthermore, a revised Drude-Lorentz model was proposed to explain the peaked feature of permittivity, and simulated results were demonstrated to agree well with the experimental measurements. It was concluded that the hopping migration between Ti3C2 MXene nanosheets with longer relaxation time than “micro-dipole” relaxation within nanosheets makes a superior contribution to overall absorbing performance.

The aim of this article is to study the effect of oxide functionalisation on the failure morphology of bicrystalline graphene. Molecular dynamics based simulations in conjunction with reactive force field were performed to study the mechanical properties as well as failure morphology of different configurations of bicrystalline graphene oxide. Separate simulations were performed with hydroxyl and epoxide functionalisation, and later on the same simulations were extended to study the graphene oxide as a whole. The authors have predicted that epoxide functionalisation helps in transforming the catastrophic brittle behaviour into ductile. Failure morphologies depict that epoxide groups tend to boost the ductility through altering the fracture path and not affecting the grain boundaries either. Also, the epoxide to ether transformations were found to be the decisive mechanism behind the plastic response shown by epoxide groups. Simulations help in concluding a ductile failure for bicrystalline graphene in conjunction with oxidation of selective atoms in the nanosheet, which further…

The formation of octonary DyErGdHoLuScTbY and senary DyGdHoLaTbY and ErGdHoLaTbY high-entropy alloys (HEAs) with the hexagonal close-packed (HCP) structure was reported in this study. Experiments using scanning electron microscopy and x-ray diffraction confirmed the single HCP solid solution in the as-cast state for these three HEAs if the presence of minor rare-earth oxides due to contamination from processing is ignored. The measured compressive yield stress values for these HEAs at room temperature are 245, 205, and 360 MPa for the ErGdHoLaTbY, DyGdHoLaTbY, and DyErGdHoLuScTbY HEAs, respectively. The corresponding solid solution strengthening contributions for these HEAs were estimated using a simple elastic model, and the resulting contributions were 28 MPa, 27 MPa, and 42 MPa for the three aforementioned HEAs.

Transient negative capacitance effects in epitaxial ferroelectric Pb(Zr0.2Ti0.8)O3 capacitors are investigated with a focus on the dynamical switching behavior governed by domain nucleation and growth. Voltage pulses are applied to a series connection of the ferroelectric capacitor and a resistor to directly measure the ferroelectric negative capacitance during switching. A time-dependent Ginzburg-Landau approach is used to investigate the underlying domain dynamics. The transient negative capacitance is shown to originate from reverse domain nucleation and unrestricted domain growth. However, with the onset of domain coalescence, the capacitance becomes positive again. The persistence of the negative capacitance state is therefore limited by the speed of domain wall motion. By changing the applied electric field, capacitor area or external resistance, this domain wall velocity can be varied predictably over several orders of magnitude. Additionally, detailed insights into the intrinsic material properties of the ferroelectric are obtainable through these measurements. A new method…

We investigate the effect of blend host polymer on solid polymer electrolyte (SPE) films doped with ammonium iodide (NH4I) salt using a variety of experimental techniques. Structural studies on the composite SPEs show that the blending of Poly(ethylene oxide) (PEO)–Poly(vinylidene fluoride) (PVDF) polymers in a suitable ratio enhances the amorphous fraction of the polymer matrix and facilitates fast ion conduction through it. We observe that the addition of a small amount of PVDF in the PEO host polymer enhances the ion – polymer interaction leading to more ion dissociation. As a result, the effective number of mobile charge carriers within the polymer matrix increases. Systematic investigation in these blend SPEs shows that the maximum conductivity (1.01 × 10–3 S/cm) is obtained for PEO – rich (80 wt. % PEO, 20 wt. % PVDF) composites at 35 wt. % NH4I concentration at room temperature. Interestingly, at higher salt concentrations (above 35 wt. %), the conductivity is found to decrease in this system. The reduction of conductivity at higher salt concentrations is the consequence of…

Low-cost shortwave infrared detectors have great potential for emerging civilian night-vision applications. This paper reports the characteristics of Ge0.89Sn0.11 photodiodes monolithically grown on a Si substrate that holds great promise for those applications. At room temperature, the 500 μm diameter active area device demonstrated a longwave cutoff of 2.65 μm and a responsivity of 0.32 A/W at 2 μm, which corresponds to an external quantum efficiency of 20% without any contribution from the Ge buffer layer. The measured peak specific detectivity at 300 K and 77 K is 1.7 × 109 Jones and 4.3 × 109 Jones, respectively. The specific detectivity at 77 K is only one-order-of-magnitude lower than that of the market dominating extended-InGaAs photodiode. The detailed device analysis indicated that the 700-nm thick fully relaxed high-quality GeSn absorbing layer and the modified depletion region lead to the above-mentioned device performance.

This work investigates the reaction dynamics of metastable intermolecular composites through high speed spectrometry, pressure measurements, and high-speed color camera pyrometry. Eight mixtures including Al/CuO and Al/Fe2O3/xWO3 (x being the oxidizer mol. %) were reacted in a constant volume pressure cell as a means of tuning gas release and adiabatic temperature. A direct correlation between gas release, peak pressure, and pressurization rate was observed, but it did not correlate with temperature. When WO3 was varied as part of the stoichiometric oxidizer content, it was found that Al/Fe2O3/70% WO3 achieved the highest pressures and shortest burn time despite a fairly constant temperature between mixtures, suggesting an interplay between the endothermic Fe2O3 decomposition and the higher adiabatic flame temperature sustained by the Al/WO3 reaction in the composite. It is proposed that the lower ignition temperature of Al/WO3 leads to the initiation of the composite and its higher flame temperature enhances the gasification of Fe2O3, thus improving advection and propagation as…

In this paper, a new type of low-frequency broadband elastic metamaterial is proposed, which is constituted of periodic I-shaped pillars arranged on a two-dimensional ground surface. The propagation of elastic waves through 3D samples of the seismic metamaterial is investigated in the harmonic regime by a combination of the finite element method and Bloch theory. In contrast, with two typical pillar structures consisting of circular and rectangle pillars, the proposed I-shaped seismic metamaterial can yield wider bandgaps and significantly attenuate the seismic surface waves at the frequencies ranging from 5.8 Hz to 8.8 Hz and from 9.2 Hz to 13.0 Hz, respectively. The generation mechanism of the bandgaps is further explored by computing the transmission spectra through a finite array and analyzing the displacement fields of the surface at 6.1 Hz and 10.2 Hz in two different bandgaps. The optimal geometric parameters of the I-shaped seismic metamaterial are also provided. Although we focus on the geophysical structure, the proposed metamaterial provides a possible alternative for…

We performed a series of systematic spall experiments on single-crystal copper in an effort to determine and isolate the effects of crystal orientation, peak stress, and unloading strain rate on the tensile spall strength. Strain rates ranging from 0.62 to 2.2 × 106 s−1 and peak shock stresses in the 5–14 GPa range, with one additional experiment near 50 GPa, were explored as part of this work. Gun-driven impactors, called flyer plates, generated flat top shocks followed by spall. This work highlights the effect of crystal anisotropy on the spall strength by showing that the spall strength decreases in the following order: [100], [110], and [111]. Over the range of stresses and strain rates explored, the spall strength of [100] copper depends strongly on both the strain rate and shock stress. Except at the very highest shock stress, the results for the [100] orientation show linear relationships between the spall strength and both the applied compressive stress and the strain rate. In addition, hydrodynamic computer code simulations of the spall experiments were performed to…

The development of an efficient neuromorphic computing system requires the use of nanodevices that intrinsically emulate the biological behavior of neurons and synapses. While numerous artificial synapses have been shown to store weights in a manner analogous to biological synapses, the challenge of developing an artificial neuron is impeded by the necessity to include leaking, integrating, firing, and lateral inhibition features. In particular, previous proposals for artificial neurons have required the use of external circuits to perform lateral inhibition, thereby decreasing the efficiency of the resulting neuromorphic computing system. This work therefore proposes a leaky integrate-and-fire neuron that intrinsically provides lateral inhibition, without requiring any additional circuitry. The proposed neuron is based on the previously proposed domain-wall magnetic tunnel junction devices, which have been proposed as artificial synapses and experimentally demonstrated for non-volatile logic. Single-neuron micromagnetic simulations are provided that demonstrate the ability of this…

This tutorial article provides a concise and pedagogical overview on negatively charged nitrogen-vacancy (NV) centers in diamond. The research on the NV centers has attracted enormous attention for its application to quantum sensing, encompassing the areas of not only physics and applied physics but also chemistry, biology, and life sciences. Nonetheless, its key technical aspects can be understood from the viewpoint of magnetic resonance. We focus on three facets of this ever-expanding research field, to which our viewpoint is especially relevant: microwave engineering, materials science, and magnetometry. In explaining these aspects, we provide a technical basis and up-to-date technologies for research on the NV centers.

The impact of intrinsic amorphous silicon bilayers in amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells is investigated. The microstructure factor R* of the interfacial a-Si:H layer, which is related to the Si-H bond microstructure and determined by infrared absorption spectroscopy, is controlled in a wide range by varying the growth pressure and the power density in plasma-enhanced chemical vapor deposition process. Surface passivation at the a-Si:H/c-Si interface is significantly improved by using an intrinsic a-Si:H bilayer, i.e., a stack of an interfacial layer with a large R* and an additional dense layer, particularly after the deposition of an overlying p-type a-Si:H layer. Consequently, the conversion efficiency of a-Si:H/c-Si heterojunction solar cells is markedly increased. However, it is also revealed that such an interfacial layer causes some negative effects including the increase in the series resistance and the current loss at the front side, depending on the growth condition. This result indicates that the interfacial layer has a…

Locally resonant metamaterials leveraging shape memory alloy (SMA) springs are explored in this work in an effort to develop adaptive metamaterial configurations that can exhibit tunable bandgap properties as well as enhanced damping capabilities. An analytical model for a locally resonant metamaterial beam in transverse vibrations is combined with an SMA model for the resonator springs to investigate and leverage the potential of temperature-induced phase transformations and stress-induced hysteretic behavior of the springs. Two case studies are presented for this new class of smart metamaterials and the resulting finite metastructures. In one case, SMA resonators operate in the linear elastic regime, first at low temperature (martensitic behavior) and then at high temperature (austenitic behavior), demonstrating how the bandgap can be tuned to a different frequency range by altering the SMA elastic modulus with temperature. In the second case, the SMA springs are kept at high temperature at all times to operate in the nonlinear regime, so that the hysteresis associated with the…

A new acoustic metamaterial plate (AMP) is proposed herein. The plate incorporates Helmholtz resonators that are periodically embedded at intervals shorter than acoustic wavelengths. This metamaterial plate exhibits extraordinary sound transmission loss (STL) at the resonance frequency of the Helmholtz resonators compared to a conventional flat plate. The STL of the AMP can be theoretically analyzed using the effective mass density and flexural rigidity. At the resonant frequency, the dynamic density of the AMP becomes much larger than that of a conventional solid flat plate with the same mass. When the Helmholtz resonant frequency is tuned to the coincidence frequency of the AMP, the dip in transmission loss owing to the coincidence effect is not observed. The frequency band, wherein high STL occurs, is narrow; however, the frequency band can be widened by embedding multiple resonators with slightly different resonant frequencies. Numerical experiments are also performed to demonstrate the acoustic performance of the proposed system. In the simulation, Helmholtz resonators with the…

We investigated the relationship between the dislocation velocity and the injected carrier concentration on the expansion of single Shockley-type stacking faults by monitoring the electroluminescence from 4H-SiC PiN diodes with various anode Al concentrations. The injected carrier concentration was calculated using a device simulation that took into account the measured accumulated charge in the drift layer during diode turn-off. The dislocation velocity was strongly dependent on the injected hole concentration, which represents the excess carrier concentration. The activation energy of the dislocation velocity was quite small (below 0.001 eV between 310 and 386 K) over a fixed range of hole concentrations. The average threshold hole concentration required for the expansion of bar-shaped single Shockley-type stacking faults at the interface between the buffer layer and the substrate was determined to be 1.6–2.5 × 1016 cm−3 for diodes with a p-type epitaxial anode with various Al concentrations.

This paper determines all the 13 elastic constants Cij of monoclinic β-Ga2O3, which has never been achieved since the discovery of this crystal about 100 years ago. We used resonant ultrasound spectroscopy with laser-Doppler interferometry, where the resonant-mode identification was unambiguously made by comparing measured and calculated displacement distributions on the vibrating specimen surface. Using more than 110 resonance frequencies, we inversely determined the Cij: C11 = 242.8 ± 2.9, C22 = 343.8 ± 3.8, C33 = 347.4 ± 2.5, C44 = 47.8 ± 0.2, C55 = 88.6 ± 0.5, C66 = 104.0 ± 0.5, C12 = 128 ± 0.1, C13 = 160 ± 1.5, C23 = 70.9 ± 2.1, C15 = –1.62 ± 0.05, C25 = 0.36 ± 0.01, C35 = 0.97 ± 0.03, and C46 = 5.59 ± 0.69 GPa. We also performed a density-functional-theory calculation and found that the local density approximation yields both the lattice parameters and Cij closer to the measurements than the generalized gradient approximation. Strong elastic-stiffness anisotropy is found in the diagonal elastic constants: C11 is smaller than C22 and C33 by ∼30%, and the difference between C44…

In recent years, the fundamental physics of spin-lattice (e.g., magnon-phonon) interaction has attracted significant experimental and theoretical interests given its potential paradigm-shifting impacts in areas like spin-thermoelectrics, spin-caloritronics, and spintronics. Modelling studies of the transport of magnons and phonons in magnetic crystals are very rare. In this paper, we use spin-lattice dynamics (SLD) simulations to model ferromagnetic crystalline iron, where the spin and lattice systems are coupled through the atomic position-dependent exchange function, and thus the interaction between magnons and phonons is naturally considered. We then present a method combining SLD simulations with spectral energy analysis to calculate the magnon and phonon harmonic (e.g., dispersion, specific heat, and group velocity) and anharmonic (e.g., scattering rate) properties, based on which their thermal conductivity values are calculated. This work represents an example of using SLD simulations to understand the transport properties involving coupled magnon and phonon dynamics.

By treating a set of equations governing transient heat and mass transfer simultaneously, here we develop the transformation theory for thermal convection with unsteady creeping flow in porous media, whose steady counterpart has been previously studied. We find that the transformation theory can still be valid when the temperature, density, and velocity of fluids vary with time. As applications, we design thermal cloaks, concentrators, and rotators at transient states examined by finite-element simulations, which can be used to control the magnitude or direction of heat flux in convection. Also, we discuss both the effects of natural or mixed convection and the differences between steady and unsteady states. This work develops a theory for dynamically controlling the flow of heat associated with thermal convection.

Since ferroelectric HfxZr1−xO2 is different from other ferroelectric doped-HfO2 materials in terms of the dopant sensitivity of the formation of a ferroelectric phase, the mechanism of formation of a ferroelectric phase is investigated in comparison with that in ZrO2. It is found that ferroelectric HfxZr1−xO2 follows a unique phase-transition pathway from tetragonal to monoclinic phases similar to that in other doped HfO2 materials. The phase diagram of the HfO2–ZrO2 solid-solution system is reconsidered thermodynamically by taking into account the effects of both film thickness and nonequilibrium fabrication conditions on structural phase stability. The formation mechanism of a ferroelectric phase in the HfxZr1−xO2 system over a wide range of Hf/Zr concentration ratios is qualitatively (but clearly) understandable from these thermodynamic considerations. Finally, it is demonstrated that both ferroelectric and antiferroelectric ZrO2 films can be formed on the same substrate by controlling the nucleation of monoclinic and tetragonal phases, respectively.

In order to tailor the multiferroic as well as dielectric properties, we have synthesized BiFeO3 (BFO) and Ni-doped BFO nanoparticles of composition BiFe1-xNixO3 (0 ≤ x ≤ 0.07) via the sol-gel method. The influence of Ni-doping at the Fe site of BiFeO3 on the structural, dielectric, and multiferroic properties was investigated via various characterization techniques such as Raman spectroscopy, vibrating sample magnetometer, polarization-electric field (P-E) loop tracer, and LCR meter. The Raman spectra reveal the rhombohedral distorted perovskite structure of the samples in the R3c space group. The frequency dependent dielectric measurements at room temperature show that the dielectric constant for the pristine sample is an order of magnitude lower than the 1% Ni-doped BFO sample. It is also evident that the ac conductivity increases with the increase in frequency as well as Ni content. Further, the universal dielectric response (UDR) phenomenon is responsible for the dielectric behavior of all the samples in the whole frequency range except for the BiFeO3 and BiFe0.97Ni0.03O3…

The effect of the surface roughness on the electrical resistivity of metallic thin films is described by electron reflection at discrete step edges. A Landauer formalism for incoherent scattering leads to a parameter-free expression for the resistivity contribution from surface mound-valley undulations that is additive to the resistivity associated with bulk and surface scattering. In the classical limit where the electron reflection probability matches the ratio of the step height h divided by the film thickness d, the additional resistivity Δρ = 3/2/(g0d) × ω/ξ, where g0 is the specific ballistic conductance and ω/ξ is the ratio of the root-mean-square surface roughness divided by the lateral correlation length of the surface morphology. First-principles non-equilibrium Green's function density functional theory transport simulations on 1-nm-thick Cu(001) layers validate the model, confirming that the electron reflection probability is equal to h/d and that the incoherent formalism matches the coherent scattering simulations for surface step separations ≥2 nm. Experimental…

We investigate theoretically an acoustic metasurface with a high absorption coefficient at two frequencies and design it from subwavelength structures. We propose the use of a two-dimensional periodic array of four Helmholtz resonators in two types to obtain a metasurface with nearly perfect sound absorption at given target frequencies via interactions between waves emanating from different resonators. By considering how fluid viscosity affects acoustic energy dissipation in the narrow necks of the Helmholtz resonators, we obtain effective complex-valued material properties that depend on frequency and on the geometrical parameters of the resonators. We furthermore derive the effective acoustic impedance of the metasurface from the effective material properties and calculate the absorption spectra from the theoretical model, which we compare with the spectra obtained from a finite-element simulation. As a practical application of the theoretical model, we derive empirical formulas for the geometrical parameters of a metasurface which would yield perfect absorption at a given…

We investigated traps in lightly Mg-doped (2 × 1017 cm−3) p-GaN fabricated by metalorganic vapor phase epitaxy (MOVPE) on a freestanding GaN substrate and the subsequent post-growth annealing, using deep level transient spectroscopy. We identified four hole traps with energy levels of EV + 0.46, 0.88, 1.0, and 1.3 eV and one electron trap at EC − 0.57 eV in a p-type GaN layer uniformly doped with magnesium (Mg). The Arrhenius plot of hole traps with the highest concentration (∼3 × 1016 cm−3) located at EV + 0.88 eV corresponded to those of hole traps ascribed to carbon on nitrogen sites in n-type GaN samples grown by MOVPE. In fact, the range of the hole trap concentrations at EV + 0.88 eV was close to the carbon concentration detected by secondary ion mass spectroscopy. Moreover, the electron trap at EC − 0.57 eV was also identical to the dominant electron traps commonly observed in n-type GaN. Together, these results suggest that the trap states in the lightly Mg-doped GaN grown by MOVPE show a strong similarity to those in n-type GaN, which can be explained by the Fermi level…

Defects such as cracks, pores, and particle-matrix interface debonding affect the sensitivity of energetic materials by reducing the time-to-ignition and the threshold pressure to initiate an explosion. Frictional sliding of preexisting cracks is considered to be one of the most important causes of localized heating. Therefore, understanding the dynamic fracture of crystalline energetic materials is of extreme importance to assess the reliability and safety of polymer-bonded explosives. Phase field damage model simulations, based on the regularization of the crack surface as a diffuse delta function, are used to describe crack propagation in cyclotetramethylene-tetranitramine crystals embedded in a Sylgard matrix. A thermal transport model that includes heat generation by friction at crack interfaces is coupled to the solution of crack propagation. 2D and 3D dynamic compression simulations are performed with different boundary velocities and initial distributions of cracks and interface defects to understand their effect on crack propagation and heat generation. It is found that, at…

We consider adiabatic superconducting cells operating as an artificial neuron and synapse of a multilayer perceptron (MLP). Their compact circuits contain just one and two Josephson junctions, respectively. While the signal is represented as magnetic flux, the proposed cells are inherently nonlinear and close-to-linear magnetic flux transformers. The neuron is capable of providing the one-shot calculation of sigmoid and hyperbolic tangent activation functions most commonly used in MLP. The synapse features both positive and negative signal transfer coefficients in the range ∼(−0.5,0.5). We briefly discuss implementation issues and further steps toward the multilayer adiabatic superconducting artificial neural network, which promises to be a compact and the most energy-efficient implementation of MLP.

Spin-waves are excellent data carriers with a perspective use in neuronal networks: Their lifetime gives the spin-wave system an intrinsic memory, they feature strong nonlinearity, and they can be guided and steered through extended magnonic networks. In this work, we present a magnon adder that integrates over incoming spin-wave pulses in an analog fashion. Such an adder is a linear prequel to a magnonic neuron, which would integrate over the incoming pulses until a certain nonlinearity is reached. In this work, the adder is realized by a resonator in combination with a parametric amplifier which is just compensating the resonator losses.

Transition metal oxides (TMOs) are of increasing importance for many applications reaching from thin-film transistors and non-volatile memory to novel contact layers in photovoltaics. Due to their tunable electrical properties and high transparency, TMOs are also promising candidates as contact layers in silicon heterojunction solar cells already leading to cell efficiencies of about 22%. However, the current extraction of charge carriers via these thin contact layers is still not fully understood. To assist the engineering of novel silicon heterojunctions, numerical device simulations are used to improve knowledge regarding relevant heterojunction and thin film properties. The efficient current extraction from a silicon absorber is investigated with Sentaurus TCAD for a TMO-based hole contact. It is shown that for an ideal hole extraction from the induced crystalline silicon pn-junction via the amorphous silicon buffer and the TMO into the external metal electrode, two requirements have to be fulfilled: (A) A sufficiently high TMO work function is needed to ensure a high hole…

A reactive flow model for the tri-amino-tri-nitro-benzene (TATB) based plastic bonded explosive PBX 9502 (95% TATB, 5% polymeric binder Kel-F 800) is presented. This newly devised model is based primarily on the shock temperature of the material, along with local pressure, and accurately models a broader range of detonation and initiation scenarios. Specifically, sensitivity changes to the initial explosive temperature are accounted for naturally and with a single set of parameters. The equation of state forms for the reactants and products, as well as the thermodynamic closure of pressure and temperature equilibration, are carried over from the Wescott-Stewart-Davis (WSD) model [Wescott et al., J. Appl. Phys. 98, 053514 (2005) and “Modeling detonation diffraction and dead zones in PBX-9502,” in Proceedings of the Thirteenth International Detonation Symposium (2006)]. This newly devised model, with Arrhenius state dependence on the shock temperature, based on the WSD equation of states, is denoted by AWSD. Modifying an existing implementation of the WSD model to the AWSD model in a…

We theoretically study the structural stability of RFe12 with the ThMn12 structure (R: rare-earth element, La, Pr, Nd, Sm, Gd, Dy, Ho, Er, Tm, Lu, Y, or Sc, or group-IV element, Zr or Hf) based on density functional theory. The formation energy has a strong correlation with the atomic radius of R. The formation energy relative to simple substances decreases as the atomic radius decreases, except for R= Sc and Hf, while that relative to R2Fe17 and bcc Fe has a minimum for R= Dy. The present results are consistent with recent experimental reports in which the partial substitution of Zr at R sites stabilizes RFe12-type compounds with R= Nd or Sm. Our results also suggest that the partial substitution of Y, Dy, Ho, Er, or Tm for Nd or Sm is a possible way to enhance the stability of the ThMn12 structure. Under hydrostatic pressure, the formation enthalpy decreases up to ≈6 GPa and then starts to increase at higher pressures.

We propose an analysis of the emission properties of anatase and rutile titanium dioxide (TiO2) that emphasizes the role of the strong electron-phonon interaction. We performed measurements of photoluminescence (PL) spectra of bulk monocrystals under continuous wave-laser excitation and of their temperature dependence. We show that in both anatase and rutile, weakly bound self-trapped excitons are actually made out from carrier polarons and give rise to a broad emission band in the visible spectral range. The thermal activation of carrier motion allows their hopping to distant sites that leads to the observed quenching of luminescence. In the specific case of rutile TiO2, the PL spectral shape and its intensity-quenching scenario reveal the presence of dark trap states. Moreover, an additional narrow line structure shows up at low temperatures. The latter is due to localized impurity states that can be attributed to oxygen vacancies and can be fitted with a large Huang-Rhys parameter S = 2.5 within a Franck-Condon model. Both phases show thus a very strong interaction between the…

In this work, we analyze theoretically and observe experimentally the frequency mixing response induced by the collinear interaction of two primary Lamb waves with different frequencies in an isotropic and homogenous plate. The internal resonance conditions for generating the combined harmonics are analyzed by using a perturbation approach and a normal-mode-expansion technique for waveguide excitation. The second- and third-order combined harmonics generated by the collinear wave mixing of two primary Lamb waves propagating in a specimen are analyzed, and the existence of the combined harmonics at specific frequencies is predicted. An experimental procedure is proposed to measure the combined harmonics induced by the collinear cross-interaction of the two specific primary Lamb waves at given frequencies. The theoretical prediction of the appearance of the combined harmonics at the specific mixing frequencies is consistent with experiment. The theoretical analysis and experimental observation provide a clear physical insight into the frequency-mixing response induced by the collinear…

Membrane-type metamaterials have shown a fantastic capacity for manipulating acoustic waves in the low frequency range. They have the advantages of simple geometry, light weight, and active tunability. In general, these membrane-type metamaterials contain a rigid frame support, leading to a fixed configuration. However, in some instances, flexible and reconfigurable devices may be desirable. A soft membrane-type acoustic metamaterial that is highly flexible and controllable is designed here. Different from the previously designed membrane-type metamaterials, the stiff supporting frame is removed and the stiff mass at the center of each unit cell is replaced by the soft mass, realized by bonding fine metallic particles in the central region. In contrast to the previous studies, the propagation of elastic transverse waves in such a soft metamaterial is investigated by employing the plane wave expansion method. Both the Bragg scattering bandgaps and locally resonant bandgaps are found to coexist in the soft metamaterial. The influences of structural parameters and finite biaxial…

An extensive analysis of the degradation characteristics of AlGaN-based ultraviolet light-emitting diodes emitting around 265 nm is presented. The optical power of LEDs stressed at a constant dc current of 100 mA (current density = 67 A/cm2 and heatsink temperature = 20 °C) decreased to about 58% of its initial value after 250 h of operation. The origin of this degradation effect has been studied using capacitance-voltage and photocurrent spectroscopy measurements conducted before and after aging. The overall device capacitance decreased, which indicates a reduction of the net charges within the space-charge region of the pn-junction during operation. In parallel, the photocurrent at excitation energies between 3.8 eV and 4.5 eV and the photocurrent induced by band-to-band absorption in the quantum barriers at 5.25 eV increased during operation. The latter effect can be explained by a reduction of the donor concentration in the active region of the device. This effect could be attributed to the compensation of donors by the activation or diffusion of acceptors, such as magnesium…

A first-principles informed approach to the calculation of Lorenz numbers for complex thermoelectric materials is presented and discussed. Example calculations illustrate the importance of using accurate band structures and energy-dependent scattering times. Results obtained by assuming that the scattering rate follows the density-of-states show that in the non-degenerate limit, Lorenz numbers below the commonly assumed lower limit of 2(kB/q)2 can occur. The physical cause of low Lorenz numbers is explained by the shape of the transport distribution. The numerical and physical issues that need to be addressed in order to produce accurate calculations of the Lorenz number are identified. The results of this study provide a general method that should contribute to the interpretation of measurements of total thermal conductivity and to the search for materials with low Lorenz numbers, which may provide improved thermoelectric figures of merit, zT.

Undoped 0.5-μm-thick BaSi2 epitaxial films were grown on Si(111) substrates with various ratios of the Ba deposition rate to the Si deposition rate (RBa/RSi) ranging from 1.0 to 5.1, and their electrical and optical properties were characterized. The photoresponse spectra drastically changed as a function of RBa/RSi, and the quantum efficiency reached a maximum at RBa/RSi = 2.2. Hall measurements and capacitance versus voltage measurements revealed that the electron concentration drastically decreased as RBa/RSi approached 2.2, and the BaSi2 films with RBa/RSi= 2.0, 2.2, and 2.6 exhibited p-type conductivity. The lowest hole concentration of approximately 1 × 1015 cm−3 was obtained for the BaSi2 grown with RBa/RSi = 2.2, which is the lowest value ever reported. First-principles calculations suggest that Si vacancies give rise to localized states within the bandgap of BaSi2 and therefore degrade the minority-carrier properties.

The spatial distribution of electron-hole pair generation in β-Ga2O3 as a function of scanning electron microscope (SEM) beam energy has been calculated by a Monte Carlo method. This spatial distribution is then used to obtain the diffusion length of charge carriers in high-quality epitaxial Ga2O3 films from the dependence of the electron beam induced current (EBIC) collection efficiency on the accelerating voltage of a SEM. The experimental results show, contrary to earlier theory, that holes are mobile in β-Ga2O3 and to a large extent determine the diffusion length of charge carriers. Diffusion lengths in the range 350–400 nm are determined for the as-grown Ga2O3, while processes like exposing the samples to proton irradiation essentially halve this value, showing the role of point defects in controlling minority carrier transport. The pitfalls related to using other popular EBIC-based methods assuming a point-like excitation function are demonstrated. Since the point defect type and the concentration in currently available Ga2O3 are dependent on the growth method and the doping…

Understanding thermal transport in nanofiber networks is essential for their applications in thermal management, which are used extensively as mechanically sturdy thermal insulation or high thermal conductivity materials. In this study, using the statistical theory and Fourier's law of heat conduction while accounting for both the inter-fiber contact thermal resistance and the intrinsic thermal resistance of nanofibers, an analytical model is developed to predict the thermal conductivity of nanofiber networks as a function of their geometric and thermal properties. A scaling relation between the thermal conductivity and the geometric properties including volume fraction and nanofiber length of the network is revealed. This model agrees well with both numerical simulations and experimental measurements found in the literature. This model may prove useful in analyzing the experimental results and designing nanofiber networks for both high and low thermal conductivity applications.

The real and imaginary parts of the dielectric function of VO2 thin films, deposited on r-plane sapphire via pulsed laser deposition, are measured by means of visible-infrared ellipsometry for wavelengths ranging from 0.4 to 15 μm and temperatures within its phase transition. For both the insulator-to-metal (heating) and metal-to-insulator (cooling) transitions, it is shown that the two ellipsometric signals exhibit three temperature-driven behaviors, which are well described by appropriate combinations of the Tauc-Lorentz, Gaussian, and Drude oscillator models. By fitting Bruggeman's effective medium model for the dielectric function to the corresponding measured experimental values, using the volumetric fraction of the VO2 metallic domains as a fitting parameter for different temperatures within the VO2 phase transition, we have found that this model is suitable for describing the dielectric function in visible and near-infrared wavelengths (∼0.4 to ∼3.0 μm), but it generally fails for longer infrared ones. Furthermore, the hysteresis loop of the VO2 emissivity averaged over a…

This work presents a multiscale modeling framework for predictive simulations of shock-to-detonation transition (SDT) in pressed energetic (HMX) materials. The macro-scale computations of SDT are performed using an ignition and growth (IG) model. However, unlike in the traditional semi-empirical ignition-and-growth model, which relies on empirical fits, in this work meso-scale void collapse simulations are used to supply the ignition and growth rates. This results in a macro-scale model which is sensitive to the meso-structure of the energetic material. Energy localization at the meso-scale due to hotspot ignition and growth is reflected in the shock response of the energetic material via surrogate models for ignition and growth rates. Ensembles of meso-scale reactive void collapse simulations are used to train the surrogate model using a Bayesian Kriging approach. This meso-informed Ignition and Growth (MES-IG) model is applied to perform SDT simulations of pressed HMXs with different porosity and void diameters. The computations are successfully validated against experimental…

Nano-crystalline LaFe1−xZnxO3 (0 ≤ x ≤ 0.3) samples synthesized through the sol-gel auto-combustion technique have been studied. X-ray diffraction patterns reveal that all the samples have an orthorhombic crystal structure and exist in a single phase. Bond lengths and bond angles determined using Rietveld refinement change with the concentration, leading to the modification of the parent structure. The change in the structure is reflected in the thermal measurements too. The morphological studies reveal a wide-range of particle size distribution, consisting of nanometer-sized particles to macro-agglomerations. FTIR spectra show two strong active vibrational bands present at 564 cm−1 and 410 cm−1, which are related to the functional groups present in the material. The optical bandgap has been determined and is found to decrease with the increase in the concentration of Zn. Raman spectroscopy rules out the symmetry breaking or any other structural phase transition in the system. The Raman spectra reveal the strain present in the samples on increasing the concentration of Zn doping.…

The conformality of thin films grown by atomic layer deposition (ALD) is studied using all-silicon test structures with long narrow lateral channels. A diffusion model, developed in this work, is used for studying the propagation of ALD growth in narrow channels. The diffusion model takes into account the gas transportation at low pressures, the dynamic Langmuir adsorption model for the film growth and the effect of channel narrowing due to film growth. The film growth is calculated by solving the diffusion equation with surface reactions. An efficient analytic approximate solution of the diffusion equation is developed for fitting the model to the measured thickness profile. The fitting gives the equilibrium constant of adsorption and the sticking coefficient. This model and Gordon's plug flow model are compared. The simulations predict the experimental measurement results quite well for Al2O3 and TiO2 ALD processes.

Hafnium oxide is widely used for resistive switching devices, and recently it has been discovered that ferroelectricity can be established in (un-)doped hafnium oxide as well. Previous studies showed that both switching mechanisms are influenced by oxygen vacancies. For resistive switching, typically amorphous oxide layers with an asymmetric electrode configuration are used to create a gradient of oxygen vacancies. On the other hand, ferroelectric switching is performed by having symmetric electrodes and requires crystalline structures. The coexistence of both effects has recently been demonstrated. In this work, a detailed analysis of the reversible interplay of both switching mechanisms within a single capacitor cell is investigated. First, ferroelectric switching cycles were applied in order to drive the sample into the fatigued stage characterized by increased concentration of oxygen vacancies in the oxide layer. Afterwards, a forming step that is typical for the resistive switching devices was utilized to achieve a soft breakdown. In the next step, twofold alternation between…

We theoretically investigate a possibility of improving the thermoelectric performance of monolayer InSe through convergence of multivalley energy bands, in which some distinct valleys become almost degenerate. The convergence of energy bands is achieved by applying mechanical strain. We find that the thermoelectric power factor of monolayer InSe can be significantly enhanced by nearly a factor of 3 through the band convergence in both valence (p-type) and conduction (n-type) bands under a biaxial compressive stress of about 1.16 GPa. However, the maximum enhancement of the figure of merit ZT in the p-type and n-type InSe differs each other depending on how the valleys converge in each case. The optimal scenario is that the heavy valleys approach the light valleys in the band convergence, which leads to an increase in the power factor and, at the same time, a decrease in the thermal conductivity of an electron. This optimal condition can be obtained in the strained n-type InSe that gives the largest enhancement of ZT as high as 230% ZT of unstrained InSe. In contrast, the…

There is potential in applying conjugated polymers in novel organic optoelectronic devices, where a comprehensive understanding of the fundamental processes and energetics involved during transport and recombination is still lacking, limiting further device optimization. The electronic transport modeling and its optimization need the energy distribution of transport and defect states, expressed by the energy distribution of the Density of States (DOS) function, as input/comparative parameters. We present the Energy Resolved-Electrochemical Impedance Spectroscopy (ER-EIS) method for the study of transport and defect electronic states in organic materials. The method allows mapping over unprecedentedly wide energy and DOS ranges. The ER-EIS spectroscopic method is based on the small signal interaction between the surface of the organic film and the liquid electrolyte containing reduction-oxidation (redox) species, which is similar to the extraction of an electron by an acceptor and capture of an electron by a donor at a semiconductor surface. The desired DOS of electronic transport…

The coefficients of thermal expansion (CTE) of the ternary borides, MoAlB, Cr2AlB2, Mn2AlB2, and Fe2AlB2, and the binary boride, CrB, were calculated from in situ high-temperature powder X-ray diffraction patterns. The order—from low to high—of the average linear thermal expansions was MoAlB (7.3 × 10−6 K−1), CrB (8.6 × 10−6 K−1), Fe2AlB2 (10.3 × 10−6 K−1), Cr2AlB2 (10.5 × 10−6 K−1), and Mn2AlB2 (14.0 × 10−6 K−1). Despite their structural and chemical similarities, the CTE anisotropies of these phases were different and could be grouped into two groups. In the first group, MoAlB and Fe2AlB2, the highest CTE values were along the stacking direction and the lowest were parallel to the B–B chains. In the second group, Cr2AlB2, Mn2AlB2, and CrB, the lowest CTE was along the stacking direction and the highest was normal to the chains. The thermal expansions parallel to the B–B chains were significantly lower (32% to 103%) than those perpendicular to the chains in all the ternaries except for MoAlB. In general, the relative CTEs parallel to the B–B chains in both the M2AlB2 ternaries and…

Phase change materials (PCMs) provide a high energy density for thermal storage systems but often suffer from limited power densities due to the low PCM thermal conductivity. Much like their electrochemical analogs, an ideal thermal energy storage medium combines the energy density of a thermal battery with the power density of a thermal capacitor. Here, we define the design rules and identify the performance limits for rationally-designed composites that combine an energy dense PCM with a thermally conductive material. Beginning with the Stefan-Neumann model, we establish the material design space using a Ragone framework and identify regimes where hybrid conductive-capacitive composites have thermal power densities exceeding that of copper and other high conductivity materials. We invoke the mathematical bounds on isotropic conductivity to optimize and define the theoretical limits for transient cooling using PCM composites. We then demonstrate the impact of power density on thermal transients using copper inverse opals infiltrated with paraffin wax to suppress the temperature…

The root cause of light- and elevated temperature-induced degradation (LeTID) in multicrystalline silicon p-type passivated emitter and rear cell (PERC) devices is still unknown. Microwave-induced remote hydrogen plasma (MIRHP) is employed to vary the concentration of bulk hydrogen and to separate the effects of hydrogen and firing temperature in LeTID-affected wafers. We find that hydrogen is required for degradation to occur, and that samples fired prior to the introduction of hydrogen do not degrade. Importantly, samples with hydrogen that have not been fired do degrade, implying that the firing time-temperature profile does not cause LeTID. Together, these results suggest that the LeTID defect reaction consists of at least two reactants: hydrogen and one or more defects that can be separately modified by high-temperature firing. We assess the leading hypotheses for LeTID in the context of our new understanding of the necessary reactants.

Since the resurgence of multiferroics research, significant advancement has been made in the theoretical and experimental investigation of the electric field control of magnetization, magnetic anisotropy, magnetic phase, magnetic domains, and Curie temperature in multiferroic heterostructures. As a result of these advances, multiferroic heterostructures are on a trajectory to impact spintronics applications through the significantly reduced energy consumption per unit area for magnetization switching (1–500 μJ cm−2) when compared to that of current-driven magnetization switching (0.2–10 mJ cm−2). Considering this potential impact, it becomes necessary to understand magnetoelectric switching dynamics and characteristic switching times. The body of experimental work investigating magnetoelectric switching dynamics is rather limited, with the majority of room temperature converse magnetoelectric switching measurements reported having employed relatively long voltage pulses. Recently, however, the field has started to consider the kinetics of the switching path in multiferroic (and…

Neutron detectors fabricated from 10B enriched hexagonal boron nitride (h-10BN or h-BN) epilayers have demonstrated the highest thermal neutron detection efficiency among solid-state neutron detectors to date at about 53%. In this work, photoconductive-like vertical detectors with a detection area of 1 × 1 mm2 were fabricated from 50 μm thick free-standing h-BN epilayers using Ni/Au and Ti/Al bilayers as ohmic contacts. Leakage currents, mobility-lifetime (μτ) products under UV photoexcitation, and neutron detection efficiencies have been measured for a total of 16 different device configurations. The results have unambiguously identified that detectors incorporating the Ni/Au bilayer on both surfaces as ohmic contacts and using the negatively biased top surface for neutron irradiation are the most desired device configurations. It was noted that high growth temperatures of h-10BN epilayers on sapphire substrates tend to yield a higher concentration of oxygen impurities near the bottom surface, leading to a better device performance by the chosen top surface for irradiation than by…

Using the transfer matrix method, we study the transmittance of 1D photonic crystals made of alternated layers of a semiconductor (GaAs) and a high-Tc superconductor (HgBa2Ca2Cu3O8+δ) under the effects of temperature, applied hydrostatic pressure, and thickness of the layers. The frequency-dependent dispersion formula according to the two-fluid model was adopted to describe the optical response of the superconducting system. We found that increasing the superconductor (semiconductor) layer thickness results in a shift to higher (lower) values of the transmittance cutoff frequency. Additionally, this cutoff frequency is shifted to lower values with the temperature increase. Furthermore, we found that the width of the photonic bandgaps varies with the applied pressure. The most notorious variation is presented near the 17 THz region, where a new gap appears with the increase in pressure. We hope this work may be taken into consideration for the development of new perspectives in the design of new optical devices.

Contactless ultrasonic acoustic energy transfer (UAET) is a new technology that eliminates risks or impracticalities associated with wired electrical connections or batteries that need to be replaced on a regular basis. This technology, which is based on the reception of acoustic waves at ultrasonic frequencies by piezoelectric receivers, can be used to wirelessly charge low-power electronics. The execution and efficiency of this technology can be significantly enhanced through patterning and focusing of the transmitted acoustic energy in space to simultaneously power distributed sensors or devices. This work investigates the use of an acoustic hologram to create a multifocal pressure pattern in a plane where target receivers are located at specific focal points or regions. First, a phase-shifting hologram is designed using an iterative angular spectrum approach. Then, a multi-physics acoustic-electro-elastic model is presented for an axially vibrating cylindrical transmitter used to power multiple piezoelectric receivers, in conjunction with the hologram. Experiments are also…

We demonstrated a wide range of magnesium (Mg) doping control (1016–1020 cm−3) in a GaN layer grown by metalorganic vapor phase epitaxy on a freestanding GaN substrate and investigated the defect states at low and high Mg concentrations ([Mg]). Hydrogen concentrations ([H]) in as-grown GaN samples showed a one-to-one relationship with [Mg] over the wide Mg doping range of 6 × 1016–3 × 1019 cm−3 due to the formation of Mg-H complexes but exhibited gaps between [Mg] and [H] at the low and the high ends of the doping range. At low [Mg], we found that [H] was in good agreement with the sum of [Mg] and carbon (C) concentrations, indicating the formation of C-H complexes. The acceptor concentration (Na) was significantly decreased for heavily Mg-doped samples with lower [H], while Na values close to [Mg] were obtained for samples having [H] close to [Mg]. These suggest that an Mg atom forming an Mg–H bond in the as-grown samples plays as an acceptor after annealing while an Mg atom not forming an Mg-H complex has other states. In the heavily Mg-doped layers, transmission electron…

The basic building blocks of every neural network are neurons and their inter-cellular connections, called synapses. In nature, synapses play a crucial role in learning and memory, since they are plastic, which means that they change their state depending on the neural activity of the respectively coupled neurons. In neuromorphic systems, the functionality of neurons and synapses is emulated in hardware systems by employing very-large-scale integration technology. In this context, it seems rather natural to use non-volatile memory technology to mimic synaptic functionality. In particular, memristive devices are promising candidates for neuromorphic computing, since they allow one to emulate synaptic functionalities in a detailed way with a significantly reduced power usage and a high packing density. This tutorial aims to provide insight on current investigations in the field to address the following fundamental questions: How can functionalities of synapses be emulated with memristive devices? What are the basic requirements to realize artificial inorganic neurons and synapses?…

Optical communication achieves high fanout and short delay advantageous for information integration in neural systems. Superconducting detectors enable signaling with single photons for maximal energy efficiency. We present designs of superconducting optoelectronic neurons based on superconducting single-photon detectors, Josephson junctions, semiconductor light sources, and multi-planar dielectric waveguides. These circuits achieve complex synaptic and neuronal functions with high energy efficiency, leveraging the strengths of light for communication and superconducting electronics for computation. The neurons send few-photon signals to synaptic connections. These signals communicate neuronal firing events as well as update synaptic weights. Spike-timing-dependent plasticity is implemented with a single photon triggering each step of the process. Microscale light-emitting diodes and waveguide networks enable connectivity from a neuron to thousands of synaptic connections, and the use of light for communication enables synchronization of neurons across an area limited only by the…

Metal magnetic memory (MMM) testing (also known as micro-magnetic testing) is a new non-destructive electromagnetic testing method that can diagnose ferromagnetic materials at an early stage by measuring the MMM signal directly on the material surface. Previous experiments have shown that many factors affect MMM signals, in particular, the temperature, the elastoplastic state, and the complex environmental magnetic field. However, the fact that there have been only a few studies of either how these factors affect the signals or the physical coupling mechanisms among them seriously limits the industrial applications of MMM testing. In this paper, a nonlinear constitutive relation for a ferromagnetic material considering the influences of temperature and elastoplastic state is established under a weak magnetic field and is used to establish a nonlinear thermo-magneto-elastoplastic coupling model of MMM testing. Comparing with experimental data verifies that the proposed theoretical model can accurately describe the thermo-magneto-elastoplastic coupling influence on MMM signals. The…