The 50 most-cited articles in Journal of the Physical Society of Japan, 2020

Quite recently, a pronounced drop of $^{17}$O NMR Knight shift in the superconducting (SC) state of an unstrained Sr$_2$RuO$_4$ was reported by Pustogow and Luo ${\it et al.}$ They revealed such behavior from the free-induction decay (FID) signal after a weak RF pulse. We examined this behavior with our single-crystalline Sr$_2$RuO$_4$, and reproduced their result: the peaks of the $^{17}$O-NMR spectra shift in the SC state as long as the RF-pulse power is smaller than a threshold. Furthermore, we measured the temperature variation of the Knight shift by a standard spin-echo method with small-power RF pulses, and found that the spin susceptibility decreases in the SC state. We conclude that our previous results of the invariance of the Knight shift in the SC state were due to instantaneous destruction of superconductivity by the RF pulses. The heat-up effect was characterized by the temperature variation of the Knight shift under various measurement conditions.

High-temperature superconductivity and a wide variety of exotic superconducting states discovered in FeSe-based materials have been at the frontier of research on condensed matter physics over the past decade. Unique properties originating from the multiband electronic structure, strongly orbital-dependent phenomena, extremely small Fermi energy, electronic nematicity, and topological aspects give rise to many distinct and fascinating superconducting states. Here, we provide an overview of our current understanding of the superconductivity of {\it bulk} FeSe-based materials, focusing on FeSe and the isovalent substituted FeSe$_{1-x}$S$_{x}$ and FeSe$_{1-x}$Te$_{x}$. We discuss the highly nontrivial superconducting properties in FeSe, including extremely anisotropic pairing states, crossover phenomena from Bardeen--Cooper--Schrieffer (BCS) to Bose--Einstein condensation (BEC) states, a novel field-induced superconducting phase, and broken time-reversal symmetry. We also discuss the evolution of the superconducting gap function with sulfur and tellurium doping, paying particular…

We performed AC calorimetry and magnetoresistance measurements under pressure for H || a-axis (easy-magnetization axis) in the novel heavy-fermion superconductor UTe2. Thanks to the thermodynamic information, multiple superconducting phases have been revealed under pressure and magnetic field. The (H,T) phase diagram of superconductivity under pressure displays an abrupt increase of the upper critical field (Hc2) at low temperature and in the high field region, and a strong convex curvature of Hc2 at high temperature. This behavior of Hc2 and the multiple superconducting phases require a state for the superconducting order parameter more complex than a spin-triplet equal spin pairing. Above the superconducting critical pressure, Pc, we find strong indications that the possible magnetic order is closer to antiferromagnetism than to ferromagnetism.

A whole series of expressions for four species of multipoles (electric, magnetic, magnetic toroidal, and electric toroidal) is provided as a complete basis set to describe arbitrary single-centered spinful electron systems. A compact formula to calculate matrix elements of these multipoles is also derived. A visualization method of an electronic state characterized in terms of multipoles is proposed. The complete basis set is useful to narrow down a candidate order parameter of electron systems in phase transition, to describe a property of cross-correlated phenomena, to analyze spectra of x-ray scattering in magnetically ordered states, and so on. We demonstrate a usage of the complete basis set by taking monopole and toroidal dipole orderings, and the mutual relationship among three distinct magnetic dipoles (orbital, spin angular momenta, and anisotropic dipole) in a spin–orbit coupled system as prime examples.

A skyrmion crystal has internal degrees of freedom in terms of vorticity and helicity, which gives rise to fascinating types of skyrmion phases (Néel, Bloch, and anti-skyrmion). We show how the degeneracy lifting in tetragonal skyrmion crystals occurs in centrosymmetric itinerant magnets by focusing on microscopic interactions through the Fermi surface instability. We demonstrate that four types of skyrmion crystals can be realized according to the signs of the anisotropic Ruderman-Kittel-Kasuya-Yosida interaction, which are sensitive to band structures, electron fillings, atomic orbitals, and sublattice positions. Our findings might provide a possibility of controlling the types of the skyrmion crystals by electron doping and external pressure rather than relying on the noncentrosymmetric lattice structures.

The development of organic ferroelectrics and antiferroelectrics with significantly improved performances has recently increased. Their attractive features are environmentally benign nature and technological advantages such as the light weight, flexibility, biocompatibility, and low-temperature processability. This review is dedicated to the 100th anniversary of the discovery of the first ferroelectric material, Rochelle salt, which contains hydrogen-bonded organic molecules. Approximately one third of more than 80 ferroelectric and 30 antiferroelectric small-molecular crystals are hydrogen-bonded (prototropic, acid–base supramolecular, and zwitterionic) proton-transfer-type crystals. The synergistic interplay of the intermolecular proton dynamics and switchable π-bond dipoles strengthens the switchable polarization and thermal stability, as represented by croconic acid with the highest spontaneous polarization among organic ferroelectrics. Highly polarizable antiferroelectric counterparts are also in the focus, especially the high density and highly efficient electrostatic energy…

We derive general rogue wave solutions of arbitrary orders in the Boussinesq equation by the bilinear Kadomtsev-Petviashvili (KP) reduction method. These rogue solutions are given as Gram determinants with $2N-2$ free irreducible real parameters, where $N$ is the order of the rogue wave. Tuning these free parameters, rogue waves of various patterns are obtained, many of which have not been seen before. Compared to rogue waves in other integrable equations, a new feature of rogue waves in the Boussinesq equation is that the rogue wave of maximum amplitude at each order is generally asymmetric in space. On the technical aspect, our contribution to the bilinear KP-reduction method for rogue waves is a new judicious choice of differential operators in the procedure, which drastically simplifies the dimension reduction calculation as well as the analytical expressions of rogue wave solutions.

Pressure is a powerful tool for exploring novel phenomena in solid-state physics. We review the electronic states of single-crystalline EuT2X2 (T: transition metal, X: Si or Ge) and related compounds, together with the effect of pressure on these compounds. Eu ions in most of the Eu compounds are divalent (Eu2+), and the Eu2+ compounds order magnetically at low temperatures. The valence of the Eu ions in EuT2X2 and their related compounds can be, however, changed from divalent to nearly trivalent (Eu3+) by applying pressure. Several characteristic features were observed under pressure: the magnetic order is suppressed, as revealed by the Doniach phase diagram, the magnetic order changes to the first-order valence transition above a critical pressure, and the moderate heavy-fermion state appears around the magnetic quantum critical point. Typical electronic properties for the Eu3+ state were observed in EuPd3. In addition, a skyrmion lattice state was found for the Eu-based cubic chiral antiferromagnet EuPtSi.

We studied the anisotropy of the superconducting upper critical field $H_{\rm c2}$ in the heavy-fermion superconductor UTe$_2$ under hydrostatic pressure by magnetoresistivity measurements. In agreement with previous experiments we confirm that superconductivity disappears near a critical pressure $p_{\rm c} \approx 1.5$~GPa, and a magnetically ordered state appears. The unusual $H_{\rm c2}(T)$ at low temperatures for $H \parallel a$ suggests that the multiple superconducting phases which appear under pressure have quite different $H_{\rm c2}$. For a field applied along the hard magnetization $b$ axis $H_{\rm c2} (0)$ is glued to the metamagnetic transition $H_{\rm m}$ which is suppressed near $p_{\rm c}$. The suppression of $H_{\rm m}$ with pressure follows the decrease of temperature $T_χ^{\rm max}$, at the maximum in the susceptibility along $b$. The strong reinforcement of $H_{\rm c2}$ at ambient pressure for $H \parallel b$ above 16~T is rapidly suppressed under pressure due to the increase of $T_{\rm sc}$ and the decrease of $H_{\rm m}$. The change in the hierarchy of the…

Superconductivity is a quantum phenomenon caused by bound pairs of electrons. In diverse families of strongly correlated electron systems, the electron pairs are not bound together by phonon exchange but instead by some other kind of bosonic fluctuations. In these systems, superconductivity is often found near a magnetic quantum critical point (QCP) where a magnetic phase vanishes in the zero-temperature limit. Moreover, the maximum of superconducting transition temperature Tc frequently locates near the magnetic QCP, suggesting that the proliferation of critical spin fluctuations emanating from the QCP plays an important role in Cooper pairing. In cuprate superconductors, however, the superconducting dome is usually separated from the antiferromagnetic phase and Tc attains its maximum value near the verge of enigmatic pseudogap state that appears below doping-dependent temperature T*. Thus a clue to the pairing mechanism resides in the pseudogap and associated anomalous transport properties. Recent experiments suggested a phase transition at T*, yet, most importantly, relevant…

Based on the Darboux transformation and N-soliton solutions, we obtain the explicit formulas of arbitrary-order multi-pole (MP) solutions of the Hirota equation via some limit technique. Then, by an improved asymptotic analysis method relying on the balance between exponential and algebraic terms, we derive the accurate expressions of all asymptotic solitons in the double- and triple-pole solutions. Moreover, we study the soliton interactions in the MP solutions and especially emphasize some unusual properties, like the interacting solitons separate from each other in a logarithmical law, the strengths of interaction forces decrease exponentially with their relative distances, and the constant–velocity asymptotic soliton does not experience a phase shift upon an interaction.

Hydrogen is playing a key role in the transition to a sustainable economy and in a variety of industrial processes.For its safe handling, the detection of hydrogen in a fast, reliable and accurate manner is crucial.Thin film metal hydride based optical hydrogen sensors provide an attractive option to sense hydrogen in a variety of conditions and have an attractive safety benefit over other methods of detection: They do not require the presence of electrical leads near the sensing area.These sensors rely on a change of the optical properties arising from a change in the hydrogenation of the metal hydride sensing layer in response to a different partial hydrogen pressure in the environment of the sensor.In this paper, we review how the performance and characteristics of an optical hydrogen sensor are related to the material properties of the metal hydride sensing layer and we discuss previously considered materials and challenges and opportunities left for the future.

We review and extend, in a self-contained way, the mathematical foundations of numerical simulation methods that are based on the use of random states.The power and versatility of this simulation technology is illustrated by calculations of physically relevant properties such as the density of states of large single particle systems, the specific heat, current-current correlations, density-density correlations, and electron spin resonance spectra of many-body systems.We explore a new field of applications of the random state technology by showing that it can be used to analyze numerical simulations and experiments that aim to realize quantum supremacy on a noisy intermediate-scale quantum processor.Additionally, we show that concepts of the random state technology prove useful in quantum information theory.

In this study, we investigate the magnetoelectric (ME) effect of the newly discovered antiferromagnetic (AFM) compound Ce$_{3}$TiBi$_{5}$ with a hexagonal structure (space group $P6_{3}$/$mcm$). In this system, Ce ions form zig-zag chains that extend along the $c$-axis. We focus on the lack of the local inversion symmetry at the Ce site, although the crystal structure has an inversion center. A theoretical study has predicted that magnetization can be induced by an electric current in the AFM ordering on the zig-zag chain. We conducted magnetization measurements on Ce$_{3}$TiBi$_{5}$ under an applied constant electric current and a static magnetic field around the AFM ordering temperature of $T_{\rm N}$ = 5.0 K. We successfully observed of the current-induced magnetization below $T_{\rm N}$. The magnitude of the current-induced magnetization has a linear electric current dependence and exhibits no magnetic field dependence. This behavior is consistent with the theoretical prediction of the ME effect in the ferrotoroidal ordered state.

The superconductivity of palladium-hydride (Pd-H) system was discovered by Skoskiewicz in 1972.Thereafter, many studies have been carried out on Pd-H and palladium deuteride (Pd-D) systems along with their alloyed (Pdmetal-H) systems.In this paper, we present a brief overview of superconducting properties of these systems.First, we describe methods for H loading into Pd, in which three principal techniques, i.e., gas loading, electrochemical loading, and H implantation are introduced.Next, we briefly summarize the superconducting properties of Pd-H(D) systems, e.g., the concentration dependence of the transition temperature T c , inverse isotope effect, critical field, pressure dependence of T c , and the impurity effects.Subsequently, we describe our recent in-situ magnetization measurements of superconducting Pd-H and Pd-D powders with 1-2 µm diameters prepared by low-temperature H absorption, in which the absorption was performed below 200 K under hydrogen (H 2 ) or deuterium (D 2 ) gas atmosphere.Additionally, we report resistance measurements of a Pd-H film with ∼100 nm…

We predict that surface acoustic waves are generated preferentially in one direction in a heterostructure of a thin magnetic film on a non-magnetic substrate. The non-reciprocity arises from magneto-elastic coupling and magneto-rotation coupling, the former being dominant for YIG/GGG heterostructures. For YIG films thinner than about 100 nm, the surface acoustic wave amplitude is nearly unidirectional at certain angles of the in-plane equilibrium magnetisation. We compute the full magnetic field dependence of the effect for a selected device.

The magneto-optical Kerr effect (MOKE) has recently been discovered in antiferromagnetic Kagome lattice Mn3Sn. Since the compound exhibits a coplanar $120^\circ$ antiferromagnetic (AFM) order, the magnetic moments cancel each other, and the net magnetization is almost zero. However, the MOKE is allowed due to the lack of the time-reversal symmetry (TRS) of the AFM order. X-ray magnetic circular dichroism (XMCD) can detect a difference in up and down the spin density of states (DOS), and thus XMCD originating from the spin operator, the so-called Sz term, should be negligibly small as well as the net magnetization. Nonetheless, we show that XMCD originating from the magnetic dipole operator, the so-called Tz term, should remain uncancelled in the AFM with the broken TRS. Moreover, we clarify that there is a strong link between the Tz term and the augmented magnetic octupole (AMO), which suggests that XMCD can be an effective approach for analyzing multipole moments in antiferromagnets.

Conventional quantum annealing does not sample all ground states fairly. We demonstrate that fair sampling can be achieved by performing simulated annealing on a quantum annealer. We discuss the problems that occur when implementing this method and propose an alternative way to overcome them. We numerically verify the fair sampling ability of our method in a small-scale toy model.

In this study, we investigated the structural, electronic, magnetic and elastic properties of the full-Heusler Cr2VSb alloy in the framework of density functional theory with generalized gradient approximation (GGA). The calculated results showed that the Cr2VSb alloy is a half-metallic material at the equilibrium of lattice parameter a = 6.136 Å, with a lattice-parameter limit of 5.936–6.286 Å. The magnetic moments of the Cr2VSb alloy follow the relationship Mt = (Zt − 24)μB. The full-Heusler Cr2VSb alloy is a ductile alloy and anisotropic material.

To analyze quantum many-body Hamiltonians, recently, machine learning techniques have been shown to be quite useful and powerful. However, the applicability of such machine learning solvers is still limited. Here, we propose schemes that make it possible to apply machine learning techniques to analyze fermion-boson coupled Hamiltonians and to calculate excited states. As for the extension to fermion-boson coupled systems, we study the Holstein model as a representative of the fermion-boson coupled Hamiltonians. We show that the machine-learning solver achieves highly accurate ground-state energy, improving the accuracy substantially compared to that obtained by the variational Monte Carlo method. As for the calculations of excited states, we propose a different approach than that proposed in K. Choo et al., Phys. Rev. Lett. 121 (2018) 167204. We discuss the difference in detail and compare the accuracy of two methods using the one-dimensional $S=1/2$ Heisenberg chain. We also show the benchmark for the frustrated two-dimensional $S=1/2$ $J_1$-$J_2$ Heisenberg model and show an…

It is shown theoretically that ferromagnetic superconductors, UGe$_2$, URhGe, and UCoGe can be described in terms of the A-phase like triplet pairing similar to superfluid $^3$He in a unified way, including peculiar reentrant, S-shape, or L-shape $H_{\rm c2}$ curves. The associated double transition inevitable between the A$_1$ and A$_2$-phases in the $H$-$T$ plane is predicted, both of which are characterized by non-unitary state with broken time reversal symmetry and the half-gap. UTe$_2$, which has been discovered quite recently to be a spin-polarized superconductor, is analyzed successively in the same view point, pointing out that the expected A$_1$-A$_2$ transition is indeed emerging experimentally. Thus the four heavy Fermion compounds all together are entitled to be topologically rich solid state materials worth further investigating together with superfluid $^3$He A-phase.

To show the properties and existence of vector nonlinear waves in a one-dimensional two-component Bose–Einstein condensate system, we investigate the pair-transition-coupled nonlinear Schrödinger equation. Through the two forms for Darboux transformations, we obtain a family of nonlinear wave solutions describing the extreme events. This family of solutions contains Akhmediev breather, Kuznetsov–Ma breather, famous vector rogue waves, bright-dark-rogue waves, beak-shaped rogue waves, and novel freak waves. Moreover, we successfully reveal different types of the distributions for the second-order vector rogue waves. Our results show that more abundant and novel localized waves may exist in the Bose–Einstein condensate system than in the Manakov system.

We report a study of the second-order phase transition at 105 K in the geometrically frustrated iridate Ca5Ir3O12 using a Raman scattering method. The Raman scattering spectra of a single crystal were measured from 4 K to room temperature. Ab initio phonon calculations that consider the spin–orbit interaction were also conducted and compared with the experimental spectra. Agreement between the theoretical and experimental results at room temperature is reasonably good. At room temperature, \(6\text{A'}_{1} + 9\text{E'} + 5\text{E''}\) were assigned among the Raman active modes, \(6\text{A'}_{1} + 13\text{E'} + 6\text{E''}\), based on the reported \(P\bar{6}2m\) crystal structure. Below Ts, 23 additional peaks were observed, suggesting the appearance of a superlattice structure. The polarization dependence of Raman spectra below Ts indicates the existence of \(\bar{6}\) symmetry. We observed at least one additional mode as a broad weak-intensity peak at temperatures higher than Ts. This suggests possible local distortion around the Ir ions, which would be expected for Ir ions with…

The valence state of UTe$_2$ was studied by core-level photoelectron spectroscopy. The main peak position of the U $4f$ core-level spectrum of UTe$_2$ coincides with that of UB$_2$, which is an itinerant compound with a nearly $5f^3$ configuration. However, the main peak of UTe$_2$ is broader than that of UB$_2$, and satellite structures are observed in the higher binding energy side of the main peak, which are characteristics of mixed-valence uranium compounds. These results suggest that the U 5$f$ state in UTe$_2$ is in a mixed valence state with a dominant contribution from the itinerant $5f^3$ configuration.

We propose the ultra-fast numerical approach to large-scale inhomogeneous superconductors, which we call the Localized Krylov Bogoliubov-de Gennes method (LK-BdG). In the LK-BdG method, the computational complexity of the local Green's function, which is used to calculate the local density of states and the mean-fields, does not depend on the system size $N$. The calculation cost of self-consistent calculations is ${\cal O}(N)$, which enables us to open a new avenue for treating extremely large systems with millions of lattice sites. To show the power of the LK-BdG method, we demonstrate a self-consistent calculation on the 143806-site Penrose quasicrystal lattice with a vortex and a calculation on 1016064-site two-dimensional nearest-neighbor square-lattice tight-binding model with many vortices. We also demonstrate that it takes less than 30 seconds with one CPU core to calculate the local density of states with whole energy range in 100-millions-site tight-binding model.

Proton conducting materials that can work without humidification and above 100 °C have been highly desired for future fuel cells, and thus development of high-performance anhydrous proton conductors and elucidation of proton conduction mechanism are pivotal issues. Those based on organic crystals allow us to investigate the proton conduction mechanism in detail by utilizing rich material designability of organic molecules. In this study, we investigated structure–property relationship and effect of molecular dynamics of constituent molecules on the proton conductivity using high-quality single crystals of a series of imidazolium hydrogen dicarboxylates. As a result, “intrinsic” proton conductivities of these salts without contributions from grain boundaries were observed for the first time. Interestingly, our investigation revealed that not only the “static” H-bond network structures but also the “dynamics” of the constituent molecules synergistically play important roles in the anhydrous proton conduction of these salts, which is thought as peculiar nature of Grotthuss-type…

We study the spin-fluctuation-mediated $s\pm$-wave superconductivity in the bilayer Hubbard model with vertical and diagonal interlayer hoppings. As in the two-leg ladder model with diagonal hoppings, studied previously by the present authors, superconductivity is strongly enhanced when one of the bands lies just below (or touches) the Fermi level, that is, when the band is incipient. The strong enhancement of superconductivity is because large weight of the spin fluctuations lies in an appropriate energy range, whereas the low energy, pair-breaking spin fluctuations are suppressed. The optimized eigenvalue of the linearized Eliashberg equation, a measure for the strength of superconductivity, is not strongly affected by the bare width of the incipient band, but the parameter regime where superconductivity is optimized is wide when the incipient band is narrow, and in this sense, the coexistence of narrow and wide bands is favorable for superconductivity.

We experimentally and theoretically investigate the Hahn echo decay curve for nitrogen vacancy centers in diamond with different spin concentrations. The Hahn echo results show a non-exponential decay for low spin concentrations, while an exponential decay is dominant for high spin concentrations. By fitting the decay curve with a theoretical model, we show that both the amplitude and correlation time of the environmental noise have a clear dependence on the spin concentration. These results are essential for optimizing the NV center concentration in high-performance quantum devices, particularly quantum sensors.

The effect of pressure on the unique electronic state of the antiferromagnetic (AF) compound EuCu2Ge2 has been measured in a wide temperature range from 10 mK to 300 K by electrical resistivity measurements up to 10 GPa. The Neel temperature of TN = 15 K at ambient pressure increases monotonically with increasing pressure and becomes a maximum of TN = 27 K at 6.2 GPa but suddenly drops to zero at Pc = 6.5 GPa, suggesting the quantum critical point (QCP) of the valence transition of Eu from a nearly divalent state to that with trivalent weight. The rhomag0 and A values obtained from the low-temperature electrical resistivity based on the Fermi liquid relation of rhomag = rhomag0 + AT^2 exhibit huge and sharp peaks around Pc. The exponent n obtained from the power law dependence rhomag = rhomag0 + BT^n is clearly less than 1.5 at P = Pc = 6. 5 GPa, which is expected at the AF-QCP. These results indicate that Pc coincides with Pv, corresponding to the quantum criticality of the valence transition pressure Pv. The electronic specific heat coefficient, estimated from the generalized…

Alkali and alkaline-earth metal borohydrides are intensely studied in view of their high hydrogen content and morerecently, due to the observed decent ionic conductivities which would make them interesting candidates for solid statebatteries. Neutron scattering techniques offer a unique tool to study both the structure and the dynamics of borohydridematerials. This review summarizes recent work in this field, illuminating the rich and interconnectivity betweenhydrogen dynamic and crystallographic phases.

We investigated the low-energy incommensurate (IC) magnetic fluctuations in Sr$_2$RuO$_4$ by the high-resolution inelastic neutron scattering measurements and random phase approximation (RPA) calculations. We observed a spin resonance with energy of $\hbarω_\text{res}=0.56$~meV centered at a characteristic wavevector $\mathbf{Q}_\text{res}=(0.3, 0.3, 0.5)$. The resonance energy corresponds well to the superconducting gap $2Δ=0.56$~meV estimated by the tunneling spectroscopy. The spin resonance shows the $L$ modulation with a maximum at around $L = 0.5$. The $L$ modulated intensity of the spin resonance and our RPA calculations indicate that the superconducting gaps regarding the quasi-one-dimensional $α$ and $β$ sheets at the Fermi surfaces have the horizontal line nodes. These results may set a strong constraint on the pairing symmetry of Sr$_2$RuO$_4$. We also discuss the implications on possible superconducting order parameters.

Iron-based superconductors have grown to be a new continent of high Tc superconductors comparable to cuprates. The optimal critical temperature (Tc) of 56 K in electron-doped 1111-type iron oxypnictides attracts considerable attention of physicists and chemists. Carrier doping is not only essential to induce superconductivity but also is a critical parameter that governs the electronic, magnetic, and crystallographic properties of ground states in high-Tc superconductors. Hydride ion (H-) which is an anionic state of hydrogen acts as an efficient electron donor in the 1111-type, leading to several important discoveries such as two-dome structure of superconducting phase and bipartite parent phase. This article summarizes the synthesis, physical properties, and electronic structure of the H- bearing 1111-type iron-based superconductors along with the relevant phenomena of other superconductors. We show several common characteristics of iron-based superconductors with high-Tc over 50 K and suggest a way to achieve higher Tc.

Extended X-ray absorption fine structure (EXAFS) is one of the common tools used to determine the local atomic coordination and bond length around an objective element via data conversion from the EXAFS profile to a radial distribution function (RDF). Thus, EXAFS has been widely used for the investigation of catalytic reactions, battery degradation, and other material developments. However, the data conversion from EXAFS to RDF involves three difficulties: weak signals, necessity of a wide energy range (∼1000 eV), and fitting parameters in reference samples. Recently, the direct estimation technique of RDF from EXAFS using machine learning was successfully developed, which overcomes the third problem. Here, we used an artificial neural network (ANN) to directly predict RDF from the near-edge region of the spectrum (X-ray absorption near-edge structure: XANES), which enables overcoming all of the difficulties. The ANN can correctly generate RDF only from XANES. XANES includes information about the bond length and coordination numbers in the range of ∼5 Å. We applied our prediction…

An open quantum system is a system coupled to an environment that can describe time-irreversible dynamics through which the system evolves toward the thermal equilibrium state.We present a quantum mechanically rigorous theory in order to help the analysis of spectra obtained from the advanced nuclear magnetic resonance (NMR) and muon spin rotation, relaxation or resonance (μSR) techniques.Our approach is based on the numerically "Exact" hierarchical equations of motion (HEOM) approach, which allows us to study the reduced system dynamics for non-perturbative and non-Markovian system-bath interactions at finite temperature even under strong time-dependent perturbations.We demonstrate the present theory to analyze μSR and low-field NMR spectra, as an extension of the Kubo-Toyabe theory focusing on the effects of temperature and anisotropy of a local magnetic field on spectra, to help further the development of these experimental means.

The quasi-two-dimensional molecular semimetallic conductor $α$-STF$_2$I$_3$ is isostructural with $α$-ET$_2$I$_3$.The latter possesses a unique band structure showing a zero-gap state with Dirac cones under high pressure, whereas the band structure of the former has been elusive because of heavy disorder at all the donor sites. To elucidate the band structure of $α$-STF$_2$I$_3$, a theoretical method based on the observed atomic parameters at 296 K and 1 bar has been proposed. The results suggest that the STF salt should have a band structure with Dirac cones under ambient pressure and temperature, which should promote future experimental studies on this system.Using the extended Hückel method, we demonstrate that the Dirac points of $α$-STF$_2$I$_3$ are aligned to be symmetric with respect to a time reversal invariant momentum (TRIM). Such novel Dirac cones, where the energy difference between the conduction and valence bands has considerable anisotropy, are clarified in terms of the parity of the wavefunction at the TRIM. We propose the conductivity measurement on the present…

We report time-resolved angle-resolved photoemission study on Ta2Ni1−xCoxSe5 (x = 0.10) in which the excitonic insulator (EI) transition temperature is reduced by Co substitution for Ni. In the photoinduced phase, the conduction band is clearly observed and the energy gap between the conduction and valence bands is undetectably small within the energy resolution. Assuming that the energy gap is closed, the conduction band crosses or overlaps with the valence band near the Fermi level resembling a nodal semimetal.

We report that the volume of an orbital-ordered Mott insulator Ca2RuO4 is purely changed by an electrical current at constant temperature and pressure. The observed giant electromechanical response is an intrinsic non-equilibrium property in currents, not from an extrinsic self-heating effect, as is clearly demonstrated by combined transport and synchrotron radiation X-ray diffraction experiments. The underlying mechanism is ascribed to a suppression of orbital order by currents, significantly differing from conventional piezoelectric or electrostrictive effect in dielectric materials. Our results imply a novel non-equilibrium equation of state, in which an equilibrium quantity is described by a non-equilibrium variable, and thus provide fundamental data for investigating the evolving field of non-equilibrium thermodynamics.

We investigate the thermoelectric properties of CaAl$_2$Si$_2$-type Zintl phase compounds $AB_2X_2$ ($A$ = Mg, Ca, Sr, Ba, $B$ = Mg, Zn, Cd, and $X$ = P, As, Sb) using first principles band calculations within the Boltzmann transport theory assuming the constant relaxation time approximation. We introduce the effective degree of valley degeneracy $n_{TE}$ to focus on the relationship between the thermoelectric properties and the multivalley character of the electronic band structure around the Fermi level. We also introduce a quantity $γ_{TE}$, which takes into account $n_{TE}$ and anisotropy of the valley structure, and it is found that $γ_{TE}$ enables us to well understand the overall trend of the material dependence of the power factor. We finally suggest promising thermoelectric materials, e.g. BaMg$_2$P$_2$ for PF $\sim 20μ$W/cmK$^2$ and $ZT > 0.2$ at 300K and SrZn$_2$As$_2$ for PF $\sim 35μ$W/cmK$^2$ and $ZT > 0.35$ at 300K assuming a relaxation time of 10 fs and a lattice thermal conductivity value of 2 W/mK.

An osmium chloride with the chemical formula of OsxCl3 (x = 0.81) was synthesized and its crystal structure and thermodynamic properties were investigated. OsxCl3 crystallizes in a layered CdCl2-type structure with the triangular lattice partially occupied by Os ions on average. However, on microscopic length scales, the triangular lattice is composed of nano-domains with a honeycomb arrangement of Os ions, as observed by electron microscopy and Raman scattering experiments. Magnetization and heat capacity measurements revealed an absence of magnetic long-range order down to 0.08 K, while a broad peak in heat capacity at 0.15 K may indicate a short-range order in the local honeycomb lattice. OsxCl3 may exhibit certain aspects of the Kitaev spin liquid that are expected for a perfect honeycomb lattice of osmium trichloride.

Magnetic properties of an Yb-based semiconductor YbCuS2 with a zigzag chain of Yb3+ ions were studied. The Curie–Weiss behavior of the magnetic susceptibility χ(T) indicates antiferromagnetic interaction and an isolated Kramers doublet ground state with effective spin-1/2. Magnetic specific heat Cm/T exhibits a sharp peak at TO = 0.95 K, where the magnetic entropy is 20% of R ln 2, indicating strong magnetic fluctuations. In magnetic fields, TO is robust for B < 4 T, and it shifts to higher temperatures but reverses downward for B ≥ 7 T. Below TO, a magnetization plateau manifests itself at around 6 T which is followed by transitions at 9.1 and 15.6 T. The phase diagram suggests that nontrivial phases arise from magnetic frustration in the Yb zigzag chain.

We study the spin dynamics of a ferromagnetic insulator on which graphene is placed. We show that the Gilbert damping is enhanced by the proximity exchange coupling at the interface. The modulation of the Gilbert damping constant is proportional to the product of the spin-up and spin-down densities of states of graphene. Consequently, the Gilbert damping constant in a strong magnetic field oscillates as a function of the external magnetic field that originates from the Landau level structure of graphene. We find that a measurement of the oscillation period enables the strength of the exchange coupling constant to be determined. The results demonstrate in theory that the ferromagnetic resonance measurements may be used to detect the spin resolved electronic structure of the adjacent materials, which is critically important for future spin device evaluations.

The piezoelectric biosensor is small, light-weight, and its implementation and use are low-cost. Therefore, sensitivity, selectivity, and challenges of a DNA based piezoelectric sensor are discussed. The variation of the flowing electrical current through DNA concerning the applied force confirms the piezoelectric property of DNA. DNA chain can behave as a sensor to detect some deterministic drives via the variation of electrical current as an output signal. The characteristic peaks in the current–voltage characteristic diagram of poly(CG) appeared at V = 0.8 and 3.4 V for F = 0.3 and 0.01 pN, respectively can determine different sensing behaviors of DNA chains. The distinguishable peaks of the electrical current can be predicted via the multifractal analysis. The analogues heat capacity in multifractal DNA shows a peak position that resembles a phase transition at the critical point. So, one can design a piezoelectric biosensor with specific performance using the appropriate parameters.

The spontaneous motion of a camphor particle with a slight modification from a circle is investigated. The effect of the shape on the motion is examined by the perturbation method. We introduce a slight $n$-mode modification from a circle, where the profile is described by $r = R(1 + ε\cos nθ)$ in polar coordinates. The results predict that a camphor particle with an $n=3$ mode modification from a circle, i.e., a triangular modification, moves in the direction of a corner for a small particle, while it moves in the direction of a side for a large particle. The numerical simulation results well reproduce the theoretical prediction. The present study will help understand the effect of the particle shape on spontaneous motion.

We study theoretically the wave packet dynamics in borophene, a two-dimensional boron crystal that hosts symmetry protected an anisotropic tilted Dirac cone where the low energy quasi-particle excitations are described by the massless Dirac equation. We find pronounced zitterbewegung of the wave packet, depending strongly on its spin orientation. An interesting dynamical scenario is exhibited by the wave packet in borophene when the system is subjected to a homogeneous in-plane electric field. In particular, the particle wave packet undergoes pronounced side-jump motion perpendicular to the field direction in addition to zitterbewegung. The amplitude of such motion can be estimated analytically which increases with decreasing the field strength for wave packet of sufficiently large width. Moreover, remarkable dynamical features are observed under the influence of an applied magnetic field, where effects of zitterbewegung due to dispersion branches of the spectrum dominate the irregular quantum motion at low energies in borophene. Interestingly, disorder induced broadening of Landau…

In this study, the weakly nonlinear propagation of pressure waves in bubbly flows is theoretically investigated, with a particular focus on different types of initial flow patterns. At the initial state, the gas and liquid phases exhibit a high velocity with uniform distribution and a low velocity with nonuniform distribution, respectively. Using basic conservation equations based on a two-fluid model and the method of multiple scales with perturbation expansions, we can derive three cases of nonlinear wave equations: (i) Korteweg–de Vries Burgers equation for long waves; (ii) Nonlinear Schrödinger (NLS)-I equation for short waves in slow nonuniform flows; (iii) NLS-II equation for short waves in fast nonuniform flows. Consequently, the initial velocities contribute to an advection effect of the waves, the initial nonuniform flow distribution induces a variable coefficient of advection, and the prominent role of relative velocity is elucidated.

We study the Kibble-Zurek mechanism in the transverse Ising chain coupled to a dissipative boson bath, making use of a new numerical method with the infinite time evolving block decimation combined with the discrete-time path integral. We first show the ground-state phase diagram and confirm that a quantum phase transition takes place in the presence of the system-bath coupling. Then we present the time dependence of the energy expectation value of the spin Hamiltonian and the scaling of the kink density with respect to the time period over which the spin Hamiltonian crosses a quantum phase transition. The energy of spins starts to grow from the energy at the ground state of the full system near a quantum phase transition. The kink density decays as a power law with respect to the time period. These results confirm that the Kibble-Zurek mechanism happens. We discuss the exponent for the decay of the kink density in comparison with a theoretical result with the quantum Monte-Carlo simulation. A comparison to an experimental study is also briefly mentioned.

We investigate the real-time spin response of the $S=1$ Kitaev model upon stimuli of a pulsed magnetic field in one of the edges using the exact diagonalization method. It is found that the pulsed magnetic field has no effect on the appearance of the spin moments in the quantum spin liquid region, but induces the spin oscillations in the other edge region with a small magnetic field. This is understood by the existence of the itinerant quasiparticles, which carry the spin excitations without the spin polarization in the quantum spin liquid state. This suggests that the spin fractionalizations occur in the $S=1$ Kitaev model as well as the exactly solvable $S=1/2$ Kitaev one and the fractionalized quasiparticles play an essential role in the spin transport.

Using cryogenic Lorentz transmission electron microscopy, we investigate the temperature evolution of magnetic structures in single crystal samples of K2CuF4 with a magnetic phase transition at 6.1 K. This material is known as one of the candidates for a two-dimensional (2D) XY magnet that may exhibit a Berezinskii–Kosterlitz–Thouless (BKT) phase transition. A fine magnetic stripe pattern was found with a period of about 120 nm in a direction along the c axis below 7.3 K in a thin sample with the c axis in the plane. Magnetic columns of vortices and antivortices with a separation of about three micrometers were observed below 6 K in a c-plane sample approximately 150 nm in thickness. The formation of two different types of magnetic structures at different threshold temperatures is likely to be consistent with a picture of two-step dimensional crossover in spin and real spaces previously derived from magnetization and neutron experiments in K2CuF4. These results indicate how the 2D XY character of K2CuF4 is incorporated in three-dimensional magnetic structures. Based on the…

We report thermal expansion and magnetostriction in breathing pyrochlore magnets LiACr$_4$X$_8$ (A = Ga, In, X = O, S) measured by a dilatometric method on sintered samples. All four of these compounds showed a large volume contraction associated with antiferromagnetic order with decreasing temperature. Above the Neél temperature, LiGaCr4S8 showed negative thermal expansion, LiInCr4O8 showed positive thermal expansion with concave-downward temperature dependence, and LiInCr4S8 showed positive forced volume magnetostriction. All these phenomena are likely caused by the complex structure-magnetism correlation within the breathing pyrochlore structure with J and J'. These results suggested that breathing pyrochlore magnets are promising for the realization of various volumetric phenomena related to their magnetism not only in the magnetically-ordered phase but also in the paramagnetic phase.

Synchrotron X-ray powder diffraction experiments were carried out at T = 300 K under pressures on a tetragonal metallic compound CeCoSi exhibiting a possible antiferroquadrupole (AFQ) ordering at low temperatures, together with nonmagnetic counterparts of LaCoSi and PrCoSi, in order to clarify a relationship between structural properties of RCoSi (R = La, Ce, Pr) and the AFQ ordering. From the results, a structural phase transition was found at Ps ∼ 4.9 GPa only in CeCoSi. However, the other two compounds retain their tetragonal structure within the present pressure range (up to P ∼ 9 GPa). This suggests that a valence instability of Ce-4f ion could be related to the structural transition at Ps. When studying the ratio of lattice parameters c/a, a marked decrease up to Ps is observed in CeCoSi and a saturation behavior at high pressures in the other two compounds. In this regards, a uniaxial compression along the c-axis could also be key to understand the pressure-induced structural transition in CeCoSi. At Ps, c/a ∼ 1.69 in CeCoSi is shown to give a threshold for maintaining P4/nmm…