The 50 most-cited articles in Journal of the Physical Society of Japan, 2017

We design a Convolutional Neural Network (CNN) which studies correlation between discretized inverse temperature and spin configuration of 2D Ising model and show that it can find a feature of the phase transition without teaching any a priori information for it. We also define a new order parameter via the CNN and show that it provides well approximated critical inverse temperature. In addition, we compare the activation functions for convolution layer and find that the Rectified Linear Unit (ReLU) is important to detect the phase transition of 2D Ising model.

Motivated by the recent successful application of artificial neural networks to quantum many-body problems [G. Carleo and M. Troyer, Science {\bf 355}, 602 (2017)], a method to calculate the ground state of the Bose-Hubbard model using a feedforward neural network is proposed. The results are in good agreement with those obtained by exact diagonalization and the Gutzwiller approximation. The method of neural-network quantum states is promising for solving quantum many-body problems of ultracold atoms in optical lattices.

Active matter exhibits a wealth of emerging nonequilibrium behaviours. A paradigmatic example is the interior of cells, where active components, such as the cytoskeleton, are responsible for its structural organization and the dynamics of the various components. Of particular interest are the properties of polymers and filaments. The intimate coupling of thermal and active noise, hydrodynamic interactions, and polymer conformations implies the emergence of novel structural and dynamical features. In this article, we review recent theoretical and simulation developments and results for the structural and dynamical properties of polymers exposed to activity. Two- and three-dimensional filaments are considered propelled by different mechanisms such as active Brownian particles or hydrodynamically-coupled force dipoles.

EuPtSi crystallizes in the cubic chiral structure (P213, No. 198), which is the same as the non-centrosymmetric space group of MnSi with the skyrmion structure, and orders antiferromagnetically below a Néel temperature TN = 4.05 K. The magnetization at 2 K for the [111] direction indicates two metamagnetic transitions at the magnetic fields HA1 = 9.2 kOe and HA2 = 13.8 kOe and saturates above Hc = 26.6 kOe. The present magnetic phase between HA1 and HA2 is most likely closed in the \((H,T)\) phase and is observed in a wide temperature range from 3.6 to 0.5 K. In this magnetic phase known as the A-phase, we found giant additional Hall resistivity ΔρH(H) and magnetoresistance Δρ(H), reaching ΔρH(H) = 0.12 µΩ·cm and Δρ(H) = 1.4 µΩ·cm, respectively. These findings are obtained for H || [111] and [100], but not for H || [110] and [112], revealing an anisotropic behavior in the new material EuPtSi.

We develop a variational method to obtain many-body ground states of the Bose-Hubbard model using feedforward artificial neural networks. A fully-connected network with a single hidden layer works better than a fully-connected network with multiple hidden layers, and a multi-layer convolutional network is more efficient than a fully-connected network. AdaGrad and Adam are optimization methods that work well. Moreover, we show that many-body ground states with different numbers of atoms can be generated by a single network.

Helical magnetic structures and its responses to external magnetic fields in Yb(Ni$_x$Cu$_{1-x}$)$_3$Al$_9$, with a chiral crystal structure of the space group $R32$, have been investigated by resonant X-ray diffraction. It is shown that the crystal chirality is reflected to the helicity of the magnetic structure by a one to one relationship, indicating that there exists an antisymmetric exchange interaction mediated via the conduction electrons. When a magnetic field is applied perpendicular to the helical axis ($c$ axis), the second harmonic peak of $(0, 0, 2q)$ develops with increasing the field. The third harmonic peak of $(0, 0, 3q)$ has also been observed for the $x$=0.06 sample. This result provides a strong evidence for the formation of a chiral magnetic soliton lattice state, a periodic array of the chiral twist of spins, which has been suggested by the characteristic magnetization curve. The helical ordering of magnetic octupole moments, accompanying with the magnetic dipole order, has also been detected.

Three-dimensional random electron systems undergo quantum phase transitions and show rich phase diagrams. Examples of the phases are the band gap insulator, Anderson insulator, strong and weak topological insulators, Weyl semimetal, and diffusive metal. As in the previous paper on two-dimensional quantum phase transitions [J. Phys. Soc. Jpn. vol. 85, 123706 (2016)], we use an image recognition algorithm based on a multilayered convolutional neural network to identify which phase the eigenfunction belongs to. The Anderson model for localization-delocalization transition, the Wilson--Dirac model for topological insulators, and the layered Chern insulator model for Weyl semimetal are studied. The situation where the standard transfer matrix approach is not applicable is also treated by this method.

The superconducting pyrochlore oxide Cd2Re2O7 is revisited with a particular emphasis on the sample-quality issue. The compound has drawn attention as the only superconductor (Tc = 1.0 K) that has been found in the family of α-pyrochlore oxides since its discovery in 2001. Moreover, it exhibits two characteristic structural transitions from the cubic pyrochlore structure, with the inversion symmetry broken at the first one at 200 K. Recently, it has attracted increasing attention as a candidate spin-orbit coupled metal (SOCM), in which specific Fermi liquid instability is expected to lead to an odd-parity order with spontaneous inversion-symmetry breaking [L. Fu, Phys. Rev. Lett. 115, 026401 (2015)] and parity-mixing superconductivity [V. Kozii and L. Fu: Phys. Rev. Lett. 115 (2015) 207002; Y. Wang et al., Phys. Rev. B 93 (2016) 134512]. We review our previous experimental results in comparison with those of other groups in the light of the theoretical prediction of the SOCM, which we consider meaningful and helpful for future progress in understanding this unique compound.

The Sasa–Satsuma equation is an integrable high-order nonlinear Schrödinger (NLS) equation, and also is a complex modified KdV-type equation. It can describe the propagation of femtosecond pulses in optical fibers. Very recently, Ablowitz and Mussliman introduced a class of reverse space-time and reverse time nonlinear integrable equations, including the reverse space nonlocal NLS equation, the real and complex reverse space-time nonlocal mKdV, sine-Gordon, Davey–Stewartson equations, etc. So, what is nonlocal version of high-order NLS? In this paper, we introduce a reverse space-time nonlocal Sasa–Satsuma equation, i.e., a reverse space-time nonlocal high-order NLS equation, and derive its solutions with the binary Darboux transformation method. Periodic solutions, and some localized solutions, such as dark soliton, W-shaped soliton, M-shaped soliton and breather soliton of the reverse space-time nonlocal Sasa–Satsuma equation are constructed.

A search for new isotopes in the neutron-rich rare-earth region has been carried out using a 345 MeV=nucleon 238U beam at the RIKEN Nishina Center RI Beam Factory. Fragments produced were analyzed and identified using the BigRIPS in-flight separator. We observed a total of 29 new neutron-rich isotopes: 153Ba, 154,155,156La, 156,157,158Ce, 156,157,158,159,160,161Pr, 162,163Nd, 164,165Pm, 166,167Sm, 169Eu, 171Gd, 173,174Tb, 175,176Dy, 177,178Ho, and 179,180Er.

The search for new isotopes using the in-flight fission of a 238U beam has been conducted concurrently with decay measurements, during the so-called EURICA campaigns, at the RIKEN Nishina Center RI Beam Factory. Fission fragments were analyzed and identified in flight using the BigRIPS separator. We have identified the following 36 new neutron-rich isotopes: 104Rb, 113Zr, 116Nb, 118,119Mo, 121,122Tc, 125Ru, 127,128Rh, 129,130,131Pd, 132Ag, 134Cd, 136,137In, 139,140Sn, 141,142Sb, 144,145Te, 146,147I, 149,150Xe, 149,150,151Cs, 153,154Ba, and 154,155,156,157La.

Berry curvature, the geometric counterpart of the magnetic field in momentum space, has been applied to various topological materials, such as topological insulators and Weyl semimetals, to give rise to robust edge states that have applications in electron transport and quantum computing. In this work, we show that under zero Berry curvature a honeycomb lattice with a Kekulé-like hopping texture possesses topological edge states, which is analogous to the scenario of the Aharonov–Bohm effect. Our results serve for the design of solid-state materials with topological edge states.

We investigate the ground-state and finite-temperature properties of the bond-random $s=1/2$ Heisenberg model on a honeycomb lattice with frustrated nearest- and next-nearest-neighbor antiferromagnetic interactions, $J_1$ and $J_2$, by the exact diagonalization and the Hams--de Raedt methods. The ground-state phase diagram of the model is constructed in the randomness versus the frustration ($J_2/J_1$) plane, with the aim of clarifying the effects of randomness and frustration in stabilizing a variety of distinct phases. We find that the randomness induces the gapless quantum spin liquid (QSL)-like state, the random-singlet state, in a wide range of parameter space. The observed robustness of the random-singlet state suggests that the gapless QSL-like behaviors might be realized in a wide class of frustrated quantum magnets possessing a certain amount of randomness or inhomogeneity, without fine-tuning the interaction parameters. Possible implications to recent experiments on the honeycomb-lattice magnets Ba$_3$CuSb$_2$O$_9$ and 6HB-Ba$_3$NiSb$_2$O$_9$ exhibiting the gapless…

The fusion reaction 48Ca + 248Cm → 296Lv* was investigated using the gas-filled recoil ion separator GARIS at RIKEN. The reaction was studied at excitation energies of 41.3 and 38.2 MeV. A total of seven decay chains were observed. Three of the chains were assigned to the decay of 292Lv and three to the decay of 293Lv. A possible observation of an α-decay branch of 284Cn populating the new spontaneously fissioning nucleus 280Ds is discussed. The assignment of one of the decay chains to the 3n evaporation channel, resulting in a spontaneous fission branch of 285Cn, or to the 2n channel, resulting in the population of the new fissioning nucleus 286Cn, is ambiguous. Measured cross sections are in agreement with previously published values.

We present the group-thoretical classification of gap functions in superconductors coexisting with some magnetic order in non-symmorphic magnetic space groups. Based on the weak-coupling BCS theory, we show that UCoGe-type ferromagnetic superconductors must have horizontal line nodes on either $k_z=0$ or $\pmπ/c$ plane. Moreover, it is likely that additional Weyl point nodes exist at the axial point. On the other hand, in UPd$_2$Al$_3$-type antiferromagnetic superconductors, gap functions with $A_g$ symmetry possess horizontal line nodes in antiferromagnetic Brillouin zone boundary perpendicular to $c$-axis. In other words, the conventional fully-gapped $s$-wave superconductivity is forbidden in this type of antiferromagnetic superconductors, irrelevant to the pairing mechanism, as long as the Fermi surface crosses a zone boundary. UCoGe and UPd$_2$Al$_3$ are candidates for unconventional superconductors possessing hidden symmetry-protected line nodes, peculiar to non-symmorphic magnetic space groups.

Superconductivity with exotic properties has often been discovered in materials with a layered (two-dimensional) crystal structure. The low dimensionality affects the electronic structure of materials, which could realize a high transition temperature (Tc) and/or unconventional pairing mechanisms. Here, we report the superconductivity in a layered tin arsenide NaSn2As2. The crystal structure consists of (Sn2As2)2- bilayers, which is bound by van-der-Waals forces, separated by Na+ ions. Measurements of electrical resistivity and specific heat confirm the bulk nature of superconductivity of NaSn2As2 with Tc of 1.3 K. Our results propose that the SnAs layers will be a basic structure providing another universality class of a layered superconducting family, and it provides a new platform for the physics and chemistry of low-dimensional superconductors with lone pair electrons.

Surface plasmon polariton (SPP) resonance spectra for noble metals (Au, Ag, and Cu) were comprehensively studied in the Kretschmann attenuated total reflection (ATR) geometry, in the wavelength (λ) range from 300 to 1000 nm with the angle of incidence (θ) ranging from 45 to 60° and the film thickness (d) ranging from 41 to 76 nm. The experimental plasmon resonance spectra were reproduced by a calculation that included the broadening effects as follows: (1) the imaginary part of the bulk dielectric constant, (2) the thickness-dependent radiative coupling of the SPP at the metal–air interface to the prism, (3) the lack of conservation of the wavevector parallel to the interface kx(k||) caused by the surface roughness, (4) scanning λ at a fixed θ (changing both energy and kx at the same time) over the SPP dispersion relation. For Au and Ag, the experimental results were in good agreement with the calculated results using the bulk dielectric constants, showing no film thickness dependence of the plasmon resonance energy. A method to extract the true width of the plasmon resonance from…

We have investigated the crystal structure and physical properties of LaO1-xFxBiSSe to reveal the intrinsic superconductivity phase diagram of the BiCh2-based layered compound family. From synchrotron X-ray diffraction and Rietveld refinements with anisotropic displacement parameters, we clearly found that the in-plane disorder in the BiSSe layer was fully suppressed for all x. In LaO1-xFxBiSSe, metallic conductivity and superconductivity are suddenly induced by electron doping even at x = 0.05 and 0.1 with a monoclinic structure. In addition, x (F concentration) dependence of the transition temperature (Tc) for x = 0.2-0.5 with a tetragonal structure shows an anomalously flat phase diagram. With these experimental facts, we have proposed the intrinsic phase diagram of the ideal BiCh2-based superconductors with less in-plane disorder.

The superconducting ground state of the newly discovered superconductor Cs$_2$Cr$_3$As$_3$ with a quasi-one-dimensional crystal structure ($T_{\bf c}\sim$ 2.1(1) K) has been investigated using magnetization and muon-spin relaxation or rotation ($μ$SR), both zero-field (ZF) and transverse-field (TF), measurements. Our ZF $μ$SR measurements reveal the presence of spin fluctuations below 4 K and the ZF relaxation rate ($λ$) shows enhancement below $T_{\bf c}\sim$ 2.1 K, which might indicate that the superconducting state is unconventional. This observation suggests that the electrons are paired via unconventional channels such as spin fluctuations, as proposed on the basis of theoretical models. Our analysis of the TF $μ$SR results shows that the temperature dependence of the superfluid density is fitted better with a nodal gap structure than an isotropic s-wave model for the superconducting gap. The observation of a nodal gap in Cs$_2$Cr$_3$As$_3$ is consistent with that observed in the isostructural K$_2$Cr$_3$As$_3$ compound through TF $μ$SR measurements. Furthermore, from our TF…

We have succeeded in preparing single crystals of CaCu3(OH)6Cl2 ∙ 0.6H2O, a candidate for the S = 1/2 Kagome lattice antiferromagnet. Magnetic properties of the compound are dominated by the nearest neighbor antiferromagnetic interaction J1, and the next nearest neighbor ferromagnetic J2 and an antiferromagnetic Jd across a hexagon, which is different from related compounds Kapellasite and Haydeeite with ferromagnetic J1. Magnetic susceptibility exhibits a sudden increase below 13 K and a cusp anomaly at T* = 7.2 K in the ab-plane, whereas only a moderate enhancement is observed below T* along the c-axis. A tiny peak detected in heat capacity at T* indicates the occurrence of a magnetic phase transition. The low temperature magnetic heat capacity was reproduced by assuming a two-dimensional spin-wave component and a temperature-linear term. The spin-wave contribution suggests a magnon excitation in a short-range ordered region, whereas the relatively large T-linear term 5.9 mJ/(Cu-mol·K2) at H = 0 T of this insulating compound suggests the existence of an unusual quasi-particle…

Understanding atomistic details of diffusion processes in amorphous structures is becoming increasingly important due to the recent advances in various information and energy devices. Atomistic simulations based on the density functional theory (DFT) represent a powerful approach; however, the development of a method characterized by both high reliability and computational efficiency remains a challenge. In this study, a simple neural network (NN) interatomic potential is constructed from the results of DFT simulations to investigate the diffusion of a single Cu atom in amorphous Ta2O5. The proposed technique is as accurate as the DFT in predicting hopping paths and the corresponding barrier energies in a given amorphous structure. Although the developed NN-based approach exhibited some limitations since it was constructed specifically for Cu, the obtained results showed that the NN potential was able to satisfactorily describe the Cu diffusion behavior. Thus, the Cu diffusion activation energy calculated at low temperatures (between 500 and 800 K) using kinetic Monte Carlo…

We study the anomalous Hall effect and orbital magnetization in chiral antiferromagnets, constructing a simple tight-binding model on a stacked Kagome lattice structure with spin-orbit coupling and the exchange interaction between the localized spins and itinerant electrons. It is shown that the ground state is an orbital ferromagnet with the easy-plane anisotropy. The relation between the orbital magnetization and the configuration of the Weyl points are discussed.

Spin-dependent electric dipole operators are investigated group-theoretically for the emergence of an electric dipole induced by a single spin or by two spins, where the spin dependences are completely classified up to the quadratic order. For a single spin, a product of spin operators behaves as an even-parity electric quadrupole operator, which differs from an odd-parity electric dipole. The lack of the inversion symmetry allows the even- and odd-parity mixing, which leads to the electric dipole described by the electric quadruple operators. Point-group tables are given for classification of the possible spin-dependent electric dipoles and for the qualitative analysis of multiferroic properties, such as an emergent electric dipole moment coexisting with a magnetic moment, electromagnon excitation, and directional dichroism. The results can be applied to a magnetic ion in crystals or embedded in molecules at a site without the inversion symmetry. In the presence of an inversion symmetry, the electric dipole does not appear for a single spin. This is not the case for the electric…

We investigate the third-harmonic generation (THG) of s-wave superconductors under microwave pulse irradiation. We consider the effect of paramagnetic impurities on the THG intensity of dirty superconductors. The nonlinear response function is calculated using the method of the quasiclassical Green function. It is shown that the amplitude mode is included as the vertex correction and makes a predominant contribution to the THG intensity. When the effect of paramagnetic impurities is weak, the THG intensity shows a peak at the temperature at which the superconducting gap is about the same as the frequency of the incident pulse, similarly to in experiments. As the effect of paramagnetic impurities is strengthened, the peak of the THG intensity disappears. This indicates that time-reversal symmetry breaking due to paramagnetic impurities eliminates the well-defined amplitude mode. The result of our calculation shows that the existence of the amplitude mode can be confirmed through the THG intensity. The result of a semiquantitative calculation is in good agreement with the experimental…

In this review we discuss the dynamics of deformable active particles mainly from the theoretical point of view. Fluid droplets in a surfactant solution undergo translational motion under certain conditions by generating a gradient of the surface tension. This migration induces shape deformation due to the nonlinear coupling between the migration and shape degrees of freedom. On the other hand, migration is induced by shape deformation in living matter. The swimming of bacteria is a pertinent example. A repeated shape change is generally necessary for the crawling motion of eukaryotic cells on substrates. We introduce a set of time-evolution equations for deformable active particles in a manner as general as possible by a symmetry argument focusing on the interplay between the shape deformation and translational motion. The spinning of a particle is also considered as the active dynamics. Our basic assumption is that the migration velocity and lower modes of the shape deformation of a spherical particle (a circular particle in two dimensions) are the relevant slow degrees of freedom…

We review our recent results on a simple droplet system consisting of droplets of a mixture of two liquids. Although the system is composed of simple chemicals, it is capable of exhibiting complex collective behaviors as a cluster dynamics. We propose a simple inhomogeneous force model to analyze the experimental observations and discuss the mechanisms of the dynamic ordering in our system. The relevance of the observations to hierarchical ordering generally observed in systems of active elements is also discussed.

We theoretically study the competition among different electronic phases in molecular conductors $κ$-(BEDT-TTF)$_2$X. The ground-state properties of a 3/4-filled extended Hubbard model with the $κ$-type geometry are investigated by a variational Monte Carlo method. We find various competing phases: dimer-Mott insulator, polar charge-ordered insulator, 3-fold charge-ordered metal, and superconductivity, whose pairing symmetry is an "extended-$s$+$d_{x^2-y^2}$"-wave type. Our results show that the superconducting phase is stabilized not on the verge of the Mott metal-insulator transition but near charge order instabilities, clearly indicating the importance of the intradimer charge degree of freedom and the intermolecular Coulomb interactions, beyond the simple description of the half-filled Hubbard model.

In order to study the nucleus–nucleus interactions for syntheses of superheavy nuclei, we measured excitation functions for the quasielastic scattering of 48Ca+208Pb, 50Ti+208Pb, and 48Ca+248Cm using the gas-filled-type recoil ion separator GARIS. The quasielastic scattering events were clearly separated from deep-inelastic events by using GARIS and its focal plan detectors, except for high-incident-energy points. The quasielastic barrier distributions were successfully extracted for these systems, and compared with coupled-channels calculations. The results of the calculations indicate that vibrational and rotational excitations of the colliding nuclei, as well as neutron transfers before contact, strongly affect the structure of the barrier distribution. For the reactions of 48Ca+208Pb and 50Ti+208Pb, a local maximum of the barrier distribution occurred at the same energy as the peak of the 2n evaporation cross section of the system. On the other hand, for the hot fusion reaction of 48Ca+248Cm, the 4n evaporation cross section of the system peaks at energies well above the maximum…

Interfaces have atomic structures that are significantly different from those in the bulk, and play crucial roles in material properties. The central structures at the interfaces that provide properties have been extensively investigated. However, determination of even one interface structure requires searching for the stable configuration among many thousands of candidates. Here, a powerful combination of machine learning techniques based on kriging and transfer learning (TL) is proposed as a method for unveiling the interface structures. Using the kriging+TL method, thirty-three grain boundaries were systematically determined from 1,650,660 candidates in only 462 calculations, representing an increase in efficiency over conventional all-candidate calculation methods, by a factor of approximately 3,600.

We have investigated optical and scintillation properties of LiGaO2 and LiAlO2 crystals. From the transmittance spectra, the absorption edges were found to be around 200 nm but wider band gap was confirmed for LiAlO2 than LiGaO2. Both photoluminescence and scintillation spectra showed a broad emission band peaking at 340 nm in the both crystals. The scintillation decay times of LiGaO2 were 460 ns and 2.8 µs while that of LiAlO2 was 2 µs. Under neutron irradiation from 252Cf, the absolute scintillation light yields of LiGaO2 and LiAlO2 were 3300 ± 300 and 7200 ± 700 ph/n, respectively.

As simple and easily controllable objects among various self-propelled particles, camphor floats on the water surface have been widely recognized. In this paper, we introduce characteristic behaviors and discuss the background mechanism of camphor floats on water, both in isolated and non-isolated conditions. In particular, we focus on: (i) the transition of dynamical characters through bifurcations exhibited by systems with small number of camphor floats and (ii) the emergence of a rich variety of complex dynamics observed in systems with large number camphor floats, and attempt to elucidate these phenomena through mathematical modeling as well as experimental analysis. Finally, we discuss the connection of the dynamics of camphor floats to that of a wider class of complex and sophisticated dynamics exhibited by various types of self-propelled particles.

In many classes of unconventional superconductors, the question of whether the superconductivity is enhanced by the quantum-critical fluctuations on the verge of an ordered phase remains elusive. One of the most direct ways of addressing this issue is to investigate how the superconducting dome traces a shift of the ordered phase. Here, we study how the phase diagram of the iron-based superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ changes with disorder via electron irradiation, which keeps the carrier concentrations intact. With increasing disorder, we find that the magneto-structural transition is suppressed, indicating that the critical concentration is shifted to the lower side. Although the superconducting transition temperature $T_c$ is depressed at high concentrations ($x\gtrsim$0.28), it shows an initial increase at lower $x$. This implies that the superconducting dome tracks the shift of the antiferromagnetic phase, supporting the view of the crucial role played by quantum-critical fluctuations in enhancing superconductivity in this iron-based high-$T_c$ family.

We conducted $^{77}$Se-nuclear magnetic resonance studies of the iron-based superconductor FeSe in magnetic fields of 0.6 to 19 T to investigate the superconducting and normal-state properties. The nuclear spin-lattice relaxation rate divided by the temperature $(T_1T)^{-1}$ increases below the structural transition temperature $T_\mathrm{s}$ but starts to be suppressed below $T^*$, well above the superconducting transition temperature $T_\mathrm{c}(H)$, resulting in a broad maximum of $(T_1T)^{-1}$ at $T_\mathrm{p}(H)$. This is similar to the pseudogap behavior in optimally doped cuprate superconductors. Because $T^*$ and $T_\mathrm{p}(H)$ decrease in the same manner as $T_\mathrm{c}(H)$ with increasing $H$, the pseudogap behavior in FeSe is ascribed to superconducting fluctuations, which presumably originate from the theoretically predicted preformed pair above $T_\mathrm{c}(H)$.

The three-dimensional Anderson model is a well-studied model of disordered electron systems that shows the delocalization--localization transition. As in our previous papers on two- and three-dimensional (2D, 3D) quantum phase transitions [J. Phys. Soc. Jpn. {\bf 85}, 123706 (2016), {\bf 86}, 044708 (2017)], we used an image recognition algorithm based on a multilayered convolutional neural network. However, in contrast to previous papers in which 2D image recognition was used, we applied 3D image recognition to analyze entire 3D wave functions. We show that a full phase diagram of the disorder-energy plane is obtained once the 3D convolutional neural network has been trained at the band center. We further demonstrate that the full phase diagram for 3D quantum bond and site percolations can be drawn by training the 3D Anderson model at the band center.

The dynamical spin susceptibility in the new-structure-type iron-based superconductor CaKFe$_4$As$_4$ was investigated by using a combination of inelastic neutron scattering (INS) measurements and random phase approximation (RPA) calculations. Powder INS measurements show that the spin resonance at $Q_\text{res}=1.17(1)$~$\textÅ^{-1}$, corresponding to the $(π,π)$ nesting wave vector in tetragonal notation, evolves below $T_\text{c}$. The characteristic energy of the spin resonance $E_\text{res}=12.5$~meV is smaller than twice the size of the superconducting gap ($2Δ$). The broad energy feature of the dynamical susceptibility of the spin resonance can be explained by the RPA calculations, in which the different superconducting gaps on different Fermi surfaces are taken into account. Our INS and PRA studies demonstrate that the superconducting pairing nature in CaKFe$_4$As$_4$ is the $s_\pm$ symmetry.

We report a high-resolution laser-based angle-resolved photoemission spectroscopy (laser-ARPES) study of single crystals of FeSe, focusing on the temperature-dependence of the hole-like bands around the ${\rm Γ}$ point. As the system cools through the tetragonal-orthorhombic "nematic" structural transition at 90~K, the splitting of the $d_{xz}$/$d_{yz}$ bands is observed to increase by a magnitude of 13 meV. Moreover, the onset of a $\sim$10 meV downward shift of the $d_{xy}$ band is also at 90~K. These measurements provide clarity on the nature, magnitude and temperature-dependence of the band shifts at the ${\rm Γ}$ point in the nematic phase of FeSe.

To understand the origin of unconventional charge-density-wave (CDW) states in cuprate superconductors, we establish the self-consistent CDW equation, and analyze the CDW instabilities based on the realistic Hubbard model, without assuming any $q$-dependence and the form factor. Many higher-order many-body processes, which are called the vertex corrections, are systematically generated by solving the CDW equation. When the spin fluctuations are strong, the uniform $q=0$ nematic CDW with $d$-form factor shows the leading instability. The axial nematic CDW instability at $q = Q_a = (δ,0)$ ($δ\approx π/2$) is the second strongest, and its strength increases under the static uniform CDW order. The present theory predicts that uniform CDW transition emerges at a high temperature, and it stabilize the axial $q = Q_a$ CDW at $T = T_{CDW}$. It is confirmed that the higher-order Aslamazov-Larkin processes cause the CDW orders at both $q = 0$ and $Q_a$.

The X-ray polarization dependent valence band HAXPES spectra of 3d transition metals (TMs) of Ti–Zn were measured to investigate the orbital resolved electronic structures by utilizing that the fact the photoionization cross-section of the atomic orbitals strongly depends on the experimental geometry. We have calculated the HAXPES spectra, which correspond to the cross-section weighted densities of states (CSW-DOSs), where the DOSs were obtained by the density functional theory calculations, and we have determined the relative photoionization cross-sections of the 4s and 4p orbitals to the 3d orbital in the 3d TMs. The experimentally obtained bulk-sensitive 3d and 4s DOSs were good agreement with the calculated DOSs in Ti, V, Cr, and Cu. In contrast, the deviations between the experimental and calculated 3d DOSs for Mn, Fe, Co, Ni were found, suggesting that the electron correlation plays an important role in the electronic structures for these materials.

High entropy alloys show a variety of fascinating properties like high hardness, wear resistance, corrosion resistance, etc. They are random solid solutions of many components with rather high concentrations. We perform ab-initio calculations for the high entropy alloy CrFeCoNi, which equal concentration of 25% for each element. By the KKRnano program package, which is based on an order-N screened Korringa–Kohn–Rostoker Green’s function method, we consider a face-centered cubic (FCC) supercell with 1372 randomly distributed elements, and in addition also smaller supercells with 500 and 256 atoms. It is found from our calculations that the local moments of the Cr atoms show a large environmental variation, ranging from −1.70 μB to +1.01 μB with an average of about −0.51 μB. We present a new method to calculate “local energies” of all atoms. This is based on the partitioning of the whole space into Voronoi cells and allows to calculate the energetic contribution of each atomic cell to the total energy of the supercell. The supercell calculations show very large variations of the local…

The magnetization process of the Kondo insulator YbB12 has been unveiled in ultrahigh magnetic fields of up to 120 T at 4.2 K. We have found a novel metamagnetic transition at Bc2 = 102 T in addition to the previously known transition at Bc1 = 55 T. It has also been observed that the magnetization tends to saturate at around 112 T. Within the rigid band model, the two-energy-gap structure in the density of states (DOS) explains the successive two-step metamagnetism as a result of the Zeeman effect of the DOS. The metamagnetic transition at Bc1 occurs along with an insulator–metal transition and the field-induced phase is expected to be a heavy fermion metallic state. The Kondo effect can weaken at the second transition of Bc2, as theoretically found in the successive two-metamagnetic-transition process in the Kondo semimetal CeNiSn.

We apply conformal field theory analysis to the $k$-channel SU($N$) Kondo system, and find a peculiar behavior in the cases $N > k > 1$, which we call Fermi/non-Fermi mixing: The low temperature scaling is described as the Fermi liquid, while the zero temperature IR fixed point exhibits the non-Fermi liquid signature. We also show that the Wilson ratio is no longer universal for the cases $N > k > 1$. The deviation from the universal value of the Wilson ratio could be used as an experimental signal of the Fermi/non-Fermi mixing.

We have investigated the low temperature quadrupolar phenomena of the non-Kramers system PrRh2Zn20 under magnetic fields in the [100] and [110] directions. Our experiments reveal the B-T phase diagram of PrRh2Zn20 involving four electronic states regardless of the field direction, namely, a non-Fermi liquid (NFL) state, an antiferro-quadrupolar (AFQ) ordered state, a novel heavy-fermion (HF) state, and a field-induced singlet (FIS) state. In the wide range of the NFL state, the resistivity can be well scaled by a characteristic temperature, suggesting the realization of the quadrupole Kondo effect. The HF state exhibits a Fermi liquid behavior with a large A coefficient of the T^2 term in the resistivity, suggesting the formation of nontrivial heavy quasi-particles. The FIS state results from the considerable splitting of a non-Kramers doublet by a magnetic field. The phase diagram shows a large anisotropy with respect to the field direction. It is found that the anisotropy of the phase diagram can be explained in terms of that of the energy splitting of the non-Kramers doublet by a…

The electrical resistivity, magnetization, and specific heat of the caged compounds ROs2Zn20 (R = La, Ce, Pr, and Nd) have been measured to study their structural, magnetic, and superconducting properties. These measurements indicate that the compounds undergo structural transitions at Ts = 151, 109, 87, and 62 K, respectively. The decrease in Ts along with the lanthanoid contraction suggests that the high-temperature phase is more stable for a smaller lattice volume. Analysis of the lattice specific heat of LaOs2Zn20 reveals that the Zn atom at the 16c site encapsulated in the R2Zn12 cage vibrates at a low energy of 3 meV. For CeOs2Zn20, the small magnetic susceptibility with a broad maximum indicates the valence-fluctuating state of the Ce ions. In PrOs2Zn20, the crystalline electric field ground state of the 4f2 state of the Pr3+ ion remains in a non-Kramers doublet at T > Ts, which is lifted by symmetry lowering of the Pr site at T < Ts. Thereby, the quadrupolar degrees of freedom are quenched, avoiding the long-range quadrupole order. PrOs2Zn20 and LaOs2Zn20 exhibit…

We have discovered a novel candidate for a spin liquid state in a ruthenium oxide composed of dimers of $S = $ 3/2 spins of Ru$^{5+}$,Ba$_3$ZnRu$_2$O$_9$. This compound lacks a long range order down to 37 mK, which is a temperature 5000-times lower than the magnetic interaction scale of around 200 K. Partial substitution for Zn can continuously vary the magnetic ground state from an antiferromagnetic order to a spin-gapped state through the liquid state. This indicates that the spin-liquid state emerges from a delicate balance of inter- and intra-dimer interactions, and the spin state of the dimer plays a vital role. This unique feature should realize a new type of quantum magnetism.

We report the discovery of nonlinear conductivity along the c-axis in a single crystal of Ca5Ir3O12, which indicates a semiconducting behavior with a narrow band gap of ∼0.2 eV. The resistivity decreases with increase in the applied current. This nonlinearity is reversible with the direction of current. We also show the ab initio density functional band structures and the Fermi surface. We found that the spin–orbit interactions result in an appreciable change in the low-energy electronic structure; the interaction splits the metallic bands and leads to a pocket-like band structure, thus reducing the metallic trend. The size of the spin–orbit interaction is estimated as ∼0.3 eV, which is large enough to be comparable to the valence bandwidth of ∼0.5 eV. The Fermi surface exhibits a sheet structure along the c*-axis, due to the 1D chain structure of edge-sharing IrO6.

The strategy of shortcuts to adiabaticity enables us to realize adiabatic dynamics in finite time. In the counter-diabatic driving approach, an auxiliary Hamiltonian which is called the counter-diabatic Hamiltonian is appended to an original Hamiltonian to cancel out diabatic transitions. The counter-diabatic Hamiltonian is constructed by using the eigenstates of the original Hamiltonian. Therefore, it is in general difficult to construct the counter-diabatic Hamiltonian for quantum many-body systems. Even if the counter-diabatic Hamiltonian for quantum many-body systems is obtained, it is generally non-local and even diverges at critical points. We construct an approximated counter-diabatic Hamiltonian for the infinite-range Ising model by making use of the mean-field approximation. An advantage of this method is that the mean-field counter-diabatic Hamiltonian is constructed by only local operators. We numerically demonstrate the effectiveness of this method through quantum annealing processes going the vicinity of the critical point. It is also confirmed that the mean-field…

We discuss Kondo phenomena in a seven-orbital impurity Anderson model hybridized with $Γ_8$ conduction electrons by employing a numerical renormalization group method. In particular, we focus on the case with three local $f$ electrons, corresponding to a Nd$^{3+}$ ion. For realistic values of Coulomb interactions, spin-orbit coupling, cubic crystalline electric field potentials, and hybridization, we find a residual entropy of $0.5\log 2$, a characteristic of two-channel Kondo phenomena, for the wide range of parameters of the local $Γ_6$ ground state. This is considered to be the magnetic two-channel Kondo effect, consistent with the result from an extended $s$-$d$ model constructed on the basis of the $j$-$j$ coupling scheme. Finally, we briefly discuss candidates of Nd compounds to observe the two-channel Kondo effect.

A general form of $N$-dark soliton solutions of the multi-component Mel'nikov system is presented. Taking the coupled Mel'nikov system comprised of two-component short waves and one-component long wave as an example, its general $N$-dark-dark soliton solutions in Gram determinant form are constructed through the KP hierarchy reduction method. The dynamics of single dark-dark soliton and two dark-dark solitons are discussed in detail. It can be shown that the collisions of dark-dark solitons are elastic and energies of the solitons in different components completely transmit through. In addition, the dark-dark soliton bound states including both stationary and moving cases are also investigated. An interesting feature for the coupled Mel'nikov system is that the stationary dark-dark soliton bound states can exist for all possible combinations of nonlinearity coefficients including all-positive, all-negative and mixed types, while the moving case are possible when they take opposite signs or they are both negative. The dynamics and several interesting structures of the solutions are…

Dirac electrons in a single-component molecular conductor [Pd(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) under pressure have been examined using a tight-binding model which consists of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) functions in four molecules per unit cell. The Dirac cone between the conduction and valence bands originates from the property that the HOMO has ungerade symmetry and the LUMO has gerade symmetry. The Dirac point forms a loop in the three-dimensional Brillouin zone, which is symmetric with respect to the plane of ky = 0, where ky is the intralayer momentum along the molecular stacking direction, i.e., with the largest (HOMO–HOMO, LUMO–LUMO) transfer energy. The parity at time reversal invariant momentum (TRIM) is calculated using the inversion symmetry around the lattice point of the crystal. It is shown that such an exotic Dirac electron can be understood from the parity of the wave function at the TRIM and also from an effective Hamiltonian.

The ground state of the spin-$1/2$ Heisenberg antiferromagnet on a distorted triangular lattice is studied using a numerical-diagonalization method. The network of interactions is the $\sqrt{3}\times\sqrt{3}$ type; the interactions are continuously controlled between the undistorted triangular lattice and the dice lattice. We find new states between the nonmagnetic 120-degree-structured state of the undistorted triangular case and the up-up-down state of the dice case. The intermediate states show spontaneous magnetizations that are smaller than one third of the saturated magntization corresponding to the up-up-down state.