The 50 most-cited articles in Journal of the Physical Society of Japan, 2019

We clarify the macroscopic symmetry and microscopic model-parameter conditions for emergence of spin-split electronic band structure in collinear antiferromagnets without atomic spin-orbit coupling. By using the microscopic multipole descriptions, we elucidate the fundamental degree of freedom in a cluster unit of an antiferromagnet giving rise to an effective spin-orbit interaction through the anisotropic kinetic motions of electrons. We show a correspondence of the ordering patterns and resultant momentum-dependent spin splitting for 32 crystallographic point groups after demonstrating two intuitive examples of four-sublattice pyrochlore and tetragonal systems. Our study unveils potential features of collinear antiferromagnets with considerably weak spin-orbit coupling in light-element materials and 3$d$ transition metal oxides, which can be utilized for a spin-current generation by electric (thermal) current and a magneto-striction effect.

We grew single crystals of the recently discovered heavy fermion superconductor UTe2, and measured the resistivity, specific heat and magnetoresistance. Superconductivity (SC) was clearly detected at Tsc=1.65K as sharp drop of the resistivity in a high quality sample of RRR=35. The specific heat shows a large jump at Tsc indicating strong coupling. The large Sommerfeld coefficient, 117mJ K-2mol-1 extrapolated in the normal state and the temperature dependence of C/T below Tsc are the signature of unconventional SC. The discrepancy in the entropy balance at Tsc between SC and normal states points out that hidden features must occur. Surprisingly, a large residual value of the Sommerfeld coefficient seems quite robust (gamma_0/gamma ~ 0.5). The large upper critical field Hc2 along the three principal axes favors spin-triplet SC. For H // b-axis, our experiments do not reproduce the huge upturn of Hc2 reported previously. This discrepancy may reflect that Hc2 is very sensitive to the sample quality. A new perspective in UTe2 is the proximity of a Kondo semiconducting phase predicted by…

A Majorana fermion is a fermionic particle that is its own antiparticle. Since the theoretical discovery in 1937, the exotic particle has long been searched in particle physics. In the last few decades, however, it has attracted renewed interest in condensed matter physics, where it can be realized as an elementary excitation (quasiparticle) in quantum states of matter. In this review, we discuss another platform for Majorana fermions, the quantum spin liquid, in which interacting magnetic moments remain disordered down to the lowest temperature under strong quantum fluctuations. They are characterized by topological entanglement and fractional excitations, whose possible application to topological quantum computation is recently discussed intensively. As a prime candidate for such exotic states, we here focus on the Kitaev magnets, a subgroup of the spin-orbit Mott insulators. After a brief overview of the Kitaev model and the fractionalization of spins in the exact ground state, we review recent explosive development in this rapidly growing field, with a focus on numerical…

The discovery in 2000 that the ferromagnetic (FM) compound UGe2 (T_Curie = 52 K at ambient pressure) becomes superconducting under a pressure of P = 1.1 GPa until it enters the paramagnetic (PM) phase above Pc = 1.6 GPa was a surprise. Successive searches for new materials (URhGe and UCoGe) led to the discovery of the coexistence of superconductivity (SC) and ferromagnetism at ambient pressure. Furthermore in UCoGe, it was found that SC survives in the PM regime from P_c = 1.1 to 4 GPa. Focus has been on low-temperature experiments under extreme conditions of magnetic field (H), pressure, and uniaxial stress. In UGe2, strong interplay exists between Fermi surface (FS) reconstructions in the cascade of different FM and PM ground states and their magnetic fluctuations. Similar phenomena occur in URhGe and UCoGe but, at first glance, the SC seems to be driven by the FM fluctuations. In UCoGe, a longitudinal field scan leads to a drastic decrease in the FM fluctuations, while a transverse field scan leads to suppression of the Curie temperature, T_Curie; the consequence is a boost in FM…

We present a study of the upper critical field of the newly discovered heavy fermion superconductor UTe$_2$ by magnetoresistivity measurements in pulsed magnetic fields up to 60~T and static magnetic fields up to 35~T. We show that superconductivity survives up to the metamagnetic transition at $H_{\rm m} \approx 35$~T at low temperature. Above $H_{\rm m}$ superconductivity is suppressed. At higher temperature superconductivity is enhanced under magnetic field leading to reentrance of superconductivity or an almost temperature independent increase of $H_{\rm c2}$. By studying the angular dependence of the upper critical field close to the $b$ axis (hard magnetization axis) we show that the maximum of the reentrant superconductivity temperature is depinned from the metamagnetic field. A key ingredient for the field-reinforcement of superconductivity on approaching $H_{\rm m}$ appears to be an immediate interplay with magnetic fluctuations and a possible Fermi-surface reconstruction.

An online advertisement optimization, which can be represented by a combinatorial optimization problem is performed using D-Wave 2000Q, a quantum annealing machine. To optimize the online advertisement allocation optimization, we introduce a generalized version of the Markowitz mean-variance model which is a basic model of portfolio optimization. The obtained optimization performance using D-Wave 2000Q is higher than that using the greedy method which is a conventional method. Additionally, to conveniently use Ising machines including a quantum annealing machine, new software called PyQUBO is developed. The first half of the paper gives a review of several combinatorial optimization problems and how to represent them using the Ising model or the quadratic unconstrained binary optimization (QUBO) form. We show the results of the online advertisement allocation optimization and the explanation of PyQUBO in the last half of the paper.

We have studied the magnetization of the recently discovered heavy fermion superconductor UTe$_2$ up to 56 T in pulsed-magnetic fields. A first-order metamagnetic transition has been clearly observed at $H_{\rm m}$ =34.9 T when the magnetic field $H$ is applied along the orthorhombic hard-magnetization $b$-axis. The transition has a critical end point at $\sim$11 K and 34.8 T, where the first order transition terminates and changes into a crossover regime. Using the thermodynamic Maxwell relation, we have evaluated the field dependence of the Sommerfeld coefficient of the specific heat directly related to the superconducting pairing. From the analysis, we found a significant enhancement of the effective mass centered at $H_{\rm m}$, which is reminiscent of the field-reentrant superconductivity of the ferromagnet URhGe in transverse fields. We discuss the origin of their field-robust superconductivity.

Quantum computers with Kerr-nonlinear parametric oscillators (KPOs) have recently been proposed by the author and others. Quantum computation using KPOs is based on quantum adiabatic bifurcations of the KPOs, which lead to quantum superpositions of coherent states, such as Schrodinger cat states. Therefore, these quantum computers are referred to as "quantum bifurcation machines (QbMs)." QbMs can be used for qauntum adiabatic optimization and universal quantum computation. Superconducting circuits with Josephson junctions, Josephson parametric oscillators (JPOs) in particular, are promising for physical implementation of KPOs. Thus, KPOs and QbMs offer not only a new path toward the realization of quantum bits (qubits) and quantum computers, but also a new application of JPOs. Here we theoretically explain the physics of KPOs and QbMs, comparing them with their dissipative counterparts. Their physical implementations with superconducting circuits are also presented.

We have performed the $^{125}$Te-nuclear magnetic resonance (NMR) measurement in the field along the $b$ axis on the newly discovered superconductor UTe$_2$, which is a candidate of a spin-triplet superconductor. The nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ abruptly decreases below a superconducting (SC) transition temperature $T_c$ without showing a coherence peak, indicative of UTe$_2$ being an unconventional superconductor. It was found that the temperature dependence of $1/T_1T$ in the SC state cannot be understood by a single SC gap behavior but can be explained by a two SC gap model. The Knight shift, proportional to the spin susceptibility, decreases below $T_c$, but the magnitude of the decrease is much smaller than the decrease expected in the spin-singlet pairing. Rather, the small Knight-shift decrease as well as the absence of the Pauli-depairing effect can be interpreted by the spin triplet scenario.

We report 125Te-NMR studies on a newly discovered heavy fermion superconductor UTe2. Using a single crystal, we have measured the 125Te-NMR Knight shift K and spin-lattice relaxation rate 1/T1 for fields along the three orthorhombic crystal axes. The data confirm a moderate Ising anisotropy for both the static (K) and dynamical susceptibilities (1/T1) in the paramagnetic state above about 20 K. Around 20 K, however, we have observed a sudden loss of NMR spin-echo signal due to sudden enhancement of the NMR spin-spin relaxation rate 1/T2, when the field is applied along the easy axis of magnetization (=a axis). This behavior suggests the development of longitudinal magnetic fluctuations along the a axis at very low frequencies below 20 K.

A recent Google study [Phys. Rev. X, 6:031015 (2016)] compared a D-Wave 2X quantum processing unit (QPU) to two classical Monte Carlo algorithms: simulated annealing (SA) and quantum Monte Carlo (QMC). The study showed the D-Wave 2X to be up to 100 million times faster than the classical algorithms. The Google inputs are designed to demonstrate the value of collective multiqubit tunneling, a resource available to D-Wave QPUs but not to simulated annealing. But the computational hardness in these inputs is highly localized in gadgets, with only a small amount of complexity coming from global interactions, meaning that the relevance to real-world problems is limited. In this study we provide a new synthetic problem class that addresses the limitations of the Google inputs while retaining their strengths. We use simple clusters instead of more complex gadgets and more emphasis is placed on creating computational hardness through frustrated global interactions like those seen in interesting real-world inputs. The logical problems used to generate these inputs can be solved in polynomial…

In 2012, we discovered superconductivity in Bi4O4S3 and LaO1-xFxBiS2, whose crystal structures are composed of alternate stacks of a BiS2-type conduction layer and a blocking layer. Since the discovery, many related superconductors and functional materials have been synthesized. Furthermore, the possibility of unconventional superconductivity has been proposed in recent theoretical and experimental studies. In addition, notable correlations between local structure and physical properties have been revealed in the BiS2-based systems. In this review article, we summarize the material development (probably all the compounds) and physical properties of BiS2-related materials developed in the last six years. The key parameters for the emergence of bulk superconductivity in the BiS2-based compounds are carrier doping to the parent phase (band insulator), pressure-induced structural transition, in-plane chemical pressure, and the suppression of in-plane disorder. We systematically review those parameters and the method of designing new superconductors with BiS2 layers. In addition, studies…

Magnetic order in the magnetic-field-induced so-called A phase of cubic chiral antiferromagnet EuPtSi was investigated by resonant X-ray scattering (RXS) near the Eu L2 absorption edge. The high-intensity RXS signal revealed the triple-q magnetic order of the A phase in a magnetic field applied along the [111] direction. The triple-q order is described by three propagation vectors lying on the plane normal to the magnetic field direction. The triple-q magnetic order is discussed in the context of a magnetic skyrmion lattice where geometrical frustration may play a crucial role.

We present magnetoresistivity measurements on the heavy-fermion superconductor UTe$_{2}$ in pulsed magnetic fields $μ_0H$ up to 68~T and temperatures $T$ from 1.4 to 80~K. Magnetic fields applied along the three crystallographic directions $\mathbf{a}$ (easy magnetic axis), $\mathbf{b}$, and $\mathbf{c}$ (hard magnetic axes), are found to induce different phenomena - depending on the field direction - beyond the low-field suppression of the superconducting state. For $\mathbf{H}\parallel\mathbf{a}$, a broad anomaly in the resistivity is observed at $μ_0H^*\simeq10$~T and $T = 1.4$~K. For $\mathbf{H}\parallel\mathbf{c}$, no magnetic transition nor crossover are observed. For $\mathbf{H}\parallel\mathbf{b}$, a sharp first-order-like step in the resistivity indicates a metamagnetic transition at the field $μ_0H_m \simeq 35$~T. When the temperature is raised signature of first-order metamagnetism is observed up to a critical endpoint at $T_{CEP}\simeq7$~K. At higher temperatures a crossover persists up to 28~K, i.e., below the temperature $T_χ^{max} = 35$~K where the magnetic…

We measured the magnetization, ac susceptibility, Hall resistivity, and magnetoresistance for a cubic chiral antiferromagnet EuPtSi, and found the skyrmion lattice of the so-called "A-phase" in a wide field direction, except around H ∥ [110], together with a new skyrmion-related phase named the "B-phase" for H ∥ [001]. A large magnitude of a skyrmion-derived additional Hall resistivity is discussed on the basis of a small size of the skyrmion, 18 Å for H ∥ [111]. We also observed the de Haas–van Alphen oscillations and clarified the Fermi surface properties. Many nearly spherical Fermi surfaces are observed in EuPtSi, which are well explained by the results of full-potential linearized augmented plane wave band calculations for a non-4f reference compound SrPtSi. The splitting energy of main two split Fermi surfaces named α and α′ is determined as a large value of 1200 K, mainly reflecting the chiral (noncentrosymmetric) crystal structure and a large spin–orbit coupling of Pt-5d electrons.

A polycrystalline sample of the high-entropy-alloy-type telluride AgInSnPbBiTe5 was synthesized using high-pressure synthesis. Superconductivity with a transition temperature (Tc) of 2.6 K was observed in AgInSnPbBiTe5. Elemental and structural analyses revealed that five metals are mixed in a metal site of an NaCl-type structure. Since AgInSnPbBiTe5 has a cation site (Ag, In, Sn, Pb, and Bi) and an anion site (Te), this is the first example of a high-entropy-alloy (HEA) pseudo-binary superconductor.

We report on the cubic double perovskite Ba2MgReO6 containing Re6+ ions with the 5d1 electron configuration. Resistivity, magnetization, and heat capacity measurements using single crystals show that the compound is a Mott insulator with a magnetic transition at Tm = 18 K, which is accompanied by a weak ferromagnetic moment with [110] anisotropy. Another transition is observed at Tq = 33 K in heat capacity, where the inverse of magnetic susceptibility changes its slope, indicating a substantial change in the electronic state. The significance of spin-orbit coupling is revealed by the reduced effective magnetic moment of ~0.68μB at high temperature above Tq and the total electronic entropy close to Rln4. These features indicate that Ba2MgReO6 is a spin-orbit coupled Mott insulator possessing a Jeff = 3/2 quartet state, which exhibits quadrupolar and dipolar orders at Tq and Tm, respectively.

We investigate the first-order phase transitions of the $q$-state Potts models with $q = 5, 6, 7$, and $8$ on the two-dimensional square lattice, using Monte Carlo simulations. At the very weakly first-order transition of the $q=5$ system, the standard data-collapse procedure for the order parameter, carried out with results for a broad range of system sizes, works deceptively well and produces non-trivial critical exponents different from the trivial values expected for first-order transitions. However, a more systematic study reveals significant drifts in the `pseudo-critical' exponents as a function of the system size. For this purpose, we employ two methods of analysis: the data-collapse procedure with narrow range of the system size, and the Binder-cumulant crossing technique for pairs of system sizes. In both methods, the estimates start to drift toward the trivial values as the system size used in the analysis exceeds a certain `cross-over' length scale. This length scale is remarkably smaller than the correlation length at the transition point for weakly first-order…

Superconductive H3S synthesized from H2S is very poorly crystallized, and has excess sulfur as impurity (3H2S -> 2H3S + S). The phase transition process undergoes in sulfur excess condition, which might cause hydrogen deficiency. The influence of hydrogen deficiency is not clear. Therefore investigation on the superconductivity in H3S with no hydrogen deficiency is demanded. Two groups performed synthesis of H3S from elemental sulfur and hydrogen (direct synthesis) and their results have shown that no hydrogen deficiency is caused when the direct synthesis is performed under hydrogen excessive condition. However, no measurements of superconductivity has been carried out because of the major technical difficulties in hydrogen experiments in diamond anvil cells (DACs). Here, we report the first electrical resistance measurements in superconductive H3S synthesized from elemental sulfur and hydrogen (3H2 + 2S -> 2H3S). Our powder X-ray diffraction (PXRD) using a synchrotron X-ray revealed that synthesized H3S has much improved crystalline quality and no sulfur as reported in previous…

The electronic structure of the unconventional superconductor UTe$_2$ was studied by resonant photoelectron spectroscopy (RPES) and angle-resolved photoelectron spectroscopy (ARPES) with soft X-ray synchrotron radiation. The partial $\mathrm{U}~5f$ density of states of UTe$_2$ were imaged by the $\mathrm{U}~4d$--$5f$ RPES and it was found that the $\mathrm{U}~5f$ state has an itinerant character, but there exists an incoherent peak due to the strong electron correlation effects. Furthermore, an anomalous admixture of the $\mathrm{U}~5f$ states into the $\mathrm{Te}~5p$ bands was observed at a higher binding energy, which cannot be explained by band structure calculations. On the other hand, the band structure of UTe$_2$ was obtained by ARPES and its overall band structure were mostly explained by band structure calculations. These results suggest that the $\mathrm{U}~5f$ states of UTe$_2$ have itinerant but strongly-correlated nature with enhanced hybridization with the $\mathrm{Te}~5p$ states.

The coupled higher order nonlinear Schrödinger equation is investigated by using the Darboux-dressing transformation with the Lax pair and asymptotic expansion method. Based on the Lax pair, we first construct the generalized Darboux transformation of the equation. Then, we construct the first-order breather wave solution expressed in terms of hyperbolic function and exponential function. Moreover, using a taylor expansion method, we naturally obtain the first, second, third even higher order rogue wave solutions. Actually, when one of \(a_{1},a_{2}\) equates to zero, the corresponding breather wave and rogue wave will propagate along with multiple bright-dark soliton background, which are also respectively called the superposition of breather wave and rogue wave with multiple bright-dark solitons. By applying a variable separation transformation with the third-order constant vector w, the higher-order rogue wave will transfer to the multiple independent rogue waves and present a triangular. These nonlinear phenomena can be used to understand the dynamics of the rogue wave…

Based on the general framework of the linear response theory to thermal perturbation by Luttinger together with the method developed by Jonson and Mahan, range of validity of Sommerfeld–Bethe relation between two correlation functions L11 and L12 associated with the Seebeck coefficient S = L12/TL11 with T being temperature is clarified for a model with both periodic and random potentials, electron–phonon interaction, and electron–electron interaction. The contributions which do not satisfy the Sommerfeld–Bethe relation are identified, including well-known phonon drag, a part of electron–phonon interaction noted by Jonson and Mahan, and the finite-range part of electron–electron interactions for the first time. The general expression of the contribution of phonon drag to L12 is derived which is valid even under the strong disorder, i.e., beyond the Boltzmann transport regime.

Applications of neural networks to condensed matter physics are becoming popular and beginning to be well accepted. Obtaining and representing the ground and excited state wave functions are examples of such applications. Another application is analyzing the wave functions and determining their quantum phases. Here, we review the recent progress of using the multilayer convolutional neural network, so-called deep learning, to determine the quantum phases in random electron systems. After training the neural network by the supervised learning of wave functions in restricted parameter regions in known phases, the neural networks can determine the phases of the wave functions in wide parameter regions in unknown phases; hence, the phase diagrams are obtained. We demonstrate the validity and generality of this method by drawing the phase diagrams of two- and higher dimensional Anderson metal-insulator transitions and quantum percolations as well as disordered topological systems such as three-dimensional topological insulators and Weyl semimetals. Both real-space and Fourier space wave…

The ion acoustic cnoidal waves (IACWs) are investigated in a magnetized anisotropic electron ion plasma with superthermal electron distribution. The ions are warm and manifest pressure anisotropy. The reductive perturbation technique is used to explicitly derive the Korteweg–de Vries (KdV) equation which shows the existence and evolution of cnoidal waves (CWs). The upshot of various physical plasma parameters on the properties of IACWs is considered at length. The relevance of present investigation to the magnetosphere and near Earth magnetosheath is pointed out.

This review paper describes the basic concept and technical details of sparse modeling and its applications to quantum many-body problems. Sparse modeling refers to methodologies for finding a small number of relevant parameters that well explain a given dataset. This concept reminds us physics, where the goal is to find a small number of physical laws that are hidden behind complicated phenomena. Sparse modeling extends the target of physics from natural phenomena to data, and may be interpreted as "physics for data". The first half of this review introduces sparse modeling for physicists. It is assumed that readers have physics background but no expertise in data science. The second half reviews applications. Matsubara Green's function, which plays a central role in descriptions of correlated systems, has been found to be sparse, meaning that it contains little information. This leads to (i) a new method for solving the ill-conditioned inverse problem for analytical continuation, and (ii) a highly compact representation of Matsubara Green's function, which enables efficient…

Based on linear response theory, we study phonon drag effects on Seebeck coefficient beyond Boltzmann transport regime by focusing on the roles of impurity bands with the aim to explore the possible microscopic understanding of unusual behavior observed in FeSb2. We find that the phonon drag in the presence of impurity bands with the narrow band gap below the conduction band with large effective mass will be the key factors behind the remarkable phenomenon.

We propose a prime factorizer operated in a framework of quantum annealing (QA). The idea is inverse operation of a multiplier implemented with QA-based Boolean logic circuits. We designed the QA machine on an application-specific-annealing-computing architecture which efficiently increases available hardware budgets at the cost of restricted functionality. The invertible operation of QA logic gates consisting of superconducting flux qubits was confirmed by circuit simulation with classical noise sources. The circuits were implemented and fabricated by using superconducting integrated circuit technologies with Nb/AlOx/Nb Josephson junctions. We also propose a 2.5Dimensional packaging scheme of a qubit-chip/interpose /package-substrate structure for realizing practically large-scale QA systems.

We investigated the thermodynamic property of the heavy fermion superconductor UTe$_2$ in pulsed high magnetic fields. The superconducting transition in zero field was observed at $T_{\rm c}$=1.65 K as a sharp heat capacity jump. Magnetocaloric effect measurements in pulsed-magnetic fields obviously detected a thermodynamic anomaly accompanied by a first-order metamagnetic transition at $μ$$_{0}$$H_{\rm m}$=36.0 T when the fields are applied nearly along the hard-magnetization $b$-axis. From the results of heat capacity measurements in magnetic fields, we found a drastic diverging electronic heat capacity coefficient of the normal state $γ$$_{\rm N}$ with approaching $H_{\rm m}$. Comparing with the previous works via the magnetic Clausius-Clapeyron relation, we unveil the thermodynamic details of the metamagnetic transition. The enhancement of the effective mass observed as the development of $γ_{\rm N}$ indicates that quantum fluctuation strongly evolves around $H_{\rm m}$; it assists the superconductivity emerging even in extremely high fields.

For large-scale computation of the exchange coupling constants $J_{ij}$, we reconstruct the Liechtenstein formula for localized orbital representation and simplify the energy integrations by adopting the finite pole approximation of the Fermi function proposed by Ozaki [Phys. Rev. B 75, 035123 (2007)]. We calculate the exchange coupling constant $J_{\mathrm{1NN}}$ of the first-nearest-neighbor sites in body-centered-cubic Fe systems of various sizes to estimate the optimal computational parameters that yield appropriate values at the lowest computational cost. It is shown that the number of poles needed for a computational accuracy of 0.05 meV is determined as $\sim$ 60, whereas the number of necessary Matsubara poles needed to obtain similar accuracy, which was determined in previous studies, is on the order of 1000. Finally, we show $J_{ij}$ as a function of atomic distance, and compared it with one derived from Korringa-Kohn-Rostoker Green's function formalism. The distance profile of $J_{ij}$ derived by KKR formalism agrees well with that derived by our study, and this agreement…

We construct a two-orbital effective model for a ferromagnetic Kagome-lattice shandite, $\rm{{Co}_3{Sn}_2{S}_2}$, a candidate material of magnetic Weyl semimetals, by considering one $d$ orbital from Co, and one $p$ orbital from interlayer Sn. The energy spectrum near the Fermi level, and the configurations of the Weyl points, computed by using our model, are similar to those obtained by first principle calculations. We show also that nodal rings appear even with spin-orbit coupling when the magnetization points in-plane direction. Additionally, magnetic properties of $\rm{{Co}_3{Sn}_2{S}_2}$ and other shandite materials are discussed.

Higher-order topological insulators have attracted considerable interests as a novel topological phase of matter, where topologically non-trivial nature of bulk protects boundary states whose co-dimension is larger than one. It has been revealed that the alternating pattern of hopping amplitudes in two-dimensional lattices provides a promising route to realization of the higher-order topological insulators. In this paper, we propose that a honeycomb-lattice model with anti-Kekulé distortion hosts a higher-order topological phase. Here, the term anti-Kekulé distortion means that the pattern of strong and weak hoppings is opposite to that for the conventional Kekulé distortion. We demonstrate the existence of the higher-order topological phase by calculating the $\mathbb{Z}_6$ Berry phase that serves as a bulk topological invariant of the higher-order topological phase, and by showing the existence of corner states.

Photoinduced nonequilibrium phenomena in magnets with strong electron correlation are reviewed. Owing to the strong competition between electron itineracy and localization and the existence of multiple degrees of freedom, a wide variety of drastic changes in electron and lattice structures are brought about by photoirradiation in correlated electron magnets. Recent rapid progress in ultrafast light sources, time-resolved probe techniques, and theoretical calculation methods for the real-time dynamics have promoted studies of photoinduced nonequilibrium dynamics in the systems. We mainly focus on three classes of systems: i) spin–charge coupled magnets where magnetism and electric transport are correlated with each other, ii) correlated electron systems with the spin-state degree of freedom, i.e., multiple spin amplitudes in magnetic ions, and iii) photogenerated carrier dynamics in low-dimensional antiferromagnetic ordered Mott insulating systems. Prototypical target materials in the three classes of systems are manganites, cobaltates, and cuprate superconductors with a perovskite…

Magnetic field versus temperature (H–T) phase diagram in the trillium lattice antiferromagnet EuPtSi for H ∥ [001] was studied by precise measurements of specific heat, thermal expansion, magnetostriction, magnetic susceptibility, and magnetization. In addition to the A-phase, the B-phase was newly observed in the field region above the A-phase in the H–T plane. The anisotropic H–T phase diagrams in EuPtSi are in sharp contrast to the almost isotropic H–T phase diagrams in prototypical skyrmion compounds, such as MnSi and Cu2OSeO3. A very sharp peak of the specific heat and jumps of the magnetic entropy, thermal expansion, and magnetic susceptibility at the ordering temperature TN = 4.0 K support the first-order transition at μ0H = 0 T. The first-order nature is rapidly suppressed by applying magnetic field and changed to the second-order transition above μ0H \( \simeq \) 0.5 T for H ∥ [001].

We investigate a possibility of odd-parity multipole orderings in a locally noncentrosymmetric tetragonal compound CeCoSi. By performing symmetrical and microscopic mean-field analyses on a two-orbital tight-binding model, we propose potential odd-parity multipoles hidden in staggered antiferromagnetic and antiferroquadrupole orderings in CeCoSi. We show that $3z^2-r^2$ type of the magnetic quadrupole is induced by the staggered magnetic dipole ordering for a large crystal-field splitting between two orbitals, while $xy$ type of the electric toroidal quadrupole is emergent by the staggered electric quadrupole ordering for a small crystal-field splitting. Furthermore, we discuss a magneto-electric effect and elastic-electric effect due to the odd-parity multipoles, which will be useful to identify order parameters in CeCoSi.

Under investigation in this work is the Kundu-nonlinear Schrödinger equation, which can be reduced to the standard nonlinear Schrödinger equation. Through the Darboux-dressing transformation, we obtain and discuss the localized wave solutions of the equation. This family of solutions can include known Peregrine soliton (i.e., Rogue waves), Akhmediev breather, Kuznetsov–Ma soliton. Moreover, the main characteristics of these rational solutions are discussed with some graphics. These solutions should be of great interest in a variety of complex systems, from optics and fluid dynamics to Bose–Einstein condensates and finance.

We report resistivity measurements under pressure for Kondo-lattice ferromagnet CeRh 6 Ge 4 , and present that a quantum ferromagnetic (FM) phase transition is easily achieved.In most clean metallic ferromagnets, a quantum critical point (QCP) at zero field is avoided by changing the FM transition to discontinuous transition or to antiferromagnetic transition.In CeRh 6 Ge 4 , to the contrary, the Curie temperature of 2.5 K decreases continuously as increasing pressure without any clear signature that the transition changes to first order.The obvious non Fermi liquid behavior is observed in the vicinity of the quantum FM phase transition.The experimental data do not contradict a picture in which CeRh 6 Ge 4 shows the FM QCP at zero field.Band structure calculation suggests unusual electronic state of CeRh 6 Ge 4 among Ce-based Kondo lattices.CeRh 6 Ge 4 deserves further investigations and will be a key material to understand the matter of a FM QCP.A quantum critical point (QCP) is a phase transition at zero temperature accompanied by a continuous change of the order parameter.A…

Recent progress in neutron spin-echo spectroscopy by means of longitudinal Modulation of IntEnsity with Zero Effort (MIEZE) is reviewed. Key technical characteristics are summarized which highlight that the parameter range accessible in momentum and energy, as well as its limitations, are extremely well understood and controlled. Typical experimental data comprising quasi-elastic and inelastic scattering are presented, featuring magneto-elastic coupling and crystal field excitations in Ho2Ti2O7, the skyrmion lattice to paramagnetic transition under applied magnetic field in MnSi, ferromagnetic criticality and spin waves in Fe. In addition bench marking studies of the molecular dynamics in H2O are reported. Taken together, the advantages of MIEZE spectroscopy in studies at small and intermediate momentum transfers comprise an exceptionally wide dynamic range of over seven orders of magnitude, the capability to perform straight forward studies on depolarizing samples or under depolarizing sample environments, as well as on incoherently scattering materials.

We study magnetoelectric (magneto-current) effects in a diamond structure under antiferro quadrupole (AFQ) orders. The AFQ orders break the spacial inversion symmetry and cause the current-induced magnetization. The current-induced magnetization strongly depends on the types of the order parameters and the direction of the current. This gives a way to the experimental identification of AFQ order parameters. We also discuss the current-induced magnetization under the AFQ orders in the diamond structure can be intuitively understood in terms of charge-imbalanced solenoids.

We succeeded in synthesizing single crystals of the tetragonal antiferromagnet CeCoSi, which shows an anomalous pressure induced ordered phase (PIOP) with an extraordinary high transition temperature of T ∼ 40 K. In this paper, we present the results of the magnetization, specific heat, and electrical resistivity at ambient pressure using single crystals. Magnetization curves up to H = 55 T were also examined on a polycrystalline sample. From the results, we found a new phase transition at T0 ∼ 12 K above the antiferromagnetic phase transition at TN = 9.4 K. At T0, the magnetization shows a small kink, followed by a further increase with decreasing temperature, which is reminiscent of characteristics reported previously under pressures with respect to the PIOP. Also, the value of T0 quantitatively coincides well with that expected by extrapolating the phase boundary of the PIOP from high pressures to ambient pressure in the P–T phase diagram. Therefore, we conclude that the phase transition at T0 ∼ 12 K found at ambient pressure is identical to the phase transition of the PIOP. In…

Today, world-wide efforts are being undertaken to explore non-von Neumann computation methods to overcome "the end of Moore's law". A promising approach is the coherent Ising machine (CIM), which utilizes a network of degenerate optical parametric oscillators for solving ground-state-search problems of the Ising model. Since its proposal in 2013 and the first proof-of-principle experiment in 2014, research on experimental CIMs has made rapid progress and now reached the level where we can find solutions to large-scale combinatorial optimization problems using a CIM system. In this paper, we review recent progress on CIMs, mainly focusing on the experimental side.

The cubic chiral magnet EuPtSi undergoes a helimagnetic transition at TN = 4.0 K. We examined the magnetic field (H) evolution of this phase transition by means of high-precision magnetization (M) measurements on a high-quality single crystal. At low fields, the temperature derivative of the magnetization dM/dT exhibits a sharp symmetric peak at TN(H), reminiscent of a first-order transition. By increasing H, the shape of dM/dT drastically changes above a certain field to an asymmetric step-like function. The results provide strong evidence for a tricritical point, whose location is strongly field-orientation dependent. Field variation of M in the paramagnetic state above TN indicates that strong antiferromagnetic fluctuations exist and persist up to ∼7 K, in accordance with a fluctuation-induced first-order transition.

We discuss applications of shortcuts to adiabaticity (STA) to adiabatic quantum computation. After reviewing the fundamental properties and the present status of STA from the author's personal point of view, we apply the method to the adiabatic algorithm of the Grover's problem. We discuss two possible implementations of STA for the adiabatic quantum computations: the method of quantum adiabatic brachistochrone and the Lewis-Riesenfeld invariant-based inverse engineering.

Using a nonequilibrium implementation of the Lanczos-based exact diagonalisation technique we study the possibility of the light-induced superconducting phase coherence in a solid state system after an ultrafast optical excitation. In particular, we investigate the buildup of superconducting correlations by calculating an exact time-dependent wave function reflecting the properties of the system in non-equilibrium and the corresponding transient response functions. Within our picture we identify a possible transient Meissner effect after dynamical quenching of the non-superconducting wavefunction and extract a characteristic superfluid density that we compare to experimental data. Finally, we find that the stability of the induced superconducting state depends crucially on the nature of the excitation quench: namely, a pure interaction quench induces a long-lived superconducting state, whereas a phase quench leads to a short-lived transient superconductor.

Traffic efficiency and safety in work zone are major concerns for traffic engineers, highway agencies and road users. Microscopic traffic simulation is an efficient method to study work zone traffic and cellular automaton (CA) model is one of the most widely adopted microscopic traffic simulation methods. Existing CA-based simulations of work zone traffic face limitations of unrealistic deceleration behaviors. Firstly, an improved CA model for heterogeneous traffic flow in work zone is proposed based on the two-lane safety driving (SD) model. Secondly, the superiority of the proposed methodology is demonstrated through comparison of driving behaviors simulated by the proposed model and conventional CA model. With the proposed model, simulated vehicles can accelerate, decelerate, keep their velocity, and change lane in a more realistic way during the whole process of passing the work zone by avoiding the limitations of the existing models. Furthermore, parametric analyses of some key parameters are performed for work zone traffic simulation with the proposed model. An optimal work…

Accurate, reliable sampling from fully-connected graphs with arbitrary correlations is a difficult problem. Such sampling requires knowledge of the probabilities of observing every possible state of a graph. As graph size grows, the number of model states becomes intractably large and efficient computation requires full sampling be replaced with heuristics and algorithms that are only approximations of full sampling. This work investigates the potential impact of adiabatic quantum computation for sampling purposes, building on recent successes training Boltzmann machines using a quantum device. We investigate the use case of quantum computation to train Boltzmann machines for predicting the 2016 Presidential election.

BiS2-based layered superconductors have attracted considerable interest owing to the relatively large spin–orbit coupling of bismuth 6p orbitals, the strong coupling nature by anharmonic phonons, and some evidence of an anisotropic superconducting gap structure. In the early stage of study, both conventional electron–phonon and unconventional mechanisms were examined as the superconducting pairing mechanism. Most experimental results can be well explained by the fully gapped strong-coupling s-wave state, which is driven by the electron–phonon interactions. Nevertheless, the recent angle-resolved photoemission measurement has indicated the possibility of a sign change of the superconducting gap. Furthermore, the absence of the isotope effect has been reported. These recent findings are at odds with the conventional s-wave pairing state, and rather imply unconventional pairing states. This means that the pairing glue also remains unclear. In this review article, we provide an overview of the crystal structures, electronic band structures, phonon properties, and candidates for the…

In this paper, we propose a new method of Bayesian measurement for spectral deconvolution, which regresses spectral data into the sum of unimodal basis function such as Gaussian or Lorentzian functions. Bayesian measurement is a framework for considering not only the target physical model but also the measurement model as a probabilistic model, and enables us to estimate the parameter of a physical model with its confidence interval through a Bayesian posterior distribution given a measurement data set. The measurement with Poisson noise is one of the most effective system to apply our proposed method. Since the measurement time is strongly related to the signal-to-noise ratio for the Poisson noise model, Bayesian measurement with Poisson noise model enables us to clarify the relationship between the measurement time and the limit of estimation. In this study, we establish the probabilistic model with Poisson noise for spectral deconvolution. Bayesian measurement enables us to perform virtual and computer simulation for a certain measurement through the established probabilistic…

Core-level X-ray photoelectron spectroscopy (XPS) is a useful measurement technique for investigating the electronic states of a strongly correlated electron system. Usually, to extract physical information of a target object from a core-level XPS spectrum, we need to set an effective Hamiltonian by physical consideration so as to express complicated electron-to-electron interactions in the transition of core-level XPS, and manually tune the physical parameters of the effective Hamiltonian so as to represent the XPS spectrum. Then, we can extract physical information from the tuned parameters. In this paper, we propose an automated method for analyzing core-level XPS spectra based on the Bayesian model selection framework, which selects the effective Hamiltonian and estimates its parameters automatically. The Bayesian model selection, which often has a large computational cost, was carried out by the exchange Monte Carlo sampling method. By applying our proposed method to the 3$d$ core-level XPS spectra of Ce and La compounds, we confirmed that our proposed method selected an…

Successive magnetic phase transitions at $T_1$=17.5 K and $T_2$=18.5 K in Gd$_3$Ru$_4$Al$_{12}$, with a distorted kagome lattice of Gd ions, is studied using resonant X-ray diffraction with polarization analysis. It has been suggested that in this compound the $S=7/2$ spins on the nearest-neighbor Gd-triangle form a ferromagnetic trimer and the Gd lattice can be effectively considered as an antiferromagnetic triangular lattice of $S=21/2$ spin trimers [S. Nakamura et al., Phys. Rev. B 98, 054410 (2018)]. We show that the magnetic order in this system is described by an incommensurate wave vector $q$~(0.27, 0, 0), which varies slightly with temperature. In the low temperature phase below $T_1$, the experimental results are well explained by considering that the spin trimers form a helical order with both the $c$-axis and $c$-plane components. In the intermediate phase above $T_1$, the $c$-axis component vanishes, resulting in a sinusoical structure within the $c$-plane. The sinusoidal-helical transition at $T_1$ can be regarded as an ordering of chiral degree of freedom, which is…