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久保 勝規
Journal of the Physical Society of Japan, 94(7), p.074701_1 - 074701_5, 2025/07
Surface states of a tight-binding model with nearest-neighbor hopping on a diamond lattice of finite thickness are investigated. We consider systems with (001), (110), and (111) surfaces. Even if the surface direction is fixed, there is freedom to choose the surface structure due to the two-sublattice nature of a diamond lattice. The existence of surface states is governed by the topology of the matrix elements of the bulk Hamiltonian. In this sense, the existence of surface states is determined by the bulk Hamiltonian. However, the matrix elements depend on the choice of the unit cell, which should be chosen to conform to the surface structure. Thus, the surface states depend on the surface structure. We find that for each surface direction, there are two choices of surface structure, and depending on the chosen structure, the corresponding surface states emerge in distinct regions of the surface Brillouin zone.
orbital system on a simple cubic lattice久保 勝規
Journal of the Physical Society of Japan, 94(5), p.054703_1 - 054703_7, 2025/05
A tight-binding model for orbitals on a simple cubic lattice with finite thickness is investigated. The hopping integrals for nearest-neighboring sites are considered. We examine the electronic band structures for systems with (001), (110), and (111) surfaces. Electronic states well localized around the surfaces are found for the (110) and (111) surfaces. In particular, the surface state is flat and extends in the entire Brillouin zone for the (111) surface, provided the bulk band projected onto the surface Brillouin zone is gapped. We also find that these surface states possess octupole moments in both the (110) and (111) surface cases.
orbital system on a square lattice久保 勝規
Physical Review B, 110(7), p.075110_1 - 075110_7, 2024/08
被引用回数：0 パーセンタイル：0.00(Materials Science, Multidisciplinary)A tight-binding model for orbitals on a square lattice is investigated. We consider only the nearest-neighbor hopping and the model is characterized by two hopping parameters, 
and 
. There are Dirac points in the electronic band structure and the type of the Dirac points (type-I or type-II) depends on the ratio 
. For the case of the type-I Dirac points, edge states appear for a lattice with edges perpendicular to the ![$$[11]$$](/search/images/ERNTCMK3EQ.gif)
direction. The edge states at a certain momentum along the edges have octupole moments with opposite signs between the right and left edges. Thus, these edge states can be called helical octupolar edge states. This study bridges the research fields of topological phenomena and multipole physics.
久保 勝規
Journal of the Physical Society of Japan, 93(2), p.024708_1 - 024708_8, 2024/02
被引用回数：4 パーセンタイル：82.50(Physics, Multidisciplinary)We investigate the Hubbard model with the Rashba spin-orbit coupling on a square lattice. The Rashba spin-orbit coupling generates two-dimensional Weyl points in the band dispersion. In a system with edges along [11] direction, zero-energy edge states appear, while no edge state exists for a system with edges along an axis direction. The zero-energy edge states with a certain momentum along the edges are predominantly in the up-spin state on the right edge, while they are predominantly in the down-spin state on the left edge. Thus, the zero-energy edge states are helical. By using a variational Monte Carlo method for finite Coulomb interaction cases, we find that the Weyl points can move toward the Fermi level by the correlation effects. We also investigate the magnetism of the model by the Hartree-Fock approximation and discuss weak magnetic order in the weak-coupling region.
久保 勝規
JPS Conference Proceedings (Internet), 38, p.011161_1 - 011161_6, 2023/05
In recent years, phenomena originating from spin-orbit coupling (SOC), such as topological phenomena, have been intensively studied. To realize these phenomena, materials containing heavy elements have been investigated because of large SOC. On the other hand, to enhance the effect of the SOC, it may be possible to use enhancement of the effective SOC by the Coulomb interaction between electrons. In this study, to investigate such enhancement of the SOC, we consider a two-orbital model, which is a minimal model to include SOC. By using the variational Monte Carlo method, we find that the effective SOC can become several times larger than the bare value by the Coulomb interaction. This study will expand the target materials for the SOC-originated phenomena to strongly correlated metals without heavy elements.
久保 勝規
Journal of the Physical Society of Japan, 91(12), p.124707_1 - 124707_7, 2022/12
被引用回数：4 パーセンタイル：46.56(Physics, Multidisciplinary)We investigate the correlation effects on spin-orbit coupling (SOC) in a two-orbital Hubbard model on a square lattice by applying the variational Monte Carlo method. We consider an effective SOC constant 
in the one-body part of the variational wave function and mainly discuss the cases of the electron number per site 
, that is, quarter filling. interaction. Increasing the electron-electron interactions in the paramagnetic phase leads to a transition to an effective one-band state, in which the upper band becomes empty due to the enhanced . We also construct phase diagrams considering magnetic order. The carrier doping effect on is also investigated. We find that enhances in a strongly correlated region around the Mott transition point and it is necessary to include the correlation effects beyond the Hartree-Fock approximation to describe the enhanced SOC properly.
大島 宏之; 森下 正樹*; 相澤 康介; 安藤 勝訓; 芦田 貴志; 近澤 佳隆; 堂田 哲広; 江沼 康弘; 江連 俊樹; 深野 義隆; et al.
Sodium-cooled Fast Reactors; JSME Series in Thermal and Nuclear Power Generation, Vol.3, 631 Pages, 2022/07
ナトリウム冷却高速炉(SFR: Sodium-cooled Fast Reactor)の歴史や、利点、課題を踏まえた安全性、設計、運用、メンテナンスなどについて解説する。AIを利用した設計手法など、SFRの実用化に向けた設計や研究開発についても述べる。
久保 勝規
Physical Review B, 103(8), p.085118_1 - 085118_7, 2021/02
被引用回数：4 パーセンタイル：23.69(Materials Science, Multidisciplinary)The two-orbital Hubbard model on a square lattice at quarter filling (electron number per site 
= 1) is investigated by the variational Monte Carlo method. For the variational wave function, we include short-range doublon-holon binding factors. We find that the energy of this wave function is lower than that of the density-density Jastrow wave function partially including long-range correlations used in a previous study. We introduce frustration to the model by the next-nearest-neighbor hopping 
in addition to the nearest-neighbor hopping 
. For 
, a ferromagnetic state with staggered orbital order occurs by increasing the Coulomb interaction 
before the Mott transition takes place. By increasing , the region of this ferromagnetic phase shrinks, and the Mott transition without magnetic order occurs.
system久保 勝規
JPS Conference Proceedings (Internet), 30, p.011041_1 - 011041_6, 2020/03
Superconductivity in a model on a cubic lattice for a non-Kramers system is discussed. In our previous studies, we have found that 
-wave superconductivity with 
symmetry occurs in a wide parameter range in a system. In such an anisotropic superconducting state, the cubic symmetry of the system can be broken. Since the quadrupole degrees of freedom is active in a system, the effect of cubic symmetry breaking should be important. In 
or 
superconducting state, a ferroquadrupole moment of 
becomes finite. In this study, we investigate the coexisting states of the superconductivity and ferroquadrupole order of 
by a mean-field theory. Then, we find that superconducting transition temperature can be increased by the quadrupole ordering.
system久保 勝規
Physical Review B, 101(6), p.064512_1 - 064512_10, 2020/02
被引用回数：4 パーセンタイル：14.79(Materials Science, Multidisciplinary)We discuss superconductivity in a model on a cubic lattice for a non-Kramers system. In previous studies, it is revealed that -wave superconductivity with symmetry occurs in a wide parameter range in a system. Such anisotropic superconductivity can break the cubic symmetry of the lattice. In a system, the quadrupole degrees of freedom are active and the effect of the cubic symmetry breaking should be important. Here, we investigate the coexisting states of the -wave superconductivity and quadrupole order by a mean-field theory. In particular, we discuss possible competition and cooperation between the superconductivity and quadrupole order depending on types of them. We find nematic superconductivity breaking the cubic symmetry and coexisting with quadrupole order. In the present model, we also find 
superconductivity, which breaks time-reversal symmetry but retains the cubic symmetry. We also discuss the effects of uniaxial stress on these superconducting states.
crystalline electric field state久保 勝規
AIP Advances (Internet), 8(10), p.101313_1 - 101313_6, 2018/10
被引用回数：4 パーセンタイル：17.00(Nanoscience & Nanotechnology)A cubic system with the cubic crystalline electric field (CEF) state is an ideal system to investigate multipole physics, since it does not have dipole but has higher-order multipoles such as quadrupole and octupole. In addition, superconductivity has been observed in PrT
X
(T = Ir, Rh, X = Zn; T = Ti, V, X = Al) systems with the CEF ground state. To approach these phenomena theoretically, we consider a model composed of f electrons with the total angular momentum 
. From this model, we have derived multipole interactions for the systems. In the present study, we investigate superconductivity in the same model. We apply random phase approximation for superconductivity in a multiorbital system to the present model. Then, we have obtained a d-wave spin-singlet state. We will discuss the relevance of the present theory to the PrTX superconductors.
non-Kramers doublet system久保 勝規
Journal of the Physical Society of Japan, 87(7), p.073701_1 - 073701_4, 2018/07
被引用回数：7 パーセンタイル：46.75(Physics, Multidisciplinary)We study superconductivity in a three-orbital model for 
ions with the crystalline electric field (CEF) ground state. An antiferromagnetic interaction between the 
and 
orbitals is introduced to stabilize the CEF state. This interaction also works as an on-site attractive interaction for spin-singlet pairing between electrons in these orbitals. The interorbital pairing state composed of the and orbitals on the same site has the symmetry. Indeed, by applying the random phase approximation, we find that the spin-singlet superconducting state is realized over a wide parameter range.
non-Kramers doublet systems on cubic lattices久保 勝規; 堀田 貴嗣*
Journal of Physics; Conference Series, 969(1), p.012096_1 - 012096_6, 2018/04
被引用回数：4 パーセンタイル：85.11(Physics, Applied)In this study, we investigate the multipole interactions between ions with the crystalline electric field ground state. We construct the doublet state of the electrons with the total angular momentum . To derive the multipole interactions, we apply the second-order perturbation theory with respect to the intersite hopping. We obtain a quadrupole interaction for a simple cubic lattice, an octupole interaction for a bcc lattice, and both quadrupole and octupole interactions for an fcc lattice. To discuss general tendencies of the multipole interactions, we compare the present results with those for the quartet systems of 
ions.
non-Kramers doublet systems久保 勝規; 堀田 貴嗣*
Physical Review B, 95(5), p.054425_1 - 054425_6, 2017/02
被引用回数：29 パーセンタイル：73.88(Materials Science, Multidisciplinary)We study the multipole interactions between ions with the non-Kramers doublet ground state under a cubic crystalline electric field. We construct the doublet state of the electrons with the total angular momentum . By applying the second-order perturbation theory with respect to the intersite hopping, we derive the multipole interactions. We obtain a quadrupole interaction for a simple cubic lattice, an octupole interaction for a bcc lattice, and both quadrupole and octupole interactions for an fcc lattice. We also discuss general tendencies of the multipole interactions depending on the lattice structure by comparing the results with those for the quartet systems of ions.
久保 勝規; 大西 弘明
Journal of the Physical Society of Japan, 86(1), p.013701_1 - 013701_4, 2017/01
被引用回数：3 パーセンタイル：26.57(Physics, Multidisciplinary)We propose a variational wavefunction containing parameters to tune the probabilities of all the possible onsite configurations for the periodic Anderson model. We call it the full onsite-correlation wavefunction (FOWF). This is a simple extension of the Gutzwiller wavefunction (GWF), in which one parameter is included to tune the double occupancy of the 
electrons at the same site. We compare the energy of the GWF and the FOWF evaluated by the variational Monte Carlo method and that obtained with the density-matrix renormalization group method. We find that the energy is considerably improved in the FOWF. On the other hand, the physical quantities do not change significantly between these two wavefunctions as long as they describe the same phase, such as the paramagnetic phase. From these results, we not only demonstrate the improvement by the FOWF, but we also gain insights on the applicability and limitation of the GWF to the periodic Anderson model.
久保 勝規
Physics Procedia, 75, p.214 - 220, 2015/12
被引用回数：2 パーセンタイル：61.16(Physics, Applied)In recent years, Fermi-surface reconstruction, which is called a Lifshitz transition, has been discussed as a possible origin of anomalies in magnetic phases of several f-electron materials. To clarify the characteristics of the Lifshitz transitions, we investigate magnetically ordered states of the periodic Anderson model by applying the variational Monte Carlo method. Around half-filling, we find an antiferromagnetic phase, and far away from half-filling, we find a ferromagnetic phase as the ground state. Inside the both magnetic phases, Lifshitz transitions take place. At the Lifshitz transitions, the sizes of the ordered moments change. We find that, in the large ordered-moment states, the f-electron contribution to the Fermi surface becomes small. This observation clearly shows that these Lifshitz transitions are itinerant-localized transitions of the f electrons.
久保 勝規
Journal of the Physical Society of Japan, 84(9), p.094702_1 - 094702_8, 2015/09
被引用回数：20 パーセンタイル：72.33(Physics, Multidisciplinary)We investigate the reconstruction of a Fermi surface, which is called a Lifshitz transition, in magnetically ordered phases of the periodic Anderson model on a square lattice with a finite Coulomb interaction between electrons. We apply the variational Monte Carlo method to the model by using the Gutzwiller wave functions for the paramagnetic, antiferromagnetic, ferromagnetic, and charge-density-wave states. We find that an antiferromagnetic phase is realized around half-filling and a ferromagnetic phase is realized when the system is far away from half-filling. In both magnetic phases, Lifshitz transitions take place. By analyzing the electronic states, we conclude that the Lifshitz transitions to large ordered-moment states can be regarded as itinerant-localized transitions of the electrons.
久保 勝規
Journal of Physics; Conference Series, 592(1), p.012039_1 - 012039_6, 2015/03
被引用回数：4 パーセンタイル：77.21(Physics, Atomic, Molecular & Chemical)The periodic Anderson model has been studied as a typical model for f-electron systems. However, the knowledge on magnetism of this model is still limited. Here, we investigate magnetic states of the periodic Anderson model on a square lattice by applying the variational Monte Carlo method. We consider the Gutzwiller wavefunctions for the paramagnetic, antiferromagnetic, ferromagnetic, and charge density wave states as variational wavefunctions. Then, we evaluate energy of each state by the Monte Carlo method and determine the ground state. Around half-filling (the number of electrons n per site is two), we find an antiferromagnetic phase for a small hybridization matrix element V and a deep f level. When carriers are doped enough, for example, n=1.5, we find a half-metallic phase in a wide parameter region. It is consistent with the previous study for the ferromagnetism by the Gutzwiller approximation. We will also discuss the physical quantities in these phases.
久保 勝規
JPS Conference Proceedings (Internet), 3, p.011023_1 - 011023_6, 2014/06
We study ferromagnetic states in the periodic Anderson model by using the Gutzwiller method. We find three ferromagnetic states: a weakly polarized state, a half-metallic state, and a ferromagnetic state with an almost completely polarized f-electron state. Between these ferromagnetic states, the structure of the Fermi surface is different. In the paramagnetic state, a large Fermi-surface is realized. The topology of the Fermi surface in the weakly polarized state is the same as in the paramagnetic state. In the half-metallic state, the Fermi surface for the majority spin disappears. In the ferromagnetic state with large f-electron polarization, a small Fermi-surface is realized. Then, the phase transitions between these ferromagnetic states accompany change in the Fermi-surface topology, called Lifshitz transitions.
久保 勝規
Physical Review B, 87(19), p.195127_1 - 195127_9, 2013/05
被引用回数：22 パーセンタイル：64.39(Materials Science, Multidisciplinary)We study ferromagnetism in the periodic Anderson model with and without a magnetic field by the Gutzwiller theory. We find three ferromagnetic phases: a weak ferromagnetic phase (FM0), a half-metallic phase without Fermi surface for the majority spin (FM1), and a ferromagnetic phase with almost completely polarized -electrons (FM2). The Fermi surface changes from the large Fermi-surface in the paramagnetic state to the small Fermi-surface in FM2. We also find that the transitions between the ferromagnetic phases can be second-order phase transitions in spite of the absence of symmetry breaking. While we cannot define an order parameter for such transitions in an ordinary way, the topology of the Fermi surface characterizes the transitions, i.e., they are Lifshitz transitions.
