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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：1 パーセンタイル：35.17(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.

久保 勝規

Physical Review B, 103(8), p.085118_1 - 085118_7, 2021/02

被引用回数：2 パーセンタイル：21.24(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.

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.

久保 勝規

Physical Review B, 101(6), p.064512_1 - 064512_10, 2020/02

被引用回数：1 パーセンタイル：6.74(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.

久保 勝規

AIP Advances (Internet), 8(10), p.101313_1 - 101313_6, 2018/10

被引用回数：3 パーセンタイル：15.57(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 PrTX (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.

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：6 パーセンタイル：47.83(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.

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：3 パーセンタイル：83.09In 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.

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：26 パーセンタイル：77.13(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：2 パーセンタイル：21.78(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：1 パーセンタイル：46.97In 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：18 パーセンタイル：72.83(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：3 パーセンタイル：72.71The 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：67.09(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.

久保 勝規

Physica Status Solidi (C), 10(3), p.544 - 548, 2013/03

被引用回数：5 パーセンタイル：90.18We study the periodic Anderson model under a magnetic field by the Gutzwiller method. In this study, we set the Coulomb interaction between electrons infinity, and we consider the variational wave function given by where excludes the double occupancy of electrons at the same site and is the number operator of the electron with spin at site . denotes the one-electron part of the wave function. We evaluate energy of the variational wave function by using Gutzwiller approximation, and determine the variational parameters in the one-electron part which minimize energy. Then, spin-dependent effective mass and magnetization under a magnetic field are obtained.

杉田 裕; 真田 昌慶; 藤田 朝雄; 羽柴 公博*; 福井 勝則*; 大久保 誠介*

第13回岩の力学国内シンポジウム講演論文集(CD-ROM), p.207 - 212, 2013/01

高レベル放射性廃棄物の地層処分においては、廃棄体を埋設した坑道の周囲に発生する掘削影響領域の特性が放射性核種の移行挙動評価のうえで重要となる。強度が小さい岩盤の場合、坑道を掘削することにより発生する掘削影響領域では岩盤の破壊も生じると考えられる。しかしながら、坑道閉鎖後の長期においては、破壊した岩盤に支保内圧と地圧の双方が作用することにより、破壊により低下した岩盤の物性が回復することが考えられる。本報告は、破壊により低下した岩盤の物性の回復を把握するために実施した室内試験の結果を示すものである。試験の結果、載荷した荷重の大きさ・時間に応じて、強度及び遮水性が回復することが明らかとなった。

久保 勝規

Journal of Physics; Conference Series, 391, p.012159_1 - 012159_4, 2012/12

被引用回数：0 パーセンタイル：0.03In recent years, valence fluctuation mediated superconductivity has been discussed. Such valence fluctuations are inferred, e.g., for CeCuSi, from a rapid change of the effective mass through the relation: , where is the free-electron mass and is the number of electrons per site. However, this relation is derived for the periodic Anderson model (PAM), in which a sharp valence change does not occur. In addition, varies non-monotonically in CeCuSi under pressure, which is not expected from that equation. Thus, we extend this relation to a model with a sharp valence change. We study an extended PAM with the Coulomb interaction between the conduction and electrons by the Gutzwiller method. Then, we find that becomes large in the intermediate valent regime by an effect of , and changes non-monotonically as a function of . Thus our theory seems relevant to CeCuSi.

大久保 誠介*; 福井 勝則*; 羽柴 公博*; 引間 亮一; 丹野 剛男; 真田 祐幸; 松井 裕哉; 佐藤 稔紀

JAEA-Research 2011-040, 54 Pages, 2012/02

高レベル放射性廃棄物の地層処分時においては、建設時及び操業時は言うまでもなく、坑道埋め戻し後も千年程度の長期に渡る坑道の安定性の評価が要求される。そこで、岩石や岩盤の時間依存性挙動を、精密な試験や観察・計測から直接的に検討する手法(現象論的方法)で解明し、岩盤構造物の長期挙動予測評価手法を開発する研究を行ってきた。本報告書は、2010年度に実施した研究をまとめたものである。第1章では、研究内容とその背景を概括した。第2章では、1997年度から継続している田下凝灰岩のクリープ試験結果について報告した。第3章では、岩石の時間依存性の程度を表すnの値の意味をより明確に示すため、強度の載荷速度依存性とクリープ寿命の応力依存性との関係及び時間依存性,強度とクリープ寿命の分布特性,寸法効果の相互関係について論じた。さらに、既往の研究事例をレビューして、土岐花崗岩の力学特性や時間依存性について、今後実施すべき試験について検討した。第4章では岩盤の破壊基準の設定に関する検討として、十分長い時間をかけて岩石を壊したときの強度(長期強度)及び、いつまで経っても破壊に至らない応力条件について検討した。第5章では、周圧下でのnの値の変化及び岩盤強度のばらつきを考慮した二次元有限要素解析により、土岐花崗岩の長期挙動に関する予察的検討を行った。最後に、数値解析結果にもとづいて原位置試験計画に関する所見を述べた。

久保 勝規

Journal of the Physical Society of Japan, 80(11), p.114711_1 - 114711_8, 2011/11

被引用回数：13 パーセンタイル：62.53(Physics, Multidisciplinary)We study an extended periodic Anderson model with the Coulomb interaction between the conduction and f electrons by the Gutzwiller method. The crossovers between the Kondo, intermediate-valence, and almost empty f-electron regimes become sharp by , and for a sufficiently large , these crossovers become first-order phase transitions. In the Kondo regime, a large enhancement in the effective mass occurs as in the ordinary periodic Anderson model without . In addition, we find that a large mass enhancement occurs also in the intermediate-valence regime by an effect of .