Enhancement of an effective spin-orbit coupling in a correlated metal

久保 勝規

JPS Conference Proceedings (Internet), 38, p.011161_1 - 011161_6, 2023/05

https://doi.org/10.7566/JPSCP.38.011161

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.

Destabilization of ferromagnetism by frustration and realization of a nonmagnetic Mott transition in the quarter-filled two-orbital Hubbard model

久保 勝規

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

https://doi.org/10.1103/PhysRevB.103.085118

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.

Nematic and time-reversal breaking superconductivities coexisting with quadrupole order in a system

久保 勝規

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

https://doi.org/10.1103/PhysRevB.101.064512

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.

Anisotropic superconductivity emerging from the orbital degrees of freedom in a non-Kramers doublet system

久保 勝規

Journal of the Physical Society of Japan, 87(7), p.073701_1 - 073701_4, 2018/07

https://doi.org/10.7566/JPSJ.87.073701

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.

Influence of lattice structure on multipole interactions in non-Kramers doublet systems

久保 勝規; 堀田 貴嗣*

Physical Review B, 95(5), p.054425_1 - 054425_6, 2017/02

https://doi.org/10.1103/PhysRevB.95.054425

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.

Itinerant-localized transitions in magnetic phases of the periodic Anderson model

久保 勝規

Physics Procedia, 75, p.214 - 220, 2015/12

https://doi.org/10.1016/j.phpro.2015.12.027

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.

Variational Monte Carlo study of magnetic states in the periodic Anderson model

久保 勝規

Journal of Physics; Conference Series, 592(1), p.012039_1 - 012039_6, 2015/03

https://doi.org/10.1088/1742-6596/592/1/012039

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.

Ferromagnetism and Fermi surface transition in the periodic Anderson model; Second-order phase transition without symmetry breaking

久保 勝規

Physical Review B, 87(19), p.195127_1 - 195127_9, 2013/05

https://doi.org/10.1103/PhysRevB.87.195127

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.

破砕した珪質泥岩の強度および遮水性の回復

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。

結晶質岩を対象とした長期岩盤挙動評価のための現象論的研究(委託研究)

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

JAEA-Research-2011-040.pdf:2.85MB

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。