Showing 1–50 of 82 results for author: Ochi, M

Fluctuation exchange study on the electron-hole asymmetry of the superconductivity across 1/3 filling in the trilayer Hubbard model

Authors: Yushi Yamada, Masataka Kakoi, Masayuki Ochi, Kazuhiko Kuroki

Abstract: We study within the fluctuation exchange approximation the trilayer Hubbard model where three layers of the Hubbard model are coupled by large interlayer hoppings so that the overlap of the bonding, nonbonding, and antibonding bands is relatively small. We pay special attention to the band fillings close to 1/3, for which the bonding and nonbonding bands as a whole are close to half filling. For r… ▽ More We study within the fluctuation exchange approximation the trilayer Hubbard model where three layers of the Hubbard model are coupled by large interlayer hoppings so that the overlap of the bonding, nonbonding, and antibonding bands is relatively small. We pay special attention to the band fillings close to 1/3, for which the bonding and nonbonding bands as a whole are close to half filling. For relatively small values of the onsite $U$, superconductivity roughly exhibits electron-hole symmetric behavior, as expected for a nearly half-filled two-band system. By contrast, an asymmetry appears when $U$ becomes large, where superconductivity is more favored in the hole-doped regime, i.e., in the regime where electrons are removed from 1/3 filling. We attribute this asymmetry to the asymmetric renormalization of the bonding, nonbonding, and antibonding bands when $U$ is large. △ Less

Submitted 23 June, 2026; originally announced June 2026.

Comments: 10 pages, 8 figures

Study on the validity of IPT+parquet method as an impurity solver in DMFT focusing on orbital fluctuations

Authors: Aira Yamada, Ryota Mizuno, Masayuki Ochi, Kazuhiko Kuroki, Takuma Ohashi

Abstract: A breakdown of calculations with exact impurity solvers in the dynamical mean field theory in multiband systems easily occurs due to the expensive numerical cost. To overcome this practical difficulty, three of the present authors developed an inexpensive and reliable impurity solver by combining the iterative perturbation theory (IPT) and parquet equation, and named it IPT+parquet [R. Mizuno, et… ▽ More A breakdown of calculations with exact impurity solvers in the dynamical mean field theory in multiband systems easily occurs due to the expensive numerical cost. To overcome this practical difficulty, three of the present authors developed an inexpensive and reliable impurity solver by combining the iterative perturbation theory (IPT) and parquet equation, and named it IPT+parquet [R. Mizuno, et al., Phys. Rev. B 104, 035160 (2021).]. In this study, we validate IPT+parquet focusing on the orbital fluctuation by comparing the numerically exact impurity solvers. We confirm that IPT+parquet can capture competition between orbital fluctuation channels, which the conventional IPT cannot capture. △ Less

Submitted 4 June, 2026; originally announced June 2026.

Comments: 7 pages, 5 figures

Theoretical study of superconductivity in freestanding infinite-layer nickelate membranes under pressure: mitigation of excess correlation enhances $T_c$

Authors: Mahiru Seki, Reo Kono, Naotaka Tanaka, Kensei Ushio, Daiki Nakaoka, Masayuki Ochi, Kazuhiko Kuroki, Hirofumi Sakakibara

Abstract: We theoretically investigate a freestanding membrane of infinite-layer nickelate Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ under pressure by constructing a seven-orbital effective model based on first-principles calculations. By performing the fluctuation exchange (FLEX) approximation, we demonstrate that the seven-orbital model explains a monotonic increase in $T_c$ reported in a recent experiment. This en… ▽ More We theoretically investigate a freestanding membrane of infinite-layer nickelate Nd$_{0.85}$Sr$_{0.15}$NiO$_2$ under pressure by constructing a seven-orbital effective model based on first-principles calculations. By performing the fluctuation exchange (FLEX) approximation, we demonstrate that the seven-orbital model explains a monotonic increase in $T_c$ reported in a recent experiment. This enhancement of superconductivity is attributed to the mitigation of excessively strong electron correlations caused by exceptionally low valence of Ni atom. Furthermore, we examine the dynamical stability of the crystal structure under pressure through phonon calculation. △ Less

Submitted 29 May, 2026; v1 submitted 23 May, 2026; originally announced May 2026.

Comments: 9 pages, 5 figures,

Nonlinear phononics in LaFeAsO: Optical control of the crystal structure toward possible enhancement of superconductivity

Authors: Shu Kamiyama, Tatsuya Kaneko, Kazuhiko Kuroki, Masayuki Ochi

Abstract: Nonlinear phononics provides a route to control crystal structures through light-induced phonon excitation. In this study, we apply nonlinear phononics to an iron-based superconductor, LaFeAsO, with the aim of tuning its crystal structure toward the ideal one to enhance superconductivity. We simulate light-induced phonon dynamics on the anharmonic lattice potential determined by first-principles c… ▽ More Nonlinear phononics provides a route to control crystal structures through light-induced phonon excitation. In this study, we apply nonlinear phononics to an iron-based superconductor, LaFeAsO, with the aim of tuning its crystal structure toward the ideal one to enhance superconductivity. We simulate light-induced phonon dynamics on the anharmonic lattice potential determined by first-principles calculations. We find that the anion height $h$, a key structural parameter in iron-based superconductors, approaches its ideal value when an appropriate infrared-active phonon mode is selectively excited. This result suggests the possibility of controlling crystal structures and enhancing superconductivity in iron-based superconductors based on the concept of nonlinear phononics. △ Less

Submitted 8 April, 2026; originally announced April 2026.

Comments: 14 pages, 9 figures

Valley-dependent electron-phonon scattering in thermoelectric semimetal Ta$_2$PdSe$_6$

Authors: Masayuki Ochi, Hitoshi Mori, Akitoshi Nakano

Abstract: Quasi-one-dimensional transition-metal chalcogenide Ta$_2$PdSe$_6$ is a promising thermoelectric semimetal due to the strong electron-hole asymmetry in the carrier lifetime. However, the microscopic origin of such a strong asymmetry remains unclear. In this study, we theoretically investigate electron-phonon scattering in Ta$_2$PdSe$_6$. There is a soft phonon mode mainly consisting of atomic disp… ▽ More Quasi-one-dimensional transition-metal chalcogenide Ta$_2$PdSe$_6$ is a promising thermoelectric semimetal due to the strong electron-hole asymmetry in the carrier lifetime. However, the microscopic origin of such a strong asymmetry remains unclear. In this study, we theoretically investigate electron-phonon scattering in Ta$_2$PdSe$_6$. There is a soft phonon mode mainly consisting of atomic displacements in PdSe$_4$ chains. This soft mode is strongly coupled with the highest valence band at the $Γ$ point, which lies slightly below the Fermi energy, and causes strong electron-phonon scattering. The bottom of the electron pocket energetically overlapped with that band also suffers from strong intervalley scattering, by which the imaginary part of the electron self-energy exhibits a sharp change near the Fermi level. On the other hand, the imaginary part of the self-energy for carriers in the hole pocket shows a moderate energy dependence. Thus, we find that electron-phonon scattering is strongly valley-dependent. Our finding will help us to understand the distinctive transport properties observed in Ta$_2$PdSe$_6$. △ Less

Submitted 4 June, 2026; v1 submitted 11 March, 2026; originally announced March 2026.

Comments: 9 pages, 9 figures

Journal ref: Phys. Rev. B 113, 245112 (2026)

Impact of the out-of-plane conductivity on spin transport evaluation in a van der Waals material

Authors: Ryoya Nakamura, Futo Tokuda, Yoshinobu Ono, Nan Jiang, Hideaki Sakai, Masayuki Ochi, Hiroaki Ishizuka, Yasuhiro Niimi

Abstract: Layered materials are promising candidates for spintronic applications due to their unique electronic structures and spin transport properties. However, the strong anisotropic conductivity inherent in these materials complicates the quantitative evaluation of spin Hall conductivity and spin diffusion length. In this work, we present a comprehensive study of spin transport in a transition metal dic… ▽ More Layered materials are promising candidates for spintronic applications due to their unique electronic structures and spin transport properties. However, the strong anisotropic conductivity inherent in these materials complicates the quantitative evaluation of spin Hall conductivity and spin diffusion length. In this work, we present a comprehensive study of spin transport in a transition metal dichalcogenide PtTe$_2$ by combining a three-dimensional finite element model with nonlocal spin valve structures. We developed a theoretical model that treats an anisotropic spin diffusion in the same way as the conventional isotropic model, enabling the extraction of spin diffusion lengths along both the in-plane and out-of-plane directions. Our analysis revealed that the conventional isotropic assumption tends to overestimate some values, particularly for the out-of-plane spin diffusion length and spin Hall conductivity. These findings provide new insight into anisotropic spin diffusion and spin-charge conversions in layered materials and emphasize the importance of accounting for anisotropic conductivity in the design of spintronic devices. △ Less

Submitted 3 March, 2026; originally announced March 2026.

Comments: 15 pages, 11 figures

Journal ref: Physical Review B 113, 104408 (2026)

Electronic band structure, phonon dispersion, and magnetic triple-q state in GdGaI

Authors: Tatsuya Kaneko, Ryota Mizuno, Shu Kamiyama, Hideo Miyamoto, Masayuki Ochi

Abstract: We theoretically investigate the physical properties of the magnetic van der Waals material GdGaI. Using first-principles calculations, we compute the phonon dispersion of GdGaI and show no imaginary phonons, suggesting that phonon-driven phase transitions are unlikely to occur in GdGaI. Our band calculation reveals that the electronic bands near the Fermi energy are composed of Gd 5d and Ga 4p or… ▽ More We theoretically investigate the physical properties of the magnetic van der Waals material GdGaI. Using first-principles calculations, we compute the phonon dispersion of GdGaI and show no imaginary phonons, suggesting that phonon-driven phase transitions are unlikely to occur in GdGaI. Our band calculation reveals that the electronic bands near the Fermi energy are composed of Gd 5d and Ga 4p orbitals. We construct a tight-binding model that incorporates the Gd 5d and Ga 4p orbitals to investigate the magnetic structure. We introduce Kondo coupling between electrons in Gd 5d orbitals and localized spins in Gd 4f orbitals and present the modified band structure when localized spins form a magnetic order characterized by three q vectors that connect the valence and conduction bands. We discuss the origin of the spin order based on the Ruderman-Kittel-Kasuya-Yosida mechanism and suggest that Coulomb interactions acting on electrons near the Fermi level can contribute to the ordering of localized spins. △ Less

Submitted 29 January, 2026; originally announced January 2026.

Comments: 12 pages, 13 figures

Journal ref: Phys. Rev. B 113, 045156 (2026)

Transport evidence of current-induced nematic Dirac valleys in a parity-time-symmetric antiferromagnet

Authors: H. Sakai, Y. Miyamoto, M. Kimata, H. Watanabe, Y. Yanase, M. Ochi, M. Kondo, H. Murakawa, N. Hanasaki

Abstract: Itinerant antiferromagnets with broken time-reversal symmetry have recently attracted attention, since their spin-split bands enable large magnetotransport responses comparable to ferromagnets despite the negligible spontaneous magnetisation. When the inversion symmetry is further broken by the antiferromagnetic order, the emerging odd-parity multipole order renders the bands spin-degenerate but a… ▽ More Itinerant antiferromagnets with broken time-reversal symmetry have recently attracted attention, since their spin-split bands enable large magnetotransport responses comparable to ferromagnets despite the negligible spontaneous magnetisation. When the inversion symmetry is further broken by the antiferromagnetic order, the emerging odd-parity multipole order renders the bands spin-degenerate but asymmetric in the momentum space. For such parity-time-symmetric antiferromagnets, it has been predicted that electronic nematicity is induced by current, allowing unconventional nonlinear transport phenomena. However, their experimental evidence has been lacking. Here, we report nonreciprocal angular magnetoresistance in the layered Dirac material SrMnBi$_2$ with parity-time-symmetric antiferromagnetic order in its Mn-Bi layers. By quantitatively modelling the angular and field dependencies using a phenomenological framework, we reveal that the observed nonreciprocal interlayer resistivity arises from the broken four-fold symmetry of the Dirac valleys in the Bi square net adjacent to the Mn-Bi layer. Furthermore, we demonstrate the alignment of parity-time-symmetric antiferromagnetic domains via current-field cooling, achieving electric-magnetic control of the $f$-wave polarity in momentum space. The observed switchable nonreciprocal transport associated with current-induced valley symmetry breaking paves the way for novel antiferromagnetic spintronic and valleytronic applications. △ Less

Submitted 17 August, 2025; originally announced August 2025.

Comments: 17 pages, 4 figures

Theoretical study on ambient pressure superconductivity in La$_3$Ni$_2$O$_7$ thin films : structural analysis, model construction, and robustness of $s\pm$-wave pairing

Authors: Kensei Ushio, Shu Kamiyama, Yuto Hoshi, Ryota Mizuno, Masayuki Ochi, Kazuhiko Kuroki, Hirofumi Sakakibara

Abstract: We theoretically study ambient pressure superconductivity in thin films of La$_3$Ni$_2$O$_7$. We construct model Hamiltonians adopting the crystal structure theoretically determined by fixing the in-plane lattice constant to those substrates examined in the experiment. We also construct a model based on the experimentally determined lattice structure. To the models obtained, we apply the fluctuati… ▽ More We theoretically study ambient pressure superconductivity in thin films of La$_3$Ni$_2$O$_7$. We construct model Hamiltonians adopting the crystal structure theoretically determined by fixing the in-plane lattice constant to those substrates examined in the experiment. We also construct a model based on the experimentally determined lattice structure. To the models obtained, we apply the fluctuation exchange approximation, which takes into account the full momentum and frequency dependencies of the Green function and the pairing interaction. We find that the electronic structure, including the presence/absence of the so-called $γ$-pocket (the Fermi surface originating from the top of the $d_{3z^2-r^2}$ bonding band) depends on the crystal structure adopted and/or the presence/absence of $+U$ correction in the band structure calculation. Nonetheless, $s\pm$-wave pairing symmetry remains robust regardless of these details in the band structure. The robustness of the $s\pm$-wave pairing mainly owes to the fact that it is mediated by finite energy spin fluctuations, which are insensitive to the details of the Fermi surface topology and give rise to a nearly-momentum-independent gap function for the interlayer $d_{3z^2-r^2}$ pairing in the orbital representation. On the other hand, $T_c$ being halved from that of the pressurized bulk can only be understood by adopting the model with small $|t_{\perp}|$ derived from the experimentally determined crystal structure, at least within the present FLEX approach, although there may remain some other possibilities beyond this approach for the origin of the reduced $T_c$. △ Less

Submitted 25 May, 2026; v1 submitted 25 June, 2025; originally announced June 2025.

Comments: 15 pages, 10 figures

Improvement of the simplification method for the local two-particle full-vertex towards precise frequency behavior

Authors: Ryota Mizuno, Kazuhiko Kuroki, Masayuki Ochi

Abstract: Estimating the local two-particle vertex functions, which are crucial for capturing the spatial fluctuation of the effective field beyond the single-site DMFT, is still challenging. In our previous work, we developed a computationally efficient method for estimating the local full-vertex in DMFT, where we can obtain the local two-particle full-vertex from the one-particle self-energy. In this stud… ▽ More Estimating the local two-particle vertex functions, which are crucial for capturing the spatial fluctuation of the effective field beyond the single-site DMFT, is still challenging. In our previous work, we developed a computationally efficient method for estimating the local full-vertex in DMFT, where we can obtain the local two-particle full-vertex from the one-particle self-energy. In this study, we further enhance our method by refining its formulation to be more faithful to the diagrammatic structure of the full-vertex. With this improvement, we can qualitatively reproduce the characteristic frequency structures of the full-vertex obtained by the numerically exact methods. In particular, the improved version of the simplified full-vertex captures a sharp value change in the cross structure. △ Less

Submitted 26 May, 2025; originally announced May 2025.

Comments: 14 pages, 10 figures, 1 table

Journal ref: Phys. Rev. B 111, 205136 (2025)

Possible high thermoelectric power factor in alkali-metal-intercalated BC$_3$: anisotropic multiple valleys originating from the van Hove singularity of graphene

Authors: Ryutaro Enami, Kazuhiko Kuroki, Masayuki Ochi

Abstract: We theoretically investigate the electronic structure of monolayer BC$_3$ and find that it hosts anisotropic multiple valleys originating from the splitting of the van Hove singularity in graphene. To make use of its favorable electronic structure, we investigate the electronic structure of alkali-metal-intercalated BC$_3$, where intercalated atoms not only introduce electron carriers but also sup… ▽ More We theoretically investigate the electronic structure of monolayer BC$_3$ and find that it hosts anisotropic multiple valleys originating from the splitting of the van Hove singularity in graphene. To make use of its favorable electronic structure, we investigate the electronic structure of alkali-metal-intercalated BC$_3$, where intercalated atoms not only introduce electron carriers but also suppress interlayer coupling. We find that the interlayer transfer is effectively suppressed by potassium intercalation, by which the favorable electronic structure of monolayer BC$_3$ is preserved. Finally, we perform model calculation with the onsite-energy offset, and we verify that the strategy of introducing the splitting to the van Hove singularity works well. △ Less

Submitted 5 August, 2025; v1 submitted 24 March, 2025; originally announced March 2025.

Comments: 9 pages, 8 figures

Journal ref: Phys. Rev. Mater. 9, 084001 (2025)

Engineered substrates for domain control in CrSe thin-film growth: Single-domain formation on lattice-matched YSZ(111) substrate

Authors: Yusuke Tajima, Junichi Shiogai, Masayuki Ochi, Kazutaka Kudo, Jobu Matsuno

Abstract: Epitaxial thin-film growth is a versatile and powerful technique for achieving a precise control of composition, stabilizing non-equilibrium phases, tailoring growth orientation, as well as forming heterointerfaces of various quantum materials. For synthesis of highly crystalline thin films, in-depth understanding of epitaxial relationship between the desired thin film and the single-crystalline s… ▽ More Epitaxial thin-film growth is a versatile and powerful technique for achieving a precise control of composition, stabilizing non-equilibrium phases, tailoring growth orientation, as well as forming heterointerfaces of various quantum materials. For synthesis of highly crystalline thin films, in-depth understanding of epitaxial relationship between the desired thin film and the single-crystalline substrates is necessary. In this study, we investigate epitaxial relationship in thin-film growth of triangular-lattice antiferromagnet CrSe on the (001) plane of Al2O3 and the lattice-matched (111) plane of yttria-stabilized zirconia (YSZ) substrates. Structural characterization using out-of-plane and in-plane x-ray diffraction shows that the presence of 19.1o-twisted domains of CrSe significantly dominates the aligned domain on the Al2O3 substrate while it reveals a single-domain formation on the YSZ substrate. The stability of the 19.1o-twisted domain rather than the aligned domain can be explained by rotational commensurate epitaxy, which is well reproduced by density functional theory calculations. The single-domain CrSe thin film on the YSZ substrate exhibits a superior metallic conductivity compared to the twisted-domain thin film on the Al2O3 substrate, implying contribution of the grain boundary scattering mechanism to electrical transport. △ Less

Submitted 12 March, 2025; originally announced March 2025.

Comments: 24 pages, 4 figures

Mass enhancement and metal-nonmetal transition driven by d-f hybridization in perovskites La1-xPrxCuO3

Authors: H. Takahashi, M. Ito, J. Fujioka, M. Ochi, S. Sakai, R. Arita, H. Sagayama, Y. Yamasaki, S. Ishiwata

Abstract: We report the large electron-mass enhancement and the metal to nonmetal transition upon the Pr doping in perovskite-type La1-xPrxCuO3. With increasing the Pr content x around 0.6, the LaCuO3-type three-dimensional structure with trivalent Cu ions changes to the quasi-one-dimensional structure with nearly divalent Cu ions, which accompanies significant changes in the electronic properties. Based on… ▽ More We report the large electron-mass enhancement and the metal to nonmetal transition upon the Pr doping in perovskite-type La1-xPrxCuO3. With increasing the Pr content x around 0.6, the LaCuO3-type three-dimensional structure with trivalent Cu ions changes to the quasi-one-dimensional structure with nearly divalent Cu ions, which accompanies significant changes in the electronic properties. Based on the resistivity, optical conductivity, specific heat measurements and the first-principles calculations, we discuss the formation of a nearly localized nonmetallic state stabilized by the hybridization between Cu 3d, O 2p, and Pr 4f orbitals in the quasi-one-dimensional lattice. The present perovskite-type cuprates offer a unique opportunity to explore novel quantum phases of correlated electrons in low-dimensional lattice, where the spin/charge/orbital degrees of freedom of A- and B-site ions are entangled. △ Less

Submitted 21 February, 2025; originally announced February 2025.

Comments: 5 pages,4 figures

Journal ref: Physical Review B 111, 085153 (2025)

First-principles Study of Metallic-atom Diffusion in Thermoelectric Material Mg$_3$Sb$_2$

Authors: Masayuki Ochi, Kazutaka Nishiguchi, Chul-Ho Lee, Kazuhiko Kuroki

Abstract: Mg$_3$Sb$_2$ is a promising thermoelectric material that consists of nontoxic and earth-abundant elements. We investigate metallic-atom diffusion in Mg$_3$Sb$_2$ by calculating the defect formation energy and the diffusion energy barrier for several kinds of metallic-atom impurities. We find that early transition metals, including $4d$ elements, with a large atomic radius have a high defect format… ▽ More Mg$_3$Sb$_2$ is a promising thermoelectric material that consists of nontoxic and earth-abundant elements. We investigate metallic-atom diffusion in Mg$_3$Sb$_2$ by calculating the defect formation energy and the diffusion energy barrier for several kinds of metallic-atom impurities. We find that early transition metals, including $4d$ elements, with a large atomic radius have a high defect formation energy, whereas Mg and late transition metals such as Ni, Cu, and Zn have relatively low formation energies as interstitial impurities. Interstitial Ni, which is found to have a very low defect formation energy, might diffuse in the $ab$ plane at high temperatures with the energy barrier of 0.7 eV, while it seems difficult to diffuse in the $c$ direction. Interstitial Cu has a higher defect formation energy than Ni but has a low energy barrier of $\sim$0.4 eV for diffusion in the $ab$ plane. This study will offer important knowledge for developing a thermoelectric device of Mg$_3$Sb$_2$. △ Less

Submitted 23 January, 2025; originally announced January 2025.

Comments: 7 pages, 5 figures

Journal ref: J. Phys. Soc. Jpn. 94, 024704 (2025)

Optical control of the crystal structure in the bilayer nickelate superconductor La3Ni2O7 via nonlinear phononics

Authors: Shu Kamiyama, Tatsuya Kaneko, Kazuhiko Kuroki, Masayuki Ochi

Abstract: Superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_7$ occurs when the interlayer Ni-O-Ni bond angle becomes straight under pressure, suggesting a strong relationship between the crystal structure and the emergence of superconductivity. In this study, we theoretically propose a way to control the crystal structure of La$_3$Ni$_2$O$_7$ toward the tetragonal symmetry via light irradiation inst… ▽ More Superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_7$ occurs when the interlayer Ni-O-Ni bond angle becomes straight under pressure, suggesting a strong relationship between the crystal structure and the emergence of superconductivity. In this study, we theoretically propose a way to control the crystal structure of La$_3$Ni$_2$O$_7$ toward the tetragonal symmetry via light irradiation instead of pressure using the idea of nonlinear phononics. Here, resonant optical excitation of an infrared-active (IR) lattice vibration induces a nonlinear Raman-mode displacement through the anharmonic phonon-phonon coupling. We calculate the light-induced phonon dynamics on the anharmonic lattice potential determined by first-principles calculation. We find that the interlayer Ni-O-Ni bond angle gets slightly closer to straight when an appropriate IR mode is selectively excited. Our study suggests that light irradiation can be a promising way for structural control of La$_3$Ni$_2$O$_7$. △ Less

Submitted 8 April, 2026; v1 submitted 20 January, 2025; originally announced January 2025.

Comments: 14 pages, 12 figures

Journal ref: Phys. Rev. B 112, 094115 (2025)

Nonreciprocal charge transport in polar Dirac metals with tunable spin-valley coupling

Authors: M. Kondo, M. Kimata, M. Ochi, T. Kaneko, K. Kuroki, K. Sudo, S. Sakaguchi, H. Murakawa, N. Hanasaki, H. Sakai

Abstract: Nonreciprocal charge transport in solids, where resistance is different between rightward and leftward currents, is a key function of rectifying devices in the modern electronics, as exemplified by $p$-$n$ semiconductor junctions. Recently, this was also demonstrated in noncentrosymmetric materials in magnetic fields, since their band structure exhibits spin polarization coupled to the position of… ▽ More Nonreciprocal charge transport in solids, where resistance is different between rightward and leftward currents, is a key function of rectifying devices in the modern electronics, as exemplified by $p$-$n$ semiconductor junctions. Recently, this was also demonstrated in noncentrosymmetric materials in magnetic fields, since their band structure exhibits spin polarization coupled to the position of momentum space due to the antisymmetric spin-orbit coupling. To enhance the magnitude of nonreciprocal effect, it is essential to tune such spin-momentum coupling, which has been hampered in the conventional materials owing to the difficulty in controlling the broken inversion symmetry built into the lattice and interfacial structures. Here we report large nonreciprocal resistivity in layered polar metal BaMn$X_2$ ($X$=Sb, Bi), where the spin-polarized Dirac dispersion depends on the in-plane polarization tunable by chemical substitution of the $X$ site. For $X$=Sb with a pair of single-type valleys, the nonreciprocal resistivity increases monotonically with decreasing temperature, while for $X$=Bi with multiple types of valleys it is reduced by about an order of magnitude and exhibits a peak at a low temperature. Theoretical calculations indicate that the nonreciprocal resistivity is sensitive not only to the spin-momentum (spin-valley) coupling, but also to the Fermi energy and the Dirac dispersion. The observed significant variation of nonreciprocal transport in the same series of materials might be of great use in the design of junction-free rectifying devices and circuits. △ Less

Submitted 13 January, 2025; originally announced January 2025.

Comments: 10 pages, 7 figures. This article is already published in Physical Review Research

Journal ref: Phys. Rev. Research 7, 013041 (2025)

Theoretical study of the crystal structure of the bilayer nickel oxychloride Sr$_3$Ni$_2$O$_5$Cl$_2$ and analysis of possible unconventional superconductivity

Authors: Masayuki Ochi, Hirofumi Sakakibara, Hidetomo Usui, Kazuhiko Kuroki

Abstract: The discovery of superconductivity under high pressure with $T_c$ exceeding 80 K in a bilayer nickelate La$_3$Ni$_2$O$_7$ has led to a strong desire to realize similar high $T_c$ phenomena at ambient pressure. As one possible path toward realizing superconductivity at ambient pressure, we here propose to consider Sr$_3$Ni$_2$O$_5$Cl$_2$ as a possible candidate. In this study, we theoretically inve… ▽ More The discovery of superconductivity under high pressure with $T_c$ exceeding 80 K in a bilayer nickelate La$_3$Ni$_2$O$_7$ has led to a strong desire to realize similar high $T_c$ phenomena at ambient pressure. As one possible path toward realizing superconductivity at ambient pressure, we here propose to consider Sr$_3$Ni$_2$O$_5$Cl$_2$ as a possible candidate. In this study, we theoretically investigate the electronic structure of Sr$_3$Ni$_2$O$_5$Cl$_2$ and its structural stability. Our phonon calculation shows that this compound with the $I4/mmm$ tetragonal structure is dynamically stable even at ambient pressure. The characteristic crystal field in this compound lowers the Ni-$d_{3z^2-r^2}$ orbital energy, by which the Ni-$d_{3z^2-r^2}$ orbital becomes rather closer to the half-filling in Sr$_3$Ni$_2$O$_5$Cl$_2$ than La$_3$Ni$_2$O$_7$. As a result, we find that superconductivity is enhanced even though a relatively strong orbital hybridization between the $t_{2g}$ and $e_g$ orbitals is somewhat detrimental for superconductivity. We also check the formation enthalpy, which shows that the high-pressure synthesis can be a good way to actually produce Sr$_3$Ni$_2$O$_5$Cl$_2$. We find that Sr$_3$Ni$_2$O$_5$Cl$_2$ is a promising new candidate of bilayer-nickelate superconductors, which can possess even higher $T_c$ than pressurized La$_3$Ni$_2$O$_7$, at ambient pressure. △ Less

Submitted 18 February, 2025; v1 submitted 10 September, 2024; originally announced September 2024.

Comments: 12 pages, 8 figures

Journal ref: Phys. Rev. B 111, 064511 (2025)

Theoretical study of spin-fluctuation-mediated superconductivity in two-dimensional Hubbard models with an incipient flat band

Authors: Tetsuaki Aida, Karin Matsumoto, Daisuke Ogura, Masayuki Ochi, Kazuhiko Kuroki

Abstract: One promising way to enhance superconductivity is to have coexisting wide and incipient narrow bands, where the Fermi level intersecting the wide band lies just above the narrow band, by which finite-energy spin fluctuations act as glue to mediate pair scattering. As an extreme case of the narrow band dispersion, we investigate spin-fluctuation-mediated superconductivity in two-dimensional Hubbard… ▽ More One promising way to enhance superconductivity is to have coexisting wide and incipient narrow bands, where the Fermi level intersecting the wide band lies just above the narrow band, by which finite-energy spin fluctuations act as glue to mediate pair scattering. As an extreme case of the narrow band dispersion, we investigate spin-fluctuation-mediated superconductivity in two-dimensional Hubbard models with an incipient flat band. For all of the systems investigated in this study, the Kagome, Lieb, and bilayer square lattices with a flat band, we find that spin-singlet pairing superconductivity is enhanced when the flat band is nearly fully filled, due to the interband pair scattering even when the flat band becomes dispersive by correlation effects. Among these models, enhancement of superconductivity is weak in the Lieb lattice, possibly because the density of states of the wide band goes to zero at the Dirac point where the flat and wide bands intersect. Also, when the electron density is smaller so that the flat band approaches half filling, ferromagnetic spin fluctuations and spin-triplet pairing arises, which does not develop strongly compared to the case of the spin-singlet pairing for the incipient band situation. △ Less

Submitted 19 August, 2024; originally announced August 2024.

Comments: 9 pages, 6 figures

Journal ref: Phys. Rev. B 110, 054516 (2024)

Theoretical study on the possibility of high $T_c$ s$\pm$-wave superconductivity in the heavily hole-doped infinite layer nickelates

Authors: Hirofumi Sakakibara, Ryota Mizuno, Masayuki Ochi, Hidetomo Usui, Kazuhiko Kuroki

Abstract: We theoretically propose a possibility of realizing high $T_c$ superconductivity having $s\pm$-wave symmetry in the heavily hole-doped infinite layer nickelates La$_{1-x}$Sr$_x$NiO$_2$. We consider situations where the original $P4/mmm$ symmetry of LaNiO$_2$ is maintained even for a significant amount of Sr substitution by growing thin films on substrates having tetragonal symmetry. Considering su… ▽ More We theoretically propose a possibility of realizing high $T_c$ superconductivity having $s\pm$-wave symmetry in the heavily hole-doped infinite layer nickelates La$_{1-x}$Sr$_x$NiO$_2$. We consider situations where the original $P4/mmm$ symmetry of LaNiO$_2$ is maintained even for a significant amount of Sr substitution by growing thin films on substrates having tetragonal symmetry. Considering such cases is indeed justified by our phonon calculations. For electron configurations somewhat close to $d^8$, the interaction between the $d_{x^2-y^2}$ band and the other $3d$ bands that lie just below the Fermi level results in an enhancement of superconductivity where the sign of the gap function is reversed between the former and the latter bands. The strong enhancement of superconductivity can be attributed to the large energy level offset between $d_{x^2-y^2}$ and other orbitals due to the absence of the apical oxygens, as has been pointed out in previous studies. △ Less

Submitted 14 June, 2025; v1 submitted 31 July, 2024; originally announced July 2024.

Comments: 7 pages, 4 figures

Theoretical analysis of the origin of the double-well band dispersion in the CuO double chains of Pr$_2$Ba$_4$Cu$_7$O$_{15-δ}$ and its impact on superconductivity

Authors: Toshiki Yagi, Masayuki Ochi, Kazuhiko Kuroki

Abstract: Pr$_2$Ba$_4$Cu$_7$O$_{15-δ}$ is a unique member of cuprate superconductors where many studies suggest that CuO double chains are responsible for superconductivity. One characteristic and non-trivial feature of its electronic structure is a relatively large electron hopping $t$ between nearest neighbor Cu sites with a Cu-O-Cu angle of around 90 degrees. In this study, we have theoretically pinned d… ▽ More Pr$_2$Ba$_4$Cu$_7$O$_{15-δ}$ is a unique member of cuprate superconductors where many studies suggest that CuO double chains are responsible for superconductivity. One characteristic and non-trivial feature of its electronic structure is a relatively large electron hopping $t$ between nearest neighbor Cu sites with a Cu-O-Cu angle of around 90 degrees. In this study, we have theoretically pinned down the origin of a large $|t|$ in the double-chain structure of Pr$_2$Ba$_4$Cu$_7$O$_{15-δ}$ using first-principles calculation and tight-binding-model analysis. We have found that, in the nearest neighbor hopping $t$, $d$-$d$ and $d$-$p$-$p$-$d$ contributions roughly cancel each other out and the $d$-$p$-$d$ hopping path enhanced by the local distortion of the double chain is a key to get the large $|t|$. Double-well band dispersion arising from the relatively large $|t/t'|$ allows the enhancement of spin-fluctuation-mediated superconductivity by the incipient-band mechanism, where the one band bottom plays a role of the incipient valley. Our study provides the important knowledge to understand the unique superconductivity in Pr$_2$Ba$_4$Cu$_7$O$_{15-δ}$. △ Less

Submitted 21 November, 2024; v1 submitted 24 July, 2024; originally announced July 2024.

Comments: 9 pages, 9 figures

Journal ref: Phys. Rev. B 110, 184516 (2024)

Fermi Surface Nesting Driving the RKKY Interaction in the Centrosymmetric Skyrmion Magnet Gd2PdSi3

Authors: Yuyang Dong, Yosuke Arai, Kenta Kuroda, Masayuki Ochi, Natsumi Tanaka, Yuxuan Wan, Matthew D. Watson, Timur K. Kim, Cephise Cacho, Makoto Hashimoto, Donghui Lu, Yuji Aoki, Tatsuma D. Matsuda, Takeshi Kondo

Abstract: The magnetic skyrmions generated in a centrosymmetric crystal were recently first discovered in Gd2PdSi3. In light of this, we observe the electronic structure by angle-resolved photoemission spectroscopy (ARPES) and unveil its direct relationship with the magnetism in this compound. The Fermi surface and band dispersions are demonstrated to have a good agreement with the density functional theory… ▽ More The magnetic skyrmions generated in a centrosymmetric crystal were recently first discovered in Gd2PdSi3. In light of this, we observe the electronic structure by angle-resolved photoemission spectroscopy (ARPES) and unveil its direct relationship with the magnetism in this compound. The Fermi surface and band dispersions are demonstrated to have a good agreement with the density functional theory (DFT) calculations carried out with careful consideration of the crystal superstructure. Most importantly, we find that the three-dimensional Fermi surface has extended nesting which matches well the q-vector of the magnetic order detected by recent scattering measurements. The consistency we find among ARPES, DFT, and the scattering measurements suggests the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction involving itinerant electrons to be the formation mechanism of skyrmions in Gd2PdSi3. △ Less

Submitted 3 July, 2024; originally announced July 2024.

Journal ref: Phys. Rev. Lett. 133, 016401 (2024)

Anomalous Fermi pockets on Hund's metal surface of Sr2RuO4 induced by the correlation-enhanced spin-orbit coupling

Authors: Takeshi Kondo, Masayuki Ochi, Shuntaro Akebi, Yuyang Dong, Haruka Taniguchi, Yoshiteru Maeno, Shik Shin

Abstract: The electronic structure of the topmost layer in Sr2RuO4 in the close vicinity of the Fermi level is investigated by angle-resolved photoemission spectroscopy (ARPES) with a 7-eV laser. We find that the spin-orbit coupling (SOC) predicted as 100 meV by the density functional theory (DFT) calculations is enormously enhanced in a real material up to 250 meV, even more than that of bulk state (200 me… ▽ More The electronic structure of the topmost layer in Sr2RuO4 in the close vicinity of the Fermi level is investigated by angle-resolved photoemission spectroscopy (ARPES) with a 7-eV laser. We find that the spin-orbit coupling (SOC) predicted as 100 meV by the density functional theory (DFT) calculations is enormously enhanced in a real material up to 250 meV, even more than that of bulk state (200 meV), by the electron-correlation effect increased by the octahedral rotation in the crystal structure. This causes the formation of highly orbital-mixing small Fermi pockets and reasonably explains why the orbital-selective Mott transition (OSMT) is not realized in perovskite oxides with crystal distortion. Interestingly, Hund's metal feature allows the quasiparticle generation only near EF, restricting the spectral gap opening derived by band hybridization within an extremely small binding energy (< 10 meV). Furthermore, it causes coherent-incoherent crossover, making the Fermi pockets disappear at elevated temperatures. The anomalous Fermi pockets are characterized by the dichotomy of the orbital-isolating Hund's coupling and the orbital-mixing SOC, which is key to understanding the nature of Sr2RuO4. △ Less

Submitted 19 June, 2024; originally announced June 2024.

Journal ref: Phys. Rev. B 109, L241107 (2024)

Electronic band structure from quasiparticle interference and Landau quantization in WTe$_2$

Authors: Raquel Sánchez-Barquilla, Francisco Martín Vega, Alberto M. Ruiz, Na Hyun Jo, Edwin Herrera, José J. Baldoví, Masayuki Ochi, Ryotaro Arita, Sergey L. Bud'ko, Paul C. Canfield, Isabel Guillamón, Hermann Suderow

Abstract: WTe$_2$ stands out as a semimetal presenting Fermi level quantum oscillations in most measured quantities under magnetic fields. However, the electronic band structure above and below the Fermi level has not been explored completely. Here we study the electronic band structure of WTe$_2$ by quasiparticle interference with Scanning Tunneling Microscopy (STM) and observe, with the support of Density… ▽ More WTe$_2$ stands out as a semimetal presenting Fermi level quantum oscillations in most measured quantities under magnetic fields. However, the electronic band structure above and below the Fermi level has not been explored completely. Here we study the electronic band structure of WTe$_2$ by quasiparticle interference with Scanning Tunneling Microscopy (STM) and observe, with the support of Density Functional Theory (DFT), the electron and hole bands around the Fermi level. We also report on the observation of Landau quantization in atomically resolved measurements and discuss the possible connection with band structure calculations. △ Less

Submitted 6 October, 2025; v1 submitted 22 May, 2024; originally announced May 2024.

Comments: 18 pages, 10 figures

Journal ref: Phys. Rev. B 112, 165401 (2025)

Pair correlations of the hybridized orbitals in a ladder model for the bilayer nickelate La$_3$Ni$_2$O$_7$

Authors: Masataka Kakoi, Tatsuya Kaneko, Hirofumi Sakakibara, Masayuki Ochi, Kazuhiko Kuroki

Abstract: To clarify the nature of high-temperature superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_7$ under pressure, we investigate, using the density-matrix renormalization group method, the pair correlations in the two-orbital $t$-$J$ ladder model. While the interchain-intraorbital pair correlations exhibit a slow power-law decay in both orbitals, the interorbital pair correlation also develop… ▽ More To clarify the nature of high-temperature superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_7$ under pressure, we investigate, using the density-matrix renormalization group method, the pair correlations in the two-orbital $t$-$J$ ladder model. While the interchain-intraorbital pair correlations exhibit a slow power-law decay in both orbitals, the interorbital pair correlation also develops strongly enough to be comparable with the intraorbital correlations. These intra and interorbital pair correlations are enhanced by Hund's coupling, but more importantly, the interorbital pair correlation develops even when interorbital pairing glue mediated by Hund's coupling is absent. Our finding suggests that the pair correlation in the present system develops as a hybridized two-orbital entity, which may have some implications on the superconductivity in the bilayer nickelate. △ Less

Submitted 24 May, 2024; v1 submitted 7 December, 2023; originally announced December 2023.

Comments: 7+6 pages, 5+9 figures

Journal ref: Phys. Rev. B 109, L201124 (2024)

Pair correlations in the two-orbital Hubbard ladder: Implications on superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_7$

Authors: Tatsuya Kaneko, Hirofumi Sakakibara, Masayuki Ochi, Kazuhiko Kuroki

Abstract: Motivated by high-temperature superconductivity in pressurized La$_3$Ni$_2$O$_7$, we investigate the pair correlations in the two-orbital Hubbard ladder, which consists of the nearly half-filled and nearly quarter-filled orbitals. By employing the density matrix renormalization group method, we demonstrate that the pair correlation exhibits a power-law decay against the distance while the spin cor… ▽ More Motivated by high-temperature superconductivity in pressurized La$_3$Ni$_2$O$_7$, we investigate the pair correlations in the two-orbital Hubbard ladder, which consists of the nearly half-filled and nearly quarter-filled orbitals. By employing the density matrix renormalization group method, we demonstrate that the pair correlation exhibits a power-law decay against the distance while the spin correlation decays exponentially. The decay exponent of the pair correlation of the nearly half-filled orbital is comparable to the exponent of the quasi-long-range superconducting correlation in the doped single-orbital Hubbard ladder, which suggests the importance of the $d_{3z^2-r^2}$ orbital in La$_3$Ni$_2$O$_7$. △ Less

Submitted 31 January, 2024; v1 submitted 3 October, 2023; originally announced October 2023.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 109, 045154 (2024)

Theoretical analysis on the possibility of superconductivity in a trilayer Ruddlesden-Popper nickelate La$_4$Ni$_3$O$_{10}$ under pressure and its experimental examination: comparison with La$_3$Ni$_2$O$_7$

Authors: Hirofumi Sakakibara, Masayuki Ochi, Hibiki Nagata, Yuta Ueki, Hiroya Sakurai, Ryo Matsumoto, Kensei Terashima, Keisuke Hirose, Hiroto Ohta, Masaki Kato, Yoshihiko Takano, Kazuhiko Kuroki

Abstract: We study the possibility of superconductivity in a trilayer Ruddlesden-Popper nickelate La$_4$Ni$_3$O$_{10}$ under pressure both theoretically and experimentally, making comparison with the recently discovered high $T_c$ superconductor La$_3$Ni$_2$O$_7$, a bilayer nickelate. Through DFT calculations, we find that a structural phase transition from monoclinic to tetragonal takes place around 10 - 1… ▽ More We study the possibility of superconductivity in a trilayer Ruddlesden-Popper nickelate La$_4$Ni$_3$O$_{10}$ under pressure both theoretically and experimentally, making comparison with the recently discovered high $T_c$ superconductor La$_3$Ni$_2$O$_7$, a bilayer nickelate. Through DFT calculations, we find that a structural phase transition from monoclinic to tetragonal takes place around 10 - 15 GPa. Using the tetragonal crystal structure, we theoretically investigate the possibility of superconductivity, where a combination of fluctuation exchange approximation and linearized Eliashberg equation is applied to a six-orbital model constructed from first principles band calculation. The obtained results suggests that La$_4$Ni$_3$O$_{10}$ may also become superconducting under high pressure with $T_c$ comparable to some cuprates, although it is not as high as La$_3$Ni$_2$O$_7$. We also perform experimental studies using our polycrystalline samples of La$_3$Ni$_2$O$_{7.01}$ and La$_4$Ni$_3$O$_{9.99}$. The superconducting transition of La$_3$Ni$_2$O$_{7.01}$, with a maximum onset $T_c$ of 67.0 K at a pressure of 26.5 GPa, is confirmed by a drop in the electrical resistance, as well as the magnetic field dependence of the resistance. Quite interestingly, similar temperature and magnetic field dependencies of the resistance are observed also for La$_4$Ni$_3$O$_{9.99}$, where a drop in the resistance is observed at lower temperatures compared to La$_3$Ni$_2$O$_{7.01}$, under pressures of 32.8 GPa and above. Given the theoretical expectation, the reduction in the resistance can most likely be attributed to the occurrence of superconductivity in La$_4$Ni$_3$O$_{9.99}$. The temperature at which the resistance deviates from a linear behavior, considered as the onset $T_c$, monotonically increases up to 23 K at 79.2 GPa, which is opposite to the pressure dependence of $T_c$ in La3Ni2O7.01. △ Less

Submitted 29 February, 2024; v1 submitted 17 September, 2023; originally announced September 2023.

Comments: 9 pages, 5 figures, Fig.4 added

Theoretical designing of multiband Nickelate and Palladate superconductors with $d^{8+δ}$ configuration

Authors: Naoya Kitamine, Masayuki Ochi, Kazuhiko Kuroki

Abstract: In a previous study, we proposed a possibility of high $T_c$ superconductivity in mixed-anion nickelates with $d^{8+δ}$ electron configuration. The theory was based on the fact that the two-orbital Hubbard model, when all the intra- and interorbital interactions have the same magnitude, is equivalent to the bilayer Hubbard model, which has been suggested to exhibit high $T_c$ superconductivity. Th… ▽ More In a previous study, we proposed a possibility of high $T_c$ superconductivity in mixed-anion nickelates with $d^{8+δ}$ electron configuration. The theory was based on the fact that the two-orbital Hubbard model, when all the intra- and interorbital interactions have the same magnitude, is equivalent to the bilayer Hubbard model, which has been suggested to exhibit high $T_c$ superconductivity. The energy level offset $ΔE$ in the two-orbital model is transformed to twice the interlayer hopping in the bilayer model, and hence appropriately large $ΔE$ is favorable for superconductivity in the former. Extending this idea to multiorbital systems, we previously suggested materials with large energy level offset between $d_{x^2-y^2}$ and other $d$ orbitals, such as Ca$_2$NiO$_2$Cl$_2$, to be good candidates for high $T_c$ superconductivity, but such materials have not been synthesized to our knowledge. In the present study, we first focus on Sr$_2$NiO$_2$Cl$_2$, which has been synthesized in the past but has small $ΔE$, and study the effect of applying pressure, which enhances $ΔE$. We also study a 4d analogue of Sr$_2$NiO$_2$Cl$_2$, namely, Sr$_2$PdO$_2$X$_2$ ($X=$ Cl, F, H) , in which $ΔE$ turns out to be large. The analysis using fluctuation exchange approximation suggests possibility of superconductivity in these systems with large $ΔE$. We also study the effect of electron doping of these material, which is expected to enhance superconductivity, within the virtual crystal approximation. △ Less

Submitted 24 August, 2023; originally announced August 2023.

Comments: 8 pages, 15 figures

Pseudogap and Fermi arc induced by Fermi surface nesting in a centrosymmetric skyrmion magnet

Authors: Yuyang Dong, Yuto Kinoshita, Masayuki Ochi, Ryu Nakachi, Ryuji Higashinaka, Satoru Hayami, Yuxuan Wan, Yosuke Arai, Soonsang Huh, Makoto Hashimoto, Donghui Lu, Masashi Tokunaga, Yuji Aoki, Tatsuma D. Matsuda, Takeshi Kondo

Abstract: Skyrmions in noncentrosymmetric materials are believed to occur due to the Dzyaloshinskii-Moriya interaction. By contrast, the skyrmion formation mechanism in centrosymmetric materials remains elusive. Here, we reveal the intrinsic electronic structure of the centrosymmetric GdRu2Si2 by selectively measuring magnetic domains using angle-resolved photoemission spectroscopy (ARPES). We found robust… ▽ More Skyrmions in noncentrosymmetric materials are believed to occur due to the Dzyaloshinskii-Moriya interaction. By contrast, the skyrmion formation mechanism in centrosymmetric materials remains elusive. Here, we reveal the intrinsic electronic structure of the centrosymmetric GdRu2Si2 by selectively measuring magnetic domains using angle-resolved photoemission spectroscopy (ARPES). We found robust Fermi surface (FS) nesting, consistent with the magnetic modulation q-vector detected by the previous resonant x-ray scattering measurements. The pseudogap opens at the nested FS portions, which vary for different magnetic domains. The anomalous pseudogap disconnects the FS to generate Fermi arcs with twofold symmetry. These results indicate that the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction plays a decisive role in generating the screw spin modulation responsible for the skyrmion formation in GdRu2Si2. Furthermore, we demonstrate the flexible nature of magnetism in GdRu2Si2 by manipulating magnetic domains with magnetic field and temperature cyclings, providing potential future applications for data storage and processing devices. △ Less

Submitted 31 July, 2025; v1 submitted 16 July, 2023; originally announced July 2023.

Journal ref: Science 388, 624-630 (2025)

Possible high $T_c$ Superconductivity in La$_3$Ni$_2$O$_7$ under High Pressure through Manifestation of a Nearly-Half-Filled Bilayer Hubbard Model

Authors: Hirofumi Sakakibara, Naoya Kitamine, Masayuki Ochi, Kazuhiko Kuroki

Abstract: Inspired by a recent experiment showing that La$_3$Ni$_2$O$_7$ exhibits high $T_c$ superconductivity under high pressure, we theoretically revisit the possibility of superconductivity in this material. We find that superconductivity can take place which is essentially similar to that of the bilayer Hubbard model consisting of the Ni $3d_{3z^2-r^2}$ orbitals. Although the coupling with the… ▽ More Inspired by a recent experiment showing that La$_3$Ni$_2$O$_7$ exhibits high $T_c$ superconductivity under high pressure, we theoretically revisit the possibility of superconductivity in this material. We find that superconductivity can take place which is essentially similar to that of the bilayer Hubbard model consisting of the Ni $3d_{3z^2-r^2}$ orbitals. Although the coupling with the $3d_{x^2-y^2}$ orbitals degrades superconductivity, $T_c$ can still be high enough to understand the experiment thanks to the very high $T_c$ reached in the bilayer Hubbard model. △ Less

Submitted 6 March, 2024; v1 submitted 9 June, 2023; originally announced June 2023.

Comments: 3 figures, 6 pages

Journal ref: Phys. Rev. Lett. 132, 106002 (2024)

Electron-hole dichotomy and enhancement of thermoelectric power factor by electron-hole-asymmetric relaxation time: a model study on a two-valley system with strong intervalley scattering

Authors: Masayuki Ochi

Abstract: The role of electron-phonon scattering in thermoelectric transport has been paid much attention, especially in multivalley systems. By investigating a two-valley model with electron-phonon coupling, we find three electron transport regimes realized by electron-hole asymmetry of electron relaxation time due to the strong intervalley scattering. Seebeck coefficient denotes an electron-hole dichotomy… ▽ More The role of electron-phonon scattering in thermoelectric transport has been paid much attention, especially in multivalley systems. By investigating a two-valley model with electron-phonon coupling, we find three electron transport regimes realized by electron-hole asymmetry of electron relaxation time due to the strong intervalley scattering. Seebeck coefficient denotes an electron-hole dichotomy due to this asymmetry. Also, the strong intervalley scattering can enhance power factor. Our finding sheds light on unexplored thermoelectric transport under the strong electron-phonon scattering. △ Less

Submitted 30 October, 2023; v1 submitted 6 June, 2023; originally announced June 2023.

Comments: 10 pages, 7 figures

Journal ref: Phys. Rev. B 108, 195139 (2023)

Spin-polarized saddle points in the topological surface states of the elemental Bismuth revealed by a pump-probe spin-resolved ARPES

Authors: Yuto Fukushima, Kaishu Kawaguchi, Kenta Kuroda, Masayuki Ochi, Hiroaki Tanaka, Ayumi Harasawa, Takushi Iimori, Zhigang Zhao, Shuntaro Tani, Koichiro Yaji, Shik Shin, Fumio Komori, Yohei Kobayashi, Takeshi Kondo

Abstract: We use a pump-probe, spin-, and angle-resolved photoemission spectroscopy (ARPES) with a 10.7 eV laser accessible up to the Brillouin zone edge, and reveal for the first time the entire band structure, including the unoccupied side, for the elemental bismuth (Bi) with the spin-polarized surface states. Our data identify Bi as in a strong topological insulator phase ($Z_2$=1) against the prediction… ▽ More We use a pump-probe, spin-, and angle-resolved photoemission spectroscopy (ARPES) with a 10.7 eV laser accessible up to the Brillouin zone edge, and reveal for the first time the entire band structure, including the unoccupied side, for the elemental bismuth (Bi) with the spin-polarized surface states. Our data identify Bi as in a strong topological insulator phase ($Z_2$=1) against the prediction of most band calculations. We unveil that the unoccupied topological surface states possess spin-polarized saddle points yielding the van Hove singularity, providing an excellent platform for the future development of opto-spintronics. △ Less

Submitted 31 March, 2023; originally announced March 2023.

Comments: 6 pages, 4 figures

TC++: First-principles calculation code for solids using the transcorrelated method

Authors: Masayuki Ochi

Abstract: TC++ is a free/libre open-source software of the transcorrelated (TC) method for first-principles calculation of solids. Here, the TC method is one of the promising wave-function theories that can be applied to periodic systems with reasonable computational cost and satisfactory accuracy. We present our implementation of TC++ including a detailed description of the divergence correction technique… ▽ More TC++ is a free/libre open-source software of the transcorrelated (TC) method for first-principles calculation of solids. Here, the TC method is one of the promising wave-function theories that can be applied to periodic systems with reasonable computational cost and satisfactory accuracy. We present our implementation of TC++ including a detailed description of the divergence correction technique applied to the TC effective interactions. We also present the way to use TC++ and some results of application to simple periodic systems: bulk silicon and homogeneous electron gas. △ Less

Submitted 18 April, 2023; v1 submitted 14 February, 2023; originally announced February 2023.

Comments: 56 pages, 2 figures. Signs of Eqs.(98) and (99) are corrected. Computational code is available at https://github.com/masaochi/TC

Journal ref: Comput. Phys. Commun. 287, 108687 (2023)

Superconductivity in a Magnetic Rashba Semimetal EuAuBi

Authors: Hidefumi Takahashi, Kazuto Akiba, Masayuki Takahashi, Alex H. Mayo, Masayuki Ochi, Tatsuo C. Kobayashi, Shintaro Ishiwata

Abstract: We report the observation of superconductivity with multiple magnetic ordering and Rashba-type spin-orbit coupling in a layered polar semimetal EuAuBi. Magnetic transition is observed at 4 K, followed by a superconducting transition at 2.2 K, which is sensitive to the crystal surface conditions. The upper critical field Hc2 of 9.8 T for the out-of-plane field is three times higher than that for th… ▽ More We report the observation of superconductivity with multiple magnetic ordering and Rashba-type spin-orbit coupling in a layered polar semimetal EuAuBi. Magnetic transition is observed at 4 K, followed by a superconducting transition at 2.2 K, which is sensitive to the crystal surface conditions. The upper critical field Hc2 of 9.8 T for the out-of-plane field is three times higher than that for the in-plane field, which can be associated with the two-dimensional structure or the surface state. On the basis of first-principles calculations, it is found that the characteristic Hc2 possibly reflects the anisotropic modification of the Fermi surface by the effective combination of Rashba-type spin splitting and Zeeman spin splitting enhanced by Eu moments. △ Less

Submitted 1 December, 2022; originally announced December 2022.

Comments: 12 pages, 5 figures

Journal ref: J. Phys. Soc. Jpn. 92, 013701 (2023)

Weak antilocalization induced by Se substitution in layered BiCh$_2$-based (Ch = S, Se) superconductors LaO$_{1-x}$F$_x$BiS$_{2-y}$Se$_y$

Authors: Kazuhisa Hoshi, Hiroto Arima, Noriyuki Kataoka, Masayuki Ochi, Aichi Yamashita, Anne de Visser, Takayoshi Yokoya, Kazuhiko Kuroki, Yoshikazu Mizuguchi

Abstract: We report transport properties for layered BiCh2-based (Ch = S, Se) superconductors LaO1-xFxBiS2-ySey (x = 0.2, 0.5, y = 0-1.05) and the observation of weak antilocalization (WAL). Electrical resistivity and Hall coefficients for the Se-poor samples increase with decreasing temperature. The increase becomes less pronounced with increasing Se concentration indicating a loss of insulating behavior.… ▽ More We report transport properties for layered BiCh2-based (Ch = S, Se) superconductors LaO1-xFxBiS2-ySey (x = 0.2, 0.5, y = 0-1.05) and the observation of weak antilocalization (WAL). Electrical resistivity and Hall coefficients for the Se-poor samples increase with decreasing temperature. The increase becomes less pronounced with increasing Se concentration indicating a loss of insulating behavior. Interestingly, the moderately Se-substituted samples exhibit metallic behavior in the high-temperature region and a weak increase in the resistivity in the low-temperature regions, which indicates the existence of carrier localization. The heavily Se-substituted compounds show metallic behavior in the entire-temperature region. Sign changes of the Hall coefficients are observed for the x = 0.2 samples, which possibly is related to a charge-density wave (CDW). Magnetoresistance measurements indicate that WAL is realized in the heavily Se-substituted systems. The WAL behavior is weakened by the changes in F and Se concentrations. A crossover state of the WAL and WL emerges around the moderately F-doped and Se-free LaO0.8F0.2BiS2. The change of the resistivity behavior by the F and Se substitution clearly correlates to the difference of the magnetoconductance. Moreover, the localization regions of the WAL-WL crossover and weak WAL states are possibly associated with the CDW. We propose that the BiCh2-based system is a good platform for studying relationship between WAL, superconductivity, and electronic ordering because those states are tunable by element substitutions with bulk single crystals. △ Less

Submitted 1 November, 2022; originally announced November 2022.

Field-tunable Weyl points and large anomalous Hall effects in degenerate magnetic semiconductor EuMg$_2$Bi$_2$

Authors: M. Kondo, M. Ochi, R. Kurihara, A. Miyake, Y. Yamasaki, M. Tokunaga, H. Nakao, K. Kuroki, T. Kida, M. Hagiwara, H. Murakawa, N. Hanasaki, H. Sakai

Abstract: Magnets, with topologically-nontrivial Dirac/Weyl points, have recently attracted significant attention owing to the unconventional physical properties, such as large anomalous Hall effects. However, they typically have a high carrier density and complicated band structure near the Fermi energy. In this study, we report degenerate magnetic semiconductor EuMg$_2$Bi$_2$, which exhibits a single vall… ▽ More Magnets, with topologically-nontrivial Dirac/Weyl points, have recently attracted significant attention owing to the unconventional physical properties, such as large anomalous Hall effects. However, they typically have a high carrier density and complicated band structure near the Fermi energy. In this study, we report degenerate magnetic semiconductor EuMg$_2$Bi$_2$, which exhibits a single valley at the $Γ$ point, where the field-tunable Weyl points form via the magnetic exchange interaction with the local Eu spins. By the high-field measurements on high-quality single crystals, we observed the quantum oscillations in resistivity, elastic constant, and surface impedance, which enabled us to determine the position of the Fermi energy. In combination with the first-principles calculation, we revealed that the Weyl points are located in the vicinity of the Fermi energy when the Eu spins are fully polarized. Furthermore, we observed large anomalous Hall effect (Hall angle $Θ_{\mathrm{AH}}\sim0.07$) in the forced ferromagnetic phase, which is consistent with this field variation of band structure. △ Less

Submitted 9 June, 2022; originally announced June 2022.

Comments: 18 pages, 4 figures

First-principles study of defect formation energies in LaO$X$S$_2$ ($X=$ Sb, Bi)

Authors: Masayuki Ochi, Kazuhiko Kuroki

Abstract: We theoretically investigate defect formation energies in LaO$X$S$_2$ ($X=$Sb, Bi) using first-principles calculation. We find that the oxygen vacancy is relatively stable, where its formation energy is higher in $X=$ Sb than in $X=$ Bi. An interesting feature of $X=$ Sb is that the vacancy of the in-plane sulfur atom becomes more stable than in $X=$ Bi, caused by the formation of an Sb$_2$ dimer… ▽ More We theoretically investigate defect formation energies in LaO$X$S$_2$ ($X=$Sb, Bi) using first-principles calculation. We find that the oxygen vacancy is relatively stable, where its formation energy is higher in $X=$ Sb than in $X=$ Bi. An interesting feature of $X=$ Sb is that the vacancy of the in-plane sulfur atom becomes more stable than in $X=$ Bi, caused by the formation of an Sb$_2$ dimer and the electron occupation of the impurity energy levels. The formation energies of cation defects and anion-cation antisite defects are positive for the chemical equilibrium condition used in this study. Fluorine likely replaces oxygen, and its defect formation energy is negative for both $X=$ Sb and Bi, while that for $X=$ Sb is much higher than $X=$ Bi. Our study clarifies the stability of several point defects and suggests that the in-plane structural instability is enhanced in $X=$ Sb, which seems to affect a structural change caused by some in-plane point defects. △ Less

Submitted 21 March, 2022; v1 submitted 24 October, 2021; originally announced October 2021.

Comments: 15 pages, 15 figures

Journal ref: Physical Review B 105, 094110 (2022)

Possibility of n-type doping in CaAl$_2$Si$_2$-type Zintl phase compound CaZn$_2X_2$ ($X$ = As, P)

Authors: Kazutaka Nishiguchi, Masayuki Ochi, Chul-Ho Lee, Kazuhiko Kuroki

Abstract: Motivated by a recent theoretical suggestion that doping electrons into various CaAl$_2$Si$_2$-type Zintl phase compounds may give rise to high thermoelectric performance, we explore the possibility of n-type (electron carrier) doping of CaZn$_2X_2$ ($X$ = As, P) using first principles calculation. We consider n-type doping of CaZn$_2X_2$ with the following two situations: interstitial-site doping… ▽ More Motivated by a recent theoretical suggestion that doping electrons into various CaAl$_2$Si$_2$-type Zintl phase compounds may give rise to high thermoelectric performance, we explore the possibility of n-type (electron carrier) doping of CaZn$_2X_2$ ($X$ = As, P) using first principles calculation. We consider n-type doping of CaZn$_2X_2$ with the following two situations: interstitial-site doping of alkaline earth metals $AE$ (= Mg, Ca, Sr, Ba) and group 3 elements $G3$ (= Sc, Y, La), and $G3$ substitutional doping for the Ca site. The evaluation of the formation energy of these defects in various charged states reveals that the interstitial-site doping of $AE$ = Ca/Mg or $G3$ = Sc/Y, and $G3$ = La/Y substitutional doping for the Ca site are relatively favorable among the possibilities considered. In particular, the formation energy of the La substitutional doping for the Ca site is the lowest among the considered cases both for CaZn$_2X_2$ ($X$ = As, P) and is negative, which implies that La is expected to be substituted for the Ca site and provide electron carriers spontaneously. We also find that for each doping case considered, the formation energy of the defects is smaller for $X$ = As than for $X$ = P, which suggests that former is relatively easier to realize n-type doping than the latter. △ Less

Submitted 23 September, 2021; originally announced September 2021.

Comments: 7 pages, 7 figures

Fully self-consistent optimization of the Jastrow-Slater-type wave function using a similarity-transformed Hamiltonian

Authors: Masayuki Ochi

Abstract: It has been well established that the Jastrow correlation factor can effectively capture the electron correlation effects, and thus, the efficient optimization of the many-body wave function including the Jastrow correlation factor is of great importance. For this purpose, the transcorrelated $+$ variational Monte Carlo (TC$+$VMC) method is one of the promising methods, where the one-electron orbi… ▽ More It has been well established that the Jastrow correlation factor can effectively capture the electron correlation effects, and thus, the efficient optimization of the many-body wave function including the Jastrow correlation factor is of great importance. For this purpose, the transcorrelated $+$ variational Monte Carlo (TC$+$VMC) method is one of the promising methods, where the one-electron orbitals in the Slater determinant and the Jastrow factor are self-consistently optimized in the TC and VMC methods, respectively. In particular, the TC method is based on similarity-transformation of the Hamitonian by the Jastrow factor, which enables the efficient optimization of the one-electron orbitals under the effective interaction. Through test calculations of some closed-shell atoms, He, Be, and Ne, we find that the total energy is in many cases systematically improved by using better Jastrow functions. We find that even a one-shot TC$+$VMC calculation, where the Jastrow parameters are optimized at the Hartree-Fock$+$VMC level, can yield partial benefits from orbital optimization. It is also suggested that one-shot TC$+$VMC can be a good alternative way for complex systems. Our study provides important insights for optimizing many-body wave function including the Jastrow correlation factor, which would be of great help for development of highly accurate electronic structure calculations. △ Less

Submitted 10 September, 2023; v1 submitted 13 September, 2021; originally announced September 2021.

Comments: 19 pages, 8 figures

Journal ref: Physical Review A 108, 032806 (2023)

Simplification of the local full vertex in the impurity problem in DMFT and its applications for the nonlocal correlation

Authors: Ryota Mizuno, Masayuki Ochi, Kazuhiko Kuroki

Abstract: The two-particle vertex function is crucial for the diagrammatic extensions beyond DMFT for the nonlocal fluctuation. However, estimating the two-particle quantities is still a challenging task. In this study, we propose a simplification of the local two-particle full vertex and, using the simplified full vertex, we develop two methods to take into account the nonlocal fluctuation. We apply these… ▽ More The two-particle vertex function is crucial for the diagrammatic extensions beyond DMFT for the nonlocal fluctuation. However, estimating the two-particle quantities is still a challenging task. In this study, we propose a simplification of the local two-particle full vertex and, using the simplified full vertex, we develop two methods to take into account the nonlocal fluctuation. We apply these methods to several models and confirm that our methods can capture important behaviors such as the pseudo gap in the DMFT + nonlocal calculation. In addition, the numerical costs are largely reduced compared to the conventional methods. △ Less

Submitted 21 August, 2021; originally announced August 2021.

Comments: 18 pages, 17 figures, 1table. arXiv admin note: substantial text overlap with arXiv:2101.04466

Journal ref: Journal of the Physical Society of Japan 91, 034002 (2022)

First-principles study on the electrical resistivity in zirconium dichalcogenides with multi-valley bands: mode-resolved analysis of electron-phonon scattering

Authors: Hitoshi Mori, Masayuki Ochi, Kazuhiko Kuroki

Abstract: Based on the first-principles calculations, we study the electron-phonon scattering effect on the resistivity in the zirconium dichalcogenides, $\text{Zr}_{}\text{S}_{2}$ and $\text{Zr}_{}\text{Se}_{2}$, whose electronic band structures possess multiple valleys at conduction band minimum. The computed resistivity exhibits non-linear temperature dependence, especially for… ▽ More Based on the first-principles calculations, we study the electron-phonon scattering effect on the resistivity in the zirconium dichalcogenides, $\text{Zr}_{}\text{S}_{2}$ and $\text{Zr}_{}\text{Se}_{2}$, whose electronic band structures possess multiple valleys at conduction band minimum. The computed resistivity exhibits non-linear temperature dependence, especially for $\text{Zr}_{}\text{S}_{2}$, which is also experimentally observed on some TMDCs such as $\text{Ti}_{}\text{S}_{2}$ and $\text{Zr}_{}\text{Se}_{2}$. By performing the decomposition of the contributions of scattering processes, we find that the intra-valley scattering by acoustic phonons mainly contributes to the resistivity around 50 K. Moreover, the contribution of the intra-valley scattering by optical phonons becomes dominant even above 80 K, which is a sufficiently low temperature compared with their frequencies. By contrast, the effect of the inter-valley scattering is found to be not significant. Our study identifies the characteristic scattering channels in the resistivity of the zirconium dichalcogenides, which provides critical knowledge to microscopically understand electron transport in systems with multi-valley band structure. △ Less

Submitted 1 August, 2021; originally announced August 2021.

Comments: 14 pages, 10 figures

Enhancing thermopower and Nernst signal of high-mobility Dirac carriers by Fermi level tuning in the layered magnet EuMnBi$_2$

Authors: Keigo Tsuruda, Kento Nakagawa, Masayuki Ochi, Kazuhiko Kuroki, Masashi Tokunaga, Hiroshi Murakawa, Noriaki Hanasaki, Hideaki Sakai

Abstract: Dirac/Weyl semimetals hosting linearly-dispersing bands have received recent attention for potential thermoelectric applications, since their ultrahigh-mobility carriers could generate large thermoelectric and Nernst power factors. To optimize these efficiencies, the Fermi energy needs to be chemically controlled in a wide range, which is generally difficult in bulk materials because of disorder e… ▽ More Dirac/Weyl semimetals hosting linearly-dispersing bands have received recent attention for potential thermoelectric applications, since their ultrahigh-mobility carriers could generate large thermoelectric and Nernst power factors. To optimize these efficiencies, the Fermi energy needs to be chemically controlled in a wide range, which is generally difficult in bulk materials because of disorder effects from the substituted ions. Here it is shown that the Fermi energy is tunable across the Dirac point for layered magnet EuMnBi$_2$ by partially substituting Gd$^{3+}$ for Eu$^{2+}$ in the insulating block layer, which dopes electrons into the Dirac fermion layer without degrading the mobility. Clear quantum oscillation observed even in the doped samples allows us to quantitatively estimate the Fermi energy shift and optimize the power factor (exceeding 100 $μ$W/K$^2$cm at low temperatures) in combination with the first-principles calculation. Furthermore, it is shown that Nernst signal steeply increases with decreasing carrier density beyond a simple theoretical prediction, which likely originates from the field-induced gap reduction of the Dirac band due to the exchange interaction with the Eu moments. Thus, the magnetic block layer provides high controllability for the Dirac fermions in EuMnBi$_2$, which would make this series of materials an appealing platform for novel transport phenomena. △ Less

Submitted 12 May, 2021; originally announced May 2021.

Comments: 22 pages, 4 figures; Original version submitted for peer review; Supporting Information is included at the end of the file

Journal ref: Adv. Funct. Mater. 2021, 2102275

Tunable spin-valley coupling in layered polar Dirac metals

Authors: Masaki Kondo, Masayuki Ochi, Tatsuhiro Kojima, Ryosuke Kurihara, Daiki Sekine, Masakazu Matsubara, Atsushi Miyake, Masashi Tokunaga, Kazuhiko Kuroki, Hiroshi Murakawa, Noriaki Hanasaki, Hideaki Sakai

Abstract: In non-centrosymmetric metals, spin-orbit coupling (SOC) induces momentum-dependent spin polarization at the Fermi surfaces. This is exemplified by the valley-contrasting spin polarization in monolayer transition metal dichalcogenides (TMDCs) with in-plane inversion asymmetry. However, the valley configuration of massive Dirac fermions in TMDCs is fixed by the graphene-like structure, which limits… ▽ More In non-centrosymmetric metals, spin-orbit coupling (SOC) induces momentum-dependent spin polarization at the Fermi surfaces. This is exemplified by the valley-contrasting spin polarization in monolayer transition metal dichalcogenides (TMDCs) with in-plane inversion asymmetry. However, the valley configuration of massive Dirac fermions in TMDCs is fixed by the graphene-like structure, which limits the variety of spin-valley coupling. Here, we show that the layered polar metal BaMn$X_2$ ($X =$Bi, Sb) hosts tunable spin-valley-coupled Dirac fermions, which originate from the distorted $X$ square net with in-plane lattice polarization. We found that in spite of the larger SOC, BaMnBi$_2$ has approximately one-tenth the lattice distortion of BaMnSb$_2$, from which a different configuration of spin-polarized Dirac valleys is theoretically predicted. This was experimentally observed as a clear difference in the Shubnikov-de Haas oscillation at high fields between the two materials. The chemically tunable spin-valley coupling in BaMn$X_2$ makes it a promising material for various spin-valleytronic devices. △ Less

Submitted 18 May, 2021; v1 submitted 11 March, 2021; originally announced March 2021.

Comments: 25 pages, 4 figures. Published in Communications Materials

Journal ref: Commun. Mater. 2, 49 (2021)

Development of an efficient impurity solver in dynamical mean field theory for multi-band systems: The iterative perturbation theory combined with the parquet equations

Authors: Ryota Mizuno, Masayuki Ochi, Kazuhiko Kuroki

Abstract: Although several impurity solvers in the dynamical mean field theory (DMFT) have been proposed, especially in multi-band systems, there are practical difficulties arising from a trade-off between numerical costs and reliability. In this study, we re-interpret the iterative perturbation theory (IPT) as an approximation which captures the strong correlation effects by mimicking the particular freque… ▽ More Although several impurity solvers in the dynamical mean field theory (DMFT) have been proposed, especially in multi-band systems, there are practical difficulties arising from a trade-off between numerical costs and reliability. In this study, we re-interpret the iterative perturbation theory (IPT) as an approximation which captures the strong correlation effects by mimicking the particular frequency structures of the exact full vertex, and extend it such that it can have efficiency and reliability simultaneously by modifying IPT vertex using the parquet equations. We apply this method to several models to evaluate their validity. We confirm that our method shows good agreements with the numerically exact continuous-time quantum Monte Carlo method in the single-site DMFT calculation. △ Less

Submitted 2 August, 2021; v1 submitted 12 January, 2021; originally announced January 2021.

Comments: 15 pages, 13 figures

Journal ref: Phys. Rev. B 104, 035160 (2021)

Epitaxial stabilization of SrCu$_3$O$_4$ with infinite Cu$_{3/2}$O$_2$ layers

Authors: Hiroshi Takatsu, Masayuki Ochi, Naoya Yamashina, Morito Namba, Kazuhiko Kuroki, Takahito Terashima, Hiroshi Kageyama

Abstract: We report the epitaxial thin film synthesis of SrCu$_3$O$_4$ with infinitely stacked Cu$_3$O$_4$ layers composed of edge-sharing CuO$_4$ square-planes, using molecular beam epitaxy. Experimental and theoretical characterizations showed that this material is a metastable phase that can exist by applying tensile biaxial strain from the (001)-SrTiO$_3$ substrate. SrCu$_3$O$_4$ shows an insulating ele… ▽ More We report the epitaxial thin film synthesis of SrCu$_3$O$_4$ with infinitely stacked Cu$_3$O$_4$ layers composed of edge-sharing CuO$_4$ square-planes, using molecular beam epitaxy. Experimental and theoretical characterizations showed that this material is a metastable phase that can exist by applying tensile biaxial strain from the (001)-SrTiO$_3$ substrate. SrCu$_3$O$_4$ shows an insulating electrical resistivity in accordance with the Cu$^{2+}$ valence state revealed X-ray photoelectron spectroscopy. First-principles calculations also indicated that the unoccupied $d_{3z^2-r^2}$ band becomes substantially stabilized owing to the absence of apical anions, in contrast to $A_2$Cu$_3$O$_4$Cl$_2$ ($A = $Sr, Ba) with an $A_2$Cl$_2$ block layer and therefore a trans-CuO$_4$Cl$_2$ octahedron. These results suggest that SrCu$_3$O$_4$ is a suitable parent material for electron-doped superconductivity based on the Cu$_3$O$_4$ plane. △ Less

Submitted 4 December, 2020; originally announced December 2020.

Comments: 1 column, 22 pages, 6 figures, Supporting Information

Journal ref: Inorg. Chem. 59, 10042 (2020)

Hidden Ladder in SrMoO$_3$/SrTiO$_3$ Superlattices: Experiments and Theoretical Calculations

Authors: Hiroshi Takatsu, Naoya Yamashina, Masayuki Ochi, Hsin-Hui Huang, Shunsuke Kobayashi, Akihide Kuwabara, Takahito Terashima, Kazuhiko Kuroki, Hiroshi Kageyama

Abstract: A double-layered perovskite oxide Sr$_3$Mo$_2$O$_7$ is considered a "hidden ladder" system with wide and narrow bands near the Fermi level, for which high-$T_{\rm c}$ superconductivity is expected. However, the difficulty in synthesis, especially in the preparation of samples without oxygen deficiency, can hinder the observation of superconductivity. In this study, we constructed a double-layer Sr… ▽ More A double-layered perovskite oxide Sr$_3$Mo$_2$O$_7$ is considered a "hidden ladder" system with wide and narrow bands near the Fermi level, for which high-$T_{\rm c}$ superconductivity is expected. However, the difficulty in synthesis, especially in the preparation of samples without oxygen deficiency, can hinder the observation of superconductivity. In this study, we constructed a double-layer SrMoO$_3$ block through artificial superlattices with the insulating SrTiO$_3$ block, (SrMoO$_3$)$_m$/(SrTiO$_3$)$_t$ ($m = 2, 4$; $t = 4$). First-principles calculations for bilayered SrMoO$_3$ ($m = 2$) exhibit a wide-narrow band structure near the Fermi level, which bears a close resemblance to Sr$_3$Mo$_2$O$_7$. The dispersion along the $k_z$ direction is strongly suppressed by increasing the number of the SrTiO$_3$ layers, $t$. However, no superconductivity is observed down to 0.1 K. We discuss the absence of the superconductivity for the present films on the basis of results of scanning transmission electron microscopy and band structure calculations. △ Less

Submitted 4 December, 2020; originally announced December 2020.

Comments: 7 pages, 6 figures, Supplemental Material

Journal ref: J. Phys. Soc. Jpn. 89, 074801 (2020)

Visualization of the strain-induced topological phase transition in a quasi-one-dimensional superconductor TaSe3

Authors: Chun Lin, Masayuki Ochi, Ryo Noguchi, Kenta Kuroda, Masahito Sakoda, Atsushi Nomura, Masakatsu Tsubota, Peng Zhang, Cedric Bareille, Kifu Kurokawa, Yosuke Arai, Kaishu Kawaguchi, Hiroaki Tanaka, Koichiro Yaji, Ayumi Harasawa, Makoto Hashimoto, Donghui Lu, Shik Shin, Ryotaro Arita, Satoshi Tanda, Takeshi Kondo

Abstract: Control of the phase transition from topological to normal insulators can allow for an on/off switching of spin current. While topological phase transitions have been realized by elemental substitution in semiconducting alloys, such an approach requires the preparation of materials with various compositions, thus it is quite far from a feasible device application, which demands a reversible operat… ▽ More Control of the phase transition from topological to normal insulators can allow for an on/off switching of spin current. While topological phase transitions have been realized by elemental substitution in semiconducting alloys, such an approach requires the preparation of materials with various compositions, thus it is quite far from a feasible device application, which demands a reversible operation. Here we use angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES to visualize the strain-driven band structure evolution of the quasi-1D superconductor TaSe3. We demonstrate that it undergoes reversible strain-induced topological phase transitions from a strong topological insulator phase with spin-polarized, quasi-1D topological surface states, to topologically trivial semimetal and band insulating phases. The quasi-1D superconductor TaSe3 provides a suitable platform for engineering the topological spintronics, for example as an on/off switch for spin current robust against impurity scattering. △ Less

Submitted 16 June, 2021; v1 submitted 14 September, 2020; originally announced September 2020.

Journal ref: Nature Materials (2021)

Quantifying the stability of the anion ordering in SrVO$_2$H

Authors: Masayuki Ochi, Kazuhiko Kuroki

Abstract: We investigate a text-book mixed-anion compound SrVO$_2$H using first-principles calculation, to theoretically pin down the factors that stabilize its anion ordering. We find that the $trans$ preference by the characteristic crystal field in the VO$_4$H$_2$ octahedron in addition to a coherent shrinkage along the V-H-V direction, taking place when such direction is consistent among neighboring hyd… ▽ More We investigate a text-book mixed-anion compound SrVO$_2$H using first-principles calculation, to theoretically pin down the factors that stabilize its anion ordering. We find that the $trans$ preference by the characteristic crystal field in the VO$_4$H$_2$ octahedron in addition to a coherent shrinkage along the V-H-V direction, taking place when such direction is consistent among neighboring hydrogens, stabilize the anion ordering observed in experiment. Our study gives an important clue for controlling the anion ordering in transition metal oxyhydrides. △ Less

Submitted 15 October, 2020; v1 submitted 3 August, 2020; originally announced August 2020.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. B 102, 134108 (2020)

Designing nickelate superconductors with $d^8$ configuration exploiting mixed-anion strategy

Authors: Naoya Kitamine, Masayuki Ochi, Kazuhiko Kuroki

Abstract: Inspired by a recently proposed superconducting mechanism for a new cuprate superconductor Ba$_2$CuO$_{3+δ}$, we theoretically design an unconventional nickelate superconductor with $d^8$ electron configuration. Our strategy is to enlarge the on-site energy difference between $3d_{x^2-y^2}$ and other $3d$ orbitals by adopting halogens or hydrogen as out-of-plane anions, so that the $3d$ bands othe… ▽ More Inspired by a recently proposed superconducting mechanism for a new cuprate superconductor Ba$_2$CuO$_{3+δ}$, we theoretically design an unconventional nickelate superconductor with $d^8$ electron configuration. Our strategy is to enlarge the on-site energy difference between $3d_{x^2-y^2}$ and other $3d$ orbitals by adopting halogens or hydrogen as out-of-plane anions, so that the $3d$ bands other than $d_{x^-y^2}$ lie just below the Fermi level for the $d^8$ configuration, acting as incipient bands that enhance superconductivity. We also discuss a possible relevance of the present proposal to the recently discovered superconductor (Nd,Sr)NiO$_2$. △ Less

Submitted 30 November, 2020; v1 submitted 2 July, 2020; originally announced July 2020.

Comments: published version

Journal ref: Phys. Rev. Research 2, 042032 (2020)

Superconducting mechanism for the cuprate Ba$_2$CuO$_{3+δ}$ based on a multiorbital Lieb lattice model

Authors: Kimihiro Yamazaki, Masayuki Ochi, Daisuke Ogura, Kazuhiko Kuroki, Hiroshi Eisaki, Shinichi Uchida, Hideo Aoki

Abstract: For the recently discovered cuprate superconductor $\mathrm{Ba_{2}CuO_{3+δ}}$, we propose a lattice structure which resembles the model considered by Lieb to represent the vastly oxygen-deficient material. We first investigate the stability of the Lieb-lattice structure, and then construct a multiorbital Hubbard model based on first-principles calculation. By applying the fluctuation-exchange appr… ▽ More For the recently discovered cuprate superconductor $\mathrm{Ba_{2}CuO_{3+δ}}$, we propose a lattice structure which resembles the model considered by Lieb to represent the vastly oxygen-deficient material. We first investigate the stability of the Lieb-lattice structure, and then construct a multiorbital Hubbard model based on first-principles calculation. By applying the fluctuation-exchange approximation to the model and solving the linearized Eliashberg equation, we show that $s$-wave and $d$-wave pairings closely compete with each other, and, more interestingly, that the intra-orbital and inter-orbital pairings coexist. We further show that, if the energy of the $d_{3z^2-r^2}$ band is raised to make it "incipient" with the lower edge of the band close to the Fermi level within a realistic band filling regime, $s\pm$-wave superconductivity is strongly enhanced. We reveal an intriguing relation between the Lieb model and the two-orbital model for the usual K$_2$NiF$_4$ structure where a close competition between $s-$ and $d-$wave pairings is known to occur. The enhanced superconductivity in the present model is further shown to be related to an enhancement found previously in the bilayer Hubbard model with an incipient band. △ Less

Submitted 5 September, 2020; v1 submitted 9 March, 2020; originally announced March 2020.

Comments: 19 pages, 20 figures; references and figures updated

Journal ref: Phys. Rev. Research 2, 033356 (2020)

Material design with the van der Waals stacking of bismuth-halide chains realizing a higher-order topological insulator

Authors: Ryo Noguchi, Masaru Kobayashi, Zhanzhi Jiang, Kenta Kuroda, Takanari Takahashi, Zifan Xu, Daehun Lee, Motoaki Hirayama, Masayuki Ochi, Tetsuroh Shirasawa, Peng Zhang, Chun Lin, Cédric Bareille, Shunsuke Sakuragi, Hiroaki Tanaka, So Kunisada, Kifu Kurokawa, Koichiro Yaji, Ayumi Harasawa, Viktor Kandyba, Alessio Giampietri, Alexei Barinov, Timur K. Kim, Cephise Cacho, Makoto Hashimoto , et al. (6 additional authors not shown)

Abstract: The van der Waals (vdW) materials with low dimensions have been extensively studied as a platform to generate exotic quantum properties. Advancing this view, a great deal of attention is currently paid to topological quantum materials with vdW structures. Here, we provide a new concept of designing topological materials by the vdW stacking of quantum spin Hall insulators (QSHIs). Most interestingl… ▽ More The van der Waals (vdW) materials with low dimensions have been extensively studied as a platform to generate exotic quantum properties. Advancing this view, a great deal of attention is currently paid to topological quantum materials with vdW structures. Here, we provide a new concept of designing topological materials by the vdW stacking of quantum spin Hall insulators (QSHIs). Most interestingly, a slight shift of inversion center in the unit cell caused by a modification of stacking is found to induce the topological variation from a trivial insulator to a higher-order topological insulator (HOTI). Based on that, we present the first experimental realization of a HOTI by investigating a bismuth bromide Bi4Br4 with angle-resolved photoemission spectroscopy (ARPES). The unique feature in bismuth halides capable of selecting various topology only by differently stacking chains, combined with the great advantage of the vdW structure, offers a fascinating playground for engineering topologically non-trivial edge-states toward future spintronics applications. △ Less

Submitted 10 February, 2021; v1 submitted 4 February, 2020; originally announced February 2020.

Comments: Nature Materials, in press. The final version of this article is available online at https://doi.org/10.1038/s41563-020-00871-7