arXiv subjects

Masayuki Ochi

Publications and source records attributed to Masayuki Ochi.

At least 19 recordsLinked to original sources

arXiv2026

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

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.

cond-mat.str-el↗

arXiv2026

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

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.

cond-mat.str-el↗

arXiv2026

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

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.

cond-mat.supr-con↗

arXiv2026

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

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.

cond-mat.supr-con↗

arXiv2026

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

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 $\Gamma$ 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$_6$.

cond-mat.mtrl-sci↗

arXiv2026

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

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.

cond-mat.mes-hall↗

arXiv2026

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

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.

cond-mat.str-el↗

arXiv2025

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

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.

cond-mat.mes-hall↗

arXiv2025

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

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.

cond-mat.mtrl-sci↗

arXiv2025

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

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.

cond-mat.mtrl-sci↗

arXiv2025

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

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 $\gamma$-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 $T_c$.

cond-mat.supr-con↗

arXiv2025

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

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.

cond-mat.str-el↗

arXiv2025

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

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.

cond-mat.mtrl-sci↗

arXiv2025

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

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.

cond-mat.supr-con↗

arXiv2025

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

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$_2$.

cond-mat.mtrl-sci↗

arXiv2025

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

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$_7$.

cond-mat.supr-con↗

arXiv2024

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

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$_{15-δ}$.

cond-mat.supr-con↗

arXiv2024

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

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.

cond-mat.supr-con↗