arXiv subjects

Kazuhiko Kuroki

Publications and source records attributed to Kazuhiko Kuroki.

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

Superconductivity in doped spin multimer systems

Binding energy, which quantifies pair formation, is a key factor in the emergence of superconductivity. Here, we show that even when multiple spins are complexly coupled, hole-doped systems, which can be mapped onto the universal hardcore boson model in the strong-binding-energy limit, exhibit promising signatures of superconductivity. We demonstrate this theory analytically and numerically in the double Kondo lattice model. Using the density-matrix renormalization group method, we show that a pairing state is maintained via a crossover even for parameters away from the strong-coupling regime. Additionally, we find that once binding energies are sufficiently generated, pair correlations develop similarly regardless of the details of local spin correlations. Our findings provide useful guidelines for research on superconductivity.

cond-mat.supr-con↗

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

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 $γ$-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 $T_c$.

cond-mat.supr-con↗

arXiv2026

Pairing properties of correlated three-leg ladders with strong interchain couplings near 1/3 filling

We investigate the ground-state properties of correlated three-leg ladders near 1/3 filling. We apply the density-matrix renormalization group method to the three-leg t-J ladder with strong interchain couplings and evaluate its pairing nature. When holes are doped into the spin-gapped state at 1/3 filling, we find that pair correlations develop with power-law decays while spin correlations decay exponentially. On the other hand, doping of electrons into the 1/3-filled state does not give rise to substantial pair correlations. We also discuss the hole-doped state in the three-leg Hubbard model to compare it with the pairing state in the t-J model. Our numerical demonstrations provide insights into the electronic properties of trilayer nickelate superconductors.

cond-mat.supr-con↗

arXiv2026

Doublon-Holon Pairing State in Photodoped Mott Insulators

We demonstrate the existence of an unconventional pairing state in photodoped Mott insulators on ladder and quasi-two-dimensional geometries, characterized by quasi-long-range doublon-holon correlations that signal Mott exciton condensation. The doublon-holon pairing exhibits correlations of $d$-wave-like symmetry, reminiscent of superconducting pairing in chemically doped Mott insulators. By constructing the phase diagram, using density matrix renormalization group, we reveal that the doublon-holon pairing state in the photodoped ladder emerges between the spin-singlet, charge-density-wave, and $η$-pairing phases. Our study suggests that the interplay of charge, spin, and $η$-spin degrees of freedom can give rise to exotic quantum many-body states in photodoped Mott insulators.

cond-mat.str-el↗

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

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↗

arXiv2026

Spin-fluctuation-mediated chiral $d+id'$-wave superconductivity in the $α$-$\mathcal{T}_3$ lattice with an incipient flat band

We study anisotropic superconductivity in the nearly quarter-filled $α$-$\mathcal{T}_3$ lattice. We analyze an extended Hubbard model with off-site attractive interactions within the mean-field framework and find two distinct chiral $d+id'$-wave superconducting phases characterized by different Chern numbers. We further investigate the superconducting mechanism mediated by spin fluctuations arising from purely repulsive interactions by applying the fluctuation-exchange (FLEX) approximation to the Hubbard model. The gap symmetry obtained by solving the linearized Eliashberg equation is $d$-wave, which corresponds to a $d+id'$-wave superconducting state with a Chern number of $8$, including the spin degree of freedom. The $\mathbf{q}=\mathbf{0}$ antiferromagnetic spin fluctuation, which possesses the largest spectral weight at finite energies arising from the incipient flat band, gives rise to an effective spin-singlet pairing glue between rim sites.

cond-mat.supr-con↗

arXiv2026

Hierarchical structure of primary and hybridization-induced superconducting correlations in bilayer nickelates

High-pressure superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_7$, with a transition temperature approaching 80 K, has stimulated intense debate regarding its microscopic origin. Although an $s_{\pm}$ gap symmetry has been widely proposed, the electronic degrees of freedom responsible for pairing remain unsettled. Here we investigate a bilayer two-orbital Hubbard model using the variational Monte Carlo method and reveal a hierarchical pairing structure in bilayer nickelates. The primary pairing interaction originates from the bonding--antibonding splitting of the Ni $3d_{z^2}$ orbitals, while orbital hybridization redistributes superconducting correlations to the $d_{x^2-y^2}$ channel despite its weak intrinsic pairing interaction. This distinction between the origin of pairing and resulting superconducting correlations explains why the two orbital channels exhibit comparable long-range correlations. The resulting $s_{\pm}$ state is robust against changes in Fermi-surface topology. These results reconcile apparently competing theoretical scenarios and provide a comprehensive understanding, highlighting the distinctive role of orbital hybridization in multilayer correlated superconductors.

cond-mat.supr-con↗

arXiv2026

Theoretical proposal of superconductivity in hole-doped reduced bilayer nickelate La3Ni2O6: a manifestation of orbital-space bilayer model with incipient bands

A correspondence exists between the multi-orbital Hubbard model and the bilayer Hubbard model, in which superconductivity is optimized in an incipient-band regime in both cases. In the multi-orbital system, the orbital level offset $ΔE$ plays a role analogous to the interlayer hopping in bilayer systems, and superconductivity is enhanced for large $ΔE$. We refer to such a multi-orbital model as an orbital-space bilayer model (OSBM). In this study, we theoretically propose that a reduced bilayer nickelate La$_3$Ni$_2$O$_6$ can be a candidate for a superconductor described by OSBM when an appropriate amount of holes is doped. By constructing a tight-binding model based on first-principles calculations, a large $ΔE$ between the Ni $d_{x^2-y^2}$ and the other $d$ orbitals is obtained due to the absence of outer apical oxygens. Furthermore, our fluctuation exchange approximation calculations indicate the emergence of $s\pm$-wave superconductivity driven by interorbital interactions in an incipient-band situation, where the superconducting gap function changes its sign between the $d_{x^2-y^2}$ and other $d$ orbital bands. We also investigate the energetic and dynamical stability of the crystal structure under atomic substitution and pressure. Although La$_3$Ni$_2$O$_7$ and La$_3$Ni$_2$O$_6$ share a similar chemical formula, our study shows that an entirely different pairing mechanism can take place in the latter.

cond-mat.supr-con↗

arXiv2025

Finite-Temperature $\textit{ab initio}$ Structural Optimization of the Bilayer Nickelate Superconductor La$_3$Ni$_2$O$_7$

We develop a first-principles framework for finite-temperature structural optimization that incorporates vibrational contributions to the free energy through anharmonic phonon theory. We extend and further improve the efficiency of the recent approach, enabling its application to systems in which the size of the primitive cell changes across structural phase transitions. Applying this framework to La$_3$Ni$_2$O$_7$, we establish its pressure-temperature phase diagram and find that the slope of the phase boundary between the high-symmetry and low-symmetry phases is negative, with a magnitude of approximately -60 K / GPa. The present results provide a theoretical foundation for discussing how changes in crystal symmetry influence the emergence of superconductivity.

cond-mat.supr-con↗

arXiv2025

$t$-$J$ model for strongly correlated two-orbital systems: Application to bilayer nickelate superconductors

We derive a $t$-$J$ model applicable to strongly correlated two-orbital systems including bilayer nickelate superconductors. Using the Schrieffer-Wolff transformation, we exclude the doubly occupied states, which raise the onsite Coulomb energy, and derive the resulting spin interactions from the two-orbital Hubbard model. We also introduce effective interactions attributed to the interorbital Coulomb interaction. To adapt the effective model to bilayer nickelates that exhibit high-temperature superconductivity, we quantitatively evaluate the strengths of the spin interactions based on the hopping parameters in La$_3$Ni$_2$O$_7$. Considering the evaluated effective interactions, we propose a simplified $t$-$J$ model for bilayer nickelate superconductors.

cond-mat.str-el↗

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

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

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.

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

Floquet engineering of effective pairing interactions in a doped band insulator

We investigate the pairing state in a doped band insulator under a periodic driving field. We employ a correlated fermionic model on a honeycomb lattice, in which pairing glue is obtainable via repulsive interactions, and derive an effective model under circularly polarized light. We demonstrate that the effective pairing interaction for doped fermions obtained with the second-order perturbation theory is tunable by the frequency and amplitude of the driving field. We find the optimal frequency range to enhance the pairing interaction and show that the modified effective system can strengthen the two-body bound state. Our study suggests that external driving light can reinforce superconducting pair states in strongly correlated electron systems.

cond-mat.str-el↗