arXiv subjects

Yoji Ohashi

Publications and source records attributed to Yoji Ohashi.

At least 19 recordsLinked to original sources

arXiv2026

Finite-momentum superconductivity with singlet-triplet mixing in an altermagnetic metal: A pairing instability analysis

We analyze the pairing instability of an altermagnetic metal on a square lattice driven by an attractive nearest-neighbor interaction. This interaction enables multiple pairing channels, including even-parity extended $s$-wave and $d$-wave states, as well as two odd-parity $p$-wave channels. We verify that altermagnetic spin-splitting in the single-particle dispersion gives rise to finite-momentum pairing between electrons with unlike spins, in agreement with earlier predictions. Quite unexpectedly, this pairing typically emerges across multiple channels with mixed parity. Consequently, the resulting finite-momentum Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting phase is expected to exhibit a multi-component order parameter featuring singlet-triplet mixing. We examine several forms of altermagnetism, specifically $d_{xy}$-wave and $d_{x^{2}-y^{2}}$-wave altermagnetic couplings, and present the corresponding phase diagrams. Additionally, we study triplet pairing between electrons with identical spins and find that it always occurs at zero center-of-mass momentum. Although it is unfavorable in the regime of weak altermagnetic coupling and low electron filling, it can dominate the phase diagram when the altermagnetic coupling is sufficiently strong. The influence of on-site attractive interactions on mixed-parity pairing is also explored.

cond-mat.supr-con↗

arXiv2026

Dimensionality of a strongly interacting 2D-3D Fermi-Fermi mixture from the perspective of superfluid instability and excitation properties

We theoretically investigate strong-coupling properties of an attractively interacting Fermi atomic gas, where the Cooper-pair formation occurs between atoms belonging to different dimensional bands. Including pairing fluctuations within the framework of the self-consistent $T$-matrix approximation (SCTMA), we examine how the BCS-type superfluid phase transition temperature $T_\mathrm{c}$ varies as one moves from the 3D-3D to the 2D-3D system, in the wide parameter region with respect to the strength of the pairing interaction. In the 2D-3D limit, we find that, while the mean-field BCS theory predicts $T_\mathrm{c}>0$ in the strong-coupling regime, $T_\mathrm{c}$ is remarkably suppressed down to zero by pairing fluctuations that are strongly enhanced by the mixed-dimensionality of the system. As the origin of this, we clarify that the lower-dimensional (2D) component dominates the superfluid instability, so that the vanishing $T_\mathrm{c}$ is the same phenomenon as that in the 2D-2D case. We also point out that this can already be seen in the mean-field level, when one examines the propagation of the Goldstone mode. On the other hand, we find that the pseudogap phenomenon, which is known as a precursor of Cooper-pair formation, exhibits a 3D character of the 2D-3D system. These results indicate that the dimensionality of a strongly interacting Fermi gas depends on what we observe.

cond-mat.quant-gas↗

arXiv2026

Emergence of Charge-Imbalanced BCS State and Suppression of Nonequilibrium FFLO State in Asymmetric NSN Junctions

We theoretically study nonequilibrium superconductivity in voltage-biased normal metal-superconductor-normal metal (NSN) junctions, focusing on effects of lead-coupling asymmetry and impurity scattering. Using the Keldysh Green's function technique, we extend the thermal-equilibrium mean-field BCS theory to the case where the system is out of equilibrium, to analyze superconducting properties in the nonequilibrium steady state. We find that, in close analogy with the thermal-equilibrium case, the inhomogeneous nonequilibrium Fulde-Ferrell-Larkin-Ovchinnikov (NFFLO) state induced by nonequilibrium electron distributions is highly sensitive to impurity scattering, whereas the uniform nonequilibrium BCS (NBCS) state remains robust against nonmagnetic impurities. Moreover, lead-coupling asymmetry is also found to suppress the NFFLO phase and to split the NBCS phase into two distinct regimes, characterized by the presence or absence of a chemical-potential imbalance between quasiparticles and the condensate. We identify a phase transition or a crossover between these two NBCS states, as well as parameter regimes exhibiting bistability. Our results provide a unified microscopic understanding of nonequilibrium superconductivity in NSN junctions under experimentally relevant conditions and are expected to provide a theoretical framework applicable to a broad class of nonequilibrium superconducting hybrid structures.

cond-mat.supr-con↗

arXiv2025

Engineering nonequilibrium superconducting phases in a voltage-driven superconductor under an external magnetic field

We theoretically investigate nonequilibrium properties of a normal metal-superconductor-normal metal (NSN) junction under an external magnetic field. When a bias voltage is applied between the normal-metal leads, the confined superconductor is driven out of equilibrium, resulting in a nonequilibrium quasiparticle distribution function having a two-step structure. Using the nonequilibrium Green's function technique, we determine a comprehensive phase diagram of the nonequilibrium superconductor. Our analysis reveals that the interplay between Zeeman-split energy bands and the nonequilibrium distribution function gives rise to a rich phase structure. Notably, we find that superconductivity destroyed by a strong external magnetic field revives by applying the bias voltage. This reentrant phenomenon is shown to originate from four effective "Fermi surfaces" that result from the combination of Zeeman-split energy bands and the two-step structure in the nonequilibrium distribution function. Our results demonstrate the possibility of controlling quantum states of matter through the combined engineering of energy band structures and distribution functions.

cond-mat.supr-con↗

arXiv2024

Nuclear Spin-Lattice Relaxation Rate in Odd-Frequency Superconductivity

We theoretically investigate the temperature dependence of nuclear spin-lattice relaxation rate $T_1^{-1}$ in bulk odd-frequency superconductivity. For a model odd-frequency pairing interaction, we first evaluate the superconducting order parameter, within the framework of the combined path-integral formalism with the saddle-point approximation. We then calculate $T_1^{-1}$ below the superconducting phase transition temperature $T_{\rm c}$, to see how the odd-frequency pairing affects this physical quantity. In the odd-frequency $p$-wave state, while the so-called coherence peak is suppressed as in the even-frequency $p$-wave case, $T_1^{-1}$ is found to exhibit the Korringa-law-like behavior ($T_1^{-1}\propto T$) except just below $T_{\rm c}$, even without impurity scatterings. In the odd-frequency $s$-wave case, the behavior of $T_{1}^{-1}$ is found to be sensitive to the detailed spin structure of the superconducting order parameter: In a case, $T_1^{-1}$ is enhanced far below $T_{\rm c}$, being in contrast to the conventional (even-frequency) $s$-wave BCS case, where the coherence peak appears just below $T_{\rm c}$. We also show that the calculated $T_1^{-1}$ in the odd-frequency $p$-wave case well explains the recent experiment on CeRh$_{0.5}$Ir$_{0.5}$In$_5$, where the possibility of odd-frequency $p$-wave superconductivity was recently suggested experimentally.

cond-mat.supr-con↗

arXiv2024

Nonequilibrium BCS-BEC crossover and unconventional FFLO superfluid in a strongly interacting driven-dissipative Fermi gas

We present a theoretical review of the recent progress in nonequilibrium BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover physics. As a paradigmatic example, we consider a strongly interacting driven-dissipative two-component Fermi gas where the nonequilibrium steady state is tuned by adjusting the chemical potential difference between two reservoirs that are coupled with the system. As a powerful theoretical tool to deal with this system, we employ the Schwinger-Keldysh Green's function technique. We systematically evaluate the superfluid transition, as well as the single-particle properties, in the nonequilibrium BCS-BEC crossover region, by adjusting the chemical potential difference between the reservoirs and the strength of an s-wave pairing interaction associated with a Feshbach resonance. In the weak-coupling BCS side, the chemical potential difference is shown to imprint a two-step structure on the particle momentum distribution, leading to an anomalous enhancement of pseudogap, as well as the emergence of exotic Fulde-Ferrell-Larkin-Ovchinnikov-type superfluid instability. Since various nonequilibrium situations have recently been realized in ultracold Fermi gases, the theoretical understanding of nonequilibrium BCS-BEC crossover physics would become increasingly important in this research field.

cond-mat.quant-gas↗

arXiv2024

Pairing properties of an odd-frequency superfluid Fermi gas

We theoretically investigate strong-coupling properties of an odd-frequency Fermi superfluid. This pairing state has the unique property that Cooper pairs are formed between fermions, not at the same time, but at different times. To see whether or not such unequal-time pairs still exhibit bosonic behavior, we examine the space-time structure of the odd-frequency Cooper-pair wavefunction at $T=0$, by employing the combined path-integral formalism with the BCS-Eagles-Leggett-type superfluid theory. In the strong-coupling regime, the odd-frequency pair wavefunction still has different space-time structure from that in the ordinary even-frequency $s$-wave superfluid state, their $\textit{magnitudes}$ are found to become close to each other, except for the equal-time pairing component. In this regime, we also evaluate the superfluid phase transition temperature $T_{\rm c}$ within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink. The calculated $T_{\rm c}$ in the strong-coupling regime of the odd-frequency system is found to be well described by the Bose-Einstein condensation of tightly bound Bose molecules. Our results indicates that, in spite of vanishing equal-time pairing, odd-frequency Cooper pairs still behave like bosons in the strong-coupling regime, as in the even-frequency $s$-wave superfluid case.

cond-mat.quant-gas↗

arXiv2024

Emergence of Larkin-Ovchinnikov-type superconducting state in a voltage-driven superconductor

We theoretically investigate a voltage-biased normal metal-superconductor-normal metal (N-S-N) junction. Using the nonequilibrium Green's function technique, we derive a quantum kinetic equation, to determine the superconducting order parameter self-consistently. The derived equation is an integral-differential equation with memory effects. We solve this equation by converting it into a system of ordinary differential equations with the use of a pole expansion of the Fermi-Dirac function. When the applied voltage exceeds the critical value, the superconductor switches to the normal state. We find that when the voltage is decreased from the normal phase, the system relaxes to a Larkin-Ovchinnikov (LO)-type inhomogeneous superconducting state, even in the absence of a magnetic Zeeman field. We point out that the emergence of the LO-type state can be attributed to the nonequilibrium energy distribution of electrons due to the bias voltage. We also point out that the system exhibits bistability, which leads to hysteresis in the voltage-current characteristic of the N-S-N junction.

cond-mat.supr-con↗

arXiv2023

Stable non-equilibrium Fulde-Ferrell-Larkin-Ovchinnikov state in a spin-imbalanced driven-dissipative Fermi gas loaded on a three-dimensional cubic optical lattice

We theoretically investigate a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type superfluid phase transition in a driven-dissipative two-component Fermi gas. The system is assumed to be in the non-equilibrium steady state, which is tuned by adjusting the chemical potential difference between two reservoirs that are coupled with the system. Including pairing fluctuations by extending the strong-coupling theory developed in the thermal-equilibrium state by Nozieres and Schmitt-Rink to this non-equilibrium case, we show that a non-equilibrium FFLO (NFFLO) phase transition can be realized without spin imbalance, under the conditions that (1) the two reservoirs imprint a two-edge structure on the momentum distribution of Fermi atoms, and (2) the system is loaded on a three-dimensional cubic optical lattice. While the two edges work like two Fermi surfaces with different sizes, the role of the optical lattice is to prevent the NFFLO long-range order from destruction by NFFLO pairing fluctuations. We also draw the non-equilibrium mean-field phase diagram in terms of the chemical potential difference between the two reservoirs, a fictitious magnetic field to tune the spin imbalance of the system, and the environmental temperature of the reservoirs, to clarify the relation between the NFFLO state and the ordinary thermal-equilibrium FFLO state discussed in spin-imbalanced Fermi gases.

cond-mat.quant-gas↗

arXiv2022

Feasibility of a Fulde-Ferrell-Larkin-Ovchinnikov superfluid Fermi atomic gas

We theoretically explore a promising route to achieve the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in a spin-imbalanced ultracold Fermi gas. In the current stage of cold atom physics, search for this exotic Fermi superfluid is facing two serious difficulties: One is the desperate destruction of the FFLO long-range order by FFLO pairing fluctuations, which precludes entering the phase through a second-order transition, even in three dimension. The other is the fierce competition with the phase separation into the BCS (Bardeen-Cooper-Schrieffer) state and the spin-polarized normal state. Including strong FFLO pairing fluctuations within the framework of the strong-coupling theory developed by Nozières and Schmitt-Rink, we show that the anisotropy of Fermi surface introduced by an optical lattice makes the FFLO state stable against the paring fluctuations. This stabilized FFLO state is also found to be able to overcome the competition with the phase separation under a certain condition. Since the realization of unconventional Fermi superfluids is one of the most exciting challenges in cold atom physics, our results would contribute to the further development of this field.

cond-mat.quant-gas↗

arXiv2022

Isothermal compressibility and effects of multi-body molecular interactions in a strongly interacting ultracold Fermi gas

We theoretically investigate the isothermal compressibility $κ_{T}$ in the normal state of an ultracold Fermi gas. Including pairing fluctuations, as well as preformed-pair formations, within the framework of the self-consistent $T$-matrix approximation (SCTMA), we evaluate the temperature dependence of this thermodynamic quantity over the entire BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. While $κ_T$ in the weak-coupling BCS regime is dominated by Fermi atoms near the Fermi surface, correlations between tightly bound Cooper-pair molecules are found to play crucial roles in the strong-coupling BEC regime. In the latter region, besides a two-body molecular interaction, a three-body one is shown to sizably affect $κ_T$ near the superfluid phase transition temperature. Our results indicate that the strong-coupling BEC regime of an ultracold Fermi gas would provide a unique opportunity to study multi-body correlations between Cooper-pair molecules.

cond-mat.quant-gas↗

arXiv2022

Proposed Fermi-surface reservoir-engineering and application to realizing unconventional Fermi superfluids in a driven-dissipative non-equilibrium Fermi gas

We develop a theory to describe the dynamics of a driven-dissipative many-body Fermi system, to pursue our proposal to realize exotic quantum states based on reservoir engineering. Our idea is to design the shape of a Fermi surface so as to have multiple Fermi edges, by properly attaching multiple reservoirs with different chemical potentials to a fermionic system. These emerged edges give rise to additional scattering channels that can destabilize the system into unconventional states, which is exemplified in this work by considering a driven-dissipative attractively interacting Fermi gas. By formulating a quantum kinetic equation using the Nambu-Keldysh Green's function technique, we explore nonequilibrium steady states in this system and assess their stability. We find that, in addition to the BCS-type isotropic pairing state, a Fulde-Ferrell-type anisotropic superfluid state being accompanied by Cooper pairs with non-zero center-of-mass momentum exists as a stable solution, even in the absence of a magnetic Zeeman field. Our result implies a great potential of realizing quantum matter beyond the equilibrium paradigm, by engineering the shape and topology of Fermi surfaces in both electronic and atomic systems.

cond-mat.quant-gas↗

arXiv2022

Beyond Gaussian pair fluctuation theory for strongly interacting Fermi gases II: The broken-symmetry phase

We theoretically study the thermodynamic properties of a strongly interacting Fermi gas at the crossover from a Bardeen-Cooper-Schrieffer (BCS) superfluid to a Bose-Einstein condensate (BEC), by applying a recently outlined strong-coupling theory that includes pair fluctuations beyond the commonly-used many-body $T$-matrix or ladder approximation at the Gaussian level. The beyond Gaussian pair fluctuation (GPF) theory always respects the exact thermodynamic relations and recovers the Bogoliubov theory of molecules in the BEC limit with a nearly correct molecule-molecule scattering length. We show that the beyond-GPF theory predicts quantitatively accurate ground-state properties at the BEC-BCS crossover, in good agreement with the recent measurement by Horikoshi \textit{et al.} in Phys. Rev. X \textbf{7}, 041004 (2017). In the unitary limit with infinitely large $s$-wave scattering length, the beyond-GPF theory predicts a reliable universal energy equation of state up to 0.6$T_c$, where $T_c$ is the superfluid transition temperature at unitarity. The theory predicts a Bertsch parameter $ξ\simeq 0.365$ at zero temperature, in good agreement with the latest quantum Monte Carlo result $ξ= 0.367(7)$ and the latest experimental measurement $ξ= 0.367(9)$. We attribute the excellent and wide applicability of the beyond-GPF theory in the broken-symmetry phase to the reasonable re-summation of Feynman diagrams following a dimensional $ε$-expansion analysis near four dimensions ($d=4-ε$), which gives rise to accurate predictions at the second order $\mathcal{O}(ε^2)$. Our work indicates the possibility of further improving the strong-coupling theory of strongly interacting fermions based on the systematic inclusion of large-loop Feynman diagrams at higher orders $\mathcal{O}(ε^n)$ with $n\ge 3$.

cond-mat.quant-gas↗

arXiv2021

Proposed Fermi-surface reservoir-engineering and application to realizing unconventional Fermi superfluids

We theoretically propose an idea based on reservoir engineering to process the structure of a Fermi edge to split into multiple Fermi edges, so as to be suitable for the state which we want to realize. When one appropriately tunes the chemical-potential difference between two reservoirs being coupled with the system, the system is shown to be in the non-equilibrium steady state with the momentum distribution having a two-edge structure. We argue that these edges play similar roles to two Fermi surfaces, which can be designed to realize exotic quantum many-body states. To demonstrate this, we consider a model driven-dissipative two-component Fermi gas with an attractive interaction as a paradigmatic example and show that it exhibits an unconventional Fermi superfluid. While the superfluid order parameter of this state has the same form as that in the Fulde-Ferrell state discussed in metallic superconductivity under an external magnetic field, the former non-equilibrium pairing state is not accompanied by any spin imbalance. Our proposed reservoir engineering to process the Fermi momentum distribution would provide further possibilities of many-body quantum phenomena beyond the thermal equilibrium case.

cond-mat.quant-gas↗

arXiv2021

Superfluid properties of an ultracold Fermi gas with an orbital Feshbach resonance in the BCS-BEC crossover region

We theoretically investigate superfluid properties of a two-band gas of $^{173}$Yb Fermi atoms with an orbital Feshbach resonance (OFR). To describe the BCS-BEC crossover region, we include superfluid fluctuations caused by inter-band and intra-band pairing interactions associated with OFR, by extending the strong-coupling theory developed by Nozières and Schmitt-Rink to the two-band case below the superfluid phase transition temperature; however, effects of an experimentally inaccessible deep bound state are removed, to model a real $^{173}$Yb Fermi gas near OFR. We show that the condensate fraction in the upper closed channel gradually becomes smaller than that in the lower open channel, as one moves from the strong- to the weak-coupling regime, because the OFR-pairing mechanism tunes the interaction strengths by adjusting the energy difference between the two bands. However, even when the closed-channel band is much higher in energy than the open-channel band in the weak-coupling regime, the magnitude of the superfluid order parameter in the closed channel is found to be still comparable to that in the open channel. As the reason for this, we point out a pair-tunneling effect by the OFR-induced inter-band interaction. Besides these superfluid quantities, we also examine collective modes, such as the Goldstone mode, Schmid (Higgs) mode, as well as Leggett mode, to clarify how they appear in the spectral weights of pair-correlation functions in each band. Since the realization of a multi-band superfluid Fermi gas is a crucial issue in cold Fermi gas physics, our results would contribute to the basic understanding of this type of Fermi superfluid in the BCS-BEC crossover region.

cond-mat.quant-gas↗

arXiv2021

Thermodynamic stability, compressibility matrix, and effects of mediated interactions in a strongly-interacting Bose-Fermi mixture

We theoretically investigate the thermodynamic stability of a normal-state Bose-Fermi mixture, with a tunable Bose-Fermi pairing interaction $-U_{\rm BF}<0$ associated with a hetero-nuclear Feshbach resonance, as well as a weak repulsive Bose-Bose interaction $U_{\rm BB}\ge 0$. Including strong hetero-pairing fluctuations associated with the former interaction within the self-consistent $T$-matrix approximation, as well as the latter within the mean-field level, we calculate the compressibility matrix, to assess the stability of this system against density fluctuations. In the weak- and the intermediate-coupling regime with respect $-U_{\rm BF}$, we show that an effective attractive interaction between bosons mediated by density fluctuations in the Fermi component makes the system unstable below a certain temperature $T_{\rm clp}$ (leading to density collapse). When $U_{\rm BB}=0$, $T_{\rm clp}$ is always higher than the Bose-Einstein condensation (BEC) temperature $T_{\rm c}$. When $U_{\rm BB}>0$, the density collapse is suppressed, and the BEC transition becomes possible. It is also suppressed by the formation of tightly bound Bose-Fermi molecules when the hetero-pairing interaction $-U_{\rm BF}$ is strong; however, since the system may be viewed as a molecular Fermi gas in this case, the BEC transition does not also occur. Since quantum gases involving Bose atoms are known to be sensitive to inter-particle correlations, our results would be useful for the study of many-body properties of a Bose-Fermi mixture in a stable manner, without facing the unwanted density collapse.

cond-mat.quant-gas↗

arXiv2020

Kovtun-Son-Starinets Conjecture and Effects of Mass Imbalance in the Normal State of an Ultracold Fermi Gas in the BCS-BEC Crossover Region

We theoretically assess the conjecture proposed by Kovtun, Son, and Starinets, stating that the ratio $η/s$ of the shear viscosity $η$ to the entropy density $s$ has the lower bound as $η/s\ge\hbar/(4πk_{\mathrm{B}})$. In the normal state of a mass-imbalanced ultracold Fermi gas, consistently including strong-coupling corrections to both $η$ and $s$ within the self-consistent $T$-matrix approximation, we evaluate $η/s$ over the entire BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region, in the presence of mass imbalance. We find that $η/s$ achieves the minimum value $4.5\times \hbar/(4πk_{\mathrm{B}})$, not at the unitarity, but slightly in the BEC regime, $(k_{\mathrm{F}}a_s)^{-1}\simeq 0.4>0$ (where $a_s$ is the $s$-wave scattering length, and $k_{\mathrm{F}}$ is the Fermi momentum). In contract to the previous expectation, we find that this lower bound is almost independent of mass imbalance: Our results predict that all the mass-balanced $^6$Li-$^6$Li and $^{40}$K-$^{40}$K mixtures and the mass-imbalanced $^{40}$K-$^{161}$Dy mixture give almost the same lower bound of $η/s$. We also point out that the two quantum phenomena, Pauli blocking and bound-state formation, are crucial keys for the lower bound of $η/s$.

cond-mat.quant-gas↗

arXiv2019

Non-equilibrium strong-coupling theory for a driven-dissipative ultracold Fermi gas in the BCS-BEC crossover region

We theoretically investigate strong-coupling properties of an ultracold Fermi gas in the BCS-BEC crossover regime in the non-equilibrium steady state, being coupled with two fermion baths. By developing a non-equilibrium strong-coupling theory based on the combined $T$-matrix approximation with the Keldysh Green's function technique, we show that the chemical potential bias applied by the two baths gives rise to the anomalous enhancement of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type pairing fluctuations (although the system has no spin imbalance), resulting in the re-entrant behavior of the non-equilibrium superfluid phase transition in the BCS-unitary regime. These pairing fluctuations are also found to anomalously enhance the pseudogap phenomenon. Since various non-equilibrium phenomena have recently been measured in ultracold Fermi gases, our non-equilibrium strong-coupling theory would be useful to catch up this experimental development in this research field.

cond-mat.quant-gas↗