arXiv subjects

Naoto Tsuji

Publications and source records attributed to Naoto Tsuji.

At least 37 records · Page 2Linked to original sources

arXiv2025

Nonequilibrium hysteretic phase transitions in periodically light-driven superconductors

We find nonequilibrium phase transitions accompanied by multiple (nested) hysteresis behaviors in superconductors coupled to baths under a time-periodic light driving. The transitions are demonstrated with a full phase diagram in the domain of the driving amplitude and frequency by means of the Floquet many-body theory. In the weak driving regime with a frequency smaller than half of the superconducting gap, excited quasiparticles are accumulated at the far edges of the bands, realizing a distribution reminiscent of the Eliashberg effect, which suddenly becomes unstable in the strong driving regime due to multi-photon-assisted tunneling across the gap mediated by the in-gap Floquet sidebands. We also show that superconductivity is enhanced in the weak driving regime without effective cooling, which is attributed to the modulation of the spectrum due to Floquet sidebands.

cond-mat.supr-con↗

arXiv2025

Light-driven interlayer propagation of collective-mode excitations in layered superconductors

Superconductors exhibit a nonlinear interaction with an applied light, which can resonantly excite the collective amplitude (Higgs) mode. Here we study light-induced dynamics of layered superconductors, where each layer is coupled to adjacent layers via the Josephson coupling and the first few layers near the surface are driven by an in-plane-polarized light. We study the system under the assumption that the interlayer Coulomb interactions are sufficiently screened out and that the phase-difference mode becomes available in the low-energy regime. We find that interlayer transport is induced via excitations of the collective amplitude and phase-difference modes, even when the applied electric field is parallel to the planes. We provide analytic calculations as well as numerical simulations of the real-time dynamics, and investigate the influence on the light-induced interlayer Josephson current and intralayer third-harmonic generation.

cond-mat.supr-con↗

arXiv2025

Quantum many-body scars with unconventional superconducting pairing symmetries via multibody interactions

We present a systematic framework to construct model Hamiltonians that have unconventional superconducting pairing states as exact energy eigenstates, by incorporating multibody interactions (i.e., interactions among more than two particles). The multibody interactions are introduced in a form of the local density-density coupling in such a way that any pair configuration in real space has a constant interaction energy by canceling the two-body and multibody interactions. Our approach is applicable to both spinless and spinful models in any spatial dimensions and on any bipartite lattices, facilitating an exhaustive extension of Yang's $s$-wave $η$-pairing state to various other unconventional pairing symmetries ($p$-wave, $d$-wave, $f$-wave, etc.). Particularly, the constructed eigenstates have off-site pairs with finite center-of-mass momentum, which leads to superconducting states with either even-parity and spin-triplet or odd-parity and spin-singlet symmetry. We verify that the two-dimensional spinful Hubbard model on a square lattice with the multibody interactions has the spin-triplet $d$-wave pairing state as an energy eigenstate, which can be regarded as a quantum many-body scar state as evidenced from the numerical analysis of the pair correlation function, entanglement entropy, and level statistics. We also discuss other examples, including spin-triplet $f$-wave pairing states on a honeycomb lattice and spin-singlet $p$-wave pairing states in a one-dimensional chain. These findings open up the possibility of realizing nonequilibrium unconventional superconductivity in a long-lived manner protected against thermalization.

cond-mat.supr-con↗

arXiv2024

Boundary dissipative spin chains with partial solvability inherited from system Hamiltonians

Partial solvability plays an important role in the context of statistical mechanics, since it has turned out to be closely related to the emergence of quantum many-body scar states, i.e., exceptional energy eigenstates which do not obey the strong version of the eigenstate themalization hypothesis. We show that partial solvability of a quantum many-body system can be maintained even when the system is coupled to boundary dissipators under certain conditions. We propose two mechanisms that support partially solvable structures in boundary dissipative systems: The first one is based on the restricted spectrum generating algebra, while the second one is based on the Hilbert space fragmentation. From these structures, we derive exact eigenmodes of the Gorini-Kossakowski-Sudarshan-Lindblad equation for a family of quantum spin chain models with boundary dissipators, where we find various intriguing phenomena arising from the partial solvability of the open quantum systems, including persistent oscillations (quantum synchronization) and the existence of the matrix product operator symmetry. We discuss how the presence of solvable eigenmodes affects long-time behaviors of observables in boundary dissipative spin chains based on numerical simulations using the quantum trajectory method.

cond-mat.stat-mech↗

arXiv2024

Exact steady states of the impurity-doped XXZ spin chain coupled to dissipators

We give an exact matrix product steady state and matrix product forms of local observables for the bulk impurity-doped XXZ spin model coupled to dissipators at both ends, whose dynamics is described by the Lindblad quantum master equation. We find that local magnetization is induced at the impurity site when the spin current flows, which is contrary to the usual situation where current suppresses magnetization due to heating. It is proved that this current-induced magnetization survives in the thermodynamic limit, and the spin current does not depend on the impurity strength. We discuss the role of bulk integrability by comparing the results with those of non-integrable models solved numerically by the quantum trajectory method.

cond-mat.stat-mech↗

arXiv2024

Classification of Lifshitz invariant in multiband superconductors: an application to Leggett modes in the linear response regime in Kagome lattice models

Multiband superconductors are sources of rich physics arising from multiple order parameters, which show unique collective dynamics including Leggett mode as relative phase oscillations. Previously, it has been pointed out that the Leggett mode can be optically excited in the linear response regime, as demonstrated in a one-dimensional model for multiband superconductors[T. Kamatani, et al., Phys. Rev. B 105, 094520 (2022)]. Here we identify the linear coupling term in the Ginzburg-Landau free energy to be the so-called Lifshitz invariant, which takes a form of $\boldsymbol{d}\cdot\left(Ψ^{*}_{i}\nablaΨ_{j} - Ψ_{j}\nablaΨ^{*}_{i}\right)$, where $\boldsymbol{d}$ is a constant vector and $Ψ_{i}$ and $Ψ_{j}$ $(i\neq j)$ represent superconducting order parameters. We have classified all pairs of irreducible representations of order parameters in the crystallographic point groups that allow for the existence of the Lifshitz invariant. We emphasize that the Lifshitz invariant can appear even in systems with inversion symmetry. The results are applied to a model of $s$-wave superconductors on a Kagome lattice with various bond orders, for which in some cases we confirm that the Leggett mode appears as a resonance peak in a linear optical conductivity spectrum based on microscopic calculations. We discuss a possible experimental observation of the Leggett mode by a linear optical response in multiband superconductors.

cond-mat.supr-con↗

arXiv2023

Floquet States

Quantum systems driven by a time-periodic field are a platform of condensed matter physics where effective (quasi)stationary states, termed "Floquet states", can emerge with external-field-dressed quasiparticles during driving. They appear, for example, as a prethermal intermediate state in isolated driven quantum systems or as a nonequilibrium steady state in driven open quantum systems coupled to environment. Floquet states may have various intriguing physical properties, some of which can be drastically different from those of the original undriven systems in equilibrium. In this article, we review fundamental aspects of Floquet states, and discuss recent topics and applications of Floquet states in condensed matter physics.

quant-ph↗

arXiv2023

Higgs and Nambu-Goldstone modes in condensed matter physics

Collective dynamics of many particle systems is tightly linked to their underlying symmetry and phase transitions. Higgs and Nambu-Goldstone modes are, respectively, collective amplitude and phase modes of the order parameter that are widely observed in various physical systems at different energy scales, ranging from magnets, superfluids, superconductors to our universe. The Higgs mode is a massive excitation, which is a condensed-matter analog of Higgs particle in high-energy physics, while the Nambu-Goldstone mode is a massless excitation that appears when a continuous symmetry is spontaneously broken. They provide important information on the fundamental aspects of many particle systems, such as symmetry, phases, dynamics, response to external fields, and so on. In this article, we review the physics of Higgs and Nambu-Goldstone modes in condensed matter physics. Especially, we focus on the development on the study of collective modes in superconductors and cold-atom systems.

cond-mat.supr-con↗

arXiv2023

Nonequilibrium DMFT approach to time-resolved Raman spectroscopy

Raman spectroscopy uses light scattering to extract information on low-energy excitations of solids. The Raman process is described by diagrams which are fourth order in the light-matter interaction, and in particular the resonant contribution, which involves four different space-time arguments, is difficult to evaluate. If one instead simulates explicitly the incoming (classical) light pulse, the Raman signal is given by the outgoing photon flux and can be determined from a two-point correlation function. Such a formalism can be used to compute the time-resolved Raman spectrum of non-equilibrium systems, as well as nonlinear signals which are higher order in the incoming field, such as hyper Raman scattering. Here we explain how to implement this time-dependent formalism within the dynamical mean field theory framework. The method is illustrated with applications to the Holstein-Hubbard model in the strong electron-phonon coupling regime. We demonstrate hyper Raman scattering in measurements with strong probe fields and frequency mixing signals in the presence of a pump field, and simulate the evolution of Stokes and anti-Stokes features after photo-excitations of metallic and Mott insulating systems.

cond-mat.str-el↗

arXiv2022

Optical response of the Leggett mode in multiband superconductors in the linear response regime

We study optical responses of Leggett modes in multiband superconductors in the linear response regime. The Leggett mode is a collective mode unique to multiband superconductors that arises from relative phase fluctuations of superconducting orders for different bands. We use the Ginzburg-Landau (GL) description to study the collective modes in multiband systems. We find that multiband superconductors generally allow a linear coupling between the Leggett mode and external electric fields due to the presence of a cross term between different components of superconducting orders in the GL theory. The presence of a linear coupling for the Leggett mode is in sharp contrast with the absence of that for Higgs (amplitude) modes in single-band superconductors that only support nonlinear optical responses such as third harmonic generation (THG). We further confirm such a linear coupling in multiband superconductors by a more microscopic description based on a diagrammatic approach. We study the collective modes within the random phase approximation (RPA) and compute their contribution to the linear optical conductivity. These findings suggest a new route to observe the Leggett mode by optical absorption.

cond-mat.str-el↗

arXiv2021

Collective Excitations and Nonequilibrium Phase Transition in Dissipative Fermionic Superfluids

We predict a new mechanism to induce collective excitations and a nonequilibrium phase transition of fermionic superfluids via a sudden switch-on of two-body loss, for which we extend the BCS theory to fully incorporate a change in particle number. We find that a sudden switch-on of dissipation induces an amplitude oscillation of the superfluid order parameter accompanied by a chirped phase rotation as a consequence of particle loss. We demonstrate that when dissipation is introduced to one of the two superfluids coupled via a Josephson junction, it gives rise to a nonequilibrium dynamical phase transition characterized by the vanishing dc Josephson current. The dissipation-induced collective modes and nonequilibrium phase transition can be realized with ultracold fermionic atoms subject to inelastic collisions.

cond-mat.quant-gas↗

arXiv2021

$η$ Pairing of Light-Emitting Fermions: Nonequilibrium Pairing Mechanism at High Temperatures

Strongly interacting fermionic atoms are shown to develop $η$-pairing superfluid correlations in a nonequilibrium steady state in the presence of spontaneous emission of light from atoms. On the basis of the Hubbard model subject to spontaneous decay between internal spin states, we show that prohibition of radiative decay due to the Pauli exclusion principle and destructive interference between doublon-decay processes lead to nonequilibrium $η$ pairing. Because of the non-thermal nature of the steady state, pair correlations arise even from a completely uncorrelated infinite-temperature initial state, allowing coherent atom pairs to be formed at high temperatures. Experimental implementation with fermionic atoms in an optical lattice is discussed.

cond-mat.quant-gas↗

arXiv2021

Tachyonic and Plasma Instabilities of $η$-Pairing States Coupled to Electromagnetic Fields

Cooper pairs featuring a nonzero center-of-mass crystal momentum ${\boldsymbol Q}=(π,π, \dots)$ and an off-diagonal long-range order ($η$-pairing states) constitute exact eigenstates of a Hubbard model [C. N. Yang, Phys. Rev. Lett. 63, 2144 (1989)]. Here we show that the $η$-pairing states are rendered dynamically unstable via coupling to dynamical electromagnetic fields. The instability is caused by "tachyonic" electromagnetic fields and unstable plasma modes for attractive and repulsive interactions, respectively. The typical time scale of the growth of the instability is of the order of femtoseconds for electron systems in solids, which places a strict bound on the lifetime of the $η$-pairing states. The decay of the $η$-pairing states leads to enhanced light emission with frequencies shifted from the Hubbard interaction strength for the repulsive case and unattenuated electromagnetic penetration for the attractive case.

cond-mat.supr-con↗

arXiv2020

Higgs-mode resonance in third harmonic generation in NbN superconductors: multiband electron-phonon coupling, impurity scattering, and polarization-angle dependence

We theoretically investigate the resonance of third harmonic generation (THG) that has been observed at frequency being half of the superconducting gap in a multiband disordered superconductor NbN. The central question is whether the dominant contribution to the THG resonance comes from the Higgs mode (the collective amplitude mode of the superconducting order parameter) or quasiparticle excitations. To resolve this issue, we analyze a realistic three-band model with effective intraband and interband phonon-mediated interactions estimated from first-principles together with nonmagnetic impurity scatterings. Using the estimated pairing interactions with multiband impurity scattering rates being varied from clean to dirty regimes, we calculate the THG susceptibility for NbN in a channel-resolved manner by means of the BCS and self-consistent Born approximations. In the dirty regime, which is close to the experimental situation, the leading contribution is given by the paramagnetic channel of the Higgs mode having almost no polarization-angle dependence, while the second leading contribution comes from the paramagnetic channel of quasiparticles generally showing significant polarization-angle dependence. The result is consistent with the recent experimental observation of no polarization-angle dependence of THG, giving firm evidence that the Higgs mode dominantly contributes to the THG resonance in NbN superconductors.

cond-mat.supr-con↗

arXiv2020

Dynamical sign reversal of magnetic correlations in dissipative Hubbard models

In quantum magnetism, the virtual exchange of particles mediates an interaction between spins. Here, we show that an inelastic Hubbard interaction fundamentally alters the magnetism of the Hubbard model due to dissipation in spin-exchange processes, leading to sign reversal of magnetic correlations in dissipative quantum dynamics. This mechanism is applicable to both fermionic and bosonic Mott insulators, and can naturally be realized with ultracold atoms undergoing two-body inelastic collisions. The dynamical reversal of magnetic correlations can be detected by using a double-well optical lattice or quantum-gas microscopy, the latter of which facilitates the detection of the magnetic correlations in one-dimensional systems because of spin-charge separation. Our results open a new avenue toward controlling quantum magnetism by dissipation.

cond-mat.quant-gas↗

arXiv2019

Out-of-time-ordered correlators of the Hubbard model: SYK strange metal in the spin freezing crossover region

The Sachdev-Ye-Kitaev (SYK) model describes a strange metal that shows peculiar non-Fermi liquid properties without quasiparticles. It exhibits a maximally chaotic behavior characterized by out-of-time-ordered correlators (OTOCs), and is expected to be a holographic dual to black holes. Recently, a striking similarity between the SYK model and the Hund-coupling induced spin-freezing crossover in multi-orbital Hubbard models has been pointed out. To further explore this connection, we study OTOCs for fermionic Hubbard models, which are prototypical models for strongly correlated electrons in solids. We introduce an imaginary-time four-point correlation function with an appropriate time ordering, which by means of the spectral representation and the out-of-time-order fluctuation-dissipation theorem can be analytically continued to real-time OTOCs. Based on this approach, we numerically evaluate real-time OTOCs for Hubbard models in the thermodynamic limit, using the dynamical mean-field theory in combination with a numerically exact continuous-time Monte Carlo impurity solver. The results for the single-orbital model show that a certain spin-related OTOC captures local moment formation in the vicinity of the metal-insulator transition, while the self-energy does not show SYK-like non-Fermi liquid behavior. On the other hand, for the two- and three-orbital models with nonzero Hund coupling we find that the OTOC exhibits a rapid damping at short times and an approximate power-law decay at longer times in the spin-freezing crossover regime characterized by fluctuating local moments and a non-Fermi liquid self-energy $Σ(ω) \sim \sqrtω$. These results are in a good agreement with the behavior of the SYK model, providing firm evidence for the close relation between the spin-freezing crossover physics of multi-orbital Hubbard models and the SYK strange metal.

cond-mat.str-el↗

arXiv2019

Higgs Mode in Superconductors

When a continuous symmetry of a physical system is spontaneously broken, two types of collective modes typically emerge: the amplitude and phase modes of the order-parameter fluctuation. For superconductors, the amplitude mode is recently referred to as the ''Higgs mode'' as it is a condensed-matter analogue of a Higgs boson in particle physics. Higgs mode is a scalar excitation of the order parameter, distinct from charge or spin fluctuations, and thus does not couple to electromagnetic fields linearly. This is why the Higgs mode in superconductors has evaded experimental observations over a half century after the initial theoretical prediction, except for a charge-density-wave coexisting system. With the advance of nonlinear and time-resolved terahertz spectroscopy techniques, however, it has become possible to study the Higgs mode through the nonlinear light-Higgs coupling. In this review, we overview recent progresses on the study of the Higgs mode in superconductors.

cond-mat.supr-con↗

arXiv2019

Nature of superconducting fluctuation in photo-excited systems

The photo-excited state associated with superconducting fluctuation above the superconducting critical temperature $T_c$ is studied based on the time-dependent Ginzburg-Laundau approach. The excited state is created by an electric-field pulse and is probed by a weak secondary external field, which is treated by the linear response theory mimicking pump-probe spectroscopy experiments. The behavior is basically controlled by two relaxation rates: one is $γ_1$ proportional to the temperature measured from the critical point $T - T_c$ and the other is $γ_2$ proportional to the excitation intensity of the pump pulse. The excited state approaches the equilibrium state exponentially in a long time $t \gg γ_1^{-1}$, while in the intermediate time domain we find a power-law or logarithmic decay with different exponents for $t\ll γ_2^{-1}$ and $γ_2^{-1} \ll t \ll γ_1^{-1}$, even though the system is located away from the critical point. This is interpreted as the critical point in equilibrium being extended to a finite region in the excited situation. The parameter dependences on both the pump and probe currents are also systematically studied in all dimensions.

cond-mat.supr-con↗