arXiv subjects

Soshun Ozaki

Publications and source records attributed to Soshun Ozaki.

12 recordsLinked to original sources

arXiv2026

Exact spectrum and anomalous relaxation in the open disorder-free Sachdev-Ye-Kitaev system

We study a disorder-free variant of the Sachdev-Ye-Kitaev (SYK) model with dissipation within the Gorini-Kossakowski-Sudarshan-Lindblad formalism. By utilizing the integrability of the clean SYK model, we derive an exact solution in a spectrum-resolved form, i.e., the eigenvalues and corresponding projection superoperators of the Liouvillian for arbitrary system size $N$. We determine the scaling of the gap that governs the long-time decay of the two-point correlation functions. Importantly, the gap does not vanish in the dissipationless limit when the thermodynamic limit is taken first, despite the integrability of the model. This phenomenon, known as anomalous relaxation, suggests a possible connection with chaotic dynamics and quantum Ruelle-Pollicott resonances. We also find several spectral features, such as transitions in the Liouvillian spectrum from complex to real eigenvalues with increasing dissipation strength, as well as the convergence of the dissipative form factor to the spectral form factor in the dissipationless limit. These findings indicate that the present model offers a useful platform for exploring nontrivial open dynamics of many-body quantum systems.

cond-mat.str-el↗

arXiv2026

Theory of clusterization in orbitally degenerate transition-metal compounds driven by lattice instabilities

We derive an effective orbital-lattice model with quantum $S=1$ degrees of freedom for transition-metal compounds, providing a microscopic understanding of cluster formation driven by the cooperative interplay of spin, orbital, and lattice degrees of freedom. Motivated by the trimerized phases observed in LiVS$_2$ and LiVO$_2$, we consider a triangular-lattice three-orbital system with two electrons per site occupying the threefold-degenerate $t_{2g}$ manifold. Starting from a multiorbital Kanamori-Hubbard Hamiltonian, we project the low-energy sector onto the local $S=1$ triplet manifold, in which two electrons occupy different orbitals according to Hund's coupling. The resulting effective model exhibits exchange networks whose geometry is determined by the orbital configuration. However, the orbital-driven exchange interactions alone do not stabilize the experimentally observed trimer phase. We find that by incorporating ionic lattice displacements that modulate transfer integrals and induce bond-dependent exchange couplings on shortened and elongated bonds, the phase competition is qualitatively altered, leading to the robust stabilization of a trimerized ground state within a fully quantum-mechanical framework. We further show that a simplified orbital-lattice model, in which the spin-exchange energy is replaced by effective bond energies, faithfully reproduces the essential ground-state properties of the microscopic model. This reduced description enables large-scale finite-temperature simulations and reveals a rich sequence of thermal phase transitions, including first-order, second-order, and Kosterlitz-Thouless transitions into distinct spin-, orbital-, and lattice-ordered phases.

cond-mat.str-el↗

arXiv2026

Orbital-Zeeman cross correlation in $p$- and $d$-wave altermagnets

Altermagnets are a novel class of magnets that exhibit a large spin splitting but the total magnetic moment is vanishing. This unconventional spin splitting gives rise to various characteristic phenomena, such as spin current generation. In this paper, we study the orbital-Zeeman (OZ) cross term in altermagnets. Specifically, we consider the Rashba metal and the surface Dirac cones of three-dimensional topological insulators (TIs) in the presence of the altermagnetic order parameters. For the Rashba metals, the $p$-wave order parameter exerts only a limited influence on the OZ term, whereas the $d$-wave one causes the sign change of it when the order parameter becomes sufficiently large. For the TI surface, the $p$-wave order parameter retains the step-function-type dependence of the OZ term as a function of the chemical potential ($μ$) associated with the jump at $μ=0$, observed in the TI surface without magnetism, but its magnitude is reduced. For the $d$-wave case, the magnitude of jump at $μ=0$ is preserved but the OZ term decreases as increasing $|μ|$.

cond-mat.mes-hall↗

arXiv2025

Orbital paramagnetism without density of states enhancement in nodal-line semimetal ZrSiS

Unconventional orbital paramagnetism without enhanced density of states was recently discovered in the nodal-line semimetal ZrSiS. We propose a novel interband mechanism, linked to the negative curvature of energy dispersions, which successfully accounts for the observed anomalous response. This negative curvature originates from energy variation along the nodal line, inherent in realistic nodal-line materials. Our results suggest that such orbital paramagnetism provides strong evidence for the presence of nodal lines in ZrSiS, and serves as a hallmark of other nodal-line materials.

cond-mat.mes-hall↗

arXiv2025

Orbital Magnetism in Honeycomb Ladder

We investigate the orbital magnetic susceptibility of the tight-binding model for the honeycomb ladder with the additional vertical hopping. Despite being one-dimensional, the magnetic flux penetrating the hexagonal rings affects the energetics of electrons, resulting in the finite orbital magnetic response. We find that the orbital magnetic susceptibility is sensitive to the parameters as well as the chemical potential. At half-filling, the response is diamagnetic when the system is close to the pure honeycomb ladder, whereas it turns to paramagnetic when it is close to the two-leg ladder. We also find several characteristic properties away from half-filling, such as the diamagnetic response at the band top and bottom, and the large paramagnetic response at the band gap sandwiched by the divergent density of states.

cond-mat.mes-hall↗

arXiv2025

Disorder-free Sachdev-Ye-Kitaev models: Integrability and a precursor of chaos

We introduce two disorder-free variants of the Sachdev-Ye-Kitaev (SYK) model, demonstrate their integrability, and study their static and dynamical properties. Unlike diagrammatic techniques, the integrability of these models allows us to obtain dynamical correlation functions even when the number of Majorana fermions is finite. From the solutions, we find that out-of-time-order correlators (OTOCs) in these models exhibit exponential growth at early times, resembling that of quantum chaotic systems, such as those with disorder or external kick terms, despite their large $N$ behavior differing from that of typical chaotic systems. Conversely, our analysis shows no evidence of random-matrix behavior in level statistics or the spectral form factor. Our findings illustrate that the clean versions of the SYK models represent simple but nontrivial examples of disorder-free quantum many-body systems displaying chaos-like behavior of OTOCs.

cond-mat.str-el↗

arXiv2024

Solvable toy model of negative energetic elasticity

Recent experiments have established negative energetic elasticity, the negative contribution of energy to the elastic modulus, as a universal property of polymer gels. To reveal the microscopic origin of this phenomenon, Shirai and Sakumichi investigated a polymer model on a cubic lattice with the energy effect from the solvent in finite-size calculations [Phys. Rev. Lett. 130, 148101 (2023)]. Motivated by this work, we provide a simple platform to study the elasticity of polymer chains by considering a one-dimensional random walk with the energy effect. This model can be mapped onto the classical Ising chain, leading to an exact form of the free energy in the thermodynamic or continuous limit. Our analytical results are qualitatively consistent with Shirai and Sakumichi's work. Our model serves as a fundamental benchmark for studying negative energetic elasticity.

cond-mat.stat-mech↗

arXiv2022

sparse-ir: optimal compression and sparse sampling of many-body propagators

We introduce sparse-ir, a collection of libraries to efficiently handle imaginary-time propagators, a central object in finite-temperature quantum many-body calculations. We leverage two concepts: firstly, the intermediate representation (IR), an optimal compression of the propagator with robust a-priori error estimates, and secondly, sparse sampling, near-optimal grids in imaginary time and imaginary frequency from which the propagator can be reconstructed and on which diagrammatic equations can be solved. IR and sparse sampling are packaged into stand-alone, easy-to-use Python, Julia and Fortran libraries, which can readily be included into existing software. We also include an extensive set of sample codes showcasing the library for typical many-body and ab initio methods.

physics.comp-ph↗

arXiv2022

Topological contribution to magnetism in the Kane-Mele model: An explicit wave function approach

In our previous publication [S. Ozaki and M. Ogata, Phys. Rev. Research 3, 013058], the quantization of the orbital-Zeeman (OZ) cross term in the magnetic susceptibility, or the cross term of spin Zeeman and orbital effect, was shown for the Kane-Mele model using the expansion around the Dirac points. In the present study, we accurately evaluate the orbital, spin-Zeeman, and OZ cross term of the Kane-Mele model using a recently developed formulation. This formula is written in terms of the explicit Bloch wave functions, and enables us to evaluate each contribution taking account of the integration over the whole Brillouin zone and the summation over all the bands. As a result, additional contributions such as core-electron diamagnetism are found. Furthermore, our evaluation confirms the quantization of the OZ cross term and reveals its behavior including the metallic case. The possibility of experimental detection of the quantization is discussed.

cond-mat.mes-hall↗

arXiv2022

Anomalous Spin Transport Properties of Gapped Dirac Electrons with Tilting

The anomalous spin transport coefficients of gapped Dirac electrons are studied with application to a quasi-two-dimensional organic conductor $α$-(BETS)$_2$I$_3$ in mind. In the presence of a gap induced by spin-orbit interaction, we show that the effective Hamiltonian is similar to the model considered by Kane and Mele with additional tilting. With this effective Hamiltonian, conductivity tensors up to the linear order of the applied magnetic field are obtained analytically using the microscopic linear response theory or Kubo formula. It is shown that spin Hall conductivity and anomalous diagonal spin conductivity proportional to the magnetic field become nonzero in this system, which are written in terms of the Berry curvature and orbital magnetic moment.The estimated values of spin conductivities using typical parameters turn out to be comparable to the spin Hall conductivity in Pt.

cond-mat.mes-hall↗

arXiv2021

Analytic approach to dynamics of the resonant and off-resonant Jaynes-Cummings systems with cavity losses

A new analytic approach to investigate the zero-temperature time evolution of the Jaynes-Cummings system with cavity losses is developed. With the realistic coupling between the cavity and the environment assumed, a simple master equation is derived, leading to the explicit analytic solution for the resonant case. This solution is suitable for the analyses not only on the single excitation states but also on many excitation states, which enables us to investigate the photon coherent state and to observe sharp collapses and revivals under dissipation. For the off-resonant case, on the other hand, the present study presents an analytic, systematic method instead. We examine the small and large detuning limits and discuss the condition where the widely-used phenomenological treatment is justified. Explicit evaluations of the time evolutions for various initial states with finite detuning are also presented.

quant-ph↗

arXiv2020

Universal Quantization of Magnetic Susceptibility Jump at Topological Phase Transition

We examine the magnetic susceptibility of topological insulators microscopically and find that the orbital-Zeeman (OZ) cross term, the cross term between the orbital effect and the spin Zeeman effect, is directly related to the Berry curvature when the $z$-component of spin is conserved. In particular, the OZ cross term reflects the spin Chern number, which results in the quantization of the magnetic susceptibility jump at the topological phase transition. The magnitude of the jump is in units of the universal value $4|e|μ_{\rm B}/h$. The physical origin of this quantization is clarified. We also apply the obtained formula to an explicit model and demonstrate the quantization.

cond-mat.mes-hall↗