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Takao Morinari

Publications and source records attributed to Takao Morinari.

At least 19 recordsLinked to original sources
arXiv2025

Angular Dependence of Specific Heat and Magnetization Effects in the Kitaev Model

We investigate the effect of a magnetic field on the Kitaev model using the equation of motion approach for the spin Green's function, considering both the case of suppressed magnetization ($m = 0$) and finite magnetization ($m \neq 0$). When magnetization is suppressed, the specific heat exhibits a clear $60^\circ$ periodicity in its angular dependence, with the locations of maxima and minima consistent with recent experimental observations in $α$-RuCl$_3$. A qualitative difference in their temperature dependence is observed: the minima show gap-like behavior that may signal Majorana gap formation due to time-reversal symmetry breaking, while the maxima do not exhibit the expected gapless Majorana fermion signature. In addition, a linear-in-field effect -- distinct from magnetization -- emerges, with the characteristic temperature below which angular dependence appears increasing linearly with the magnetic field. Importantly, this directional dependence becomes quantitatively significant only at very low temperatures. When finite magnetization is included, the angular dependence of the specific heat remains, and the qualitative behavior is similar to the $m = 0$ case: the minima continue to exhibit gap-like features, while the maxima do not show signatures of gapless Majorana fermions. These results suggest that suppressing magnetization alone is insufficient to realize quantum spin liquid behavior in the Kitaev model under a magnetic field.

cond-mat.str-el↗
arXiv2025

Static and Dynamical Spin Correlations in the Kitaev Model at Finite Temperatures via Green's Function Equation of Motion

The Kitaev model, renowned for its exact solvability and potential to host non-Abelian anyons, remains a focal point in the study of quantum spin liquids and topological phases. While much of the existing literature has employed Majorana fermion techniques to analyze the model, particularly at zero temperature, its finite-temperature behavior has been less thoroughly explored via alternative approaches. In this paper, we investigate the finite-temperature properties of the Kitaev model using the spin Green's function formalism. This approach enables the computation of key physical quantities such as spin correlations, magnetic susceptibility, and the dynamical spin structure factor, offering crucial insights into the system's thermal dynamics. In solving the equation of motion for the spin Green's function, we truncate the hierarchy of multi-spin Green's functions using a decoupling approximation, which proves to be particularly accurate at high temperatures. Our results show several similarities with Majorana-based numerical simulations, though notable differences emerge. Specifically, both static and dynamical spin-spin correlation functions capture not only $\mathbb{Z}_2$ flux excitations but also simple spin-flip excitations, with the latter overshadowing the former. Interestingly, without explicitly assuming fractionalization, our results for the spin susceptibility and spin relaxation rate still suggest the presence of fermionic degrees of freedom at low temperatures. This study provides a complementary approach to understanding the thermal properties of the Kitaev model, which could be relevant for future experiments and theoretical investigations.

cond-mat.str-el↗
arXiv2024

Spin-Spin Correlations in the Kitaev Model at Finite Temperatures: Approximate and Exact Results via Green's Function Equation of Motion

The Kitaev model, defined on a honeycomb lattice, features an exactly solvable ground state with fractionalized Majorana fermion excitations, which can potentially form non-Abelian anyons crucial for fault-tolerant topological quantum computing. Although Majorana fermions are essential for obtaining the exact ground state, their physical interpretation in terms of spin operators remains unclear. In this study, we employ a Green's function approach that maintains SU(2) symmetry to address this issue and explore the model's finite temperature properties. Our results demonstrate that the computed temperature dependence of the correlation functions closely approximates the exact values at zero temperature, confirming the accuracy of our method. We also present several exact results concerning the spin Green's function and spin-spin correlation functions that are specific to the Kitaev model.

cond-mat.str-el↗
arXiv2024

Time-Reversal Symmetry-Breaking Flux State in an Organic Dirac Fermion System

We investigate symmetry breaking in the Dirac fermion phase of the organic compound $α$-(BEDT-TTF)$_2$I$_3$ under pressure, where BEDT-TTF denotes bis(ethylenedithio)tetrathiafulvalene. The exchange interaction resulting from inter-molecule Coulomb repulsion leads to broken time-reversal symmetry and particle-hole symmetry while preserving translational symmetry. The system breaks time-reversal symmetry by creating fluxes in the unit cell. This symmetry-broken state exhibits a large Nernst signal as well as thermopower. We compute the Nernst signal and thermopower, demonstrating their consistency with experimental results.

cond-mat.str-el↗
arXiv2023

General Formula for the Green's Function Approach to the Spin-1/2 Antiferromagnetic Heisenberg Model

A wide range of analytical and numerical methods are available to study quantum spin systems. However, the complexity of spin correlations and interactions limits their applicability to specific temperature ranges. The analytical approach utilizing Green's function has proved advantageous, as it allows for formulation without restrictions on the presence of long-range order and facilitates estimation of the spin excitation spectrum and thermodynamic quantities across the entire temperature range. In this work, we present a generalized formulation of the Green's function method that can be applied to diverse spin systems. As specific applications, we consider the hypercubic lattice and the $J_1$-$J_2$ model. For the cubic lattice case, the Green's function approach provides a good estimation for the transition temperature. Regarding the $J_1$-$J_2$ model, we include nematic correlations in the analysis and find no signature of such correlations, though accurate numerical calculations are required in the presence of strong frustration. Although our focus is on the spin one-half antiferromagnetic Heisenberg model on an arbitrary lattice, the Green's function approach can be generalized to incorporate other interactions and higher spin values.

cond-mat.str-el↗
arXiv2023

Evidence for three-dimensional Dirac semimetal state in strongly correlated organic quasi-two-dimensional material

The three-dimensional Dirac semimetal is distinct from its two-dimensional counterpart due to its dimensionality and symmetry. Here, we observe that molecule-based quasi-two-dimensional Dirac fermion system, $α$-(BEDT-TTF)$_2$I$_3$, exhibits chiral anomaly-induced negative magnetoresistance and planar Hall effect upon entering the coherent inter-layer tunneling regime under high pressure. Time-reversal symmetry is broken due to the strong electronic correlation effect, while the spin-orbit coupling effect is negligible. The system provides an ideal platform for investigating the chiral anomaly physics by controlling dimensionality and strong electronic correlation.

cond-mat.str-el↗
arXiv2023

Theory for Planar Hall Effect in Organic Dirac Fermion System

In a recent experiment on the interlayer magnetoresistance in the quasi-two-dimensional organic salt, $α$-(BEDT-TTF)$_2$I$_3$, it has been observed that at low temperatures, interlayer tunneling attains phase coherence, leading to the emergence of a three-dimensional electronic structure. Theoretically and experimentally it has been suggested that the system exhibits characteristics of a three-dimensional Dirac semimetal as a consequence of broken time-reversal symmetry and inversion symmetry. Here, we perform a theoretical calculation of the magnetoconductivity under an in-plane magnetic field and demonstrate that the system displays a planar Hall effect. Our calculations are based on a realistic model for $α$-(BEDT-TTF)$_2$I$_3$ incorporating interlayer tunneling and the tilt of the Dirac cone. Given that the planar Hall effect is anticipated as a consequence of chiral anomaly, our findings provide support for the classification of $α$-(BEDT-TTF)$_2$I$_3$ as a three-dimensional Dirac semimetal.

cond-mat.str-el↗
arXiv2022

Coherent interlayer coupling in quasi-two-dimensional Dirac fermions in $α$-(BEDT-TTF)$_2$I$_3$

Theoretical and experimental studies have supported that the electronic structure of $α$-(BEDT-TTF)$_2$I$_3$ under pressure is described by two-dimensional Dirac fermions. When the interlayer tunneling is coherent, the electronic structure of the system becomes three-dimensional, and we expect the peak structure to appear in the interlayer resistivity under magnetic fields. We theoretically and experimentally show that the peak appears in the interlayer resistivity at low temperatures and high magnetic fields. From the experiment, we estimate that the magnitude of the interlayer tunneling is $t_1 \sim 1$ meV. Our result opens the door to investigating the three-dimensional electronic structure of $α$-(BEDT-TTF)$_2$I$_3$.

cond-mat.str-el↗
arXiv2022

Anomalous High-Field Magnetotransport in CaFeAsF due to the Quantum Hall Effect

CaFeAsF is an iron-based superconductor parent compound whose Fermi surface is quasi-two dimensional, composed of Dirac-electron and Schrödinger-hole cylinders elongated along the $c$ axis. We measured the longitudinal and Hall resistivities in CaFeAsF with the electrical current in the $ab$ plane in magnetic fields up to 45 T applied along the $c$ axis and obtained the corresponding conductivities via tensor inversion. We found that both the longitudinal and Hall conductivities approached zero above $\sim$40 T as the temperature was lowered to 0.4 K. Our analysis indicates that the Landau-level filling factor is $ν$ = 2 for both electrons and holes at these high field strengths, resulting in a total filling factor $ν$ = $ν_{hole} - ν_{electron}$ = 0. We therefore argue that the $ν$ = 0 quantum Hall state emerges under these conditions.

cond-mat.supr-con↗
arXiv2021

Intervalley Tunneling and Crossover from the Positive to Negative Interlayer Magnetoresistance in Quasi-Two-Dimensional Dirac Fermion System with or without Mass Gap

We theoretically investigate the interlayer magnetoresistance in quasi-two-dimensional Dirac fermion systems, where the Fermi energy is at the Dirac point. If there is an intermediate insulating layer that has an overlap with the wave functions in the Dirac fermion layers, there appears a positive magnetoresistance regime due to the intervalley tunneling. We show that the interlayer magnetoresistance can be used to find whether Dirac fermions are massive or not from the minimum in the interlayer magnetoresistance. As a specific system, we consider \alphaI under high pressure. We also discuss that one has to be careful in analyzing the crossover temperature from the positive to negative magnetoresistance. A simple picture is applied to the crossover in the zero temperature limit but it does not apply to the data at finite temperatures. We show that the ratio of the Fermi velocity to the scattering rate is evaluated from the zero temperature limit of the crossover temperature.

cond-mat.str-el↗
arXiv2020

Dynamical time-reversal and inversion symmetry breaking, dimensional crossover, and chiral anomaly in $α$-(BEDT-TTF)$_2$I$_3$

In most Dirac semimetals, time-reversal and inversion symmetries are believed to play a crucial role in their stability. We demonstrate that these symmetries are broken in Dirac fermions in the organic conductor $α$-(BEDT-TTF)$_2$I$_3$ due to the strong electronic correlation. The system is a three-dimensional type-II Dirac semimetal in the coherent inter-layer tunneling regime. A chiral anomaly is predicted to be observed in the magnetoresistance when the magnetic field is tuned to the inter-layer tunneling direction. Our result suggests that $α$-(BEDT-TTF)$_2$I$_3$ is a useful platform to explore the interplay between the chiral anomaly and the strong correlation and/or dimensionality.

cond-mat.str-el↗
arXiv2019

Short-range antiferromagnetic correlation effect on conduction electrons in two-dimensional strongly correlated electron systems

We investigate magnetic polarons in two-dimensional strongly correlated electron systems, where conduction electrons interact with antiferromagnetically interacting localized spins. Starting from a basic model, we derive a simplified model with the help of spin Green's function and a perturbation analysis. A strong coupling analysis is applied to the model, where the sum of the scattering wave vectors is approximated to be $(π,π)$ or zero, using the equation of motion for the conduction electron Green's function, and we discuss the pseudogap like behavior associated with the suppression of the quasiparticle weights and the transition from the large magnetic polaron to the small magnetic polaron. In the antiferromagnetic long-range ordered state, the spectral weight of the conduction electrons has a form of broad humps due to Franck-Condon broadening associated with the multi-magnon scattering. The band folding feature due to the $(π,π)$ scattering disappears as we increase the number of the magnons involved in the multi-magnon scattering. It is crucial to include long-range antiferromagnetic correlations as well as dumping of magnons.

cond-mat.str-el↗
arXiv2018

Relationship between Magnetic Anisotropy Below Pseudogap Temperature and Short-Range Antiferromagnetic Order in High-Temperature Cuprate Superconductor

The central issue in high-temperature cuprate superconductors is the pseudogap state appearing below the pseudogap temperature $T^*$, which is well above the superconducting transition temperature. In this study, we theoretically investigate the rapid increase of the magnetic anisotropy below the pseudogap temperature detected by the recent torque-magnetometry measurements on YBa$_2$Cu$_3$O$_y$ [Y. Sato et al., Nat. Phys., 13, 1074 (2017)]. Applying the spin Green's function formalism including the Dzyaloshinskii--Moriya interaction arising from the buckling of the CuO$_2$ plane, we obtain results that are in good agreement with the experiment and find a scaling relationship. Our analysis suggests that the characteristic temperature associated with the magnetic anisotropy, which coincides with $T^*$, is not a phase transition temperature but a crossover temperature associated with the short-range antiferromagnetic order.

cond-mat.str-el↗
arXiv2018

Magnetotransport in Layered Dirac Fermion System Coupled with Magnetic Moments

We theoretically investigate the magnetotransport of Dirac fermions coupled with localized moments to understand the physical properties of the Dirac material EuMnBi$_2$. Using an interlayer hopping form, which simplifies the complicated interaction between the layers of Dirac fermions and the layers of magnetic moments in EuMnBi$_2$, the theory reproduces most of the features observed in this system. The hysteresis observed in EuMnBi$_2$ can be caused by the valley splitting that is induced by the spin-orbit coupling and the external magnetic field with the molecular field created by localized moments. Our theory suggests that the magnetotransport in EuMnBi$_2$ is due to the interplay among Dirac fermions, localized moments, and spin-orbit coupling.

cond-mat.str-el↗
arXiv2016

Destruction of Magnetic Long-Range Order by Hole-Induced Skyrmions in Two-Dimensional Heisenberg Model

Motivated by the rapid destruction of antiferromagnetic long-range order in hole-doped cuprate high-temperature superconductors, we study the effect of skyrmions on the magnetic long-range order (MLRO). Here we assume that either a skyrmion or antiskyrmion is introduced by a doped hole. Our numerical simulation indicates that in the case of isolated skyrmions, there is an abrupt disappearance of MLRO for doping concentration $x < 1.0\times 10^{-4}$. In the case of skyrmion-antiskyrmion pairs, the critical doping concentration $x_c$ for the suppression of MLRO is given as a function of the separation of the pairs. For a moderate separation of $3 - 4$ lattice constants, we find that the critical doping is consistent with the experimental value.

cond-mat.supr-con↗
arXiv2015

Effect of Interlayer Spin-Flip Tunneling for Interlayer Magnetoresistance in Multilayer Massless Dirac Fermion Systems

We investigate the effect of the interlayer spin-flip tunneling for the interlayer magnetoresistance under magnetic fields in alpha-(BEDT-TTF)2I3, which is a multilayer massless Dirac fermion system under pressure. The mean field of the spin-flip correlation associated with the interlayer Coulomb interaction enables the interlayer spin-flip tunneling. Assuming the non-vertical interlayer spin-flip tunneling, we calculate the interlayer magnetoresistance using the Kubo formula. The crossover magnetic field, at which the interlayer magnetoresistance changes from positive to negative is shifted by the Zeeman energy and in good agreement with the experiment.

cond-mat.mes-hall↗
arXiv2015

Theory of spin motive force in one-dimentional antiferromagnetic domain wall

We present the theory of the spin motive force in antiferromagnets. We consider a one-dimensional antiferromagnetic domain wall strongly coupled with conduction electrons via an exchange interaction. We carry out a unitary transformation that rotates the spin coordinate system of the conduction electron locally, so that the quantization axis is in the direction of the localized spin. By numerically solving the time-dependent Schr$ö$dinger equation, we clearly demonstrate that the spin motive force acts on the conduction electron. The result suggests that there is no distinction between antiferromagnets and ferromagnets from the view point of the basic phenomenon relevant to spintronics.

cond-mat.mes-hall↗