arXiv subjects

Takahiro Misawa

Publications and source records attributed to Takahiro Misawa.

At least 37 records · Page 2Linked to original sources

arXiv2023

Comprehensive $ab$ $initio$ investigation of the phase diagram of quasi-one-dimensional molecular solids

An $ab$ $initio$ investigation of the family of molecular compounds TM$_2$$X$ is conducted, where TM is either TMTSF or TMTTF and $X$ takes centrosymmetric monovalent anions. By deriving the extended Hubbard-type Hamiltonians from first-principles band calculations and evaluating not only the intermolecular transfer integrals but also the Coulomb parameters, we discuss their material dependence in the unified phase diagram. Furthermore, we apply the many-variable variational Monte Carlo method to accurately determine the symmetry-breaking phase transitions, and show the development of the charge and spin orderings. We show that the material-dependent parameter can be taken as the correlation effect, represented by the value of the screened on-site Coulomb interaction $U$ relative to the intrachain transfer integrals, for the comprehensive understanding of the spin and charge ordering in this system.

cond-mat.str-el↗

arXiv2023

Correlated Zak insulator in organic antiferromagnets

Searching for topological insulators in solids is one of the main issues of modern condensed matter physics since robust gapless edge or surface states of the topological insulators can be used as building blocks of next-generation devices. Enhancing spin-orbit couplings is a promising way to realize topological insulators in solids, whereas the amplitude of the spin-orbit couplings is not sufficiently large in most materials. Here we show a way to realize a topological state characterized by the quantized Zak phase, termed the Zak insulator, with spin-polarized edges in organic antiferromagnetic Mott insulators without relying on the spin-orbit coupling. The obtained Zak insulator can have a large charge gap compared to the conventional topological insulators, since Coulomb interactions mainly govern the amplitude of the charge gap in the antiferromagnetic Mott insulators. Besides the mean-field analysis, we demonstrate that the Zak insulator survives against electron correlation effects by calculating the many-body Zak phase. Our finding provides an unprecedented way to realize a topological state in strongly correlated electron systems.

cond-mat.str-el↗

arXiv2023

Update of $\mathcal{H}Φ$: Newly added functions and methods in versions 2 and 3

$\mathcal{H}Φ$ [$aitch$-$phi$] is an open-source software package of numerically exact and stochastic calculations for a wide range of quantum many-body systems. In this paper, we present the newly added functions and the implemented methods in vers. 2 and 3. In ver. 2, we implement spectrum calculations by the shifted Krylov method, and low-energy excited state calculations by the locally optimal blocking preconditioned conjugate gradient (LOBPCG) method. In ver. 3, we implement the full diagonalization method using ScaLAPACK and GPGPU computing via MAGMA. We also implement a real-time evolution method and the canonical thermal pure quantum (cTPQ) state method for finite-temperature calculations. The Wannier90 format for specifying the Hamiltonians is also implemented. Using the Wannier90 format, it is possible to perform the calculations for the $ab$ $initio$ low-energy effective Hamiltonians of solids obtained by the open-source software RESPACK. We also update Standard mode $\unicode{x2014}$simplified input format in $\mathcal{H}Φ$$\unicode{x2014}$ to use these functions and methods. We explain the basics of the implemented methods and how to use them.

cond-mat.str-el↗

arXiv2023

Interface tool from Wannier90 to RESPACK: wan2respack

We develop the interface tool $\verb|wan2respack|$, which connects $\verb|RESPACK|$ (software that derives the low-energy effective Hamiltonians of solids) with $\verb|Wannier90|$ (software that constructs Wannier functions). $\verb|wan2respack|$ converts the Wannier functions obtained by $\verb|Wannier90|$ into those used in $\verb|RESPACK|$, which is then used to derive the low-energy effective Hamiltonians of solids. In this paper, we explain the basic usage of $\verb|wan2respack|$ and show its application to standard compounds of correlated materials, namely, the correlated metal SrVO$_3$ and the high-$T_{c}$ superconductor La$_2$CuO$_4$. Furthermore, we compare the low-energy effective Hamiltonians of these compounds using Wannier functions obtained by $\verb|Wannier90|$ and those obtained by $\verb|RESPACK|$. We confirm that both types of Wannier functions give the same Hamiltonians. This benchmark comparison demonstrates that $\verb|wan2respack|$ correctly converts Wannier functions in the $\verb|Wannier90|$ format into those in the $\verb|RESPACK|$ format.

cond-mat.str-el↗

arXiv2023

Fermi surface reconstruction due to the orthorhombic distortion in Dirac semimetal YbMnSb$_2$

Dirac semi-metal with magnetic atoms as constituents delivers an interesting platform to investigate the interplay of Fermi surface (FS) topology, electron correlation, and magnetism. One such family of semi-metal is YbMn$Pn_2$ ($Pn$ = Sb, Bi), which is being actively studied due to the intertwined spin and charge degrees of freedom. In this Letter, we investigate the relationship between the magnetic/crystal structures and FS topology of YbMnSb$_2$ using single crystal x-ray diffraction, neutron scattering, magnetic susceptibility, magnetotransport measurement and complimentary DFT calculation. Contrary to previous reports, the x-ray and neutron diffraction reveal that YbMnSb$_2$ crystallizes in an orthorhombic $Pnma$ structure with notable anti-phase displacement of the magnetic Mn ions that increases in magnitude upon cooling. First principles DFT calculation reveals a reduced Brillouin zone and more anisotropic FS of YbMnSb$_2$ compared to YbMnBi$_2$ as a result of the orthorhombicity. Moreover, the hole type carrier density drops by two orders of magnitude as YbMnSb$_2$ orders antiferromagnetically indicating band folding in magnetic ordered state. In addition, the Landau level fan diagram yields a non-trivial nature of the SdH quantum oscillation frequency arising from the Dirac-like Fermi pocket. These results imply that YbMnSb$_2$ is an ideal platform to explore the interplay of subtle lattice distortion, magnetic order, and topological transport arising from relativistic quasiparticles.

cond-mat.mtrl-sci↗

arXiv2023

Data analysis of $ab$ $initio$ effective Hamiltonians in iron-based superconductors $\unicode{x2014}$ Construction of predictors for superconducting critical temperature

High-temperature superconductivity occurs in strongly correlated materials such as copper oxides and iron-based superconductors. Numerous experimental and theoretical works have been done to identify the key parameters that induce high-temperature superconductivity. However, the key parameters governing the high-temperature superconductivity remain still unclear, which hamper the prediction of superconducting critical temperatures ($T_\text{c}$s) of strongly correlated materials. Here by using data-science techniques, we clarified how the microscopic parameters in the $ab$ $initio$ effective Hamiltonians correlate with the experimental $T_\text{c}$s in iron-based superconductors. We showed that a combination of microscopic parameters can characterize the compound-dependence of $T_\text{c}$ using the principal component analysis. We also constructed a linear regression model that reproduces the experimental $T_\text{c}$ from the microscopic parameters. Based on the regression model, we showed a way for increasing $T_\text{c}$ by changing the lattice parameters. The developed methodology opens a new field of materials informatics for strongly correlated electron systems.

cond-mat.supr-con↗

arXiv2023

Gap opening mechanism for correlated Dirac electrons in organic compounds $α$-(BEDT-TTF)$_2$I$_3$ and $α$-(BEDT-TSeF)$_2$I$_3$

To determine how electron correlations open a gap in two-dimensional massless Dirac electrons in the organic compounds $α$-(BEDT-TTF)$_2$I$_3$ [$α$-(ET)$_2$I$_3$] and $α$-(BEDT-TSeF)$_2$I$_3$ [$α$-(BETS)$_2$I$_3$], we derive and analyze $ab$ $initio$ low-energy effective Hamiltonians for these two compounds. We find that the horizontal stripe charge ordering opens a gap in the massless Dirac electrons in $α$-(ET)$_2$I$_3$, while an insulating phase without explicit symmetry breaking appears in $α$-(BETS)$_2$I$_3$. We clarify that the combination of the anisotropic transfer integrals and the electron correlations induces a dimensional reduction in the spin correlations, i.e., one-dimensional spin correlations develop in $α$-(BETS)$_2$I$_3$. We show that the one-dimensional spin correlations open a gap in the massless Dirac electrons. Our finding paves the way for opening gaps for massless Dirac electrons using strong electronic correlations.

cond-mat.str-el↗

arXiv2022

Zeros of Green Functions in Topological Insulators

This study demonstrates that the zeros of the diagonal components of Green functions are key quantities that can detect non-interacting topological insulators. We show that zeros of the Green functions traverse the band gap in the topological phases. The traverses induce the crosses of zeros, and the zeros' surface in the band gap, analogous to the Fermi surface of metals. By calculating the zeros of the microscopic models, we show the traverses of the zeros universally appear in all six classes of conventional non-interacting topological insulators. By utilizing the eigenvector-eigenvalue identity, which is a recently rediscovered relation in linear algebra, we prove that the traverses of the zeros in the bulk Green functions are guaranteed by the band inversions, which occur in the topological phases. The relevance of the zeros to detecting the exotic topological insulators such as the higher-order topological insulators is also discussed. For the Hamiltonians with the nearest-neighbor hoppings, we also show that the gapless edge state guarantees the zeros' surfaces in the band gap. The analysis demonstrates that the zeros can be used to detect a wide range of topological insulators and thus useful for searching new topological materials.

cond-mat.mes-hall↗

arXiv2022

$Ab$ $initio$ material design of Ag-based oxides for high-$T_c$ superconductor

We propose silver-based oxides with layered perovskite structure as candidates of exhibiting intriguing feature of strongly correlated electrons. The compounds show unique covalence between Ag d and O p orbitals with the strong electron correlation of their antibonding orbital similar to the copper oxide high-temperature superconductors, but stronger covalency and slightly smaller correlation strength. We examine $A_2$AgO$_2X_2$ with $A$ =Sr and Ba and $X$ =F, I and Cl in detail. Among them, Sr$_2$AgO$_2$F$_2$ has the largest effective onsite Coulomb repulsion and shows an antiferromagnetic insulating ground state, which competes with correlated metals. It offers features both similar and distinct from the copper oxides, and paves a new route. The possibility of superconductivity in doped systems is discussed.

cond-mat.supr-con↗

arXiv2022

Unconventional dual 1D-2D quantum spin liquid revealed by $ab$ $initio$ studies on organic solids family

Organic solids host various electronic phases. Especially, a milestone compound of organic solid, $β'$-$X$[Pd(dmit)$_2$]$_2$ with $X$=EtMe$_3$Sb shows quantum spin-liquid (QSL) properties suggesting a novel state of matter. However, nature of the QSL has been largely unknown. Here, we computationally study five compounds comprehensively with different $X$ using 2D $ab$ $initio$ Hamiltonians and correctly reproduce experimental phase diagram with antiferromagnetic order for $X$=Me$_4$P, Me$_4$As, Me$_4$Sb, Et$_2$Me$_2$As and a QSL for $X$=EtMe$_3$Sb without adjustable parameters. We find that the QSL for $X$=EtMe$_3$Sb exhibits 1D nature characterized by algebraic decay of spin correlation along one direction, while exponential decay in the other direction, indicating dimensional reduction from 2D to 1D. The 1D nature indeed accounts for the experimental specific heat, thermal conductivity and magnetic susceptibility. The identified QSL, however, preserves 2D nature as well consistently with spin fractionalization into spinon with Dirac-like gapless excitations and reveals duality bridging the 1D and 2D QSLs.

cond-mat.str-el↗

arXiv2022

$Ab$ $initio$ derivation and exact-diagonalization analysis of low-energy effective Hamiltonians for $β^\prime$-X[Pd(dmit)$_2$]$_2$

The molecular solids $β^\prime$-$X$[Pd(dmit)$_2$]$_2$ (where $X$ represents a cation) are typical compounds whose electronic structures are described by single-orbital Hubbard-type Hamiltonians with geometrical frustration. Using the $ab$ $initio$ downfolding method, we derive the low-energy effective Hamiltonians for $β^\prime$-$X$[Pd(dmit)$_2$]$_2$ with available room- and low-temperature structures. We find that the amplitudes of the Coulomb interactions and the anisotropy of the hopping parameters in the effective Hamiltonians are sensitive to the changes in the lattice constants induced by lowering the temperature. The obtained effective Hamiltonians are analyzed using the exact diagonalization method with the boundary-condition average. We find that a significant reduction of the antiferromagnetic ordered moment occurs in the effective Hamiltonian of $β^\prime$-EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ with the low-temperature structure. The reduction is consistent with the quantum spin liquid behavior observed in experiments. The comprehensive derivations of the effective Hamiltonians and exact-diagonalization analyses of them will clarify the microscopic origins of the exotic quantum states of matter found in $β^\prime$-$X$[Pd(dmit)$_2$]$_2$ such as the quantum spin liquid behavior.

cond-mat.str-el↗

arXiv2021

Magnetization Step in Spatially Distorted Heisenberg Kagome Antiferromagnets

Motivated by a recent experiment on volborthite, a typical spin-$1/2$ antiferromagnet with a kagomé lattice structure, we study the magnetization process of a classical Heisenberg model on a spatially distorted kagomé lattice using the Monte Carlo (MC) method. We find a distortion-induced magnetization step at low temperatures and low magnetic fields. The magnitude of this step is given by $Δm_z=\left|1-α\right|/3α$ at zero temperature, where $α$ denotes the spatial anisotropy in exchange constants. The magnetization step signals a first-order transition at low temperatures, between two phases distinguished by distinct and well-developed short-range spin correlations, one characterized by spin alignment of a local $120^{\circ}$ structure with a $\sqrt{3}\times\sqrt{3}$ period, and the other by a partially spin-flopped structure. We point out the relevance of our results to the unconventional steps observed in volborthite.

cond-mat.str-el↗

arXiv2021

Long-range spin transport on the surface of topological Dirac semimetal

We theoretically propose the long-range spin transport mediated by the gapless surface states of topological Dirac semimetal (TDSM). Low-dissipation spin current is a building block of next-generation spintronics devices. While conduction electrons in metals and spin waves in ferromagnetic insulators (FMIs) are the major carriers of spin current, their propagation length is inevitably limited due to the Joule heating or the Gilbert damping. In order to suppress dissipation and realize long-range spin transport, we here make use of the spin-helical surface states of TDSMs, such as $\mathrm{Cd_3 As_2}$ and $\mathrm{Na_3 Bi}$, which are robust against disorder. Based on a junction of two FMIs connected by a TDSM, we demonstrate that the magnetization dynamics in one FMI induces a spin current on the TDSM surface flowing to the other FMI. By both the analytical transport theory on the surface and the numerical simulation of real-time evolution in the bulk, we find that the induced spin current takes a universal semi-quantized value that is insensitive to the microscopic coupling structure between the FMI and the TDSM. We show that this surface spin current is robust against disorder over a long range, which indicates that the TDSM surface serves as a promising system for realizing spintronics devices.

cond-mat.mes-hall↗

arXiv2020

Electronic Correlation and Geometrical Frustration in Molecular Solids -- A Systematic ab initio Study of $β^\prime$-$X$[Pd(dmit)$_{2}$]$_{2}$

We systematically derive low-energy effective Hamiltonians for molecular solids $β^\prime$-$X$[Pd(dmit)$_{2}$]$_{2}$ ($X$ represents a cation) using ab initio density functional theory calculations and clarify how the cation controls the inter-dimer transfer integrals and the interaction parameters. The effective models are solved using the exact diagonalization method and the antiferromagnetic ordered moment is shown to be significantly suppressed around the spin-liquid candidate of $X$=EtMe$_{3}$Sb, which is reported in experiments. We also show that both the geometrical frustration and the off-site interactions play essential roles in the suppression of antiferromagnetic ordering. This systematic derivation and analysis of the low-energy effective Hamiltonians offer a firm basis to clarify the nature of the quantum spin liquid found in $β^\prime$-EtMe$_{3}$Sb[Pd(dmit)$_{2}$]$_{2}$.

cond-mat.str-el↗

arXiv2020

Asymmetric melting of one-third plateau in kagome quantum antiferromagnets

Asymmetric destruction of the one-third magnetization plateau upon heating is found in the spin-1/2 kagome Heisenberg antiferromagnets using the typical pure quantum state approach. The asymmetry originates from larger density of states of low-lying excited sates of $N_{\rm s}$ spin systems with magnetization $(1/3-2/N_{\rm s})$ than that of low-lying sates with magnetization $(1/3+2/N_{\rm s})$. The enhanced specific heat and entropy that reflect the larger density of states in the lower-field side of the plateau are detectable in candidate materials of the kagome antiferromagnets. We discuss that the asymmetry originates from the unprecedented preservation of the ice rule around the plateau.

cond-mat.str-el↗

arXiv2020

K$ω$ -- Open-source library for the shifted Krylov subspace method of the form $(zI-H)x=b$

We develop K$ω$, an open-source linear algebra library for the shifted Krylov subspace methods. The methods solve a set of shifted linear equations $(z_k I-H)x^{(k)}=b\, (k=0,1,2,...)$ for a given matrix $H$ and a vector $b$, simultaneously. The leading order of the operational cost is the same as that for a single equation. The shift invariance of the Krylov subspace is the mathematical foundation of the shifted Krylov subspace methods. Applications in materials science are presented to demonstrate the advantages of the algorithm over the standard Krylov subspace methods such as the Lanczos method. We introduce benchmark calculations of (i) an excited (optical) spectrum and (ii) intermediate eigenvalues by the contour integral on the complex plane. In combination with the quantum lattice solver $\mathcal{H} Φ$, K$ω$ can realize parallel computation of excitation spectra and intermediate eigenvalues for various quantum lattice models.

math.NA↗

arXiv2020

Dynamical spin-to-charge conversion on the edge of quantum spin Hall insulator

We theoretically manifest that the edge of a quantum spin Hall insulator (QSHI), attached to an insulating ferromagnet (FM), can realize a highly efficient spin-to-charge conversion. Based on a one-dimensional QSHI-FM junction, the electron dynamics on the QSHI edge is analyzed, driven by a magnetization dynamics in the FM. Under a large gap opening on the edge from the magnetic exchange coupling, we find that the spin injection into the QSHI edge gets suppressed while the charge current driven on the edge gets maximized, demanded by the band topology of the one-dimensional helical edge states.

cond-mat.mes-hall↗

arXiv2020

Charge dynamics of correlated electrons: Variational description with inclusion of composite fermions

We propose a method to calculate the charge dynamical structure factors for the ground states of correlated electron systems based on the variational Monte Carlo method. Our benchmarks for the one- and two-dimensional Hubbard models show that inclusion of composite-fermion excitations in the basis set greatly improves the accuracy, in reference to the exact charge dynamical structure factors for clusters. Together with examination for larger systems beyond tractable sizes by the exact diagonalization, our results indicate that the variational Monte Carlo method is a promising way for studies on the nature of charge dynamics in correlated materials such as the copper oxide superconductors, if the composite-fermion excitations are properly included in the restricted Hilbert space of intermediate states in the linear response theory. Our results are consistent with the particle-hole excitations inferred from the single-particle spectral function $A(\mathbf{k}, ω)$ in the literature. We also discuss the importance of incorporating nonlocal composite fermion for more accurate description. Future issues for further improvements are also discussed.

cond-mat.str-el↗