Kazuyoshi Yoshimi

Tuning the magnetic dimensionality by charge ordering in the molecular TMTTF salts

We theoretically investigate the interplay between charge ordering and magnetic states in quasi-one-dimensional molecular conductors TMTTF(2)X, motivated by the observation of a complex variation of competing and/or coexisting phases. We show that the ferroelectric-type charge order increases two-dimensional antiferromagnetic spin correlation, whereas in the one-dimensional regime two different spin-Peierls states are stabilized. By using first-principles band calculations for the estimation for the transfer integrals and comparing our results with the experiments, we identify the controlling parameters in the experimental phase diagram to be not only the interchain transfer integrals but also the amplitude of the charge order.

Sparse modeling approach to analytical continuation of imaginary-time quantum Monte Carlo data

A data-science approach to solving the ill-conditioned inverse problem for analytical continuation is proposed. The root of the problem lies in the fact that even tiny noise of imaginary-time input data has a serious impact on the inferred real-frequency spectra. By means of a modern regularization technique, we eliminate redundant degrees of freedom that essentially carry the noise, leaving only relevant information unaffected by the noise. The resultant spectrum is represented with minimal bases and thus a stable analytical continuation is achieved. This framework further provides a tool for analyzing to what extent the Monte Carlo data need to be accurate to resolve details of an expected spectral function.

Superconductivity in the Vicinity of Charge Ordered State in Organic Conductor β-(meso-DMBEDT-TTF)2PF6

We study theoretically competition between the charge ordering and the superconductivity in the two-dimensional organic conductor β-( meso -DMBEDT-TTF) 2 PF 6 . We analyze the extended Hubbard mode...We study theoretically competition between the charge ordering and the superconductivity in two-dimensional organic conductor beta-(meso-DMBEDT-TTF)_2PF_6. We analyze the extended Hubbard model on a weakly-dimerized lattice based on the random phase approximation and Eliashberg equations. We found reentrant behavior of the checkerbord-type charge-ordered phase in the phase diagram, and the triplet superconductivity due to the charge fluctuation in the neighboring region. In the low temperature and high pressure region, the singlet superconductivity also appears due to the enhancement of the spin fluctuation.

Compressing Green's function using intermediate representation between imaginary-time and real-frequency domains

New model-independent compact representations of imaginary-time data are presented in terms of the intermediate representation (IR) of analytical continuation. This is motivated by a recent numerical finding by the authors [J. Otsuki et al., arXiv:1702.03056]. We demonstrate the efficiency of the IR through continuous-time quantum Monte Carlo calculations of an Anderson impurity model. We find that the IR yields a significantly compact form of various types of correlation functions. The present framework will provide general ways to boost the power of cutting-edge diagrammatic/quantum Monte Carlo treatments of many-body systems.

Enhanced Spin Susceptibility toward the Charge-Ordering Transition in a Two-Dimensional Extended Hubbard Model

We investigate the charge and spin susceptibilities in the vicinity of a charge-ordering transition point on the basis of a two-dimensional extended Hubbard model with nearest-neighbor Coulomb interaction on a square lattice. In order to evaluate the leading contribution of vertex corrections, we construct a new iterative method generating a series of non-skeleton diagrammatic conserving approximations, and utilize the first approximation for the calculation of response functions. This approximation can determine a charge-ordering transition point and static susceptibilities on an equal footing in a sense that the compressibility and spin-susceptibility sum rules are satisfied. As a prominent effect of vertex corrections, we find that the uniform spin susceptibility is enhanced owing to charge fluctuations that develop toward the charge-ordering transition. We also discuss the experimental relevance of our theory for organic conductors, together with its connection to Landaus Fermi-liquid theory.

Quantum lattice model solver HΦ

Abstract H Φ [ aitch-phi ] is a program package based on the Lanczos-type eigenvalue solution applicable to a broad range of quantum lattice models, i.e., arbitrary quantum lattice models with two-body interactions, including the Heisenberg model, the Kitaev model, the Hubbard model and the Kondo-lattice model. While it works well on PCs and PC-clusters, H Φ also runs efficiently on massively parallel computers, which considerably extends the tractable range of the system size. In addition, unlike most existing packages, H Φ supports finite-temperature calculations through the method of thermal pure quantum (TPQ) states. In this paper, we explain theoretical background and user-interface of H Φ . We also show the benchmark results of H Φ on supercomputers such as the K computer at RIKEN Advanced Institute for Computational Science (AICS) and SGI ICE XA (Sekirei) at the Institute for the Solid State Physics (ISSP). Program summary Program Title: H Φ Program Files doi: http://dx.doi.org/10.17632/vnfthtyctm.1 Licensing provisions: GNU General Public License, version 3 or later Programming language: C External routines/libraries: MPI, BLAS, LAPACK Nature of problem: Physical properties (such as the magnetic moment, the specific heat, the spin susceptibility) of strongly correlated electrons at zero- and finite-temperature. Solution method: Application software based on the full diagonalization method, the exact diagonalization using the Lanczos method, and the microcanonical thermal pure quantum state for quantum lattice model such as the Hubbard model, the Heisenberg model and the Kondo model. Restrictions: System size less than about 20 sites for an itinerant electronic system and 40 site for a local spin system. Unusual features: Finite-temperature calculation of the strongly correlated electronic system based on the iterative scheme to construct the thermal pure quantum state. This method is efficient for highly frustrated system which is difficult to treat with other methods such as the unbiased quantum Monte Carlo.

Coulomb Frustrated Phase Separation in Quasi-Two-Dimensional Organic Conductors on the Verge of Charge Ordering

We study a 3/4-filled two-dimensional extended Hubbard model under the fluctuation–exchange approximation. We find that this model undergoes phase separation in a region of nonzero temperature, where the quantum critical phenomenon of charge ordering dominates. By considering the long-range Coulomb interaction that frustrates this phase separation, we present a mechanism for generating a glassy state in a quasi-two-dimensional organic conductor on the verge of charge ordering.

Fermi Surface Deformation Near Charge-Ordering Transition

We study the deformation of a Fermi surface (FS) near charge-ordering (CO) transition. By applying a fluctuation–exchange approximation to the two-dimensional extended Hubbard model, we show that the FS is largely modified by strong charge fluctuations when the wave number of the CO pattern does not match the nesting vector of the FS in a noninteracting system. We also discuss the enhanced anisotropy in quasiparticle properties in the resultant metallic state.

Spin frustration, charge ordering, and enhanced antiferromagnetism in TMTTF2SbF6

We theoretically investigate the effects of charge order and spin frustration on the spin ordering in TMTTF salts. Using first-principles band calculations, we find that a diagonal inter-chain transfer integral tq1, which causes spin frustration between the inter-chain dimers in the dimer-Mott insulating state, strongly depends on the choice of anion. Within the numerical Lanczos exact diagonalization method, we show that the ferroelectric charge order changes the role of tq1 from the spin frustration to the enhancement of the two-dimensionality in spin sector. The results indicate that tq1 assists the cooperative behavior between charge order and antiferromagnetic state observed in TMTTF2SbF6.

mVMC—Open-source software for many-variable variational Monte Carlo method

Abstract mVMC (many-variable Variational Monte Carlo) is an open-source software package based on the variational Monte Carlo method applicable for a wide range of Hamiltonians for interacting fermion systems. In mVMC, we introduce more than ten thousands variational parameters and simultaneously optimize them by using the stochastic reconfiguration (SR) method. In this paper, we explain basics and user interfaces of mVMC. By using mVMC, users can perform the calculation by preparing only one input file of about ten lines for widely studied quantum lattice models, and can also perform it for general Hamiltonians by preparing several additional input files. We show the benchmark results of mVMC for the Hubbard model, the Heisenberg model, and the Kondo-lattice model. These benchmark results demonstrate that mVMC provides ground-state and low-energy-excited-state wave functions for interacting fermion systems with high accuracy. Program summary Program title: mVMC Program Files doi: http://dx.doi.org/10.17632/xhgyp6ncvt.1 Licensing provisions: GNU General Public License version 3 Programming language: C External routines/libraries: MPI, BLAS, LAPACK, Pfapack, ScaLAPACK (optional) Nature of problem: Physical properties (such as the charge/spin structure factors) of strongly correlated electrons at zero temperature. Solution method: Application software based on the variational Monte Carlo method for quantum lattice model such as the Hubbard model, the Heisenberg model and the Kondo model. Unusual features: It is possible to perform the highly-accurate calculations for ground states in a wide range of theoretical Hamiltonians in quantum many-body systems. In addition to the conventional orders such as magnetic and/or charge orders, user can treat the anisotropic superconductivities within the same framework. This flexibility is the main advantage of mVMC.