Mitsuaki Kawamura

Dirac Fermions in Borophene

Honeycomb structures of group IV elements can host massless Dirac fermions with nontrivial Berry phases. Their potential for electronic applications has attracted great interest and spurred a broad search for new Dirac materials especially in monolayer structures. We present a detailed investigation of the β_{12} sheet, which is a borophene structure that can form spontaneously on a Ag(111) surface. Our tight-binding analysis revealed that the lattice of the β_{12} sheet could be decomposed into two triangular sublattices in a way similar to that for a honeycomb lattice, thereby hosting Dirac cones. Furthermore, each Dirac cone could be split by introducing periodic perturbations representing overlayer-substrate interactions. These unusual electronic structures were confirmed by angle-resolved photoemission spectroscopy and validated by first-principles calculations. Our results suggest monolayer boron as a new platform for realizing novel high-speed low-dissipation devices.

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.

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.

Anisotropic superconducting gaps in YNi 2 B 2 C : A first-principles investigation

We calculate superconducting gaps and quasiparticle density of states of

First-principles study of the pressure and crystal-structure dependences of the superconducting transition temperature in compressed sulfur hydrides

{\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}

Improved tetrahedron method for the Brillouin-zone integration applicable to response functions

in the framework of the density functional theory for superconductors to investigate the origin of highly anisotropic superconducting gaps in this material. Calculated phonon frequencies, the quasiparticle density of states, and the transition temperature show good agreement with experimental results. From our calculation of superconducting gaps and orbital character analysis, we establish that the orbital character variation of the Fermi surface is the key factor of the anisotropic gap. Since the electronic states that consist of mainly Ni

Fully non-empirical study on superconductivity in compressed sulfur hydrides

3d

“Visible” 5d Orbital States in a Pleochroic Oxychloride

orbitals couple weakly with phonons, the superconducting gap function is suppressed for the corresponding states, which results in the anisotropy observed in the experiments. These results are hints to increase the transition temperature of materials in the borocarbide family.