arXiv subjects

Shinji Tsuneyuki

Publications and source records attributed to Shinji Tsuneyuki.

45 records · Page 3Linked to original sources

arXiv2016

Possible "Magnéli" phases and self-alloying in the superconducting sulfur hydride

We theoretically give an infinite number of metastable crystal structures for the superconducting sulfur hydride H$_{x}$S under pressure. It has been thought that theoretically predicted structures of H$_{2}$S and H$_{3}$S exhibit low and high $T_{\rm c}$ in the experiment, respectively. The newly found structures are long-period modulated crystals where slab-like H$_{2}$S and H$_{3}$S regions intergrow in a microscopic scale. The extremely small formation enthalpy for the H$_{2}$S--H$_{3}$S boundary indicated with the first-principles calculations suggests possible alloying of these phases through formation of local H$_{3}$S regions. The modulated structures and gradual alloying transformations between them not only explain the peculiar pressure dependence of $T_{\rm c}$ in sulfur hydride observed experimentally, but also could prevail in the experimental samples under various compression schemes.

cond-mat.supr-con↗

arXiv2016

Iterative diagonalization of the non-Hermitian transcorrelated Hamiltonian using a plane-wave basis set: Application to $sp$-electron systems with deep core states

We develop an iterative diagonalization scheme in solving a one-body self-consistent-field equation in the transcorrelated (TC) method using a plane-wave basis set. Non-Hermiticity in the TC method is well handled with a block-Davidson algorithm. We verify the required computational cost is efficiently reduced by our algorithm. In addition, we apply our plane-wave-basis TC calculation to some simple $sp$-electron systems with deep core states to elucidate an impact of the pseudopotential approximation to the calculated band structures. We find a position of the deep valence bands is improved by an explicit inclusion of core states, but an overall band structure is consistent with a regular setup that includes core states into the pseudopotentials. This study offers an important understanding for the future application of the TC method to strongly correlated solids.

cond-mat.mtrl-sci↗

arXiv2015

Self-consistent phonon calculations of lattice dynamical properties in cubic SrTiO$_{3}$ with first-principles anharmonic force constants

We present an \textit{ab initio} framework to calculate anharmonic phonon frequency and phonon lifetime that is applicable to severely anharmonic systems. We employ self-consistent phonon (SCPH) theory with microscopic anharmonic force constants, which are extracted from density-functional calculations using the least absolute shrinkage and selection operator technique. We apply the method to the high-temperature phase of SrTiO$_{3}$ and obtain well-defined phonon quasiparticles that are free from imaginary frequencies. Here we show that the anharmonic phonon frequency of the antiferrodistortive mode depends significantly on the system size near the critical temperature of the cubic-to-tetragonal phase transition. By applying perturbation theory to the SCPH result, phonon lifetimes are calculated for cubic SrTiO$_{3}$, which are then employed to predict lattice thermal conductivity using the Boltzmann transport equation within the relaxation-time approximation. The presented methodology is efficient and accurate, paving the way toward a reliable description of thermodynamic, dynamic, and transport properties of systems with severe anharmonicity, including thermoelectric, ferroelectric, and superconducting materials.

cond-mat.mtrl-sci↗

arXiv2015

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

We calculate superconducting transition temperatures ($T_{\rm c}$) in sulfur hydrides H$_{2}$S and H$_{3}$S from first principles using the density functional theory for superconductors. At pressures of $\lesssim$150 GPa, the high values of $T_{\rm c}$ ($\gtrsim$130 K) observed in the recent experiment [A. P. Drozdov, M. I. Eremets, and I. A. Troyan, arXiv:1412.0460] are accurately reproduced by assuming that H$_{2}$S decomposes into $R3m$-H$_{3}$S and S. For the higher pressures, the calculated $T_{\rm c}$s for $Im3m$-H$_{3}$S are systematically higher than those for $R3m$-H$_{3}$S and the experimentally observed maximum value (190 K), which suggests the possibility of another higher-$T_{\rm c}$ phase. We also quantify the isotope effect from first principles and demonstrate that the isotope effect coefficient can be larger than the conventional value (0.5) when multiple structural phases energetically compete.

cond-mat.supr-con↗

arXiv2015

Impact of Rattlers on Thermal Conductivity of a Thermoelectric Clathrate: A First-Principles Study

We investigate the role of rattling guest atoms on the lattice thermal-conductivity of a type-I clathrate Ba$_{8}$Ga$_{16}$Ge$_{30}$ by first-principles lattice dynamics. Comparing phonon properties of filled and empty clathrates, we show that rattlers cause 10-fold reductions in the relaxation time of phonons by increasing the phonon-phonon scattering probability. Contrary to the resonant scattering scenario, the reduction in the relaxation time occurs in a wide frequency range, which is crucial for explaining unusually low thermal-conductivities of clathrates. We also find that the impact of rattlers on the group velocity of phonons is secondary because the flattening of phonon dispersion occurs only in a limited phase space in the Brillouin zone.

cond-mat.mtrl-sci↗

arXiv2007

Optical Absorption Study by Ab initio Downfolding Approach: Application to GaAs

We examine whether essence and quantitative aspects of electronic excitation spectra are correctly captured by an effective low-energy model constructed from an {\em ab initio} downfolding scheme. A global electronic structure is first calculated by {\em ab initio} density-functional calculations with the generalized gradient approximation. With the help of constrained density functional theory, the low-energy effective Hamiltonian for bands near the Fermi level is constructed by the downfolding procedure in the basis of maximally localized Wannier functions. The excited states of this low-energy effective Hamiltonian ascribed to an extended Hubbard model are calculated by using a low-energy solver. As the solver, we employ the Hartree-Fock approximation supplemented by the single-excitation configuration-interaction method considering electron-hole interactions. The present three-stage method is applied to GaAs, where eight bands are retained in the effective model after the downfolding. The resulting spectra well reproduce the experimental results, indicating that our downfolding scheme offers a satisfactory framework of the electronic structure calculation, particularly for the excitations and dynamics as well as for the ground state.

cond-mat.str-el↗

arXiv2005

First-principles Calculation of Effective Onsite Coulomb Interaction of 3d Transition Metals: Constrained Local Density Functional Approach with Maximally Localized Wannier Function

We present a new ab initio method for calculating effective onsite Coulomb interactions of itinerant and strongly correlated electron systems. The method is based on constrained local density functional theory formulated in terms of maximally localized Wannier functions. This scheme can be implemented with any basis, and thus allows us to perform the constrained calculation with plane-wave-based electronic-structure codes. We apply the developed method to the evaluation of the onsite interaction of 3d transition-matal series. The results are discussed using a heuristic formula for screened Coulomb interactions.

cond-mat.str-el↗

arXiv2001

Role of fluctuation, disorder and catalyst in graphite-diamond transition

The pressure-induced structural transition from graphite to diamond is investigated by semi-empirical molecular dynamics simulation. The result shows that the graphite-diamond transition is a cooperative process with large fluctuation. We studied catalyst-aided effect by introducing a simple model into a conventional tight binding Hamiltonian of carbon bonding structure. The obtained result suggest that weak disorder in graphite not only accelerate the transition to cubic diamond but also will be an origin of high-pressure polymorph of carbon, such as hexagonal diamond.

cond-mat.mtrl-sci↗

arXiv2001

Molecular dynamics simulation for pressure-induced structural transition from C$_{60}$ fullerene into amorphous diamond

The pressure-induced structural transition in fcc C$_{60}$ fullerene by shock compression and rapid quenching is investigated by a semi-empirical tight-binding molecular dynamics simulation, adopting a constant-pressure scheme and a method of the order N electronic structure calculation. At first, the process of the amorphization of C$_{60}$ is demonstrated. The simulated results indicated that, in the material fabricated after the quenching, the remaining dangling bonds have a large influence on physical properties, such as, the density and the presence of the band gap at the Fermi level. We have furthermore studied the formation of the short-range order, observed as amorphous diamond. In order to form the amorphous diamond phase, the bonding state of sp$^2$ must be turned into that of sp$^3$. The transition process is seriously influenced from the the external pressure, the temperature, or the presence of hydrogen. The comparison to the pressure-induced structural transition in the graphite is also executed and a brief discussion on the difference in those carbon crystals is given.

physics.comp-ph↗