arXiv subjects

Isao Tanaka

Publications and source records attributed to Isao Tanaka.

At least 73 records · Page 4Linked to original sources

arXiv2015

First principles phonon calculations in materials science

Phonon plays essential roles in dynamical behaviors and thermal properties, which are central topics in fundamental issues of materials science. The importance of first principles phonon calculations cannot be overly emphasized. Phonopy is an open source code for such calculations launched by the present authors, which has been world-widely used. Here we demonstrate phonon properties with fundamental equations and show examples how the phonon calculations are applied in materials science.

cond-mat.mtrl-sci↗

arXiv2015

C-axis electrical resistivity of PrO$_{1-a}$F$_a$BiS$_2$ single crystals

The high anisotropy in RO1-aFaBiS2 (R denotes a rare-earth element) superconductors demonstrates their potential use as intrinsic Josephson junctions, considering the weak coupling among BiS2-PrO(F)-BiS2 (superconducting-normal-superconducting) layers along the c-axis. We grew PrO1-aFaBiS2 single crystals using CsCl/KCl flux. The superconducting anisotropies of the grown single crystals were estimated to be approximately 40-50 from the effective mass model. The c-axis transport properties were characterized using single-crystal s-shaped intrinsic Josephson junctions with a focused ion beam. Along the c-axis, the crystals showed zero resistivity at 2.7 K and a critical current density of 1.33*10^3 A/cm2 at 2.0 K. The current-voltage curve along the c-axis displayed hysteresis. The c-axis transport measurements under a magnetic field parallel to the ab-plane revealed a "lock-in" state due to the Josephson vortex flow, indicating that BiS2 superconductors are promising candidates for intrinsic Josephson junctions.

cond-mat.supr-con↗

arXiv2015

Choice of basis vectors for conventional unit cells revisited

Band diagrams often pass through special $\mathbf{k}$-vector points with "irrational" fractional coordinates such as some Brillouin zone vertices and centers of edges. Use of "standard" primitive cells as defined by Setyawan and Curtarolo [Comp. Mater. Sci. 49, 299] is convenient for band diagram and effective mass calculations because the definitions and coordinates of special $\mathbf{k}$-vector points including "irrational" fractional points are provided, but their basis vectors are taken differently and/or special $\mathbf{k}$-vector point definitions differ from the crystallographic convention in many cases. On the other hand, the Bilbao Crystallographic Server [Bulg. Chem. Commun. 43, 183] defines special $\mathbf{k}$-vector points based on crystallographic convention; however, symbols for "irrational" special $\mathbf{k}$-vector points are not defined. Obviously there is a need for a crystallographic convention-friendly scheme to describe such "irrational" special $\mathbf{k}$-vector points. This paper outlines an algorithm that is relatively straightforward to implement in software to bridge between crystallographic conventional cells and "standard" primitive cells by retaking basis vectors. The procedure is expected to ease systematic electronic and phonon band calculations.

cond-mat.mtrl-sci↗

arXiv2015

Discovery of low thermal conductivity compounds with first-principles anharmonic lattice dynamics calculations and Bayesian optimization

Compounds of low lattice thermal conductivity (LTC) are essential for seeking thermoelectric materials with high conversion efficiency. Some strategies have been used to decrease LTC. However, such trials have yielded successes only within a limited exploration space. Here we report the virtual screening of a library containing 54,779 compounds. Our strategy is to search the library through Bayesian optimization using for the initial data the LTC obtained from first-principles anharmonic lattice dynamics calculations for a set of 101 compounds. We discovered 221 materials with very low LTC. Two of them have even an electronic band gap < 1 eV, what makes them exceptional candidates for thermoelectric applications. In addition to those newly discovered thermoelectric materials, the present strategy is believed to be powerful for many other applications in which chemistry of materials are required to be optimized.

cond-mat.mtrl-sci↗

arXiv2015

First-principles interatomic potentials for ten elemental metals via compressed sensing

Interatomic potentials have been widely used in atomistic simulations such as molecular dynamics. Recently, frameworks to construct accurate interatomic potentials that combine a systematic set of density functional theory (DFT) calculations with machine learning techniques have been proposed. One of these methods is to use compressed sensing to derive a sparse representation for the interatomic potential. This facilitates the control of the accuracy of interatomic potentials. In this study, we demonstrate the applicability of compressed sensing to deriving the interatomic potential of ten elemental metals, namely, Ag, Al, Au, Ca, Cu, Ga, In, K, Li and Zn. For each elemental metal, the interatomic potential is obtained from DFT calculations using elastic net regression. The interatomic potentials are found to have prediction errors of less than 3.5 meV/atom, 0.03 eV/Å and 0.15 GPa for the energy, force and the stress tensor, respectively, which enable the accurate prediction of physical properties such as lattice constants and the phonon dispersion relationship.

cond-mat.mtrl-sci↗

arXiv2015

Distribution of phonon lifetime in Brillouin zone

Lattice thermal conductivities of zincblende- and wurtzite-type compounds with 33 combinations of elements are calculated with the single-mode relaxation-time approximation and linearized phonon Boltzmann equation from first-principles anharmonic lattice dynamics calculations. In 9 zincblende-type compounds, distributions of phonon linewidths (inverse phonon lifetimes) are discussed in detail. The phonon linewidths vary non-smoothly with respect to wave vector, which is explained from the imaginary parts of the self energies. It is presented that detailed combination of phonon-phonon interaction strength and three phonon selection rule is critically important to determine phonon lifetime for these compounds. This indicates difficulty to predict phonon lifetime quantitatively without anharmonic force constants. However it is shown that joint density of states weighted by phonon numbers, which is calculated only from harmonic force constants, can be potentially used for a screening of the lattice thermal conductivity of materials.

cond-mat.mtrl-sci↗

arXiv2014

Phonon softening in paramagnetic bcc Fe and relationship with pressure-induced phase transition

Structural stability of paramagnetic (PM) body-centered cubic (bcc) Fe under pressure is investigated based on first-principles phonon calculations. Spin configurations of the PM phase are approximated using a binary special quasi-random structure (SQS) with a supercell approach. The behavior of phonon modes can be associated with pressure-induced phase transitions to the face-centered cubic (fcc) and hexagonal close-packed (hcp) structures as follows: For the PM phase, it is found that the low-frequency transverse mode at the N point (N$_4^-$ mode), which corresponds to a bcc-hcp phase transition pathway, exhibits strong softening under isotropic volume compression. The frequency of this mode becomes zero by $2\%$ volume decrease within the harmonic approximation. This result is not consistent with the experimental fact that phase transition from the PM bcc to hcp phases does not occur under volume compression. The seeming contradiction can be explained only when anharmonic behavior of the N$_4^-$ mode is taken into consideration; a potential energy curve along the N$_4^-$ mode becomes closer to a double-well shape for the PM phase under the volume compression. On the other hand, softening of the longitudinal mode at the 2/3[111] point under the volume compression is also found for the PM phase, which indicates the pressure-induced bcc-fcc phase transition along this mode. Such behaviors are not seen in ferromagnetic (FM) bcc Fe, implying that the magnetic structure plays essential roles on the phase transition mechanism.

cond-mat.mtrl-sci↗

arXiv2014

Special quasirandom structure in heterovalent ionic systems

The use of a special quasirandom structure (SQS) is a rational and efficient way to approximate random alloys. A wide variety of physical properties of metallic and semiconductor random alloys have been successfully estimated by a combination of an SQS and density functional theory (DFT) calculation. Here, we investigate the application of an SQS to the ionic multicomponent systems with configurations of heterovalent ions, including point-charge lattices, MgAl$_2$O$_4$ and ZnSnP$_2$. It is found that the physical properties do not converge with the supercell size of the SQS. This is ascribed to the fact that the correlation functions of long-range clusters larger than the period of the supercell are not optimized in the SQS. However, we demonstrate that the physical properties of the perfectly disordered structure can be estimated by linear extrapolation using the inverse of the supercell size.

cond-mat.mtrl-sci↗

arXiv2014

A sparse representation for potential energy surface

We propose a simple scheme to estimate potential energy surface (PES) with which the accuracy can be easily controlled and improved up to the level of the density functional theory (DFT) calculations. It is based on a model selection within the framework of linear regression using the least absolute shrinkage and selection operator (LASSO) technique. Basis functions are selected from a systematic large set of candidate functions. The sparsity of PES significantly reduces the computational demands for evaluation of the energy and the force in molecular dynamics simulations without losing the accuracy. The usefulness of the scheme is well demonstrated for describing elemental metals of Na and Mg.

cond-mat.mtrl-sci↗

arXiv2014

Efficient determination of alloy ground-state structures

We propose an efficient approach to accurately finding the ground-state structures in alloys based on the cluster expansion method. In this approach, a small number of candidate ground-state structures are obtained without any information of the energy. To generate the candidates, we employ the convex hull constructed from the correlation functions of all possible structures by using an efficient algorithm. This approach is applicable to not only simple lattices but also complex lattices. Firstly, we evaluate the convex hulls for binary alloys with four types of simple lattice. Then we discuss the structures on the vertices. To examine the accuracy of this approach, we perform a set of density functional theory calculations and the cluster expansion for Ag-Au alloy and compare the formation energies of the vertex structures with those of all possible structures. As applications, the ground-state structures of the intermetallic compounds CuAu, CuAg, CuPd, AuAg, AuPd, AgPd, MoTa, MoW and TaW are similarly evaluated. Finally, the energy distribution is obtained for different cation arrangements in MgAl$_2$O$_4$ spinel, for which long-range interactions are essential for the accurate description of its energetics.

cond-mat.mtrl-sci↗

arXiv2014

Growth and superconducting anisotropies of F-substituted LaOBiSe2 single crystals

F-substituted LaOBiSe2 single crystals were grown using CsCl flux. The obtained single crystals showed a plate-like shape with a size of about 1.0 mm square. The c-axis lattice constant of the grown crystals was determined to be 14.114(3) Å. The superconducting critical temperature of the single crystal was approximately 3.5 K. The superconducting anisotropies were determined to be 49 and 24 using the upper critical field and the effective mass model, respectively.

cond-mat.supr-con↗

arXiv2014

Thermal physics of the lead chalcogenides PbS, PbSe, and PbTe from first principles

The lead chalcogenides represent an important family of functional materials, in particular due to the benchmark high-temperature thermoelectric performance of PbTe. A number of recent investigations, experimental and theoretical, have aimed to gather insight into their unique lattice dynamics and electronic structure. However, the majority of first-principles modelling has been performed at fixed temperatures, and there has been no comprehensive and systematic computational study of the effect of temperature on the material properties. We report a comparative lattice-dynamics study of the temperature dependence of the properties of PbS, PbSe and PbTe, focussing particularly on those relevant to thermoelectric performance, viz. phonon frequencies, lattice thermal conductivity, and electronic band structure. Calculations are performed within the quasi-harmonic approximation, with the inclusion of phonon-phonon interactions from many-body perturbation theory, which are used to compute phonon lifetimes and predict the lattice thermal conductivity. The results are critically compared against experimental data and other calculations, and add new insight to on-going research on the PbX compounds in relation to the off-centring of Pb at high temperatures, which is shown to be related to phonon softening. The agreement with experiment suggests that this method could serve as a straightforward, powerful and generally-applicable means of investigating the temperature dependence of material properties from first principles.

cond-mat.mtrl-sci↗

arXiv2014

Crystal structures of LaO1-xFxBiS2 (x~0.23, 0.46): effect of F doping on distortion of Bi-S plane

The crystal structures of superconducting LaO1-xFxBiS2 (x ~ 0.23, 0.46) were determined by single-crystal X-ray diffraction analysis. Their space group was P4/nmm. Distortion of the Bi-S plane changed when the F content was increased from 0.23 to 0.46, and a nearly flat Bi-S plane was formed at x ~ 0.46. Computational calculations supported this effect of F doping on distortion of Bi-S plane. LaO1-xFxBiS2 with higher F contents were computationally predicted to be thermodynamically more unstable under ambient pressure. We discussed the bonding, conductivities, and synthetic routes of LaO1-xFxBiS2.

cond-mat.supr-con↗

arXiv2014

Cluster expansion of multicomponent ionic systems with controlled accuracy: Importance of long-range interactions in heterovalent ionic systems

We have been examining factors determining the accuracy of cluster expansion (CE), which is used in combination with many density functional theory (DFT) calculations. With the exception of multicomponent metallic or isovalent ionic systems, the contributions of long-range effective cluster interactions (ECIs) to configurational energetics are not negligible, which is ascribed to long-range electrostatic interactions. The truncation of ECIs in such systems leads to systematic errors. A typical problem with such errors can be seen in Monte Carlo (MC) simulations since simulation supercells composed of a larger number of atoms than those of the input DFT structures are used. The prediction errors for long-period structures beyond the cell size of the input DFT structures in addition to those for short-period structures within the cell size of the input DFT structures need to be carefully examined to control the accuracy of CE. In the present study, we quantitatively discuss the contribution of the truncation of long-range ECIs to the accuracy of CE. Two types of system, namely, a point-charge spinel lattice and a real MgAl2O4 spinel crystal, are examined.

cond-mat.mtrl-sci↗

arXiv2014

Machine learning with systematic density-functional theory calculations: Application to melting temperatures of single and binary component solids

A combination of systematic density functional theory (DFT) calculations and machine learning techniques has a wide range of potential applications. This study presents an application of the combination of systematic DFT calculations and regression techniques to the prediction of the melting temperature for single and binary compounds. Here we adopt the ordinary least-squares regression (OLSR), partial least-squares regression (PLSR), support vector regression (SVR) and Gaussian process regression (GPR). Among the four kinds of regression techniques, the SVR provides the best prediction. In addition, the inclusion of physical properties computed by the DFT calculation to a set of predictor variables makes the prediction better. Finally, a simulation to find the highest melting temperature toward the efficient materials design using kriging is demonstrated. The kriging design finds the compound with the highest melting temperature much faster than random designs. This result may stimulate the application of kriging to efficient materials design for a broad range of applications.

cond-mat.mtrl-sci↗

arXiv2013

Growth and superconducting properties of F-substituted ROBiS2 (R = La, Ce, Nd) single crystals

F-substituted ROBiS2 (R = La, Ce, Nd) superconducting single crystals with different F concentration were grown successfully using CsCl/KCl flux. All the obtained single crystals had a plate-like shape with a well-developed ab-plane of 1-2 mm in size. The flux components of Cs, K, and Cl were not detected in the obtained single crystals by electron probe microanalysis. The grown single crystals of F-substituted LaOBiS2 and CeOBiS2 showed superconducting at about 3 K while the Tc of the F-substituted NdOBiS2 exhibited approximately 5 K. The superconducting anisotropy of the single crystals of F-substituted LaOBiS2 and NdOBiS2 was estimated to be 30-45 according to the effective mass model whereas those values were 13-21 for the F-substituted CeOBiS2 single crystals. The F-substituted CeOBiS2 single crystals exhibited magnetic order at about 7 K that apparently coexisted with superconductivity below around 3 K.

cond-mat.supr-con↗

arXiv2013

Structural Analysis and Superconducting Properties of F-Substituted NdOBiS2 Single Crystals

F-substituted NdOBiS2 superconducting single crystals were grown using CsCl/KCl flux. This is the first example of the single-crystal growth of a BiS2-based superconductor. The obtained single crystals had a plate-like shape with a size of 1-2 mm and a well-developed ab-plane. The crystal structure of the grown crystals was determined by single-crystal X-ray diffraction analysis to be the tetragonal space group P4/nmm (#129) with a = 3.996(3) A and c = 13.464(6) A. The chemical formula of the grown crystals was approximately Nd0.98(0.06)O0.7(0.1)F0.3(0.1)Bi0.98(0.04)S2, and Cs, K, and Cl were not detected in the grown crystals by electron probe microanalysis. The grown crystals had a critical temperature of approximately 5 K. The superconducting anisotropy of the single crystals was estimated to be about 30 from the effective mass model and the upper critical field.

cond-mat.supr-con↗

arXiv2012

Inducement of superconductivity in Fe(Te,S) by sulfuric acid treatment

Superconductivity of Fe0.95Te0.85S0.15 was induced and enhanced by immersing the as-grown (non-superconducting) sample into sulfuric acid. Dissolved Fe was detected in the sulfuric acid treatment, indicating that excess Fe was removed from the Fe0.95Te0.85S0.15 sample.

cond-mat.supr-con↗