arXiv subjects

Takashi Miyake

Publications and source records attributed to Takashi Miyake.

At least 55 records · Page 3Linked to original sources

arXiv2017

A regression-based feature selection study of the Curie temperature of transition-metal rare-earth compounds: prediction and understanding

The Curie temperature ($T_C$) of binary alloy compounds consisting of 3$d$ transition-metal and 4$f$ rare-earth elements is analyzed by a machine learning technique. We first demonstrate that nonlinear regression can accurately reproduce $T_C$ of the compounds. The prediction accuracy for $T_C$ is maximized when five to ten descriptors are selected, with the rare-earth concentration being the most relevant. We then discuss an attempt to utilize a regression-based model selection technique to learn the relation between the descriptors and the actuation mechanism of the corresponding physical phenomenon, i.e., $T_C$ in the present case.

cond-mat.mtrl-sci↗

arXiv2017

Machine learning reveals orbital interaction in crystalline materials

We propose a novel representation of crystalline materials named orbital-field matrix (OFM) based on the distribution of valence shell electrons. We demonstrate that this new representation can be highly useful in mining material data. Our experiment shows that the formation energies of crystalline materials, the atomization energies of molecular materials, and the local magnetic moments of the constituent atoms in transition metal--rare-earth metal bimetal alloys can be predicted with high accuracy using the OFM. Knowledge regarding the role of coordination numbers of transition-metal and rare-earth metal elements in determining the local magnetic moment of transition metal sites can be acquired directly from decision tree regression analyses using the OFM.

cond-mat.mtrl-sci↗

arXiv2017

Topological Dirac Nodal Lines and Surface Charges in fcc Alkaline Earth Metals

In nodal-line semimetals, the gaps close along loops in ${\bf k}$ space, which are not at high-symmetry points. Typical mechanisms for the emergence of nodal lines involve mirror symmetry and the $π$ Berry phase. Here, we show via ab initio calculations that fcc calcium (Ca), strontium (Sr) and ytterbium (Yb) have topological nodal lines with the $π$ Berry phase near the Fermi level, when spin-orbit interaction is neglected. In particular, Ca becomes a nodal-line semimetal at high pressure. Owing to nodal lines, the Zak phase becomes either $π$ or 0 depending on the wavavector ${\bf k}$, and the $π$ Zak phase leads to surface polarization charge. Carriers eventually screen it, leaving behind large surface dipoles. In materials with nodal lines, both the large surface polarization charge and the emergent drumhead surface states enhance Rashba splitting when heavy adatoms are present, as we have shown to occur in Bi/Sr(111) and in Bi/Ag(111).

cond-mat.mtrl-sci↗

arXiv2016

First-principles study on stability and magnetism of NdFe11M and NdFe11MN for M=Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn

Recently synthesized NdFe12N has excellent magnetic properties, while it is thermodynamically unstable. Using first-principles method, we study the effect of substitutional 3d transition metal elements to the mother compound NdFe12. We find that Co has positive effect on the stability of the ThMn12 structure. In contrast with Ti substitution, Co substitution does not reduce the magnetization significantly. The crystal field parameter A_{0}^{2} is nearly unchanged by Co substitution, and nitrogenation to NdFe11Co greatly enhances A_{0}^{2}. This suggests that Co is a good candidate as a substitutional element for NdFe12N.

cond-mat.mtrl-sci↗

arXiv2016

Emergence of topological semimetals in gap closing in semiconductors without inversion symmetry

A band gap for electronic states in crystals governs various properties of solids, such as transport, optical and magnetic properties. Its estimation and control have been an important issue in solid state physics. The band gap can be controlled externally by various parameters, such as pressure, atomic compositions and external field. Sometimes, the gap even collapses by tuning some parameter. In the field of topological insulators, such closing of the gap at a time-reversal invariant momentum indicates a band inversion, i.e. it leads to a topological phase transition from a normal insulator to a topological insulator. Here we show that the gap losing in inversion-asymmetric crystals is universal, in the sense that the gap closing always leads either to a Weyl semimetal or a nodal-line semimetal, from an exhaustive study on possible space groups. We here consider three-dimensional spinful systems with time-reversal symmetry. The space group of the system and the wavevector at the gap closing uniquely determine which possibility occurs and where the gap-closing points or lines lie in the wavevector space after closing of the gap. In particular, we show that an insulator-to-insulator transition never happens, which is in sharp contrast with inversion-symmetric systems.

cond-mat.mes-hall↗

arXiv2016

Monte Carlo analysis for finite temperature magnetism of Nd$_2$Fe$_{14}$B permanent magnet

We investigate the effects of magnetic inhomogeneities and thermal fluctuations on the magnetic properties of a rare earth intermetallic compound, Nd$_2$Fe$_{14}$B. The constrained Monte Carlo method is applied to a Nd$_2$Fe$_{14}$B bulk system to realize the experimentally observed spin reorientation and magnetic anisotropy constants $K^{\rm A}_m (m=1, 2, 4)$ at finite temperatures. % Subsequently, it is found that the temperature dependence of $K^{\rm A}_1$ deviates from the Callen--Callen law, $K^{\rm A}_1(T) \propto M(T)^3$, even above room temperature, $T_{\rm R}\sim 300\rm\, K$, when the Fe (Nd) anisotropy terms are removed to leave only the Nd (Fe) anisotropy terms. This is because the exchange couplings between Nd moments and Fe spins are much smaller than those between Fe spins. % It is also found that the exponent $n$ in the external magnetic field $H_{\rm ext}$ response of barrier height $\mathcal{F}_{\rm B}=\mathcal{F}_{\rm B}^0(1-H_{\rm ext}/H_0)^n$ is less than $2$ in the low-temperature region below $T_{\rm R}$, whereas $n$ approaches $2$ when $T>T_{\rm R}$, indicating the presence of Stoner--Wohlfarth-type magnetization rotation. This reflects the fact that the magnetic anisotropy is mainly governed by the $K^{\rm A}_1$ term in the $T>T_{\rm R}$ region.

cond-mat.mtrl-sci↗

arXiv2016

Relevance of $4f$-$3d$ exchange to finite-temperature magnetism of rare-earth permanent magnets: an ab-initio-based spin model approach for NdFe$_{12}$N

A classical spin model derived ab initio for rare-earth-based permanent magnet compounds is presented. Our target compound, NdFe$_{12}$N, is a material that goes beyond today's champion magnet compound Nd$_{2}$Fe$_{14}$B in its intrinsic magnetic properties with a simpler crystal structure. Calculated temperature dependence of the magnetization and the anisotropy field agree with the latest experimental results in the leading order. Having put the realistic observables under our numerical control, we propose that engineering $5d$-electron-mediated indirect exchange coupling between $4f$-electrons in Nd and $3d$-electrons from Fe would most critically help to enhance the material's utility over the operation-temperature range.

cond-mat.mtrl-sci↗

arXiv2015

Nitrogen as the best interstitial dopant among $X$=B, C, N, O and F for strong permanent magnet NdFe$_{11}$Ti$X$: First-principles study

We study magnetic properties of NdFe$_{11}$Ti$X$, where $X$=B, C, N, O, and F, by using the first-principles calculation based on the density functional theory. Its parent compound NdFe$_{11}$Ti has the ThMn$_{12}$ structure, which has the symmetry of space group $I4/mmm$, No. 139. The magnetization increases by doping B, C, N, O, and F at the $2b$ site of the ThMn$_{12}$ structure. The amount of the increase is larger for $X$=N, O, F than for $X$=B, C. On the other hand, the crystal field parameter $\langle r^{2} \rangle A_{0}^{2}$, which controls the axial magnetic anisotropy of the Nd $4f$ magnetic moment, depends differently on the dopant. With increase of the atomic number from $X$=B, $\langle r^{2} \rangle A_{0}^{2}$ increases, takes a maximum value for $X$=N, and then turns to decrease. This suggests that in NdFe$_{11}$Ti$X$, nitrogen is the most appropriate dopant among B, C, N, O, and F for permanent magnets in terms of magnetization and anisotropy. The above calculated properties are explained based on the detailed analysis of the electronic structures of NdFe$_{11}$Ti$X$.

cond-mat.mtrl-sci↗

arXiv2015

Low-energy effective Hamiltonians for correlated electron systems beyond density functional theory

We propose a refined scheme of deriving an effective low-energy Hamiltonian for materials with strong electronic Coulomb correlations beyond density functional theory (DFT). By tracing out the electronic states away from the target degrees of freedom in a controlled way by a perturbative scheme we construct an effective model for a restricted low-energy target space incorporating the effects of high-energy degrees of freedom in an effective manner. The resulting effective model can afterwards be solved by accurate many-body solvers. We improve this "multi-scale ab initio scheme for correlated electrons" (MACE) primarily in two directions: (1) Double counting of electronic correlations between the DFT and the low-energy solver is avoided by using the constrained GW scheme. (2) The frequency dependence of the interaction emerging from the partial trace summation is taken into account as a renormalization to the low-energy dispersion. The scheme is successfully tested on the example of SrVO3. Our work opens unexplored ways to understanding the electronic structure of strongly correlated systems beyond current DFT methods.

cond-mat.str-el↗

arXiv2015

Ab initio Studies of Magnetism in the Iron Chalcogenides FeTe and FeSe

The iron chalcogenides FeTe and FeSe belong to the family of iron-based superconductors. We study the magnetism in these compounds in the normal state using the ab initio downfolding scheme developed for strongly correlated electron systems. In deriving ab initio low-energy effective models, we employ the constrained GW method to eliminate the double counting of electron correlations originating from the exchange correlations already taken into account in the density functional theory. By solving the derived ab initio effective models, we reveal that the elimination of the double counting is important in reproducing the bicollinear antiferromagnetic order in FeTe, as is observed in experiments. We also show that the elimination of the double counting induces a unique degeneracy of several magnetic orders in FeSe, which may explain the absence of the magnetic ordering. We discuss the relationship between the degeneracy and the recently found puzzling phenomena in FeSe as well as the magnetic ordering found under pressure.

cond-mat.str-el↗

arXiv2015

Weyl Node and Spin Texture in Trigonal Tellurium and Selenium

We study Weyl nodes in materials with broken inversion symmetry. We find based on first-principles calculations that trigonal Te and Se have multiple Weyl nodes near the Fermi level. The conduction bands have a spin splitting similar to the Rashba splitting around the H points, but unlike the Rashba splitting the spin directions are radial, forming a hedgehog spin texture around the H points, with a nonzero Pontryagin index for each spin-split conduction band. The Weyl semimetal phase, which has never been observed in real materials without inversion symmetry, is realized under pressure. The evolution of the spin texture by varying the pressure can be explained by the evolution of the Weyl nodes in k space.

cond-mat.mtrl-sci↗

arXiv2015

Robust flat bands in RCo5 (R=rare earth) compounds

The mechanism to realize the peculiar flat bands generally existing in RCo5 (R=rare earth) compounds is clarified by analyzing the first-principles band structures and the tight-binding model. These flat bands are constructed from the localized eigenstates, the existence of which is guaranteed by the partial cancelation between the intersite hopping amplitudes among the Co-3d states at the Kagome sites and those between the Kagome and honeycomb sites. Their relative positions to other bands can be controlled by varying the lattice parameters keeping their dispersion almost flat, which suggests the possibility of flat-band engineering.

cond-mat.mtrl-sci↗

arXiv2014

Second-order Perturbation Formula for Magnetocrystalline Anisotropy using Orbital Angular Momentum Matrix

We derive a second-order perturbation formula for an electronic system subject to spin-orbit interactions (SOI). The energy correction originates in the spin-conserving and the spin-flip transitions. The former are represented by the orbital angular momentum (OAM) acquired via the SOI. The latter come from the quantum fluctuation effect. By using our formula, we examine the relativistic electronic structures of a d orbital chain and L1_0 alloys. The appearance of OAM in the chain is understood by using a parabolic-bands model and the exact expressions of the single-particle states. The total energy is found to be accurately reproduced by the formula. The self-consistent fully relativistic first-principles calculations based on the density functional theory are performed for five L1_0 alloys. It is found that the formula reproduces qualitatively the behavior of their exact magnetocrystalline anisotropy (MCA) energies. While the MCA of FePt, CoPt, and FePd originates in the spin-conserving transitions, that in MnAl and MnGa originates in the spin-flip contributions. For FePt, CoPt, and FePd, the tendency of the MCA energy with the variation in the lattice constant obeys basically that of the spin-flip contributions. These results indicate that not only the anisotropy of OAM, but also that of spin-flip contributions must be taken into account for the understanding of the MCA of the L1_0 alloys.

cond-mat.mtrl-sci↗

arXiv2013

Asymmetric band widening by screened exchange competing with local correlations in SrVO3: new surprises on an old compound from combined GW and dynamical mean field theory GW+DMFT

The very first dynamical implementation of the combined GW and dynamical mean field scheme "GW+DMFT" for a real material was achieved recently [J.M. Tomczak et al., Europhys. Lett. 100 67001 (2012)], and applied to the ternary transition metal oxide SrVO3. Here, we review and extend that work, giving not only a detailed account of full GW+DMFT calculations, but also discussing and testing simplified approximate schemes. We give insights into the nature of exchange and correlation effects: Dynamical renormalizations in the Fermi liquid regime of SrVO3 are essentially local, and nonlocal correlations mainly act to screen the Fock exchange term. The latter substantially widens the quasi-particle band structure, while the band narrowing induced by the former is accompanied by a spectral weight transfer to higher energies. Most interestingly, the exchange broadening is much more pronounced in the unoccupied part of spectrum. As a result, the GW+DMFT electronic structure of SrVO3 resembles the conventional density functional based dynamical mean field (DFT+DMFT) description for occupied states, but is profoundly modified in the empty part. Our work leads to a reinterpretation of inverse photoemission spectroscopy (IPES) data. Indeed, we assign a prominent peak at about 2.7 eV dominantly to eg states, rather than to an upper Hubbard band of t2g character. Similar surprises can be expected for other transition metal oxides, calling for more detailed investigations of the conduction band states.

cond-mat.str-el↗

arXiv2013

Spin-orbital frustration in pyrochlores A2Mo2O7

Electronic and magnetic properties of molybdenum pyrochlores A2Mo2O7 are studied by the fully relativistic density-functional theory plus on-site repulsion (U) method, with focusing on the spin-glass insulating material Y2Mo2O7. We find that the system exhibits peculiar competition in energy between different magnetic states in the large-U insulating region. The magnetic competition cannot be explained by the conventional picture based on the geometrical frustration of isotropic Heisenberg antiferromagnetic exchange interactions. Through an analysis by using a generalized spin model, we find that the effective spin interactions are distinct from the simple Heisenberg form and strongly anisotropic in spin space. We also reveal that they give rise to keen competition between antiferromagnetic and ferromagnetic states. The complex form of the magnetic interactions indicates a crucial role of the orbital degree of freedom. Analyzing a three-orbital Hubbard model, we clarify that the magnetic competition is tightly connected with orbital frustration in the 4d2 electronic configuration through the spin-orbital interplay. The results challenge the conventional picture of the spin-glass behavior that attributes the origin to the geometrical frustration of purely antiferromagnetic exchange interactions.

cond-mat.str-el↗

arXiv2012

Combined GW and dynamical mean field theory: Dynamical screening effects in transition metal oxides

We present the first dynamical implementation of the combined GW and dynamical mean field scheme ("GW+DMFT") for first principles calculations of the electronic properties of correlated materials. The application to the ternary transition metal oxide SrVO3 demonstrates that this schemes inherits the virtues of its two parent theories: a good description of the local low energy correlation physics encoded in a renormalized quasi-particle band structure, spectral weight transfer to Hubbard bands, and the physics of screening driven by long-range Coulomb interactions. Our data is in good agreement with available photoemission and inverse photoemission spectra; our analysis leads to a reinterpretation of the commonly accepted "three-peak structure" as originating from orbital effects rather than from the electron addition peak within the t2g manifold.

cond-mat.str-el↗

arXiv2012

Derivation of Static Low-Energy Effective Models by ab initio Downfolding Method without Double Counting of Coulomb Correlations: Application to SrVO3, FeSe and FeTe

Derivation of low-energy effective models by a partial trace summation of the electronic degrees of freedom far away from the Fermi level, called downfolding, is reexamined. We propose an improved formalism free from the double-counting of electron correlation in the low-energy degrees of freedom. In this approach, the exchange-correlation energy in the local density approximation (LDA) is replaced with the constrained self-energy corrections defined by the sum of the contribution from eliminated high-energy degrees of freedom, Sigma H and that from the frequency-dependent part of the partially screened interaction, Delta Sigma L. We apply the formalism to SrVO3 as well as to two iron-based superconductors, FeSe and FeTe. The resultant bandwidths of the effective models are nearly the same as those of the previous downfolding formalism because of striking cancellations of Sigma H and Delta Sigma L. In SrVO3, the resultant bandwidth of the effective low-energy model is 2.56 eV (in comparison to LDA: 2.58 eV, full GW approximation: 2.19 eV). However, in the non-degenerate multi-band materials such as FeSe and FeTe, the momentum dependent self-energy effects yield substantial modifications of the band structures and relative shifts of orbital-energy levels of the effective models. The FeSe model indicates a substantial downward shift of the x2-y2 orbital level, which may lead to an increase in the filling of the x2-y2 orbital above half filling. It suggests a destruction of the orbital selective Mott phase of the x2-y2 orbital in agreement with the experimental absence of the antiferromagnetic phase.

cond-mat.str-el↗

arXiv2012

Ab-initio Low-Energy Model of Transition-Metal-Oxide Heterostructure LaAlO3/SrTiO3

We develop the multi-scale ab-initio scheme for correlated electrons (MACE) for transitionmetal-oxide heterostructures, and determine the parameters of the low-energy effective model. By separating Ti t2g bands near the Fermi level from the global Kohn-Sham (KS) bands of LaAlO3/SrTiO3 which are highly entangled with each other, we are able to calculate the parameters of the low-energy effective model of the interface with the help of constrained random phase approximation (cRPA). The on-site energies of the Ti t2g orbitals in the 1stlayer is about 650 meV lower than those in the 2nd-layer. In the 1st-layer, the transfer integral of the Ti t2g orbital is nearly the same as that of the bulk SrTiO3, while the effective screened Coulomb interaction becomes about 10 percent larger than that of the bulk SrTiO3. The differences of the parameters from the bulk SrTiO3 reduce rapidly with increasing distance from the interface. Our present versatile method makes it possible to derive effective ab-initio low-energy models and allows studying interfaces of strongly correlated electron systems from first principles.

cond-mat.str-el↗