arXiv subjects

Terumasa Tadano

Publications and source records attributed to Terumasa Tadano.

At least 37 records · Page 2Linked to original sources

arXiv2022

Lattice dielectric properties of rutile $\mathrm{TiO}_2$: First-principles anharmonic self-consistent phonon study

We calculate the lattice dielectric function of strongly anharmonic rutile $\mathrm{TiO}_2$ from ab initio anharmonic lattice dynamics methods. Since an accurate calculation of the $Γ$ point phonons is essential for determining optical properties, we employ the modified self-consistent approach, including third-order anharmonicity as well as fourth-order anharmonicity. The resulting optical phonon frequencies and linewidths at the $Γ$ point much better agree with experimental measurements than those from a perturbative approach. We show that the four-phonon scattering process contributes as much as the third-order anharmonic term to phonon linewidths. Furthermore, incorporating the frequency dependence of phonon linewidth reveals that experimentally known but unidentified peaks of the dielectric function are due to two-phonon process. This work emphasizes the importance of a self-consistent approach in predicting the optical properties of highly anharmonic materials.

cond-mat.mtrl-sci↗

arXiv2022

$Ab$ $initio$ material design of Ag-based oxides for high-$T_c$ superconductor

We propose silver-based oxides with layered perovskite structure as candidates of exhibiting intriguing feature of strongly correlated electrons. The compounds show unique covalence between Ag d and O p orbitals with the strong electron correlation of their antibonding orbital similar to the copper oxide high-temperature superconductors, but stronger covalency and slightly smaller correlation strength. We examine $A_2$AgO$_2X_2$ with $A$ =Sr and Ba and $X$ =F, I and Cl in detail. Among them, Sr$_2$AgO$_2$F$_2$ has the largest effective onsite Coulomb repulsion and shows an antiferromagnetic insulating ground state, which competes with correlated metals. It offers features both similar and distinct from the copper oxides, and paves a new route. The possibility of superconductivity in doped systems is discussed.

cond-mat.supr-con↗

arXiv2022

LO-mode phonon of KCl and NaCl at 300 K by inelastic X ray scattering measurements and first principles calculations

Longitudinal-optical (LO) mode phonon branches of KCl and NaCl were measured using inelastic X-ray scattering (IXS) at 300 K and calculated by the first-principles phonon calculation with the stochastic self-consistent harmonic approximation. Spectral shapes of the IXS measurements and calculated spectral functions agreed well. We analyzed the calculated spectral functions that provide higher resolutions of the spectra than the IXS measurements. Due to strong anharmonicity, the spectral functions of these phonon branches have several peaks and the LO modes along $Γ$--L paths are disconnected.

cond-mat.mtrl-sci↗

arXiv2022

First-Principles Phonon Quasiparticle Theory Applied to a Strongly Anharmonic Halide Perovskite

Understanding and predicting lattice dynamics in strongly anharmonic crystals is one of the long-standing challenges in condensed matter physics. Here we propose a first-principles method that gives accurate quasiparticle (QP) peaks of the phonon spectrum with strong anharmonic broadening. On top of the conventional first-order self-consistent phonon (SC1) dynamical matrix, the proposed method incorporates frequency renormalization effects by the bubble self-energy within the QP approximation. We apply the developed methodology to the strongly anharmonic $α$-CsPbBr$_3$ that displays phonon instability within the harmonic approximation in the whole Brillouin zone. While the SC1 theory significantly underestimates the cubic-to-tetragonal phase transition temperature (\tc) by more than 50\%, we show that our approach yields \tc = 404--423~K, in excellent agreement with the experimental value of 403~K. We also demonstrate that an accurate determination of QP peaks is paramount for quantitative prediction and elucidation of phonon linewidth..

cond-mat.mtrl-sci↗

arXiv2021

Lattice dynamics and its effects on magnetocrystalline anisotropy energy of pristine and hole-doped YCo$_5$ from first principles

We study the lattice dynamics effects on the phase stability and magnetocrystalline anisotropy (MCA) energy of CaCu$_5$-type YCo$_5$ at finite temperatures using first-principles calculations based on density functional theory (DFT). Harmonic lattice dynamics (HLD) calculations indicate that YCo$_5$ with 56 full valance electrons is dynamically unstable and this instability can be cured by reducing the number of electrons ($N_e$). Crystal orbital Hamilton population analysis reveals that the observed phonon instability originates from the large population of antibonding states near the Fermi level, which is dominated by the Co atoms in the honeycomb layer. The antibonding state depopulates with decreasing $N_e$, resulting in stable phonons for hole-doped YCo$_5$ with $N_e$ $\leq$ 55. We then evaluate the temperature-dependent MCA energy using both HLD and $ab$ $initio$ molecular dynamics (AIMD) methods. For the pristine YCo$_5$, we observe a very weak temperature decay of the MCA energy, indicating little effect of lattice dynamics. Also, the MCA energies evaluated with AIMD at all target temperatures are larger than that of the static hexagonal lattice at 0 K, which is mainly attributed to the structural distortion driven by soft phonon modes. In the hole-doped YCo$_5$, where the distortion is suppressed, a considerable temperature decay in MCA energy is obtained both in HLD and AIMD methods, showing that lattice dynamics effects on MCA energy are non-negligible.

cond-mat.mtrl-sci↗

arXiv2021

Crucial role of interfacial $s$-$d$ exchange interaction in the temperature dependence of tunnel magnetoresistance

The tunnel magnetoresistance (TMR) is one of the most important spintronic phenomena but its reduction at finite temperature is a severe drawback for applications. Here, we reveal a crucial determinant of the drawback, that is, the $s$-$d$ exchange interaction between conduction $s$ and localized $d$ electrons at interfacial ferromagnetic layers. By calculating the temperature dependence of the TMR ratio in Fe/MgO/Fe(001), we show that the obtained TMR ratio significantly decreases with increasing temperature owing to the spin-flip scattering in the $Δ_1$ state induced by the $s$-$d$ exchange interaction. The material dependence of the coupling constant $J_{sd}$ is also discussed on the basis of a nonempirical method.

cond-mat.mtrl-sci↗

arXiv2021

Descriptors of intrinsic hydrodynamic thermal transport: screening a phonon database in a machine learning approach

Machine learning techniques are used to explore the intrinsic origins of the hydrodynamic thermal transport and to find new materials interesting for science and engineering. The hydrodynamic thermal transport is governed intrinsically by the hydrodynamic scale and the thermal conductivity. The correlations between these intrinsic properties and harmonic and anharmonic properties, and a large number of compositional (290) and structural (1224) descriptors of 131 crystal compound materials are obtained, revealing some of the key descriptors that determines the magnitude of the intrinsic hydrodynamic effects, most of them related with the phonon relaxation times. Then, a trained black-box model is applied to screen more than 5000 materials. The results identify materials with potential technological applications. Understanding the properties correlated to hydrodynamic thermal transport can help to find new thermoelectric materials and on the design of new materials to ease the heat dissipation in electronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Strongly Anharmonic Organic Cation Vibrations in Hybrid Lead Halide Perovskite CH3NH3PbI3 on Electronic Ground State

Ultrafast vibrational dynamics of inorganic-organic hybrid lead halide perovskite CH3NH3PbI$ in the room-temperature tetragonal phase is investigated under moderate photoexcitation slightly below the band gap. Time-resolved optical Kerr effect signal exhibits periodic modulations due to the libration and twisting of the methylammonium molecule at 4 and 8 THz, in addition to the inorganic sublattice deformation at 1.2 THz. These oscillations are heavily damped, and their frequencies exhibit blueshifts with increasing pump density, contrary to the expectation from conventional laser heating. Our ab initio lattice dynamics simulations reveal strongly anharmonic vibrational potentials for the zone-center optical phonon modes, and thereby confirm the anharmonicity to be the origin of the experimentally observed frequency blueshifts.

cond-mat.mtrl-sci↗

arXiv2021

Ab initio Derivation of Low-Energy Hamiltonians for Systems with Strong Spin-Orbit Interaction and Its Application to Ca5Ir3O12

We present an ab initio derivation method for effective low-energy Hamiltonians of material with strong spin-orbit interactions. The effective Hamiltonian is described in terms of the Wannier function in the spinor form, and effective interactions are derived with the constrained random phase approximation (cRPA) method. Based on this formalism and the developed code, we derive an effective Hamiltonian of a strong spin-orbit interaction material Ca5Ir3O12. This system consists of three edge-shared IrO6 octahedral chains arranged along the c axis, and the three Ir atoms in the ab plane compose a triangular lattice. For such a complicated structure, we need to set up the Wannier spinor function under the local coordinate system. We found that a density-functional band structure near the Fermi level is formed by local dxy and dyz orbitals. Then, we constructed the ab initio dxy/dyz model. The estimated nearest neighbor transfer t is close to 0.2 eV, and the cRPA onsite U and neighboring V electronic interactions are found to be 2.4-2.5 eV and 1 eV, respectively. The resulting characteristic correlation strength defined by (U-V)/t is above 7, and thus this material is classified as a strongly correlated electron system. The onsite transfer integral involved in the spin-orbit interaction is 0.2 eV, which is comparable to the onsite exchange integrals near 0.2 eV, indicating that the spin-orbit-interaction physics would compete with the Hund physics. Based on these calculated results, we discuss possible rich ground-state low-energy electronic structures of spin, charge and orbitals with competing Hund, spin-orbit and strong correlation physics.

cond-mat.str-el↗

arXiv2021

Lattice dynamics effects on finite-temperature stability of $R_{1-x}$Fe$_{x}$ ($R$ = Y, Ce, Nd, Sm, and Dy) alloys from first principles

We report the effects of lattice dynamics on thermodynamic stability of binary $R_{1-x}$Fe$_x$ $(0<x<1)$ compounds ($R$: rare-earth elements, Y, Ce, Nd, Sm, and Dy) at finite temperature predicted by first-principles calculation based on density functional theory (DFT). We first demonstrate that the thermodynamic stability of $R_{1-x}$Fe$_x$ $(0<x<1)$ alloys cannot be predicted accurately by the conventional approach, where only the static DFT energy at $T = 0$ K is used. This issue can be overcome by considering the entropy contribution, including electronic and vibrational free energies, and we obtained convex hull plots at finite temperatures that successfully explain the thermodynamic stability of various known compounds. Our systematic calculation indicates that vibrational entropy helps stabilize various $R_{1-x}$Fe$_x$ compounds with increasing temperature. In particular, experimentally reported $R_2$Fe$_{17}$ compounds are predicted to become thermodynamically stable above $\sim$800 K. We also show that thermodynamic stability is rare-earth dependent and discuss its origin. Besides the experimentally reported structures, the stability of two new monoclinic $R$Fe$_{12}$ structures found by Ishikawa \textit{et al.} [Phys. Rev. Mater.~\textbf{4}, 104408 (2020)] based on a genetic algorithm are investigated. These monoclinic phases are found to be dynamically stable and have larger magnetization than the ThMn$_{12}$-type $R$Fe$_{12}$. Although they are thermodynamically unstable, the formation energies decrease significantly with increasing temperature, indicating the possibility of synthesizing these compounds at high temperatures.

cond-mat.mtrl-sci↗

arXiv2021

Evolutionary search for cobalt-rich compounds in the yttrium-cobalt-boron system

Modern high-performance permanent magnets are made from alloys of rare earth and transition metal elements, and large magnetization is achieved in the alloys with high concentration of transition metals. We applied evolutionary search scheme based on first-principles calculations to the Y-Co-B system and predicted 37 cobalt-rich compounds with high probability of being stable. Focusing on remarkably cobalt-rich compounds, YCo$_{16}$ and YCo$_{20}$, we found that, although they are metastable phases, the phase stability is increased with increase of temperature due to the contribution of vibrational entropy. The magnetization and Curie temperature are higher by 0.22 T and 204 K in YCo$_{16}$ and by 0.29 T and 204 K in YCo$_{20}$ than those of Y$_{2}$Co$_{17}$ which has been well studied as strong magnetic compounds.

cond-mat.mtrl-sci↗

arXiv2020

Materials design of dynamically stable $d^9$ layered nickelates

Motivated by the recent discovery of superconductivity in the Sr-doped layered nickelate NdNiO$_2$, we perform a systematic computational materials design of layered nickelates that are dynamically stable and whose electronic structure better mimics the electronic structure of high-$T_c$ cuprates than NdNiO$_2$. While the Ni $3d$ orbitals are self-doped from the $d^9$ configuration in NdNiO$_2$ and the Nd-layer states form Fermi pockets, we find more than 10 promising compounds for which the self-doping is almost or even completely suppressed. We derive effective single-band models for those materials and find that they are in the strongly-correlated regime. We also investigate the possibility of palladate analogues of high-$T_c$ cuprates. Once synthesized, these nickelates and palladates will provide a firm ground for studying superconductivity in the Mott-Hubbard regime of the Zaanen-Sawatzky-Allen classification.

cond-mat.supr-con↗

arXiv2020

RESPACK: An ab initio tool for derivation of effective low-energy model of material

RESPACK is a first-principles calculation software for evaluating the interaction parameters of materials and is able to calculate maximally localized Wannier functions, response functions based on the random phase approximation and related optical properties, and frequency-dependent electronic interaction parameters. RESPACK receives its input data from a band-calculation code using norm-conserving pseudopotentials with plane-wave basis sets. Automatic generation scripts that convert the band-structure results to the RESPACK inputs are prepared for xTAPP and Quantum ESPRESSO. An input file for specifying the RESPACK calculation conditions is designed pursuing simplicity and is given in the Fortran namelist format. RESPACK supports hybrid parallelization using OpenMP and MPI and can treat large systems including a few hundred atoms in the calculation cell.

cond-mat.str-el↗

arXiv2019

Formation of 2D single-component correlated electron system and band engineering in the nickelate superconductor NdNiO2

Motivated by the recent experimental discovery of superconductivity in the infinite-layer nickelate Nd0.8Sr0.2NiO2 [Li et al., Nature 572, 624 (2019)], we study how the correlated Ni 3dx2-y2 electrons in the NiO2 layer interact with the electrons in the Nd layer. We show that three orbitals are necessary to represent the electronic structure around the Fermi level: Ni 3dx2-y2, Nd 5d3z2-r2, and a bonding orbital made from an interstitial s orbital in the Nd layer and the Nd 5dxy orbital. By constructing a three-orbital model for these states, we find that the hybridization between the Ni 3dx2-y2 state and the states in the Nd layer is tiny. We also find that the metallic screening by the Nd layer is not so effective in that it reduces the Hubbard U between the Ni 3dx2-y2 electrons just by 10--20 %. On the other hand, the electron-phonon coupling is not strong enough to mediate superconductivity of Tc ~ 10 K. These results indicate that NdNiO2 hosts an almost isolated correlated 3dx2-y2 orbital system. We further study the possibility of realizing a more ideal single-orbital system in the Mott-Hubbard regime. We find that the Fermi pockets formed by the Nd-layer states dramatically shrink when the hybridization between the interstitial s state and Nd 5dxy state becomes small. By an extensive materials search, we find that the Fermi pockets almost disappear in NaNd2NiO4 and NaCa2NiO3.

cond-mat.supr-con↗

arXiv2019

Exploring diamond-like lattice thermal conductivity crystals via feature-based transfer learning

Ultrahigh lattice thermal conductivity materials hold great importance since they play a critical role in the thermal management of electronic and optical devices. Models using machine learning can search for materials with outstanding higher-order properties like thermal conductivity. However, the lack of sufficient data to train a model is a serious hurdle. Herein we show that big data can complement small data for accurate predictions when lower-order feature properties available in big data are selected properly and applied to transfer learning. The connection between the crystal information and thermal conductivity is directly built with a neural network by transferring descriptors acquired through a pre-trained model for the feature property. Successful transfer learning shows the ability of extrapolative prediction and reveals descriptors for lattice anharmonicity. Transfer learning is employed to screen over 60000 compounds to identify novel crystals that can serve as alternatives to diamond.

cond-mat.mtrl-sci↗

arXiv2019

Quantum Crystal Structure in the 250 K Superconducting Lanthanum Hydride

The discovery of superconductivity at 200 K in the hydrogen sulfide system at large pressures [1] was a clear demonstration that hydrogen-rich materials can be high-temperature superconductors. The recent synthesis of LaH$_{10}$ with a superconducting critical temperature (T$_{\text{c}}$) of 250 K [2,3] places these materials at the verge of reaching the long-dreamed room-temperature superconductivity. Electrical and x-ray diffraction measurements determined a weakly pressure-dependent T$_{\text{c}}$ for LaH$_{10}$ between 137 and 218 gigapascals in a structure with a face-centered cubic (fcc) arrangement of La atoms [3]. Here we show that quantum atomic fluctuations stabilize in all this pressure range a high-symmetry Fm-3m crystal structure consistent with experiments, which has a colossal electron-phonon coupling of $λ\sim3.5$. Even if ab initio classical calculations neglecting quantum atomic vibrations predict this structure to distort below 230 GPa yielding a complex energy landscape with many local minima, the inclusion of quantum effects simplifies the energy landscape evidencing the Fm-3m as the true ground state. The agreement between the calculated and experimental T$_{\text{c}}$ values further supports this phase as responsible for the 250 K superconductivity. The relevance of quantum fluctuations in the energy landscape found here questions many of the crystal structure predictions made for hydrides within a classical approach that at the moment guide the experimental quest for room-temperature superconductivity [4,5,6]. Furthermore, quantum effects reveal crucial to sustain solids with extraordinary electron-phonon coupling that may otherwise be unstable [7].

cond-mat.supr-con↗

arXiv2019

First-principles study of phonon anharmonicity and negative thermal expansion in ScF3

The microscopic origin of the large negative thermal expansion of cubic scandium trifluorides (ScF3) is investigated by performing a set of anharmonic free-energy calculations based on density functional theory. We demonstrate that the conventional quasiharmonic approximation (QHA) completely breaks down for ScF3 and the quartic anharmonicity, treated nonperturbatively by the self-consistent phonon theory, is essential to reproduce the observed transition from negative to positive thermal expansivity and the hardening of the R4+ soft mode with heating. In addition, we show that the contribution from the cubic anharmonicity to the vibrational free energy, evaluated by the improved self-consistent phonon theory, is significant and as important as that from the quartic anharmonicity for robust understandings of the temperature dependence of the thermal expansion coefficient. The first-principles approach of this study enables us to compute various thermodynamic properties of solids in the thermodynamic limit with the effects of cubic and quartic anharmonicities. Therefore, it is expected to solve many known issues of the QHA-based predictions particularly noticeable at high temperature and in strongly anharmonic materials.

cond-mat.mtrl-sci↗

arXiv2019

$\textit{Ab initio}$ derivation of effective Hamiltonian for La$_{2}$CuO$_{4}$/La$_{1.55}$Sr$_{0.45}$CuO$_{4}$ heterostructure

We formulate a method of deriving effective low-energy Hamiltonian for nonperiodic systems such as interfaces for strongly correlated electron systems by extending conventional multi-scale $\textit{ab initio}$ scheme for correlated electrons (MACE). We apply the formalism to copper-oxide high $T_{\rm c}$ superconductors in an example of the interface between overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ and Mott insulating La$_{2}$CuO$_{4}$ recently realized experimentally. We show that the parameters of the $E_g$ Hamiltonian derived for the La$_{2}$CuO$_{4}$/La$_{1.55}$Sr$_{0.45}$CuO$_{4}$ superlattice differ considerably from those for the bulk La$_{2}$CuO$_{4}$, particularly significant in the partially-screened Coulomb parameters and the level offset between the $d_{x^{2}-y^{2}}$ and $d_{z^{2}}$ orbitals, $ΔE$. In addition, we investigate the effect of the lattice relaxation on the $E_g$ Hamiltonian by carefully comparing the parameters derived before and after the structure optimization. We find that the CuO$_{6}$ octahedra distort after the relaxation as a consequence of the Madelung potential difference between the insulator and metal sides, by which the layer dependence of the hopping and Coulomb parameters becomes more gradual than the unrelaxed case. Furthermore, the structure relaxation dramatically changes the $ΔE$ value and the occupation number at the interface. This study not only evidences the importance of the ionic relaxation around interfaces but also provides a set of layer-dependent parameters of the $E_g$ Hamiltonian, which is expected to provide further insight into the interfacial superconductivity when solved with low-energy solvers.

cond-mat.str-el↗