arXiv subjects

Takuya Nomoto

Publications and source records attributed to Takuya Nomoto.

At least 37 records · Page 2Linked to original sources

arXiv2024

Effect of collective spin excitation on electronic transport in topological spin texture

We develop an efficient real-time simulation method for the spin-charge coupled system in the velocity gauge. This method enables us to compute the real-time simulation for the two-dimensional system with the complex spin texture. We focus on the effect of the collective excitation of the localized spins on the electronic transport properties of the non-trivial topological state in real space. To investigate this effect, we calculate the linear optical conductivity by calculating the real-time evolution of the Kondo lattice model on the triangular lattice, which hosts an all-in/all-out magnetic structure. In the linear conductivity spectra, we observe multiple peaks below the bandgap regime, attributed to the resonant contributions of collective modes similar to the skyrmionic system, alongside broadband modifications resulting from off-resonant spin dynamics. The result shows that the collective excitation, similar to the skyrmionic system, influences the optical response of the electron systems based on symmetry analysis. We elucidate the interference between the contributions from the different spin excitations to the optical conductivity in the multiple spin texture, pointing out the mode-dependent electrical activity. We show the complex interplay between the complex spin texture and the itinerant electrons in the two-dimensional spin-charge coupled system.

cond-mat.str-el↗

arXiv2024

Nernst effect of high-mobility Weyl electrons in NdAlSi enhanced by a Fermi surface nesting instability

The thermoelectric Nernst effect of solids converts heat flow to beneficial electronic voltages. Here, using a correlated topological semimetal with high carrier mobility $μ$ in presence of magnetic fluctuations, we demonstrate an enhancement of the Nernst effect close to a magnetic phase transition. A magnetic instability in NdAlSi modifies the carrier relaxation time on 'hotspots' in momentum space, causing a strong band filling dependence of $μ$. We quantitatively derive electronic band parameters from a novel two-band analysis of the Nernst effect $S_{xy}$, in good agreement with quantum oscillation measurements and band calculations. While the Nernst response of NdAlSi behaves much like conventional semimetals at high temperatures, an additional contribution $ΔS_{xy}$ from electronic correlations appears just above the magnetic transition. Our work demonstrates the engineering of the relaxation time, or the momentum-dependent self energy, to generate a large Nernst response independent of a material's carrier density, i.e. for metals, semimetals, and semiconductors with large $μ$.

cond-mat.mtrl-sci↗

arXiv2024

Ab initio exploration of short-pitch skyrmion materials: Role of orbital frustration

In recent years, the skyrmion lattice phase with a short lattice constant has attracted attention due to its high skyrmion density, making it a promising option for achieving high-density storage memory and for observing novel phenomena like the quantized topological Hall effect. Unlike conventional non-centrosymmetric systems where the Dzyaloshinsky-Moriya interaction plays a crucial role, the short pitch skyrmion phase requires a quadratic magnetic interaction $J(q)$ with a peak at finite-$Q$, and weak easy-axis magnetic anisotropy is also critical. Thus, conducting first-principles evaluations is essential for understanding the formation mechanism as well as for promoting the discovery of new skyrmion materials. In this {\it Perspective}, we focus on recent developments of the first-principles evaluations of these properties and apply them to the prototype systems Gd$T_2X_2$ and Eu$T_2X_2$, where $T$ denotes a transition metal and $X$ represents Si or Ge. In particular, based on the spin density functional theory with the Hubbard correction combined with the Liechtenstein method in the Wannier tight-binding model formalism, we first show that the Hubbard $U$ and Hund's coupling is essential to stabilize a skyrmion lattice state by enhancing the easy-axis anisotropy. We then discuss mechanisms of finite-$Q$ instability and show that competition among Gd-5$d$ orbitals determines whether ferromagnetism or a finite-$Q$ structure is favored in Gd$T_2$Si$_2$ with $T=$ Fe and Ru. Our systematic calculations reveal that GdRu$_2$$X_2$, GdOs$_2$$X_2$, and GdRe$_2X_2$ are promising, while GdAg$_2X_2$, GdAu$_2X_2$, and EuAg$_2X_2$ are possible candidates as the skyrmion host materials. Analysis based on a spin spiral calculation for the candidate materials is also presented.

cond-mat.str-el↗

arXiv2024

High-throughput calculations of antiferromagnets hosting anomalous transport phenomena

We develop a high-throughput computational scheme based on cluster multipole theory to identify new functional antiferromagnets. This approach is applied to 228 magnetic compounds listed in the AtomWork-Adv database, known for their elevated Néel temperatures. We conduct systematic investigations of both stable and metastable magnetic configurations of these materials. Our findings reveal that 34 of these compounds exhibit antiferromagnetic structures with zero propagation vectors and magnetic symmetries identical to conventional ferromagnets, rendering them potentially invaluable for spintronics applications. By cross-referencing our predictions with the existing MAGNDATA database and published literature, we verify the reliability of our findings for 26 out of 28 compounds with partially or fully elucidated magnetic structures in the experiments. These results not only affirm the reliability of our scheme but also point to its potential for broader applicability in the ongoing quest for the discovery of new functional magnets.13

cond-mat.mtrl-sci↗

arXiv2024

Continuous crossover between insulating ferroelectrics and the polar metals: \textit{Ab initio} calculation of structural phase transitions of Li$B$O$_3$ ($B$ = Ta, W, Re, Os)

Inspired by the recent discovery of a new polar metal LiReO$_3$ by K. Murayama, \textit{et al}, we calculate the temperature($T$)-dependent crystal structures of Li$B$O3 with $B$ = Ta, W, Re, Os, using the self-consistent phonon (SCPH) theory. We have reproduced the experimentally observed polar-nonpolar structural phase transitions and the transition temperatures ($T_c$) of LiTaO$_3$, LiReO$_3$, and LiOsO$_3$. From the calculation, we predict that LiWO$_3$ is a polar metal, which is yet to be tested experimentally. Upon doping electrons to the insulating LiTaO$_3$, the predicted $T_c$ is quickly suppressed and approaches those of the polar metals. Thus, there is a continuous crossover between ferroelectric insulators and polar metals if we dope electrons to the polar metals. Investigating the detailed material dependence of the interatomic force constants (IFCs), we explicitly show that the suppression of $T_c$ in polar metals can be ascribed to the screening of the long-range Li-O interaction, which is caused by the presence of the itinerant electrons.

cond-mat.mtrl-sci↗

arXiv2024

Ferromagnetic state with large magnetic moments realized in epitaxially strained Sr3Ru2O7 films

Technical advancement of oxide molecular beam epitaxy (MBE) has opened new avenues for studying various quantum transport phenomena in correlated transition-metal oxides, as exemplified by the exotic superconductivity of Sr$_2$RuO$_4$ and quantum oscillations of SrRuO$_3$. On the other hand, film research of another Ruddlesden-Popper strontium ruthenate Sr$_3$Ru$_2$O$_7$ which exhibits a unique quantum phase related to metamagnetism in bulk systems did not progress well. Here we report the fabrication of high-quality Sr$_3$Ru$_2$O$_7$ thin films by oxide MBE and the observation of a strain-induced ferromagnetic ground state. The change in magnetic exchange coupling evaluated by first-principles calculations indicates a systematic relation between the compression of the $c$-axis length and induced ferromagnetism. Giant epitaxial strain in high-quality films will be a key to a comprehensive understanding of the magnetism in Ruddlesden-Popper strontium ruthenates Sr$_{n+1}$Ru$_n$O$_{3n+1}$, which sensitively depends on the ratio of in-plane to out-of-plane Ru-Ru distances.

cond-mat.str-el↗

arXiv2024

Symmetry analysis with spin crystallographic groups: Disentangling effects free of spin-orbit coupling in emergent electromagnetism

Recent studies identified spin-order-driven phenomena such as spin-charge interconversion without relying on the relativistic spin-orbit interaction. Those physical properties can be prominent in systems containing light magnetic atoms due to sizable exchange splitting and may pave the way for realization of giant responses correlated with the spin degree of freedom. In this paper, we present a systematic symmetry analysis based on the spin crystallographic groups and identify the physical property of a vast number of magnetic materials up to 1500 in total. By decoupling the spin and orbital degrees of freedom, our analysis enables us to take a closer look into the relation between the dimensionality of spin structures and the resultant physical properties and to identify the spin and orbital contributions separately. In stark contrast to the established analysis with magnetic space groups, the spin crystallographic group manifests richer symmetry including spin-translation symmetry and leads to emergent responses. For representative examples, we discuss the geometrical nature of the anomalous Hall effect and magnetoelectric effect and classify the spin Hall effect arising from the nonrelativistic spin-charge coupling. Using the power of computational analysis, we apply our symmetry analysis to a wide range of magnets, encompassing complex magnets such as those with noncoplanar spin structures as well as collinear and coplanar magnets. We identify emergent multipoles relevant to physical responses and argue that our method provides a systematic tool for exploring sizable electromagnetic responses driven by spin order.

cond-mat.mtrl-sci↗

arXiv2024

Bulk photovoltaic effect in antiferromagnet: Role of collective spin dynamics

Inspired by recent advancements in the bulk photovoltaic effect which can extend beyond the independent particle approximation (IPA), this study delves into the influence of collective spin dynamics in an antiferromagnetic on photocurrent generation using a time domain calculation. In the linear and photocurrent conductivity spectra, we observe peaks below the bandgap regime, attributed to the resonant contributions of collective modes, alongside broadband modifications resulting from off-resonant spin dynamics. Notably, the emergence of spin dynamics allows various types of photocurrent, which are absent in the IPA framework. Furthermore, we emphasize the importance of energy scale proximity between electronic and spin degrees of freedom in enabling efficient feedback between them. These findings offer new avenues for efficient energy harvesting and optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2023

Topological Band Inversion and Chiral Majorana Mode in Hcp Thallium

The chiral Majorana fermion is an exotic particle that is its own antiparticle. It can arise in a one-dimensional edge of topological materials, and especially that in a topological superconductor can be exploited in non-Abelian quantum computation. While the chiral Majorana mode (CMM) remains elusive, a promising situation is realized when superconductivity coexists with a topologically non-trivial surface state. Here, we perform fully non-empirical calculation for the CMM considering superconductivity and surface relaxation, and show that hexagonal close-packed thallium (Tl) has an ideal electronic state that harbors the CMM. The $k_z=0$ plane of Tl is a mirror plane, realizing a full-gap band inversion corresponding to a topological crystalline insulating phase. Its surface and hinge are stable and easy to make various structures. Another notable feature is that the surface Dirac point is very close to the Fermi level, so that a small Zeeman field can induce a topological transition. Our calculation indicates that Tl will provide a new platform of the Majorana fermion.

cond-mat.supr-con↗

arXiv2023

Algorithm for spin symmetry operation search

A spin space group provides a suitable way to fully exploit the symmetry of a spin arrangement with a negligible spin-orbit coupling. There has been a growing interest in applying spin symmetry analysis with the spin space group in the field of magnetism. However, there is no established algorithm to search for spin symmetry operations of the spin space group. This paper presents an exhaustive algorithm for determining spin symmetry operations of commensurate spin arrangements. The present algorithm searches for spin symmetry operations from the symmetry operations of a corresponding nonmagnetic crystal structure and determines their spin-rotation parts by solving a Procrustes problem. An implementation is distributed under a permissive free software license in spinspg v0.1.1: https://github.com/spglib/spinspg.

cond-mat.mtrl-sci↗

arXiv2023

High-performance descriptor for magnetic materials: Accurate discrimination of magnetic structure

The magnetic structure is crucial in determining the physical properties inherent in magnetic compounds. We present an adequate descriptor for magnetic structure with proper magnetic symmetry and high discrimination performance, which does not depend on artificial choices for coordinate origin, axis, and magnetic unit cell in crystal. We extend the formalism called ``smooth overlap of atomic positions'' (SOAP), providing a numerical representation of atomic configurations to that of magnetic moment configurations. We introduce the descriptor in terms of the vector spherical harmonics to describe a magnetic moment configuration and partial spectra from the expansion coefficients. We discuss that the lowest-order partial spectrum is insufficient to discriminate the magnetic structures with different magnetic anisotropy, and a higher-order partial spectrum is required in general to differentiate detailed magnetic structures on the same atomic configuration. We then introduce the fourth-order partial spectrum and evaluate the discrimination performance for different magnetic structures, mainly focusing on the difference in magnetic symmetry. The modified partial spectra that are defined not to reflect the difference of magnetic anisotropy are also useful in evaluating magnetic structures obtained from the first-principles calculations performed without spin-orbit coupling. We apply the present method to the symmetry-classified magnetic structures for the crystals of Mn$_3$Ir and Mn$_3$Sn, which are known to exhibit anomalous transport under the antiferromagnetic order, and examine the discrimination performance of the descriptor for different magnetic structures on the same crystal.

cond-mat.mtrl-sci↗

arXiv2023

Full optimization of quasiharmonic free energy with anharmonic lattice model: Application to thermal expansion and pyroelectricity of wurtzite GaN and ZnO

We present a theory and a calculation scheme of structural optimization at finite temperatures within the quasiharmonic approximation (QHA). The theory is based on an efficient scheme of updating the interatomic force constants with the change of crystal structures, which we call the IFC renormalization. The cell shape and the atomic coordinates are treated equally and simultaneously optimized. We apply the theory to the thermal expansion and the pyroelectricity of wurtzite GaN and ZnO, which accurately reproduces the experimentally observed behaviors. Furthermore, we point out a general scheme to obtain correct $T$ dependence at the lowest order in constrained optimizations that reduce the number of effective degrees of freedom, which is helpful to perform efficient QHA calculations with little sacrificing accuracy. We show that the scheme works properly for GaN and ZnO by comparing with the optimization of all the degrees of freedom.

cond-mat.mtrl-sci↗

arXiv2022

Anharmonic Grüneisen theory based on self-consistent phonon theory: Impact of phonon-phonon interaction neglected in the quasiharmonic theory

We formulate a theory of thermal expansion based on the self-consistent phonon (SCP) theory, which nonperturbatively considers the anharmonic effect. We show that the Grünseisen formula holds within the SCP theory by replacing the phonon frequency by the SCP frequency. By comparing it with the quasiharmonic approximation (QHA), we derive explicit formulas of the correction to the QHA result. We show that the phonon anharmonicity gives a small correction of $O(\langle{\hat{U}_4\rangle}/\langle{\hat{U}_2\rangle})$ to the thermal expansion coefficient $α$, where $\hat{U}_2$ and $\hat{U}_4$ are the harmonic and the quartic terms of the potential energy surface. On the other hand, we show that the phonon anharmonicity gives two correction terms to the temperature ($T$)-dependent phonon frequency shift which are comparable to the original QHA term. In strongly anharmonic materials such as NaCl and MgO, these two correction terms tend to cancel out each other, which explains why the QHA sometimes gives reasonable values for the $T$-dependent phonon frequency shift while it fails for thermal expansion.

cond-mat.mtrl-sci↗

arXiv2022

Ab initio structural optimization at finite temperatures based on anharmonic phonon theory: Application to the structural phase transitions of BaTiO$_3$

We formulate a first-principle scheme for structural optimization at finite temperature ($T$) based on the self-consistent phonon (SCP) theory, which accurately takes into account the effect of strong phonon anharmonicity. The $T$-dependence of the shape of the unit cell and internal atomic configuration is determined by minimizing the variational free energy in the SCP theory. At each optimization step, the interatomic force constants in the new structure are calculated without running additional electronic structure calculations, which makes the method dramatically efficient. We demonstrate that the thermal expansion of silicon and the three-step structural phase transitions in BaTiO$_3$ and its pressure-temperature ($p$-$T$) phase diagram are successfully reproduced. The present formalism will open the way to the non-empirical prediction of physical properties at finite $T$ of materials having a complex structural phase diagram.

cond-mat.mtrl-sci↗

arXiv2022

Signature of topological band crossing in ferromagnetic Cr1/3NbSe2 epitaxial thin film

In intercalated transition metal dichalcogenides (I-TMDC), transition metal intercalation introduces magnetic phases which in some cases induce topological band crossing. However, evidence of the topological properties remains elusive in such materials. Here we employ angle-resolved photoemission spectroscopy to reveal the band structure of epitaxially grown ferromagnetic Cr1/3NbSe2. Experimental evidence of the Weyl crossing shows Cr1/3NbSe2 to be a topological ferromagnet. This work highlights I-TMDC as platform towards the interplay of magnetic and topological physics in low-dimensional systems.

cond-mat.mtrl-sci↗

arXiv2022

Magnetic interactions in intercalated transition metal dichalcogenides: a study based on ab initio model construction

Transition metal dichalcogenides (TMDs) are known to have a wide variety of magnetic structures by hosting other transition metal atoms in the van der Waals gaps. To understand the chemical trend of the magnetic properties of the intercalated TMDs, we perform a systematic first-principles study for 48 compounds with different hosts, guests, and composition ratios. Starting with calculations based on spin density functional theory, we derive classical spin models by applying the Liechtenstein method to the ab initio Wannier-based tight-binding model. We show that the calculated exchange couplings are overall consistent with the experiments. In particular, when the composition rate is 1/3, the chemical trend can be understood in terms of the occupation of the 3d-orbital in the intercalated transition metal. The present results give us a useful guiding principle to predict the magnetic structure of compounds that are yet to be synthesized.

cond-mat.mtrl-sci↗

arXiv2022

Local density of states as a probe for tunneling magnetoresistance effect: application to ferrimagnetic tunnel junctions

We investigate the tunneling magnetoresistance (TMR) effect using the lattice models which describe the magnetic tunnel junctions (MTJ). First, taking a conventional ferromagnetic MTJ as an example, we show that the product of the local density of states (LDOS) at the center of the barrier traces the TMR effect qualitatively. The LDOS inside the barrier has the information on the electrodes and the electron tunneling through the barrier, which enables us to easily evaluate the tunneling conductance more precisely than the conventional Julliere's picture. We then apply this method to the MTJs with collinear ferrimagnets and antiferromagnets. We find that the TMR effect in the ferrimagnetic and antiferromagnetic MTJs changes depending on the interfacial magnetic structures originating from the sublattice structure, which can also be captured by the LDOS. Our findings will reduce the computational cost for the qualitative evaluation of the TMR effect, and be useful for a broader search for the materials which work as the TMR devices showing high performance.

cond-mat.mes-hall↗

arXiv2022

Efficient ab initio many-body calculations based on sparse modeling of Matsubara Green's function

This lecture note reviews recently proposed sparse-modeling approaches for efficient ab initio many-body calculations based on the data compression of Green's functions. The sparse-modeling techniques are based on a compact orthogonal basis, an intermediate representation (IR) basis, for imaginary-time and Matsubara Green's functions. A sparse sampling method based on the IR basis enables solving diagrammatic equations efficiently. We describe the basic properties of the IR basis, the sparse sampling method and its applications to ab initio calculations based on the GW approximation and the Migdal-Eliashberg theory. We also describe a numerical library for the IR basis and the sparse sampling method, sparse-ir, and provide its sample codes. This lecture note follows the Japanese review article with major revisions [H. Shinaoka et al., Solid State Physics 56(6), 301 (2021)].

cond-mat.str-el↗