Shuichi Murakami's research while affiliated with Tokyo Institute of Technology and other places
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Publications (199)
In topological phase transitions involving a change in topological invariants such as the Chern number and the Z2 topological invariant, the gap closes, and the electric polarization becomes undefined at the transition. In this paper, we show that the jump of polarization across such topological phase transitions in two dimensions is described in t...
Chirality is an indispensable concept that pervades fundamental science and nature, manifesting itself in diverse forms, e.g., quasiparticles, and crystal structures. Of particular interest are Weyl phonons carrying specific Chern numbers and chiral phonons doing circular motions. Up to now, they have been studied independently and the interpretati...
Chirality is an indispensable concept that pervades fundamental science and nature, manifesting itself in diverse forms such as chiral quasiparticles and chiral structures. Of particular interest are Weyl phonons carrying specific Chern numbers and chiral phonons doing circular motions in crystals. Up to now, Weyl and chiral phonons have been studi...
We study equilibrium crystal shapes of a topological insulator (TI), a topological crystalline insulator (TCI) protected by mirror symmetry, and a second-order topological insulator (SOTI) protected by inversion symmetry. By adding magnetic fields to the three-dimensional TI, we can realize the mirror-symmetry-protected TCI and the inversion-symmet...
![Z-type and Z2\documentclass[12pt]{minimal} \usepackage{amsmath}...](publication/371376101/figure/fig3/AS:11431281166215007@1686193397241/Z-type-and-Z2documentclass12ptminimal-usepackageamsmath-usepackagewasysym_Q320.jpg)
![Weyl phonons in boron with I4132\documentclass[12pt]{minimal}...](publication/371376101/figure/fig4/AS:11431281166175621@1686193397450/Weyl-phonons-in-boron-with-I4132documentclass12ptminimal-usepackageamsmath_Q320.jpg)
Weyl points, carrying a Z-type monopole charge [Formula: see text], have bulk-surface correspondence (BSC) associated with helical surface states (HSSs). When |[Formula: see text]| [Formula: see text], multi-HSSs can appear in a parallel manner. However, when a pair of Weyl points carrying [Formula: see text] [Formula: see text] meet, a Dirac point...
Chiral phonons have an angular momentum which represents the microscopic local rotation of atoms in crystals. In this theoretical investigation, we establish a spin-wave model in a ferromagnet with exchange and Dzyaloshinskii-Moriya interactions on a two-dimensional kagome lattice. We then introduce chiral phonons, which modulate spin-spin interact...
In topological phase transitions involving a change in topological invariants such as the Chern number and the $\mathbb{Z}_2$ topological invariant, the gap closes, and the electric polarization becomes undefined at the transition. In this paper, we show that the jump of polarization across such topological phase transitions in two dimensions is de...
We study equilibrium crystal shapes of a topological insulator (TI), a topological crystalline insulator (TCI) protected by mirror symmetry, and a second-order topological insulator (SOTI) protected by inversion symmetry. By adding magnetic fields to the three-dimensional TI, we can realize the mirror-symmetry-protected TCI and the inversion-symmet...
We construct a general theory of Z2 topological phase transitions in two-dimensional systems with time-reversal symmetry. We investigate the possibilities of Z2 topological phase transitions at band inversions at all high-symmetry points in k space in all 80 layer groups. We exclude the layer groups with inversion symmetry because the Z2 topologica...
The electric polarization as a bulk quantity is described by the modern theory of polarization in insulating systems and cannot be defined in conducting systems. Upon a gradual change of a parameter in the system, the polarization always varies smoothly as long as the gap remains open. In this paper, we focus on the two-dimensional Weyl semimetal,...
The properties of systems with exact n-fold screw symmetry (n=2,3,4,6) have been well-studied because they can be understood in terms of space groups. On the other hand, the existence of materials with approximate screw symmetries, such as sevenfold and tenfold screw symmetries, has been predicted. In this paper, we study the properties of phonons...
Chirality is a manifestation of the asymmetry inherent in nature. It has been defined as the symmetry breaking of the parity of static objects, and the definition was extended to dynamic motion such that true and false chiralities were distinguished. Recently, rotating, yet not propagating, atomic motions were predicted and observed in two-dimensio...
We construct a general theory of $Z_2$ topological phase transitions in two-dimensional systems with time-reversal symmetry. We investigate possibilities of $Z_2$ topological phase transitions at band inversions at all high-symmetry points in $k$-space in all the 80 layer groups. We exclude the layer groups with inversion symmetry, because the $Z_2...
The electric polarization as a bulk quantity is described by the modern theory of polarization in insulating systems and cannot be defined in conducting systems. Upon a gradual change of a parameter in the system, the polarization always varies smoothly as long as the gap remains open. In this paper, we focus on the two-dimensional Weyl semimetal,...
In some Dirac systems with time-reversal (T) and glide (G) symmetries, multihelicoid surface states (MHSSs) appear, as discussed in various systems such as electronic and photonic ones. However, the topological nature and the conditions for the appearance of the MHSSs have not been understood. Here we show that MHSSs result from bulk-surface corres...
Excitons, which are composite boson quasi-particles composed of bound electrons and holes, have many fascinating properties and great potential in practical applications. Though experimental studies on exciton dynamics are well-developed, the ab initio simulation ones still remain vacant until two years ago. Here, we apply the density functional th...
Understanding crystal shapes is a fundamental subject in surface science. It is now well studied how chemical bondings determine crystal shapes via dependence of surface energies on surface orientations. Meanwhile, discoveries of topological materials have led us to a new paradigm in surface science, and one can expect that topological surface stat...
Weyl points, carrying a Z-type monopole charge C, have bulk-surface correspondence (BSC) associated with helical surface states (HSSs). When |C| > 1, multi-HSSs can appear in a parallel manner. However, when a pair of Weyl points carrying C = \pm 1 meet, a Dirac point carrying C = 0 can be obtained and the BSC vanishes. Nonetheless, a recent study...
Recently, a series of two-dimensional nonmagnetic layered materials XSi2Y4 (X=transition metals; Y = pnictogens) having similar crystal structures with transition-metal dichalcogenides (TMDs) were proposed for their potential application value. Like TMDs, we propose that chiral phonon involved valley-selective optical circular dichroism can be also...
Properties of systems with exact n-fold screw symmetry (n=2, 3, 4, 6) have been well studied because they can be understood in terms of space groups. On the other hand, existence of materials with approximate screw symmetries, such as 7-fold and 10-fold screw symmetries, has been predicted. In this paper, we study properties of phonons in crystals...
Recently, a series of two-dimensional (2D) nonmagnetic layered materials XSi2Y4 (X=transition metals; Y=pnictogens) having similar crystal structures with transition-metal dichalcogenides (TMDs) were proposed for their potential application value. Like TMDs, we propose that chiral phonon involved valley-selective optical circular dichroism can be a...
In this paper, we theoretically show that in a helical crystal, a current is induced by chiral phonons representing the microscopic local rotation of atoms. By treating the rotational motion as a perturbation, we calculate the time-dependent current by using the adiabatic Berry phase method. The time average of the current along the helical axis be...
We study magneto-optical (MO) properties of a semi-Dirac system in presence of a perpendicular magnetic field using kernel polynomial method (KPM) based on the Keldysh formalism in few hundreds of the terahertz (THz) frequency regime. For the semi-Dirac case, the band structure demonstrates a linear (Dirac-like) dispersion along the y direction, wh...
In this study, we theoretically show that in a helical crystal, a current is induced by chiral phonons representing the microscopic local rotation of atoms. By treating the rotational motion as a perturbation, we calculate the time-dependent current by using the adiabatic Berry phase method. The time average of the current along the helical axis be...
Understanding crystal shapes is a fundamental subject in surface science. It is now well studied how chemical bondings determine crystal shapes via dependence of surface energies on surface orientations. Meanwhile, discoveries of topological materials have led us to a new paradigm in surface science, and one can expect that topological surface stat...
We give a bulk-hinge correspondence for higher-order topological phases protected by rotoinversion C4I symmetry in magnetic systems. Our approach allows us to show the emergence of the chiral hinge modes only from the information of the C4I eigenvalues at the high-symmetry points in the Brillouin zone. In addition, based on the bulk-hinge correspon...
Chiral phonons have nonzero polarization and can be observed only via a selective coupling with valley electrons and circularly polarized photons. In such process, a new physical quantity, i.e., pseudoangular momentum (PAM), is required to meet the selection rule. However, phonon PAM was only studied in symmorphic systems with quantized integer val...
Symmetries are always entangled with topology. In systems with time-reversal (T) and glide(G) symmetries, Z2 monopole charge Q defined by the time-reversal-glide symmetry {\Theta} = TG for Dirac points is considered to accompany double/quad-helicoid surface states (DHSSs/QHSSs) in previous studies. Here we study the topology of Q for Z2 Dirac point...
For topological materials with a coexistence of Weyl nodes and nodal rings, their unique surface-state configuration and connection still need to be studied and discussed. In this Letter, we predict a ferromagnetic (FM) material, Cs2MoCl6, with a coexistence of Weyl and nodal ring fermions in its spinful FM electronic band structure, which is unusu...
Degenerate points/lines in the band structures of crystals have become a staple of the growing number of topological materials. The bulk-boundary correspondence provides a relation between bulk topology and surface states. While line degeneracies of bulk excitations have been extensively characterised, line degeneracies of surface states are not we...
We give a bulk-hinge correspondence for higher-order topological phases protected by rotoinversion $C_{4}\mathcal{I}$ symmetry in magnetic systems. Our approach allows us to show the emergence of the chiral hinge modes only from the information of the $C_{4}\mathcal{I}$ eigenvalues at the high-symmetry points in the Brillouin zone. In addition, bas...
Degenerate points/lines in the bulk band structures of crystals have become a staple of the growing number of topological materials. The bulk-boundary correspondence provides a relation between bulk topology and surface states. While line degeneracies of bulk excitations have been extensively characterized, line degeneracies of surface states are n...
We study magneto-optical (MO) properties of a semi-Dirac nanoribbon in presence of a perpendicular magnetic field using Kernel Polynomial Method (KPM) based on the Keldysh formalism in the experimental (terahertz frequency) regime. For comparison, we have also included results for the Dirac systems as well, so that the interplay of the band structu...
Chirality is a manifestation of the asymmetry inherent in nature. It has been defined as the symmetry breaking of the parity of static objects, and the definition was extended to dynamic motion such that true and false chiralities were distinguished. Recently, rotating, yet not propagating, atomic motions were predicted and observed in two-dimensio...
We investigate transverse charge and spin dc supercurrents in a ferromagnet coupled to a superconductor where the ferromagnet has inhomogeneous magnetic structure. These transverse supercurrents arise from nontrivial structure of the magnetization. The magnetic structure manifested in the transverse charge supercurrent is essentially different from...
The kinetic magnetoelectric effect is an orbital analog of the Edelstein effect and offers an additional degree of freedom to control magnetization via the charge current. Here we theoretically propose a gigantic kinetic magnetoelectric effect in topological insulators and interpret the results in terms of topological surface currents. We construct...
Gapped systems with glide symmetry can be characterized by a Z2 topological invariant. We study the magnetic photonic crystal with a gap between the second and third lowest bands, which is characterized by the nontrivial glide-Z2 topological invariant that can be determined by symmetry-based indicators. We show that under the space group No. $\bold...
For topological materials with coexistence of Weyl nodes and nodal rings, the surface-state configuration and connection are unique yet have never been studied and discussed before. In this paper, we predict a ferromagnetic (FM) material, Cs2MoCl6, with coexistence of Weyl and nodering fermions in its spinful FM electronic band structure, which is...
Gapped systems with glide symmetry can be characterized by a Z_2 topological invariant. We study the magnetic photonic crystal with a gap between the second and third lowest bands, which is characterized by the nontrivial glide-Z_2 topological invariant that can be determined by symmetry-based indicators. We show that under the space group No. 230...
Though symmetry-based indicators formulae are powerful in diagnosing topological states with a gapped band structure at/between any high-symmetry points, it fails in diagnosing topological degeneracies when the compatibility condition is violated. In such cases, we can only obtain information of whether there is a band degeneracy at some high-symme...
Though symmetry-based indicators formulae are powerful in diagnosing topological states with a gapped band structure at/between any high-symmetry points, it fails in diagnosing topological degeneracies when the compatibility condition is violated. In such cases, we can only obtain information of whether there is a band degeneracy at some high-symme...
Chiral phonons are the ones with nonzero polarization and can be observed only via a selective coupling with valley electrons and circularly polarized photons. In such process, a new physical quantity, i.e., pseudo-angular momentum (PAM), is required to meet the selection rule. However, phonon PAM was thought to be quantized and can be only defined...
It is known that phonons have angular momentum, and when the time-reversal symmetry (TRS) is broken, the total phonon angular momentum in the whole system becomes nonzero. In this paper, we propose that as an angular momentum of phonons for a crystal without TRS, we need to consider the canonical angular momentum, as opposed to the kinetic angular...
It is known that phonons have angular momentum, and when the time-reversal symmetry (TRS) is broken, the total phonon angular momentum in the whole system becomes nonzero. In this paper, we propose that as an angular momentum of phonons for a crystal without TRS, we need to consider the canonical angular momentum, as opposed to the kinetic angular...
Based on the similarity between the chiral nanotube and the classical solenoid, we study chiral transport along the circumferential direction in a carbon nanotube. We calculate the chiral conductivity, representing a circumferential current induced by an electric field along the nanotube axis for various chiralities of carbon nanotubes. We find tha...
In this paper, we derive a general formula for the quantized fractional corner charge in two-dimensional Cn-symmetric higher-order topological insulators. We assume that the electronic states can be described by the Wannier functions and that the edges are charge neutral, but we do not assume vanishing bulk electric polarization. We expand the scop...
We theoretically propose a gigantic orbital Edelstein effect in topological insulators and interpret the results in terms of topological surface currents. We numerically calculate the orbital Edelstein effect for a model of a three-dimensional Chern insulator as an example. Furthermore, we calculate the orbital Edelstein effect as a surface quantit...
In this paper, we derive a general formula for the quantized fractional corner charge in two-dimensional C_n-symmetric higher-order topological insulators. We assume that the electronic states can be described by the Wannier functions and that the edges are charge-neutral, but we do not assume vanishing bulk electric polarization. We expand the sco...
We study positions of chiral hinge states in higher-order topological insulators (HOTIs) with inversion symmetry. First, we exhaust all possible configurations of the hinge states in the HOTIs in all type-I magnetic space groups with inversion symmetry by studying dependence of the sign of the surface Dirac mass on surface orientations. In particul...
Perovskite oxides exhibit a rich variety of structural phases hosting different physical phenomena that generate multiple technological applications. We find that topological phonons, i.e., nodal rings, nodal lines, and Weyl points, are ubiquitous in oxide perovskites in terms of structures (tetragonal, orthorhombic, and rhombohedral), compounds (B...
We study the topological crystalline insulator phase protected by the glide symmetry, which is characterized by the Z2 topological number. In the present paper, we derive a formula for the Z2 topological invariant protected by glide symmetry in a nonprimitive lattice from that in a primitive lattice. We establish a formula for the glide-Z2 invarian...
Perovskite oxides exhibit a rich variety of structural phases hosting different physical phenomena that generate multiple technological applications. We find that topological phonons-nodal rings, nodal lines, and Weyl points-are ubiquitous in oxide perovskites in terms of structures (tetragonal, orthorhombic, and rhombohedral), compounds (BaTiO3 ,...
In systems with time-reversal symmetry, the orbital magnetization is zero in equilibrium. Recently, it has been proposed that the orbital magnetization can be induced by an electric current in a helical crystal structure in the same manner as that in a classical solenoid. In this paper, we extend this theory and study the current-induced orbital ma...
In some kind of three-dimensional higher-order topological insulators, bulk and surface electronic states are gapped, while there appear gapless hinge states protected by spatial symmetry. Here we show by ab initio calculations that the La apatite electride is a higher-order topological crystalline insulator. It is a one-dimensional electride, in w...
Unlike conventional Weyl nodes, unconventional ones carry a quantized monopole charge C>1, and their existence needs the protection of crystalline symmetries in addition to translation symmetry. There have been many studies on unconventional Weyl nodes, yet we have so far missed one, which is the twofold Weyl node with C=4. In this paper, we study...
In this paper, we introduce Berry curvature, topological Chern number, and topological chiral edge mode that emerge from a hybridization between magnon and electromagnetic wave in a ferromagnet insulator. By focusing on the energy conservation, we first reformulate the Landau-Lifshitz-Maxwell equation into a Hermitian eigenvalue equation. From the...
Here we comprehensively investigate Landau levels, Hofstadter's butterfly, and transport properties of a semi-Dirac nanoribbon in a perpendicular magnetic field using a recently developed real-space implementation of the Kubo formula based on the kernel polynomial method. A Dirac ribbon is considered to compare and contrast our results for a semi-D...
![FIG. 2. Dielectric response of the SSH model (model I). [(a) and (b)]...](publication/343563893/figure/fig1/AS:932414855585792@1599316433079/Dielectric-response-of-the-SSH-model-model-I-a-and-b-Schematic-picture-of-the_Q320.jpg)
In various topological phases, nontrivial states appear at the boundaries of the system. In this paper, we investigate anomalous dielectric response caused by such states caused by the π Zak phase. First, by using the one-dimensional Su-Schrieffer-Heeger model, we show that, when the system is insulating and the Zak phase is π, the polarization sud...
Theories of symmetry-based indicators and topological quantum chemistry, while powerful in diagnosing gapped topological materials, cannot be directly applied to diagnosing band degeneracies at/between high-symmetry momenta due to the violation of the compatibility conditions. However, the only information that compatibility condition can tell us i...
In various topological phases, nontrivial states appear at the boundaries of the system. In this paper, we investigate anomalous dielectric response caused by such states caused by the pi Zak phase. First, by using the one-dimensional Su-Schrieffer-Heeger model, we show that, when the system is insulating and the Zak phase is pi, the polarization s...
We study the topological crystalline insulator phase protected by the glide symmetry, which is characterized by the Z2 topological number. In the present paper, we derive a formula for the Z2 topological invariant protected by glide symmetry in a nonprimitive lattice, from that in a primitive lattice. We establish a formula for the glide-Z2 invaria...
We theoretically investigate the microscopic mechanism of conversion between the electron spin and the microscopic local rotation of atoms in crystals. In phonon modes with angular momenta, the atoms microscopically rotate around their equilibrium positions in crystals. In a simple toy model with phonons, we calculate the spin expectation value by...
Here we comprehensively investigate Landau levels, Hofstadter butterfly and transport properties of a semi-Dirac nanoribbon in a perpendicular magnetic field using a recently developed real-space implementation of the Kubo formula based on Kernel Polynomial Method. A Dirac ribbon is considered to compare and contrast our results for a semi-Dirac sy...
In crystals with time-reversal symmetry but without inversion symmetry, the phonon angular momentum can be generated by the temperature gradient, and it is called phonon thermal Edelstein effect. On the other hand, when both symmetries are broken and their product is conserved, the phonon angular momentum for any phonon modes at any wave vectors va...
We theoretically investigate the microscopic mechanism of conversion between the electron spin and the microscopic local rotation of atoms in crystals. In phonon modes with angular momenta, the atoms microscopically rotate around their equilibrium positions in crystals. In a simple toy model with phonons, we calculate the spin expectation value by...
We propose inversion-asymmetric hinge states, which are different from the conventional inversion-symmetric hinge states in the higher-order topological insulators (HOTIs) realized in layered antiferromagnets (AFMs). First, we find all possible configurations of the hinge states in the HOTIs in all type-I magnetic space groups with inversion symmet...
In this paper, we introduce Berry curvature, topological Chern number and topological chiral edge mode, that emerge from a hybridization between magnon and electromagnetic wave in a ferromagnet insulator. By focusing on the energy conservation, we first reformulate the Landau-Lifshitz-Maxwell equation into a Hermitian eigenvalue equation. From the...
In crystals with time-reversal symmetry but without inversion symmetry, the phonon angular momentum can be generated by the temperature gradient, and it is called the phonon thermal Edelstein effect. On the other hand, when both symmetries are broken and their product is conserved, the phonon angular momentum for any phonon modes at any wave vector...
In higher-order topological insulators, bulk and surface electronic states are gapped, while there appear gapless hinge states protected by spatial symmetry. Here we show by ab initio calculations that the La apatite electride is a higher-order topological crystalline insulator. It is a one-dimensional electride, in which the one-dimensional inters...
Conventional Weyl nodes are twofold band crossings that carry a unit monopole charge, which can exist in condensed matter systems with the protection of translation symmetry. Unconventional Weyl nodes are twofold/multifold band crossings carrying a quantized monopole charge larger than one, and their existence needs the protection of additional cry...
In recent years, second-order topological insulators have been proposed as a new class of topological insulators. Second-order topological insulators are materials with gapped bulk and surfaces, but with topologically protected gapless states at the intersection of two surfaces. These gapless states are called hinge states. In this paper, we give a...
We show that a slab of a three-dimensional inversion-symmetric higher-order topological insulator (HOTI) in class A is a 2D Chern insulator, and that in class AII is a 2D Z2 topological insulator. We prove it by considering a process of cutting the three-dimensional inversion-symmetric HOTI along a plane and study the spectral flow in the cutting p...
The Berry curvature for magnons in ferromagnetic films gives rise to new phenomena such as thermal Hall effect and a shift of a magnon wave packet at the reflection at the edge of the magnetic film. In this paper, we calculate the Berry curvature of magnetoelastic waves in ferromagnets. In order to calculate the Berry curvature, we first formulate...
Significance
Topological Weyl semimetal, which is a gapless semimetallic phase protected by symmetry, generally appears by band gap closing in noncentrosymmetric semiconductors. So far, there have been only a limited number of reports of such a topological phase transition so that many basic aspects have remained unexplored. Here we report potentia...
Theories of symmetry-based indicators and topological quantum chemistry, while powerful in diagnosing gapped topological materials, cannot be directly applied to diagnosing band degeneracies at high-symmetry momenta due to the violation of the compatibility conditions. Here we design a recursive protocol that utilizes indicators of maximal subgroup...
We show that a slab of a three-dimensional inversion-symmetric higher-order topological insulator (HOTI) in class A is a 2D Chern insulator, and that in class AII is a 2D $Z_2$ topological insulator. We prove it by considering a process of cutting the three-dimensional inversion-symmetric HOTI along a plane, and study the spectral flow in the cutti...
In recent years, second-order topological insulators have been proposed as a new class of topological insulators. Second-order topological insulators are materials with gapped bulk and surfaces, but with topologically protected gapless states at the intersection of two surfaces. These gapless states are called hinge states. In this paper, we give a...
It is known that three-dimensional magnetic systems with glide symmetry can be characterized by a Z2 topological invariant together with the Chern number associated with the normal vector of the glide plane, and they are expressed in terms of integrals of the Berry curvature. In the present paper, we study the fate of this topological invariant whe...
In the search for stable topological semimetals with clean band profiles, we have screened all the 3d metal-intercalated transition metal dichalcogenides (3dI-TMDCs) by performing hybrid-functional-based ab initio calculations. Two classes of topological materials featuring 12 Weyl points (WPs) in the kz=0 plane (without spin-orbit interactions) ar...
Recent progress in understanding the electronic band topology and emergent topological properties encourage us to reconsider the band structure of well-known materials including elemental substances. Controlling such a band topology by external field is of particular interest from both fundamental and technological view point. Here we report the pr...
In the search for stable topological semimetals with clean band profiles, we have screened all the 3$d$ metal-intercalated transition-metal dichalcogenides (3dI-TMDCs) by performing hybrid-functional-based ab initio calculations. Two classes of topological materials featuring twelve Weyl nodes in the $k_z=0$ plane (without spin-orbit interactions)...
It is known that three-dimensional systems with glide symmetry can be characterized by a $Z_2$ topological invariant, and it is expressed in terms of integrals of the Berry curvature. In the present paper, we study the fate of this topological invariant when the inversion symmetry is added. There are two ways to add the inversion symmetry, leading...
Phonon modes in crystals can have angular momenta in general. It nevertheless cancels in equilibrium when the time-reversal symmetry is preserved. In this Letter, we show that when a temperature gradient is applied and heat current flows in the crystal, the phonon distribution becomes off equilibrium, and a finite angular momentum is generated by t...
Phonon modes in crystals can have angular momenta in general. It nevertheless cancels in equilibrium when the time-reversal symmetry is preserved. In this paper we show that when a temperature gradient is applied and heat current flows in the crystal, the phonon distribution becomes off-equilibrium, and a finite angular momentum is generated by the...
Recent discoveries of topological phases realized in electronic states in solids have revealed an important role of topology, which ubiquitously appears in various materials in nature. Many well-known materials have turned out to be topological materials, and this new viewpoint of topology has opened a new horizon in material science. In this paper...
Recent discoveries of topological phases realized in electronic states in solids have revealed an important role of topology, which ubiquitously appears in various materials in nature. Many well-known materials have turned out to be topological materials, and this new viewpoint of topology has opened a new horizon in material science. In this paper...
Dipole interaction between spins induces a nonzero Berry curvature for magnons in ferromagnetic films, and this Berry curvature gives rise to various phenomena such as thermal Hall effect. In previous works, it has been calculated for magnons with dipole interaction only. Nevertheless, magnon dispersion is largely affected by the exchange interacti...
Nodal-line semimetals, one of the topological semimetals, have degeneracy along nodal lines where the band gap is closed. In many cases, the nodal lines appear accidentally, and in such cases it is impossible to determine whether the nodal lines appear or not, only from the crystal symmetry and the electron filling. In this paper, for spinless syst...
In topological semimetals such as Weyl, Dirac and nodal-line semimetals, the band gap closes at points or along lines in k space, which are not located at high-symmetry positions in the Brillouin zone. Therefore, it is not straightforward to find these topological semimetals by ab initio calculations, because the band structure is usually calculate...
Spins can act as mediators to interconvert electricity, light, sound, vibration and heat. Here, we give an overview of the recent advances in different sub-disciplines of spintronics that can be associated with the developing field of spin conversion, and discuss future prospects.
We study a general phase transition between spinless topological nodal-line semimetal and Weyl semimetal phases. We classify topological nodal lines into two types based on their positions and shapes, and their phase transitions depends on their types. We show that the topological nodal-line semimetal becomes the Weyl semimetal by breaking time-rev...
We theoretically study current-induced orbital magnetization in a chiral crystal. This phenomenon is an orbital version of the Edelstein effect. We propose an analogy between the current-induced orbital magnetization and an Amp\`ere field in a solenoid in classical electrodynamics. In order to quantify this effect, we define a dimensionless paramet...
Nodal-line semimetals, one of the topological semimetals, has line-shaped degeneracy (nodal line) where the gap is closed. Usually, nodal lines appear accidentally, and it is considered to be impossible to determine whether nodal lines appear from crystal symmetry alone. In this paper, we show that for spinless systems with certain space groups, pr...
We review recent developments in theories and experiments on the magnon Hall effect. We derive the thermal Hall conductivity of magnons in terms of the Berry curvature of magnonic bands. In addition to the Dzyaloshinskii-Moriya interaction, we show that the dipolar interaction can make the Berry curvature nonzero. We mainly discuss theoretical aspe...
In nodal-line semimetals, the gaps close along loops in 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 phas...
Supplementary Figures, Supplementary Notes and Supplementary References
Citations
... We formulate the local polarization by decomposing the Berry phase in terms of semilocal hybrid polarizations (SHPs), while also making a connection to Chern topology [44]. In particular, we consider the evolution of the local polarization in a crystal superlattice and elucidate the correspondences between the local polarization textures, local polarization jumps [45], and the changes of the bulk ...
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... Similar results have been observed and reported by Liu et al. 68 in Figures 4G-4I. ll Charge-four WP phonons Several separate groups [75][76][77][78][79][80] have recently proposed a novel type of WP in spinless systems with a maximum topological charge of jCj = 4, named the C-4 WP, which has a cubic dispersion along the [111] direction and a quadratic dispersion in any other direction. ...
Reference: Topological nodal-point phononic systems
... As we show, our formulations do not suffer from such kind of ambiguities, and furthermore allow to explicitly study TPTs which may occur in crystal supercells. While the links between topological phase transitions and associated changes in polarization captured by geometric Berry phases according to the modern theory of polarization [41,44] have been established in simple systems without supercells [45,56], we report an analogous effect in crystal supercells, e.g. provided by polar heterostructures supporting topologically non-trivial polarization textures in real space. ...
... In first-order Raman scattering, recent studies have revealed robust selection rules for circularly polarized Raman experiments on 2D transition metal dichalcogenides (TMDs) with 3-fold rotational symmetry [3,12]. Zhang et al. [2] and Kyosuke Ishito et al. [13] found that the PAM in systems with 3-fold rotational symmetry, along with the angular momentum (AM) of photons, satisfies AM conservation in circularly polarized Raman scattering. ...
Reference: Phonon Pseudoangular Momentum in α-MoO3
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... Since the monopole charge C is zero for Dirac points, the BSC are also expected to vanish. However, a recent study gives a strict proof that a Z 2 -type monopole charge Q can be defined for Dirac points when time-reversal ( T)-glide ( G ) symmetry ( ˜ =TG) is present 27 , and they are associated with anti-parallel HSSs like double/quad-helical surface states (DHSSs/ QHSSs) 21,[28][29][30][31][32][33] . Among diverse topological semimetals, Weyl and Dirac semimetals draw the most attention, not only due to their exotic transport properties from the bulk bands, but also due to their topological surface states with fascinating helical shapes. ...
... However, we can assume that these spectral signals do not correspond to the Mo-Si-N vibrational mode that exists at the Γ point of the Brillouin zone [1], because the difference in wavenumbers is sufficiently large. As can be seen in Figure 4 and 10, the activation of phonons from K point of the Brillouin zone can give rise of a spectral line, but the correct determination of the atomic displacement within this vibrational mode is a challenging task, probably including exciton-phonon dynamics investigations [73,74]. Figure 10. ...
... In addition, by comparing the changes in the shapes of the TI to that of the trivial insulator by adding magnetic fields, we show that the topological surface states affect the equilibrium crystal shape, and this is unique to the topological phases. We note that in our previous work [94], equilibrium crystal shapes of TCIs protected by glide symmetry are studied. The calculation method in the present paper is partially parallel to the previous work, but we will see that the results are quite different because the symmetries protecting the topological phases are different. ...
... Similar to CDW phase where the sliding mode is a supercurrent of charge as a result of spontaneous breaking of translation symmetry, the chiral sliding mode in the incommensurate self-twisting wave phase is a supercurrent of angular momentum instead. Also, the new ground state will be a perfect chiral crystal, where the chiral phonon is discussed in recent works in the context of chiral [24], nonsymmorphic [25], and twodimensional [26] materials. ...
Reference: Transverse Peierls Transition
... Among the possible symmetries, mirror symmetries play a special role: When they are broken, the lattice ions can display circular motion with finite angular momentum (2). These modes are called chiral phonons, and recently, they have been the subject of intense research in a large variety of materials and applications (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16). In magnetic fields, chiral phonons preferably absorb polarized light of a given handedness, resulting in magnetic circular dichroism (MCD) (17)(18)(19). ...
... As E increases (assuming E > 0), the band gap in X valley decreases while that in X ′ valley becomes lager. At a critical value E = E c , the conduction and valence bands touch at X valley, forming a semi-Driac point [49][50][51]. Interestingly, the semi-Driac point exhibites a linear dispersion along k x direction but a quadratic dispersion along k y direction [see Fig. 5(a)], and can be considered as a critical point where two conventional Dirac points merge together. ...
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