arXiv subjects

Shuichi Murakami

Publications and source records attributed to Shuichi Murakami.

At least 37 records · Page 2Linked to original sources

arXiv2025

Theory of spin magnetization driven by chiral phonons

We construct a general theory of spin magnetization driven by chiral phonons under an adiabatic process, in which atoms rotate around their equilibrium positions with a low phonon frequency. Here the spin magnetization originates from the modulated electronic states with spin-orbital coupling by atomic rotations. Under the adiabatic approximation, the time-dependent spin magnetization can be calculated by a Berry-phase method. In this paper, we focus on its time average, which is evaluated by assuming that the phonon displacement is small. As a result, the time average of the spin magnetization is concisely formulated in the form of the Berry curvature defined in the phonon-displacement space as an intrinsic property of atomic rotations. Our formula for spin magnetization reflects the chiral nature of phonons, and is convenient for $ab$ $initio$ calculations.

cond-mat.mes-hall↗

arXiv2025

Double-helicoid surface states in Dirac semimetals protected by glide-time-reversal symmetry

Recently, some $Z_2$ monopole charges were defined for Dirac semimetals with $\mathcal{GT}$ symmetry ($\mathcal{G}$: glide, $\mathcal{T}$: time-reversal) in previous works, and the charges are believed to lead to double-helicoid surface states. However, no proof of the bulk-surface correspondence is given there. In this paper, we point out one of the $Z_2$ charges in the previous works is gauge-dependent, and newly define another $Z_2$ charge. Using this new $Z_2$ charge, we give a proof of the bulk-surface correspondence. We also compare the new $Z_2$ charge with the $Z_2$ invariant for $\mathcal{G}$-protected topological crystalline insulators, and the second Stiefel-Whitney number for $\mathcal{PT}$-protected nodal line semimetals.

cond-mat.mtrl-sci↗

arXiv2025

Disorder-Induced Slow Relaxation of Phonon Polarization

The role of the polarization degree of freedom in lattice dynamics in solids has been underlined recently. We theoretically discover a relaxation mechanism for both linear and circular polarizations of acoustic phonons. In the absence of scattering, the polarization exhibits oscillatory behavior. This behavior leads to a counterintuitive result: unlike linear momentum, more frequent scattering events cause slower polarization relaxation due to motional narrowing. We validate this mechanism using the quantum kinetic equation. We derive the relaxation rates of polarizations analytically for isotropic elastic bodies and numerically for a cubic crystal. Remarkably, we reveal that linear polarizations relax more slowly than circular ones. Our findings provide a pathway to extend the lifetime of phonon angular momentum.

cond-mat.mes-hall↗

arXiv2024

Diverging shift current responses in the gapless limit of two-dimensional systems

The shift current responses of two-dimensional systems in the gapless limit are investigated. As the energy gap becomes smaller, the interband transition probability becomes larger at the band edges. We found the divergence of the shift current response in a manner proportional to the inverse of the gap size when the system becomes a two-dimensional Weyl semimetal in the gapless limit. This behavior is different from that for electric polarization, which has a jump across the gapless case. It means that the quantitative relationship between electric polarizations and shift currents is broken in this gapless limit.

cond-mat.mes-hall↗

arXiv2024

Symmetry-driven anisotropic coupling effect in antiferromagnetic topological insulator: Mechanism for high-Chern-number quantum anomalous Hall state

Antiferromagnetic (AFM) topological insulators (TIs), which host magnetically gapped Dirac-cone surface states and exhibit many exotic physical phenomena, have attracted great attention. Here, we find that the coupled surface states can be intertwined to give birth to a set of $2n$ unique new Dirac cones, dubbed intertwined Dirac cones, through the anisotropic coupling enforced by crystalline $n$-fold ($n=2, 3, 4, 6$) rotation symmetry $C_{nz}$ in the presence of a $PT$-symmetry breaking potential, for example, an electric field. Interestingly, we also find that the warping effect further drives the intertwined Dirac-cone state into a quantum anomalous Hall phase with a high Chern number ($C=n$). Then, based on first-principles calculations, we have explicitly demonstrated six intertwined Dirac cones and a Chern insulating phase with a high Chern number ($C=3$) in MnBi$_2$Te$_4$$/$(Bi$_2$Te$_3$)$_{\mathrm{m}}/$MnBi$_2$Te$_4$ heterostructures, as well as the $C=2$ and $C=4$ phases in HgS and $α$-Ag$_2$Te films, respectively. This work discovers the intertwined Dirac-cone state in AFM TI thin films, which reveals a mechanism for designing the quantum anomalous Hall state with a high Chern number and also paves a way for studying highly tunable high-Chen-number flat bands of twistronics.

cond-mat.mes-hall↗

arXiv2023

General corner charge formulas in various tetrahedral and cubic space groups

In some insulators, corner charges are fractionally quantized, due to the topological invariant called a filling anomaly. The previous theories of fractional corner charges have been mostly limited to two-dimensional systems. In three dimensions, only limited cases have been studied. In this study, we derive formulas for the filling anomaly and the corner charge in various crystals with all the tetrahedral and cubic space groups. We discuss that the quantized corner charge requires the crystal shapes to be vertex-transitive polyhedra. We show that the formula of the filling anomaly is universally given by the difference between electronic and ionic charges at the Wyckoff position 1a. The fractional corner charges appear by equally distributing the filling anomaly to all the corners of the crystal. We also derive the k-space formulas for the fractional corner charge. In some cases, the corner charge is not determined solely from the irreps at high-symmetry k-points. In such cases, we introduce a new Z2 topological invariant to determine the corner charge.

cond-mat.mtrl-sci↗

arXiv2023

Conversion of chiral phonons into magnons in ferromagnets and antiferromagnets

Chiral phonons with atomic rotations converted into electron spins result in a change of spin magnetizations in crystals. In this paper, we investigate a new conversion of chiral phonons into magnons both in ferromagnets and antiferromagnets by spin models with exchange and Dzyaloshinskii-Moriya interactions. The atomic rotations in chiral phonons are treated as slow phonons, which modulate spin-spin interaction and induce time-dependent magnon excitations due to geometric effect within adiabatic approximation. We demonstrate that a non-trivial change of the number of magnons requires breaking of the spin-rotation symmetry around the spin quantization axis. As a result, the clockwise and counterclockwise chiral phonons induce a change of the magnon number with opposite signs, which corresponds to an increasing or decreasing spin magnetizaton due to the chiral nature of the atomic rotations. In particular, in antiferromagnets, the modulation of magnons due to chiral phonons generates a non-zero net magnetization by the proposed effect, which is expect to be observed in experiments.

cond-mat.mes-hall↗

arXiv2023

Polarization Jumps across Topological Phase Transitions in Two-dimensional Systems

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 described in terms of positions and monopole charges of Weyl points in the intermediate Weyl semimetal phase. We find that the jump of polarization is described by the Weyl dipole at $\mathbb{Z}_2$ topological phase transitions and at phase transitions without any change in the value of the Chern number. Meanwhile, when the Chern number changes at the phase transition, the jump is expressed in terms of the relative positions of Weyl points measured from a reference point in the reciprocal space.

cond-mat.mes-hall↗

arXiv2023

Weyl phonons in chiral crystals

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 studied independently and the interpretations of chirality seem to be different in these two concepts, impeding our understanding. Here, we demonstrate that Weyl and chiral phonons are entangled in chiral crystals. Employing a typical chiral crystal of elementary tellurium (Te) as a case study, we expound on the intrinsic relationship between Chern number of Weyl phonons and pseudo-angular momentum (PAM) of chiral phonons. In light of the mutual coupling, we propose Raman scattering as a new technique to demonstrate the existence of Weyl phonons in Te, by detecting the chirality-induced energy splitting between the two constituent chiral phonon branches for Weyl phonons. By using the same experimental approach, we also observe the obstructed phonon surface states for the first time.

cond-mat.mtrl-sci↗

arXiv2023

Effects of first- and second-order topological phases on equilibrium crystal shapes

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-symmetry-protected SOTI. They each have topological boundary states in different positions: the TCI has gapless states on the surfaces that are invariant under the symmetry operation, and the SOTI has gapless states at the intersections between certain surfaces. In this paper, we discuss how these boundary states affect the surface energies and the equilibrium crystal shapes in terms of the calculations of the simple tight-binding model by using the Wulff construction. By comparing the changes in the shapes of the TI to that of the trivial insulator through the process of applying the magnetic fields, we show that the presence/absence of the topological boundary states affects the emergence of the specific facets in a different way from the trivial insulator.

cond-mat.mes-hall↗

arXiv2023

Non-Bloch bands in two-dimensional non-Hermitian systems

The non-Bloch band theory can describe energy bands in a one-dimensional (1D) non-Hermitian system. On the other hand, whether the non-Bloch band theory can be extended to higher-dimensional non-Hermitian systems is nontrivial. In this work, we construct the non-Bloch band theory in two classes of two-dimensional non-Hermitian systems, by reducing the problem to that for a 1D non-Hermitian model. In these classes of systems, we get the generalized Brillouin zone for a complex wavevector and investigate topological properties. In the model of the non-Hermitian Chern insulator, as an example, we show the bulk-edge correspondence between the Chern number defined from the generalized Brillouin zone and the appearance of the edge states.

cond-mat.mes-hall↗

arXiv2023

Optical non-Hermitian skin effect in two-dimensional uniform media

The non-Hermitian skin effect (NHSE) is a novel localization phenomenon in certain non-Hermitian systems with gain and/or loss. Most of previous works study the non-Hermitian skin effect in periodic systems. However, electromagnetic waves often propagate within uniform materials without periodic modulation, and it has not been clear whether the optical NHSE occurs in uniform media such as bulk materials and electromagnetic metamaterials. Here we establish the theory of the optical NHSE in non-Hermitian anisotropic media. We show that the NHSE occurs even in uniform media with appropriate anisotropy and material loss. The localization of non-Hermitian skin modes are completely determined by an effective gauge potential caused by the anisotropy of a dielectric tensor. On the basis of the theory, we propose subwavelength multilayer metamaterials as a novel platform for the optical NHSE. We also propose a new concept of stationarily-excited skin modes whose frequencies are forced to be real in non-Hermitian systems. We find that the NHSE occurs even under the condition that the frequency is forced to be real, which implies that the NHSE we propose is observable under stationary excitation. Our work presents a general theory of the NHSE in homogeneous systems, and pave the way to realize the optical NHSE in bulk materials and metamaterials.

physics.optics↗

arXiv2023

Morse theory study on the evolution of nodal lines in $\mathcal{PT}$-symmetric nodal-line semimetals

A nodal-line semimetal is a topological gapless phase containing one-dimensional degeneracies called nodal lines. The nodal lines are deformed by a continuous change of the system such as pressure and they can even change their topology, but it is not systematically understood what kind of changes of topology of nodal lines are possible. In this paper, we classify the events of topology change of nodal lines by the Morse theory and reveal that only three types of topology changes of nodal lines, i.e., creation, reconnection, and annihilation, are possible in the spinless nodal-line semimetal protected by inversion and time-reversal symmetries. They are characterized by an index having the values 0, 1, and 2 for the above three types in the Morse theory. Moreover, we extend our theory to systems with rotational symmetries and mirror symmetry and disclose the possible events of topology change of nodal lines under each symmetry.

cond-mat.mes-hall↗

arXiv2023

Systematic study for two-dimensional $Z_2$ topological phase transitions at high-symmetry points in all layer groups

We construct a general theory of $Z_2$ topological phase transitions in two-dimensional systems with time-reversal symmetry. We investigate the 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 $Z_2$ topological phase transition is known to be associated with band inversions with an exchange of parities. Among the other layer groups, we find 21 layer groups with insulator-to-insulator transitions with band inversion, and this problem is finally reduced to five point groups $C_3, C_4, C_6, S_4$, and $C_{3h}$. We show how the change of the $Z_2$ topological invariant at a band inversion is entirely determined by the irreps of occupied and unoccupied bands at the high-symmetry point. For example, in the case of $C_3$, we show that the $Z_2$ topological invariants change whenever the band inversion occurs between two Kramers pairs whose $C_3$ eigenvalues are $\{e^{πi / 3}, e^{-πi / 3}\}$ and $\{-1, -1\}$. These results are not included in the theory of symmetry-based indicators or topological quantum chemistry.

cond-mat.mes-hall↗

arXiv2022

Hidden Symmetry Protection and Topology in Surface Maxwell Waves

Since the latter half of the 20th century, the use of metal in optics has become a promising plasmonics field for controlling light at a deep subwavelength scale. Surface plasmon polaritons localized on metal surfaces are crucial in plasmonics. However, despite the long history of plasmonics, the underlying mechanism producing the surface waves is not fully understood. This study unveils the hidden symmetry protection that ensures the existence of degenerated electric zero modes. These zero modes are identified as physical origins of surface plasmon polaritons, and similar zero modes can be directly excited at a temporal boundary. In real space, the zero modes possess vector-field rotation related to surface impedance. Focusing on the surface impedance, we prove the bulk-edge correspondence, which guarantees the existence of surface plasmon polaritons even with nonuniformity. Lastly, we extract the underlying physics in the topological transition between metal and dielectric material using a minimal circuit model with duality. The transition is considered the crossover between electric and magnetic zero modes.

physics.optics↗

arXiv2022

Truly chiral phonons in α-HgS

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-dimensional materials, and they were referred to as "chiral phonons" . A natural development would be the discovery of truly chiral phonons that propagate while rotating in three-dimensional materials. Here, we used circularly polarised Raman scattering and first-principles calculations to identify truly chiral phonons in chiral bulk crystals. This approach enabled us to determine the chirality of a crystal in a non-contact and non-destructive manner. In addition, we demonstrated that the law of the conservation of pseudo-angular momentum holds between circularly polarised photons and chiral phonons. These findings are expected to help develop ways for transferring the pseudo-angular momentum from photons to electron spins via the propagating chiral phonons in opto-phononic-spintronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Physics of Phonons in Systems with Approximate Screw Symmetry

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 with approximate screw symmetries, which will lead to unique and new physical phenomena. In a crystal with an approximate screw symmetry, we propose a method to extract information of pseudoangular momentum of phonons, which is a quantum number characterizing the properties of phonon modes under screw symmetry, based on the fact that the information of the quantum numbers defined under exact screw symmetry partially remains in the eigenvectors of approximate screw symmetric systems. As a preparation, we study a one-dimensional crystal with partially broken translation symmetry to have an enlarged unit cell, and we show how to extract information of a quantum number corresponding to the pseudoangular momentum, by studying a relative phase between neighboring atoms. We also extend this method to systems with an approximate screw symmetry, and discuss properties of the pseudoangular momentum. We apply this method to results of our first-principle calculations on candidate materials with an approximate translational symmetry or with an approximate screw symmetry, and show how this approximate symmetry is reflected in the phonon wavefunctions.

cond-mat.other↗

arXiv2022

Polarization jumps by breaking symmetries of two-dimensional Weyl semimetals

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, which hosts Weyl nodes protected by symmetries, and study the behavior of the polarization when a symmetry-breaking term $M$ is introduced and a gap opens. We show that there can be a jump between $M\to0^+$ and $M\to0^-$ limits. We find that the jump is universally described by the ``Weyl dipole" representing how the Weyl nodes with monopole charges are displaced in the reciprocal space. Our result is applicable to general two-dimensional Weyl semimetals.

cond-mat.mes-hall↗