arXiv subjects

Hiroto Tanaka

Publications and source records attributed to Hiroto Tanaka.

7 recordsLinked to original sources

arXiv2026

Large spontaneous Hall effect arising from collinear antiferromagnetism in Ce$_2$PtGe$_6$

The spontaneous Hall effect, corresponding to a zero-field anomalous Hall effect (AHE), is induced by symmetry breaking associated with ferromagnetism. Studies in recent years, however, have revealed that antiferromagnetic (AFM) states characterized by magnetic point groups that allow ferromagnetism can also break the relevant symmetries and induce AHE without a large net magnetization. Here, we report that the AFM system Ce$_2$PtGe$_6$ exhibits a pronounced spontaneous Hall effect. Single-crystal neutron scattering experiments demonstrate that Ce$_2$PtGe$_6$ exhibits a collinear AFM structure with a propagation vector $q=0$. The small net magnetization of $\sim 10^{-3}$ $μ_B$/Ce indicates that the observed AHE arises from symmetry breaking inherent to its AFM structure. The anomalous Hall conductivity (AHC) reaches $300$ $Ω^{-1}$cm$^{-1}$, which exceeds the intrinsic AHC of related compounds such as Ce$_2$CuGe$_6$ and Ce$_2$PdGe$_6$. This large AHC, most likely attributed to the large spin-orbit coupling of the Pt atoms, provides a platform for understanding the interplay between the Berry curvatures and localized $f$-moments with an AFM configuration.

cond-mat.str-el↗

arXiv2025

Surface acoustic wave-driven valley current generation in intervalley coherent states

Recent experiments have reported valley-gauge-symmetry-broken phases, identified as intervalley coherent (IVC) states. Exploration of anomalous responses, particularly those analogous to superconductivity, has become an urgent theoretical issue. In this study, we show that the IVC order gives rise to anomalous valley-current generation driven by surface acoustic waves (SAWs). The anomalous valley current exhibits a characteristic power-law dependence for low-frequency SAWs. Furthermore, we demonstrate by numerical analysis that the IVC order significantly enhances valley-current generation in rhombohedral graphene. These results open a pathway toward exploring exotic phenomena emerging from valley-gauge-symmetry breaking, in close analogy with gauge-symmetry breaking in superconductors.

cond-mat.mes-hall↗

arXiv2025

Vertex correction for the linear and nonlinear optical responses in superconductors: multiband effect and topological superconductivity

Intensive research has revealed intriguing optical responses in topological materials. This paper focuses on the optical responses in $s$-wave superconductors with a Rashba spin-orbit coupling and a magnetic field, one of the platforms of topological superconductivity. On the one hand, to satisfy some conservation laws in superconducting responses, it is essential to take into account collective excitation modes. On the other hand, the optical response is a promising phenomenon for detecting hidden collective modes in superconductors. In this paper, we investigate the effect of collective excitation modes on the linear and second-order optical responses based on the self-consistent response approximation, which is formulated using the Kadanoff-Baym method. Our main results reveal that the Higgs mode enhances the optical responses when the Fermi level is close to the Dirac point. The enhancement is due to the multiband effects characterized by interband pairing. We also demonstrate the sign reversal of the photocurrent conductivity around the topological transition with increasing the Zeeman field. This finding supports the prediction in our previous work without considering collective excitation modes [H. Tanaka, et al., Phys. Rev. B 110, 014520 (2024)]. The sign reversal phenomenon is attributed to the magnetic injection current modified by the Higgs mode, and is proposed for a bulk probe of topological superconductors. We also discuss the interplay of quantum geometry and collective modes.

cond-mat.supr-con↗

arXiv2024

Large Anomalous Hall Conductivity Derived from an $f$-Electron Collinear Antiferromagnetic Structure

Appropriate symmetry breaking generates an anomalous Hall (AH) effect, even in antiferromagnetic (AFM) materials. Itinerant magnets with $d$ electrons are typical examples that show a significant response. By contrast, the process by which a response emerges from $f$-electron AFM structures remains unclear. In this study, we show that an AFM material, Ce$_2$CuGe$_6$, yields a large AH conductivity (AHC) of $550$ $Ω^{-1}$cm$^{-1}$, which exceeds the values previously reported in $d$-electron AFM materials. Observed features, including the scaling relation against electrical conductivity, suggest that this AH transport is induced cooperatively by both intrinsic and extrinsic mechanisms derived from the AFM structure.

cond-mat.str-el↗

arXiv2024

Nonlinear optical responses in superconductors under magnetic fields: quantum geometry and topological superconductivity

Noncentrosymmetric superconductors offer fascinating phenomena of quantum transport and optics such as nonreciprocal and nonlinear responses. Time-reversal symmetry breaking often plays an essential role in the emergence and enhancement of nonreciprocal transport. In this paper, we show the nonreciprocal optical responses in noncentrosymmetric superconductors arising from time-reversal symmetry breaking by demonstrating them in $s$-wave superconductors with a Rashba spin-orbit coupling and a magnetic field. Numerical results reveal the superconductivity-induced bulk photocurrent and second harmonic generation, which are forbidden at the zero magnetic field. We discuss the properties and mechanisms of the superconducting nonlinear responses emerging under the magnetic field. In particular, we investigate the magnetic-field dependence of the photocurrent conductivity and clarify the essential ingredients which give a contribution unique to superconductors under the magnetic field. This contribution is dominant in the low carrier density regime although the corresponding joint density of states is tiny. We attribute the enhancement to the quantum geometry. Moreover, the nonlinear conductivity shows peculiar sign reversal at the transition to the topological superconducting state. We propose a bulk probe of topological transition and quantum geometry in superconductors.

cond-mat.supr-con↗

arXiv2022

Nonlinear optical responses in noncentrosymmetric superconductors

The unique nonreciprocal responses of superconductors, which stem from the Cooper pairs' quantum condensation, have been attracting attention. Recently, theories of the second-order nonlinear response in noncentrosymmetric superconductors were formulated based on the Bogoliubov-de Gennes theory. In this paper, we study the mechanism and condition for second-order optical responses of time-reversal symmetric superconductors. The numerical results show the characteristic photocurrent and second harmonic generation in the superconducting state. However, the superconductivity-induced nonlinear optical responses disappear under some conditions on pair potential. We show that the coexistence of intraband and interband pairing is necessary for the second-order superconducting optical responses. In addition, the superconducting Berry curvature factor, which is related to a component of Berry curvature in the superconducting state, is essential for the nonlinear responses. Thus, we derived the microscopic conditions where the superconducting nonlinear response appears.

cond-mat.supr-con↗

arXiv2016

Added costs of insect-scale flapping flight in unsteady airflows

The aerial environment in the operating domain of small-scale natural and artificial flapping wing fliers is highly complex, unsteady and generally turbulent. Considering flapping flight in an unsteady wind environment with a periodically varying lateral velocity component, we show that body rotations experienced by flapping wing fliers result in the reorientation of the aerodynamic force vector that can render a substantial cumulative deficit in the vertical force. We derive quantitative estimates of the body roll amplitude and the related energetic requirements to maintain the weight support in free flight under such conditions. We conduct force measurements of a miniature hummingbird-inspired robotic flapper and numerical simulations of a bumblebee. In both cases, we demonstrate the loss of weight support due to body roll rotations. Using semi-restrained flight measurements, we demonstrate the increased power requirements to maintain altitude in unsteady winds, achieved by increasing the flapping frequency. Flapping fliers may increase their flapping frequency as well as the stroke amplitude to produce the required increase in aerodynamic force, both of these two types of compensatory control requiring additional energetic cost. We analyze the existing data from experiments on animals flying in von Kármán streets and find reasonable agreement with the proposed theoretical model.

physics.flu-dyn↗