arXiv subjects

Taisei Yamanaka

Publications and source records attributed to Taisei Yamanaka.

3 recordsLinked to original sources

arXiv2026

Thermodynamic Electric Toroidal Dipole and Intrinsic Longitudinal Spin Transport

Electric toroidal dipoles (ETDs) characterize ferroaxial order, yet their bulk definition in periodic crystals has remained elusive because conventional multipole operators involve the ill-defined position operator. Here we formulate a thermodynamic ETD by coupling a spatially varying electric field to the relativistic spin-induced electric polarization. The resulting expression is gauge invariant and provides a bulk order parameter for ferroaxial phases. We further establish a direct relation between the chemical-potential derivative of the ETD and the intrinsic longitudinal spin conductivity in insulating systems. To demonstrate the formulation, we construct a minimal ferroaxial extension of the Kane--Mele model. The ETD becomes finite exclusively in the ferroaxial phase and is strongly enhanced near a small band gap, accompanied by a sizable longitudinal spin current. Our results establish a thermodynamic theory of ETDs in crystalline solids and identify the longitudinal spin conductivity as a direct transport manifestation of ferroaxial order.

cond-mat.str-el↗

arXiv2026

Magnetic toroidal monopoles from relativistic polarization responses to magnetic field gradients

The magnetic toroidal monopole, a time-reversal-odd scalar, has attracted attention through its characteristic responses, such as electric-field-induced nonreciprocal directional dichroism observed in Co$_2$SiO$_4$. However, its evaluation in crystalline solids remains unresolved, as it cannot be defined within conventional multipole expansions or thermodynamic formulations. In this paper, we propose a theoretical framework to evaluate the magnetic toroidal monopole in periodic crystals based on the response of relativistic electric polarization to a magnetic field gradient. By incorporating the magnetic-field-gradient correction to the relativistic polarization, we derive an explicit expression for the magnetic toroidal monopole beyond symmetry arguments. The resulting expression is formulated in terms of geometric quantity such as Berry curvatures and orbital magnetic moment defined in an extended parameter space spanning momentum, magnetic field, and electric field. We further perform model calculations for an antiferromagnetic system hosting a magnetic toroidal monopole and confirm that the proposed quantity is finite. These results provide a practical route to characterize magnetic toroidal monopoles in crystalline solids and clarify their quantum geometric nature.

cond-mat.str-el↗

arXiv2024

Nonlinear nonreciprocal electronic conductivity driven by magnetic field gradients

We theoretically propose the emergence of nonlinear nonreciprocal conductivity in centrosymmetric paramagnetic systems when a spatially gradient magnetic field is externally applied. The key essence lies in the appearance of magnetic toroidal dipole moment under the gradient field that breaks both spatial inversion and time-reversal symmetries. By analyzing a minimal tight-binding model on a two-dimensional system, we show that an effective coupling between the magnetic toroidal dipole moment arising from the gradient field and sublattice-dependent antisymmetric spin-orbit interaction plays an important role in inducing the nonlinear nonreciprocal transport. We also discuss the favorable situation to observe the nonlinear nonreciprocal conductivity in real materials by presenting an experimental setup in order to stimulate the findings.

cond-mat.str-el↗