arXiv subjects

Akiyoshi Yamada

Publications and source records attributed to Akiyoshi Yamada.

8 recordsLinked to original sources

arXiv2026

Classical transport theory for the planar Hall effect with threefold symmetry

In recent years, the planar Hall effect (PHE) has become a key probe of Berry curvature and the anomalous Hall effect (AHE). Threefold-symmetric signals under in-plane fields are often attributed to such quantum mechanisms. Here, we establish a purely classical origin for a three-fold-symmetric PHE. The idea is simple yet decisive: a third-order expansion of the Boltzmann equation in the magnetic field reveals that the threefold component originates from the relative positions of the mirror planes in the crystals with respect to the measurement setups. Remarkably, the threefold contribution should be ubiquitous because this symmetry condition can be realized across a broad range of crystals. Numerical estimates based on concrete models further show that its amplitude is comparable to that expected from the AHE.

cond-mat.mes-hall↗

arXiv2025

Magnetoresistance in the Extreme Quantum Limit: Field-Induced Crossover to the Unitarity Limit

We theoretically investigate magnetoresistance (MR) in the extreme quantum limit (EQL), where the kinetic energy becomes significantly smaller than the cyclotron energy, using the Kubo formula with Green's functions and the $T$-matrix approximation. We uncover a magnetic-field-induced crossover in the scattering rate: $1/τ\propto B^2$ in the Born regime and $1/τ\propto B^{-2}$ in the unitarity limit. This crossover gives rise to distinct MR behaviors in the EQL, characterized by linear transverse MR ($ρ_{xx} \propto B$) and negative longitudinal MR ($ρ_{zz} \propto B^{-2}$). This dichotomy implies insulating behavior when the magnetic field is perpendicular to the current, and metallic behavior when it is parallel. In the unitarity limit, we further derive a universal relation that enables direct experimental determination of the impurity density from $ρ_{xx}$ and $ρ_{xy}$. Our results establish a quantum--classical correspondence that remains valid even in the EQL, provided that the field dependences of the scattering rate and quantum corrections are properly incorporated.

cond-mat.mtrl-sci↗

arXiv2025

Pseudo anomalous Hall effect in semiconductors and semimetals: A classical perspective

We demonstrate that the non-linear field dependence in the Hall effect, often indistinguishable from the anomalous Hall effect, can be realized entirely within the classical mechanism due to the Lorentz force by analyzing multi-valley models for semiconductors and semimetals. The non-linear component in the Hall resistivity $ρ_H^{\rm NL}$ originates from carrier mobility anisotropy or the coexistence of different charges. Since $ρ_H^{\rm NL}$ is inversely proportional to the carrier difference between electrons and holes $Δn$, it exceeds its zero-field value near charge neutrality. As a practical example, we show that the magnitude of the classical non-linear Hall response in ZrTe$_5$ is comparable to the experimental values, underscoring the importance of accounting for classical contributions before attributing non-linear Hall effects to quantum mechanisms.

cond-mat.mes-hall↗

arXiv2025

Non-Fermi liquid transport and strong mass enhancement near the nematic quantum critical point in FeSe$_x$Te$_{1-x}$ thin films

Unconventional superconductivity is often accompanied by non-Fermi liquid (NFL) behavior, which emerges near a quantum critical point (QCP) - a point where an electronic ordered phase is terminated at absolute zero under non-thermal parameters. While nematic orders, characterized by broken rotational symmetry, are sometimes found in unconventional superconductors, the role of nematic fluctuations in driving NFL transport behavior remains unclear. Here, we investigated electrical and thermoelectric transport properties in FeSe$_x$Te$_{1-x}$ thin films and observed hallmark NFL behavior: temperature-linear resistivity and logarithmic divergence of thermoelectricity at low temperatures. Notably, the thermoelectricity peaks sharply at the nematic QCP ($x$ = 0.45), highlighting the dominant role of nematic fluctuations in the NFL transport. Furthermore, we found that the pair-breaking mechanisms in the superconducting phase crosses over from orbital- to Pauli-limited effects, indicating the mass enhancement near the nematic critical regime. These findings reveal the profound impact of nematic fluctuations on both normal-state transport and superconducting properties.

cond-mat.str-el↗

arXiv2024

High-field immiscibility of electrons belonging to adjacent twinned bismuth crystals

Bulk bismuth has a complex Landau spectrum. The small effective masses and the large g-factors are anisotropic. The chemical potential drifts at high magnetic fields. Moreover, twin boundaries further complexify the interpretation of the data by producing extra anomalies in the extreme quantum limit. Here, we present a study of angle dependence of magnetoresistance up to 65 T in bismuth complemented with Nernst, ultrasound, and magneto-optic data. All observed anomalies can be explained in a single-particle picture of a sample consisting of two twinned crystals tilted by 108$^{\circ}$ and with two adjacent crystals keeping their own chemical potentials despite a shift between chemical potentials as large as 68 meV at 65 T. This implies an energy barrier between adjacent twinned crystals reminiscent of a metal-semiconductor Schottky barrier or a p-n junction. We argue that this barrier is built by accumulating charge carriers of opposite signs across a twin boundary.

cond-mat.mtrl-sci↗

arXiv2023

Quantum--classical correspondence and dissipative to dissipationless crossover in magnetotransport phenomena

The three-dimensional magneto-conductivity tensor was derived in a gauge invariant form based on the Kubo formula considering the quantum effect under a magnetic field, such as the Landau quantization and the quantum oscillations. We analytically demonstrated that the quantum formula of the magneto-conductivity can be obtained by adding a quantum oscillation factor to the classical formula. This result establishes the quantum--classical correspondence, which has long been missing in magnetotransport phenomena. Moreover, we found dissipative-to-dissipationless crossover in the Hall conductivity by paying special attention to the analytic properties of thermal Green's function. Finally, by calculating the magnetoresistance of semimetals, we identified a phase shift in quantum oscillation originating from the dissipationless transport predominant at high fields.

cond-mat.mes-hall↗

arXiv2021

Negative magnetoresistance and sign change of the planar Hall effect due to the negative off-diagonal effective-mass in Weyl semimetals

We theoretically investigated the magnetoresistance (MR) and planar Hall effect (PHE) in Weyl semimetals based on the semiclassical Boltzmann theory, focusing on the fine structure of the band dispersion. We identified that the negative longitudinal MR and sign change in the PHE occur because of the negative off-diagonal effective-mass with no topological effects or chiral anomaly physics. Our results highlight the crucial role of the off-diagonal effective-mass, which can cause anomalous galvanomagnetic effects. We propose that the PHE creates a dip in their temperature dependence, which enables the experimental detection of the Weyl point.

cond-mat.mes-hall↗

arXiv2020

Angular dependence of magnetoresistance and planar Hall effect in semimetals in strong magnetic fields

The semiclassical transport theory is especially powerful for investigating galvanomagnetic effects. Generally, the semiclassical theory is applicable only in weak fields because it does not consider Landau quantization. Herein, we extend the conventional semiclassical theory by considering Landau quantization through the field dependence of carrier density in semimetals. The extended semiclassical theory is applicable even in strong fields, where Landau quantization is noticeable. Using this new approach, we explain the qualitative change in the angular dependence of transverse magnetoresistance (TMR), anisotropic magnetoresistance (AMR), and planar Hall effect (PHE) in bismuth with an increase in the magnetic field. This unveils the puzzle of nontrivial field-induced changes in TMR, AMR, and PHE observed recently in semimetal bismuth.

cond-mat.mes-hall↗