arXiv subjects

Takuma Ogasawara

Publications and source records attributed to Takuma Ogasawara.

7 recordsLinked to original sources

arXiv2026

$\mathcal{PT}$-symmetry as Effective Time Reversal Symmetry for Anderson Localization in a Collinear Antiferromagnet

We show that $\mathcal{PT}$-symmetry acts as the effective time-reversal symmetry governing Anderson localization in the antiferromagnet BaMn$_2$Bi$_2$, whereas the general time-reversal symmetry is broken by its long-range magnetic order. The magnetoconductance follows the laws established from breaking time-reversal symmetry throuhg the orbital coupling of charge carriers to the magnet vector potential, however, the degrees of symmetry breaking are qualitatively modulated by the magnetocrystalline anisotropy stabilizing $\mathcal{PT}$-related sublattices. The quantum interferences and hoping amplitudes governing the phenomena are drastically impaired by transverse magnetic fields, which readily cant $\mathcal{PT}$-related magnetic sublattices. Accordingly, when the $\mathcal{PT}$-related texture is weakly perturbed by longitudinal fields orienting along the sublattices, the localization is minutely affected. The robustness of the $\mathcal{PT}$-enforced degeneracies is thus governed by exchange and magnetocrystalline interactions, in contrast to the exact $\mathcal{PT}$-symmetry of the full Hamiltonian, which is broken by any finite field.

cond-mat.other↗

arXiv2026

Supercritical fluid of quantum electrons in three-dimensional superconducting fullerides

The supercritical fluid (SCF) of quantum electrons at the Mott metal-insulator transition without symmetry breaking is one of the most elusive phenomena in strongly correlated electron physics. Prior studies of Cr-doped V2O3 and organic Mott systems reported discrepant critical exponents. A key limitation is that the scaling analysis relies on a single experimental observable, leaving the roles of phase coexistence, inhomogeneity, and percolation unaddressed. Here we report the first experimental identification of a thermodynamically equilibrated SCF phase and its associated Mott endpoint in the three-dimensional superconducting fullerides CsxRb3-xC60, using two independent probes of electrical conductivity and magnetic susceptibility, which reveal two distinct metal-insulator transition lines converging at a single Mott endpoint. A hypothesis-free two-particle analysis of magnetic susceptibilities yields a metal-insulator coexisting SCF by exhibiting the maximum two-phase mixing entropy, in agreement with a picture of a thermodynamically equilibrated Widom line. Simultaneously, conductivity scaling yields a critical exponent in the regime of quantum critical predictions. Our new dual-probe approach provides a unified microscopic picture of the Mott SCF with a characteristic length scale below current diffraction resolution, in addition to a new interpretation on the origin of superconducting Tc-dome.

cond-mat.str-el↗

arXiv2026

Time-resolved measurement of Seebeck effect for superionic metals during structural phase transition

We propose a new time (t)-resolved method of both vertical- and horizontal-temperature gradients in an orthogonal configuration (t-resolved T(t)-HVOT) to have real interpretations of the enhancement in thermoelectric Seebeck effect (SE) observed during the structural phase transition. We apply our new method to superionic-state semiconductors of p-type Cu2Se and n-type Ag2S. The experimental data differentiate the two types of enhancements during the phase transition: a colossal SE (Scolossal), exhibiting an enormous value of up to 5 mV/K, and a slight enhancement in SE (Sstructure), approximately 1.5-2.0 times larger than those in the absence of the phase transition. We provide critical insights that both enhancements in SE arising during the structural phase transition are not intrinsic phenomena.

cond-mat.mtrl-sci↗

arXiv2025

Longitudinal magneto-thermal conductivity and magneto-Seebeck of itinerant antiferromagnetic BaMn$_2$Bi$_2$

Thermal transport, generally mediated by the direct microscopic exchange of kinetic energy via lattice phonons, can also be modified by contributions from additional quasiparticles, such as electrons and magnons. However, a comprehensive understanding of the magnon influence has yet to be realized and remains an active research area. The most significant roadblock has been a lack of available materials in which these three quasiparticles can be clearly identified and quantitatively examined in order to provide an intrinsic understanding, not only of their independent contributions to thermal conductivity but also of the cross-correlated interactions among them. Itinerant antiferromagnetic (AFM) BaMn$_{2}$Bi$_{2}$ with PT symmetry exhibits Anderson metal-insulator localization, which can be tuned into the metallic regime via an applied magnetic field due to its unique electron-magnon interactions. We identify itinerant AFM BaMn$_{2}$Bi$_{2}$ as an ideal material for scientific investigations into how these quasiparticles participate in thermal conductivity. Here, we present the direct contribution of electrons, phonons, and magnons to thermal conductivity, as well as their interspecies interactions, supported by detailed analyses conducted in the framework of the Boltzmann transport formalism. The comparison of the magneto-thermal conductivity and magneto-electrical conductivity, as well as the magneto-Seebeck effect of itinerant antiferromagnetic BaMn$_{2}$Bi$_{2}$, gives unique insight into how magnons participate in longitudinal thermal-associated phenomena.

cond-mat.str-el↗

arXiv2022

Magnetic-field-induced Anderson localization in orbital selective antiferromagnet BaMn$_2$Bi$_2$

We report a metal-insulator transition (MIT) in the half-filled multiorbital antiferromagnet (AF) BaMn$_2$Bi$_2$ that is tunable by a magnetic field perpendicular to the AF sublattices. Instead of an Anderson-Mott mechanism usually expected in strongly correlated systems, we find by scaling analyses that the MIT is driven by an Anderson localization. Electrical and thermoelectrical transport measurements in combination with electronic band calculations reveal a strong orbital-dependent correlation effect, where both weakly and strongly correlated $3d$-derived bands coexist with decoupled charge excitations. Weakly correlated holelike carriers in the $d_{xy}$-derived band dominate the transport properties and exhibit the Anderson localization, whereas other $3d$ bands show clear Mott-like behaviors with their spins ordered into AF sublattices. The tuning role played by the perpendicular magnetic field supports a strong spin-spin coupling between itinerant holelike carriers and the AF fluctuations, which is in sharp contrast to their weak charge coupling.

cond-mat.str-el↗

arXiv2021

Large negative magnetoresistance in BaMn$_2$Bi$_2$ antiferromagnet

A very large negative magnetoresistance (LNMR) is observed in the insulating regime of the antiferromagnet BaMn$_2$Bi$_2$ when a magnetic field is applied perpendicular to the direction of the sublattice magnetization. High perpendicular magnetic field eventually suppresses the insulating behavior and allows BaMn$_2$Bi$_2$ to re-enter a metallic state. This effect is seemingly unrelated to any field induced magnetic phase transition, as measurements of magnetic susceptibility and specific heat did not find any anomaly as a function of magnetic fields at temperatures above $2\,\mathrm{K}$. The LNMR appears in both current-in-plane and current-out-of-plane settings, and Hall effects suggest that its origin lies in an extreme sensitivity of conduction processes of holelike carriers to the infinitesimal field-induced canting of the sublattice magnetization. The LNMR-induced metallic state may thus be associated with the breaking of the antiferromagnetic parity-time symmetry by perpendicular magnetic fields and/or the intricate multi-orbital electronic structure of BaMn$_2$Bi$_2$.

cond-mat.str-el↗

arXiv2018

Itinerant antiferromagnetic BaMn$_2$Pn$_2$'s showing both negative and positive magnetoresistances

We report the discovery of a novel giant magnetoresistance (GMR) phenomenon in a family of BaMn$_{2}$Pn$_{2}$ antiferromagnets (Pn stands for P, As, Sb, and Bi) with a parity-time symmetry. The resistivities of these materials are reduced by $60$ times in magnetic fields ($\vec{H}$'s), thus yielding the GMR of about $-98\%$. The GMR changes systematically along with the Pn elements, hinting that its origin is the spin orbit coupling (SOC) and/or $d$-$p$ orbital hybridization. A positive MR component emerging on top of the negative GMR at low temperatures suggests an orbital-sensitive magnetotransport as $\vec{H}$ suppresses the conduction of the electron-like carriers in the $d$-like band but enhances those of hole-like ones in the $d$-$p$ hybridized band. The anisotropy of the GMR reveals that the electrical conductivity is extremely sensitive to the minute changes in the direction of the antiferromagnetic moments induced by the parity-time breaking $\vec{H}$, which seems to be associated with a magnetoelectric effect in the dynamic regime of conduction electrons. We attribute the observed GMR to the non-trivial low energy band of BMPn's, which is governed by the parity-time symmetry and an magnetic hexadecapole ordering.

cond-mat.str-el↗