arXiv subjects

Toshio Naito

Publications and source records attributed to Toshio Naito.

6 recordsLinked to original sources

arXiv2026

Anomalous Phase-Coherence Scaling in a Quantum-Critical Dirac Semimetal

We have investigated the weak antilocalization (WAL) in the pressurized Dirac semimetal $α$-(BEDT-TTF)$_2$I$_3$ across a correlation-driven quantum phase transition to a charge-ordered insulating state and evaluated the phase coherence length $L_ϕ$ and its temperature scaling under various pressures from the low-temperature magnetoconductivity. In the high-pressure regime, the system exhibits the conventional two-dimensional dephasing behavior ($L_ϕ \propto T^{-p}$ with $p \approx 1/2$), characteristic of electron-electron scattering in diffusive conductors. As the pressure approaches the critical pressure ($P_c \sim 1.2$ GPa), the temperature exponent is suppressed to $p \sim 0.3$, while $L_ϕ$ remains large ($700\text{-}800$ nm at 0.5 K). This anomalous scaling suggests nontrivial inelastic scattering associated with Dirac electrons near the quantum critical point. The persistence of WAL across the transition supports a gapless or nearly gapless quantum phase transition.

cond-mat.str-el↗

arXiv2023

Evidence for three-dimensional Dirac semimetal state in strongly correlated organic quasi-two-dimensional material

The three-dimensional Dirac semimetal is distinct from its two-dimensional counterpart due to its dimensionality and symmetry. Here, we observe that molecule-based quasi-two-dimensional Dirac fermion system, $α$-(BEDT-TTF)$_2$I$_3$, exhibits chiral anomaly-induced negative magnetoresistance and planar Hall effect upon entering the coherent inter-layer tunneling regime under high pressure. Time-reversal symmetry is broken due to the strong electronic correlation effect, while the spin-orbit coupling effect is negligible. The system provides an ideal platform for investigating the chiral anomaly physics by controlling dimensionality and strong electronic correlation.

cond-mat.str-el↗

arXiv2022

Coherent interlayer coupling in quasi-two-dimensional Dirac fermions in $α$-(BEDT-TTF)$_2$I$_3$

Theoretical and experimental studies have supported that the electronic structure of $α$-(BEDT-TTF)$_2$I$_3$ under pressure is described by two-dimensional Dirac fermions. When the interlayer tunneling is coherent, the electronic structure of the system becomes three-dimensional, and we expect the peak structure to appear in the interlayer resistivity under magnetic fields. We theoretically and experimentally show that the peak appears in the interlayer resistivity at low temperatures and high magnetic fields. From the experiment, we estimate that the magnitude of the interlayer tunneling is $t_1 \sim 1$ meV. Our result opens the door to investigating the three-dimensional electronic structure of $α$-(BEDT-TTF)$_2$I$_3$.

cond-mat.str-el↗

arXiv2022

Conductivity of Two-dimensional Dirac Electrons Close to Merging in Organic Conductor $α$-STF$_2$I$_3$ at Ambient Pressure

The electric conductivity of Dirac electrons in the organic conductor $α$-STF$_2$I$_3$ (STF = bis(ethylenedithio)diselenadithiafulvalene), which has an isostructure of ET(=bis(ethylenedithio)tetrathiafulvalene), has been theoretically studied using a two-dimensional tight-binding model in the presence of both impurity and electron-phonon (e-p) scatterings.In contrast to ET, which has a Dirac cone with almost isotropic velocity, STF provides a large anisotropy owing to a Dirac point that is close to merging. As a result, $σ_{x}$ becomes much larger than $σ_{y}$, where $σ_y$ and $σ_x$ are diagonal conductivities parallel and perpendicular to a stacking axis of molecules, respectively. With increasing temperature ($T$), $σ_x$ takes a broad maximum because of e-p scattering and $σ_y$ remains almost constant. The ratio $σ_x/σ_y$ is analyzed in terms of the band structure. Such an exotic conductivity of STF is compared with that of an experiment showing a good correspondence. Finally, $σ_x/σ_y$ values of ET and BETS(= bis(ethylenedithio)tetraselenafulvalene) are shown to demonstrate the dissimilarity with STF.

cond-mat.mes-hall↗

arXiv2020

Universal Behavior of Magnetoresistance in Organic Dirac Electron Systems

In-plane magnetoresistance for organic massless Dirac electron system (OMDES) $α$-(BEDT-TTF)$_2$I$_3$ and $θ$-(BEDT-TTF)$_2$I$_3$ in addition to possible candidates of the OMDES $α$-(BETS)$_2$I$_3$ and $α$-(BEDT-STF)$_2$I$_3$, was investigated under hydrostatic pressure. We have found the universal behavior of the in-plane magnetoresistance under a low magnetic field perpendicular to two-dimensional plane. As for $α$-(BEDT-TTF)$_2$I$_3$, the universality was examined with the parameters of temperature, magnetic field and its direction. We suggest that the universal magnetoresistance behavior is found even for the gapped state of $α$-(BEDT-TTF)$_2$I$_3$ under intermediate pressure, when the thermal energy exceeds the gap.

cond-mat.str-el↗

arXiv2020

Exotic Dirac Cones on the Band Structure of $α$-STF$_2$I$_3$ at Ambient Temperature and Pressure

The quasi-two-dimensional molecular semimetallic conductor $α$-STF$_2$I$_3$ is isostructural with $α$-ET$_2$I$_3$.The latter possesses a unique band structure showing a zero-gap state with Dirac cones under high pressure, whereas the band structure of the former has been elusive because of heavy disorder at all the donor sites. To elucidate the band structure of $α$-STF$_2$I$_3$, a theoretical method based on the observed atomic parameters at 296 K and 1 bar has been proposed. The results suggest that the STF salt should have a band structure with Dirac cones under ambient pressure and temperature, which should promote future experimental studies on this system.Using the extended Hückel method, we demonstrate that the Dirac points of $α$-STF$_2$I$_3$ are aligned to be symmetric with respect to a time reversal invariant momentum (TRIM). Such novel Dirac cones, where the energy difference between the conduction and valence bands has considerable anisotropy, are clarified in terms of the parity of the wavefunction at the TRIM. We propose the conductivity measurement on the present $α$-STF$_2$I$_3$, which is expected to show a large anisotropy as a characteristic of the present Dirac electrons.

cond-mat.mtrl-sci↗