arXiv subjects

Toshihito Osada

Publications and source records attributed to Toshihito Osada.

At least 19 recordsLinked to original sources

arXiv2026

Hall Shift Current and Nonlinear Anomalous Hall Effect in Gapped Dirac Fermion Systems

We discuss a new mechanism for the nonlinear anomalous Hall effect in tilted two-dimensional gapped Dirac fermion systems. This mechanism originates from a side-jump displacement associated with Landau-Zener tunneling across the mass gap. In lightly doped tilted Dirac systems with a small gap, this interband mechanism can contribute to the nonlinear anomalous Hall effect in addition to the conventional intraband mechanism arising from the Berry curvature dipole. It provides a possible explanation for the nonlinear Hall effect accompanied by nonlinear longitudinal transport observed in the organic Dirac fermion system {\alpha}-(ET)2I3 with an extremely small gap.

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arXiv2025

Helical Edge Transport in the \nu = 0 Quantum Hall Ferromagnetic State of an Organic Dirac Fermion System

We experimentally confirm the \nu = 0 quantum Hall ferromagnetic (QHF) state, accompanied by helical edge states, in the layered organic Dirac-fermion system \alpha-(ET)2I3 by demonstrating helical edge transport in multilayers. The saturation of interlayer magnetoresistance (MR) in the high-magnetic-field quantum limit does not scale with the sample cross-sectional area and appears when the magnetic field is oriented along the side surface. The in-plane MR exhibits a similar angle dependence, whereas this feature disappears in the Corbino geometry where no edge channels are present. These results are consistent with helical edge transport in the multilayer QHF state. They also rule out the possibility that the observed angle-dependent MR arises from the chiral magnetic effect expected for a 3D Dirac or Weyl semimetal.

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arXiv2025

Wannier Center Analysis on Possible Three-Dimensional Topological Phases in α-Type Layered Organic Conductors

Topological features of possible three-dimensional (3D) states in α-type layered organic conductors are investigated within a unified framework based on Wannier charge centers (WCCs), aiming to identify their actual topological states. Among the 3D Dirac/Weyl semimetal states of multilayered α-(ET)2I3, the type-I Dirac semimetal state, induced by interlayer spin-orbit coupling (SOC), most effectively explains the observed chiral transport phenomena attributed to the chiral magnetic effect, which originates from the spiral structures of the WCC sheets. In multilayered α-(BETS)2I3, a 3D weak topological insulator (TI) state consistently emerges, irrespective of the presence of interlayer SOC and/or inversion symmetry breaking. The strong TI state suggested by experimental observations appears unlikely to be realized.

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arXiv2024

Three-Dimensional Topological Semimetal/Insulator States in α-Type Organic Conductors with Interlayer Spin-Orbit Interaction

We have studied the tight-binding model for the α-type layered organic conductors, α-(ET)2I3 and α-(BETS)2I3, with a uniform interlayer coupling accompanied by spin-orbit interaction originating from the I3- anion potential. The model preserves the time reversal and inversion symmetries. In α-(ET)2I3, the interlayer spin-orbit coupling realizes the experimentally suggested Dirac semimetal state with inversion symmetry. In contrast, the inversion breaking in interlayer hoppings realizes the Weyl semimetal state without spin-orbit coupling. In α-(BETS)2I3, the proposed strong topological insulator is hardly realized with inversion symmetry.

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arXiv2024

Hofstadter Butterfly and Broken-Symmetry Quantum Hall States in α-Type Organic Dirac Fermion Systems

The electronic state of α-type organic Dirac fermion systems such as α-(ET)_2I_3 or α-(BETS)_2I_3 has been studied under magnetic fields using the four-band tight-binding model with Peierls phase factors. The validity of the Dirac fermion picture in these materials was confirmed by the generated Hofstadter butterfly and its Chern numbers. The four-component envelope function of the N = 0 Landau level with valley degeneracy was studied. It was found that the two degenerate valley states have different weights on A and A' molecules connected by inversion. This feature is also recognized for the N = 0 spin-split Landau levels under the Zeeman effect and the spin-orbit interaction. The spontaneous valley symmetry breaking in the N = 0 Landau levels due to the exchange interaction results in the ν= 1 and -1 quantum Hall states accompanied by the spatial charge and spin modulations in a unit cell.

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arXiv2023

Magnetic-field periodic quantum Sondheimer oscillations in thin-film graphite

Materials with the mesoscopic scales have provided an excellent platform for quantum-mechanical studies. Among them, the periodic oscillations of the electrical resistivity against the direct and the inverse of the magnetic fields, such as the Aharonov-Bohm effect and the Shubnikov-de Haas effect, manifest the interference of the wavefunction relevant to the electron motion perpendicular to the magnetic field. In contrast, the electron motion along the magnetic field also leads to the magnetic-field periodicity, which is the so-called Sondheimer effect. However, the Sondheimer effect has been understood only in the framework of the semiclassical picture, and thereby its interpretation at the quasiquantum limit was not clear. Here, we show that thin-film graphite exhibits clear sinusoidal oscillations with a period of about 1-3 T over a wide range of the magnetic fields (from around 10 T to 30 T), where conventional quantum oscillations are absent. In addition, the sample with a designed step in the middle for eliminating the stacking disorder effect verifies that the period of the oscillations is inversely proportional to the thickness, which supports the emergence of the Sondheimer oscillations in the quasiquantum limit. These findings suggest that the Sondheimer oscillations can be reinterpreted as inter-Landau-level resonances even at the field range where the semiclassical picture fails. Our results expand the quantum oscillation family, and pave the way for the exploration of the out-of-plane wavefunction motion.

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arXiv2023

Quantized thermoelectric Hall plateau in the quantum limit of graphite as a nodal line semimetal

We performed thermoelectric Hall conductivity $α_{xy}$ measurements on single-crystal graphite in the quantum limit up to 13 T. Both electrical and thermoelectric transport measurements were performed on the same crystal to extract pure $α_{xy}$, avoiding any sample quality dependence. The $α_{xy}$ converges to a plateau in the quantum limit with a linear dependence on temperature. This behavior is analogous to the quantized thermoelectric Hall effect (QTHE) observed in three-dimensional Dirac/Weyl nodal-point semimetals, and experimentally confirms a theoretical proposal on the QTHE in semimetals with nodal lines as in graphite.

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arXiv2022

Weak localization on moiré superlattice in twisted double bilayer graphene

Moiré superlattice created by twist stacking has multiple physical properties. These physical properties depend on the twist angle, hence investigation of the twist angle dependency is important for the deep understanding of physical phenomena in moiré superlattice. In this work, negative magnetoresistance owing to weak localization (WL) was investigated in twisted double bilayer graphene (TDBG) as a function of the twist angle. The ratio of the intervalley scattering time to the intravalley scattering time, estimated using the WL formula for bilayer graphene, tended to decrease as the twist angle increased. This feature is qualitatively explained by the enhancement of intervalley scattering due to the reduction of the intervalley distance in the moiré Brillouin zone (BZ) of the TDBG. This indicates that WL in the TDBG occurs for the moiré superlattice with the reconstructed BZ.

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arXiv2022

Thermoelectric Hall Effect at High-Magnetic-Field Quantum Limit in Graphite as a Nodal-Line Semimetal

The high-magnetic-field thermoelectric effect in nodal-line semimetals with straight nodal lines was investigated. Three-dimensional (3D) Dirac/Weyl semimetals exhibit constant thermoelectric Hall conductivity at the high-magnetic-field quantum limit, resulting in a boundless linear increase of Seebeck coefficient. This is known as the quantized thermoelectric Hall effect (QTHE), and is expected to lead to high-performance thermoelectricity at low temperatures. Here, in addition to Dirac/Weyl semimetals, we demonstrated that 3D semimetals with straight nodal lines can also exhibit the QTHE under high magnetic fields. As a candidate material for experimental validation, we discussed the thermoelectric properties of bulk graphite. Furthermore, we investigated the dimensional crossover of thermoelectricity to two-dimensional behavior in thin-film graphite.

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arXiv2021

Observation of Possible Nonlinear Anomalous Hall Effect in Organic Two-Dimensional Dirac Fermion System

We report the observation of nonlinear anomalous Hall effect (NLAHE) in the multilayered organic conductor $α$-(BEDT-TTF)$_2$I$_3$ in the charge order (CO) insulating phase just under the critical pressure for transition into two-dimensional (2D) massless Dirac fermion (DF) phase. We successfully extracted the finite nonlinear Hall voltage proportional to square current at zero magnetic field. The observed NLAHE features, current direction dependence and correlation with CO, are consistent with the previous estimation assuming 2D massive DF with a pair of tilted Dirac cones. This is the first observation of topological transport in organic conductors, and also the first example of NLAHE in the electronic phase with spontaneous symmetry breaking.

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arXiv2021

Thermoelectric Effect at Quantum Limit in Two-Dimensional Organic Dirac Fermion System with Zeeman Splitting

The thermoelectric effect in a two-dimensional (2D) massless Dirac fermion (DF) system at the quantum limit is discussed to verify the prediction of high-performance thermopower in an organic conductor α-(BEDT-TTF)2I3. Because of relatively large Zeeman splitting in α-(BEDT-TTF)2I3, the boundless increase of thermopower at high magnetic fields, predicted without the Zeeman effect, is hardly expected, whereas there appears to be a broad local maximum. This is characteristic of 2D DF systems with Zeeman splitting and is recognized in the previous experiment. In contrast to 3D Dirac/Weyl semimetals with robust gapless features, it might be difficult to realize high-performance thermopower in real 2D DF systems under high magnetic fields.

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arXiv2021

Possible Nonlinear Anomalous Thermoelectric Effect in Organic Massive Dirac Fermion System

We propose a novel current-induced thermoelectric phenomenon, the nonlinear anomalous Ettingshausen effect (AEE), at zero magnetic field in inversion-asymmetric conductors. As an example, we discuss the weak charge ordering state in a layered organic conductor alpha-(BEDT-TTF)2I3, which is a two-dimensional massive Dirac fermion system with a pair of tilted Dirac cones. The nonlinear AEE is a thermoelectric analogue of the nonlinear anomalous Hall effect, which is recently observed in alpha-(BEDT-TTF)2I3, and these two effects generally appear simultaneously. The nonlinear AEE generates a transverse heat current, which exhibits rectifying characteristics, namely unidirectionality even under an AC current.

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arXiv2021

Experimental Confirmation of Massive Dirac Fermions in Weak Charge-Ordering State in α-(BEDT-TTF)_2I_3

The electronic structure of weak charge-ordering (CO) state just below the critical pressure in an organic conductor α-(BEDT-TTF)_2I_3 was experimentally investigated using peak structure in the temperature dependence of interlayer magnetoresistance (MR). Based on a minimal model considering multiple Landau levels (LLs), we discuss herein the MR peak as characteristic to multilayer massless/massive Dirac fermion (DF) systems. MR measured in the weak CO state exhibited a clear MR peak, and its magnetic-field dependence was consistent with the LL behavior of a massive DF with a small gap. Results indicate that the weak CO state in α-(BEDT-TTF)_2I_3 is a massive DF state.

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arXiv2020

Possible Current-Induced Phenomena and Domain Control in an Organic Dirac Fermion System with Weak Charge Ordering

We show that when the electron and hole densities are unbalanced, observable nonlinear anomalous Hall effect and current-induced orbital magnetization appear at zero magnetic field in the weak charge ordering (CO) state of an organic two-dimensional Dirac fermion system, a-(BEDT-TTF)2I3. These current-induced phenomena are caused by a finite Berry curvature dipole resulting from inversion symmetry breaking and Dirac cone tilting. In the actual system, however, these effects are canceled out between different types of inversion asymmetric CO domains. To avoid the cancellation, we propose a new experimental method to realize the selective formation of a single type of domain using the current-induced magnetization.

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arXiv2019

Topological Properties of τ-Type Organic Conductors with a Checkerboard Lattice

Although the topological phases are difficult to be realized in organic molecular crystals, we demonstrate here that they can emerge in the τ-type organic layered conductors, τ-(EDO-S,S-DMEDT-TTF)_2X_{1+y} and τ-(P-S,S-DMEDT-TTF)_2X_{1+y} (X=AuBr_2, I_3, IBr_2), where EDO-S,S-DMEDT-TTF and P-S,S-DMEDT-TTF denote the planar donor molecules ethylenedioxy-S,S-dimethyl(ethylenedithio)tetrathiafulvalene and pyrazino-S,S-dimethyl(ethylenedithio)tetrathiafulvalene, respectively. The conducting layers of these conductors have a highly symmetric checkerboard structure, which can be regarded as a modified Mielke lattice. Because their electronic structure inherits that of the Mielke lattice, their conduction and valence bands exhibits the quadratic band touching. The contact point splits into a pair of Dirac cones under uniaxial strain which breaks C_4-symmetry. In τ-type conductors, we can expect rather large spin-orbit coupling (SOC) as organic conductors. We show that the SOC in this case opens a topologically nontrivial gap at the band contact point, and the helical edge states exist in the gap. The actual τ-type conductors could be regarded as heavily-doped topological insulators, which could exhibit finite spin Hall effect.

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arXiv2019

Experimental Confirmation of Quantum Hall Ferromagnetic State in an Organic Dirac Fermion System

We have experimentally confirmed the quantum Hall ferromagnetic state with Chern number ν=0, characterized by the helical edge state, in a layered organic Dirac fermion system α-(BEDT-TTF)_2I_3. The interlayer resistance saturates at low temperatures and high magnetic fields. It does not scale with the sample cross-sectional area in the saturating region, and resonantly depends on the magnetic field direction. These results strongly suggest that the helical edge state dominates transport. This is the first observation of the topological phase in organic molecular crystals.

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arXiv2018

Double carrier transport in electron doped region in black phosphorus FET

The double carrier transport has been observed in thin film black phosphorus (BP) field effect transistor (FET) devices in highly electron doped region. BP thin films with typical thickness of 15 nm were encapsulated by hexagonal boron nitride (h-BN) thin films to avoid degradation by air exposure. Their Hall mobility has reached 5300 cm2/Vs and 5400 cm2/Vs at 4.2 K in the hole and electron doped regions, respectively. The gate voltage dependence of conductivity exhibits an anomalous shoulder structure in electron doped region. In addition, at gate voltages above the shoulder, the magnetoresistance changes to positive, and there appears an additional slow Shubnikov-de Haas oscillation. These results strongly suggest the appearance of the second carriers, which originate from the second subband with localized band edge.

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arXiv2018

Anisotropy of Dirac cones and van Hove singularity in an organic Dirac fermion system

We propose an experimental method to examine the in-plane anisotropy of electronic structure in layered conductors. In the method, we measure the interlayer magnetoresistance as a function of in-plane magnetic field orientation. We applied it to an organic Dirac fermion system a-(BEDT-TTF)2I3 to experimentally determine the orientation of the anisotropic Dirac cones. It is concluded that the long axis of the elliptic constant-energy contours of the Dirac cone is tilted by approximately -30 deg from the crystalline a-axis to b-axis under hydrostatic pressures. Additionally, we observed a signature of van Hove singularity (which is a saddle point of the band dispersion) at 30-40 K above or below the Dirac point. The ridgeline of the saddle point is estimated as almost parallel to the crystalline b-axis.

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