Yoshitaka Kawasugi

We investigate nonreciprocal conductivity in the superconducting state of the molecular conductor κ-(BEDT-TTF)2Cu[N(CN)2]Cl. Although the material is neither chiral nor polar, we have observed nonreciprocal resistance proportional to the applied current and in-plane magnetic field, which is comparable with that of gate-induced superconducting MoS2. However, we find that this nonreciprocal resistance exhibits significant sample dependence, with even its sign varying. Furthermore, within the same sample, its magnitude and sign varied depending on the measurement location, suggesting the existence of nonreciprocal resistance governed by a mechanism distinct from the Rashba effect in polar superconductors. The increase in nonreciprocal resistance due to current terminals with high contact resistance suggests the Nernst effect, arising from slight temperature gradients along the out-of-plane direction, as its origin. Utilizing this, we successfully increased the nonreciprocal coefficient by up to approximately twofold by processing the sample into a nonuniform shape.

We report on the off-diagonal (transverse) thermoelectric effect in the molecular conductor κ-(BEDT-TTF)2Cu[N(CN)2]Cl, originating from the coexistence of the two-dimensional hole-like and one-dimensional electron-like Fermi surfaces. The off-diagonal thermoelectric voltage corresponds to the anisotropy of the Seebeck coefficient and is proportional to the sample length perpendicular to the temperature gradient, reaching ∼90 μV/K in our sample. Also, we investigated the effect of electric-double-layer doping on the off-diagonal thermoelectric effect. Under hole doping, while the resistance significantly decreases, the off-diagonal Seebeck coefficient does not decrease probably because of pseudogap formation. As a result, hole doping increases the off-diagonal power factor by up to a factor of approximately 15 at the doped surface.

Chiral electroactive materials have attracted attention for the effects of electrical magnetochiral anisotropy (eMChA) and chirality-induced spin selectivity (CISS). The combination of tetrathiafulvalene (TTF) with chiral moieties is one way to access chiral electroactive materials. In this paper, we have focused on the fused 2,3-dimethylcyclohexene (DMCh) ring as a substituent with chiral carbon atoms and without heteroatoms, which has not been used in the field of molecular conductors, and we synthesized a new TTF derivative (rac-DMCh-EDT-TTF). We have developed novel molecular conductors (rac-DMCh-EDT-TTF)₂X (X⁻ = PF₆⁻, AsF₆⁻ and ClO₄⁻), which have bilayer conducting sheets composed of the two crystallographically independent molecules. All salts exhibited semiconducting behavior from room temperature down to low temperatures, and a resistivity anomaly was observed at 180–250 K. X-ray structure analysis at 100 K and 263 K and molecular orbital calculations using the results of X-ray structure analysis indicated the emergence of a charge disproportionation between Layers 1 and 2 at the low-temperature phase.

Uniaxial pressure can dramatically change the properties of low-dimensional systems and induce electronic phase transitions. The first-discovered molecular massless Dirac electron system α-(BEDT-TTF)₂I₃ has intensively been studied because of its unique features, such as the Fermi energy being very close to the Dirac point, strong electron correlation, and the quantum phase transition from the charge-ordered insulating state. However, the Dirac state is realized only under high pressure of 15 kbar, which limits the measurement of physical properties (e.g., experimental determination of band dispersion and density of states). Here, we demonstrate that the Dirac state of α-(BEDT-TTF)₂I₃ can be realized by applying uniaxial bending strain without confining it in a pressure cell. Uniaxial strain below 1% completely suppresses the metal–insulator transition observed at ambient pressure. Under strain, a characteristic temperature dependence of magnetoresistance associated with the formation of the n = 0 Landau level is observed, indicating the realization of the massless Dirac state.

The physics of quantum many-body systems have been studied using bulk correlated materials, and recently, moiré superlattices formed by atomic bilayers have appeared as a novel platform in which the carrier concentration and the band structures are highly tunable. In this brief review, we introduce an intermediate platform between those systems, namely, a band-filling- and bandwidth-tunable electric double-layer transistor based on a real organic Mott insulator κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl. In the proximity of the bandwidth-control Mott transition at half filling, both electron and hole doping induced superconductivity (with almost identical transition temperatures) in the same sample. The normal state under electric double-layer doping exhibited non-Fermi liquid behaviors as in many correlated materials. The doping levels for the superconductivity and the non-Fermi liquid behaviors were highly doping-asymmetric. Model calculations based on the anisotropic triangular lattice explained many phenomena and the doping asymmetry, implying the importance of the noninteracting band structure (particularly the flat part of the band).