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We investigated the magnetoresistance and the Hall effect in an organic massless Dirac fermion system $\alpha$-(BETS)$_2$I$_3$ under pressure. The Fermi energy of this system is slightly far away from the Dirac points, and thus the $\nu =1$ quantum Hall state is realized in a low magnetic field at low temperatures. Moreover, the experimental formula for chemical potential as a function of temperature is clarified. We succeeded in detecting the inter-band effects of the magnetic field on the Hall conductivity when the chemical potential passes the Dirac points., Comment: 13 pages, 4 figures

Theoretical and experimental studies have supported that the electronic structure of $\alpha$-(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 $\alpha$-(BEDT-TTF)$_2$I$_3$., Comment: 4 pages, 3 figures

Magnetocaloric effect (MCE) and magnetic torque measurements have been carried out in the π–d system λ-(BETS)2FeCl4 [BETS = bis(ethylenedithio)tetraselenafulvalene], which shows an antiferromagnetic insulating (AFI) phase below ∼8.5 K. In the magnetic torque curve, a sharp structure at ∼1.2 T and a step at ∼10 T are observed at low temperatures, which are caused by the spin-flop (SF) transition and the transition from the AFI to paramagnetic metallic (PM) phase, respectively. The MCE, directly related to the magnetic entropy, shows a small sharp peak at the SF transition and a sharp dip at the AFI–PM transition. The overall feature above 3 K is qualitatively interpreted by a simple picture: antiferromagnetic (AF) π spins and paramagnetic 3d spins at the Fe sites. However, a broad dip in the MCE is additionally found at ∼5 T below ∼3 K, which is not explained by the above picture. The results are compared with those of κ-(BETS)2FeBr4, which shows an AF order of the 3d spins at the Fe sites.

Pressure-controlled switching between an insulating state and a superconducting state has been successfully realized on a supramolecular organic conductor θ-(DIETS) 2 [Au(CN) 4 ] [DIETS = diiodo(et...

We demonstrate that an organic conductor α-(BEDT-TTF) 2 I 3 exhibits various types of transport phenomena when compressed along some crystal axes. When the sample is strained along the a -axis, the low-temperature electronic state changes from a charge-ordered insulating state under small strains to a narrow gap semiconductor state under large strains. In between these two states, there appears a superconducting state with a T c of about 7 K. When compressed along the b -axis, on the other hand, this material behaves as a typical quasi-two-dimensional metal with large Fermi surfaces.

We have found quite a new type of transport phenomenon in an organic crystal α-(BEDT-TTF) 2 I 3 under high pressure. Essentially, it is a semimetal or a narrow gap semiconductor. But, the transport property is peculiar. The conductivity of this the carrier (hole) density and mobility change by a about 6 orders of magnitude. They change in a manner so that the effects just cancel out giving rise to the temperature independent conductivity. At low temperatures, the system is in a state with high mobility (3 ×10 5 cm 2 /V·sec) and low carrier density (5 ×10 15 cm -3 ). This state has been found to be very sensitive to magnetic field. We propose a mechanism for the extremely strong temperature dependence of the carrier density. It is based on the band structure and takes the thermal effect into consideration.

Resistance and dielectric constants have been measured in the antiferromagnetic insulating phase of the quasi-two-dimensional organic conductor λ-(BETS)2FeCl4 to understand charge transport. Nonlinear current–voltage characteristics are observed at low temperatures, which are explained by a charge transport model based on the electric-field dependent Coulomb potential between the thermally excited electron and hole. A small dip in the magnetic field dependence of the resistance is found at 1.2 T, which is ascribed to a spin-flop transition. The large difference between the in-plane and out-of-plane dielectric constants shows the two-dimensionality of the Coulomb potential, which is consistent with the charge transport model. The angular dependence of the metal–insulator transition field is determined, which suggests that the Zeeman effect of the 3d spins of the Fe ions plays an essential role.

The relationship between magnetism and superconductivity has been intensively studied by many researchers. The organic conductor λ-(BETS)2FeCl4 [BETS = bis(ethylenedithio)tetraselenafulvalene] undergoes a metal–insulator (M–I) transition with antiferromagnetic (AF) ordering under zero magnetic field. The magnetic field (μ0H ∼ 11 T) induces the transition from the insulating state to the metallic state and then to the superconducting state (μ0H ∼ 17 T). The origin and mechanism of the M–I transition accompanying AF ordering have not been clarified yet. In the present study, to investigate the origin and mechanism of the M–I and AF transitions in λ-(BETS)2FeCl4, we derived key information on these phase transitions by studying the effective magnetic field on an FeCl4 site through specific heat measurements under a magnetic field.

The transport phenomena of zero-gap conductors are one of the central subjects in condensed matter physics. α-(BEDT-TTF)2I3 [BEDT-TTF = bis(ethylenedithio)-tetrathiafulvalene] is a typical zero-gap conductor consisting of two-dimensional multiple two-dimensional layers. Under high pressures of above 1.5 GPa, it undergoes a phase transition to a zero-gap state, in which it exhibits unusual transport phenomena. This paper reviews the recent progress in clarifying the physics of α-(BEDT-TTF)2I3. In particular, the effect of a magnetic field on the behavior of electrons is investigated. On the basis of the interlayer magnetoresistance, evidence of the Landau level with the index N = 0 was obtained. The collaboration of experiments and theory has opened a new field of exploring the interlayer electron transport in two-dimensional layered zero-gap conductors. Furthermore, these phenomena have been observed by NMR and specific heat measurements in contrast to the case of graphene. The paper also reviews some rec...

We studied the normal-state properties of an organic superconductor, κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br, through the investigation of the Hall effect as functions of temperature and pressure. At low pressures, the Hall coefficient shows an apparent increase with lowering temperature and a following peak structure. In this pressure range, the Hall effect possibly correlates with the spin fluctuation. At high pressures, we observed a drastic decrease and sign change in the Hall coefficient. With the dissimilarity with the behavior of similar compounds such as κ-(BEDT-TTF) 2 Cu(NCS) 2 and κ-(BEDT-TTF) 2 Cu(N[CN] 2 )Cl, we concluded that this behavior originates from a superstructure below 200 K.

Mysterious transport phenomena observed in a zero gap conductor α-(BEDT-TTF) 2 I 3 were investigated. In particular, the electron system in the quantum limit where electrons in the Landau level with index 0 dominate the transport was of interest. In order to determine the magnetic field dependence of the electron transport, we utilized a conductivity tensor. We showed how each component of the conductivity tensor changes with the magnetic field. All the components were found to be expressed by simple functions of magnetic field. Interlayer Hall resistance (Hall effect observed in the interlayer current) was observed for the first time.

We performed static susceptibility and 77 Se NMR measurements on α-(BETS) 2 I 3 and α-(BEDT-STF) 2 I 3 which are analogues of the “zero gap” system, α-(BEDT-TTF) 2 I 3 to systematically compare the electronic properties of the α-type I 3 systems, where BETS, BEDT-STF, and BEDT-TTF are bis(ethylendithio)tetraselenafulvalene, bis(ethylendithio)dithiadiselenafulvalene, and bis(ethylendithio)tetrathiafulvalene, respectively. From the analysis of the NMR lineshape for a polycrystalline sample at high temperatures, which are explained as an admixture of several powder patterns, we found that the local spin susceptibility is not uniform but at least one of the molecular sites has very small susceptibility. It was confirmed from single crystal data that there are three kinds of molecules with different local susceptibilities in a unit cell. The analysis of the angular dependence on a single crystal clarified that the relative magnitude of the local susceptibilities of A, B, and C molecules, χ loc A , χ loc B , an...

Temperature depedence of the longitudinal interlayer magnetoresistance (MR) of a bulk Dirac cone system, α-(BEDT-TTF) 2 I 3 , is examined. It exhibits a peak related to the crossover from the quantum limit regime with negative MR at low temperatures to the inter-level scattering regime with positive MR at higher temperatures. The field dependence of the MR peak temperature, which measures the energy separation between the n = 0 and ±1 Landau levels, evidences that the peculiar Landau quantization of the Dirac cone dispersion is relevant to both effects. The effective Fermi velocity, v eff , which reflects the width and spacing of the Landau levels, has been estimated from the analysis taking account of the Zeeman-split Landau levels.

We perform crystal structure analyses of the organic conductor α-(BEDT-TTF) 2 I 3 [where BEDT-TTF denotes bis(ethylene)dithiotetrathiafulvalene] and its selenium analogue α-(BEDT-TSeF) 2 I 3 [where BEDT-TSeF denotes bis(ethylene)dithiotetraselenafulvalene] at room temperature under hydrostatic pressure, where they are expected to have zero-gap points at the Fermi level with a Dirac-cone-type dispersion for a massless particle. We calculate the electronic band structures using a combination of the extended Huckel molecular orbital calculation based on the obtained crystal structures and the tight-binding band structure calculation. We find that I) the obtained Fermi surfaces of both salts under hydrostatic pressure are simply larger than those at ambient pressure owing to an increase in overlap integrals caused by the pressure, and II) electronic structures having zero-gap points with a Dirac-cone-type dispersion at the Fermi level are realized only in α-(BEDT-TTF) 2 I 3 by the introduction of suitable sit...

We investigated an out-of-plane resistivity of quasi two-dimensional conductors, κ(BEDT-TSeF) 2 FeX 4 (X=Cl, Br) which have an elliptical in-plane Fermi surface. The resistivity under a magnetic field parallel to conducting layers was found to obey a simple formula, \(\rho_{zz}(B,\phi)=\rho_{zz}(0)\sqrt{1+(\beta(\phi)B/\rho_{zz}(0)){}^{2}}\), where B is a field strength and φ is an angle specifying the field direction. β(φ) is a parameter that is inversely proportional to the Fermi wave number in a direction normal to the magnetic field. We showed that we can map the in-plane Fermi surface, using φ dependence of β. This method is convenient because it works at high temperatures and in low magnetic fields where other methods to map the Fermi surface do not work.

We succeeded in deriving an analytical expression that yields the electrical conductivity for quasi-one-dimensional electron systems, when the current is in the least conductive direction and the magnetic field is perpendicular to it. This formula is valid for the magnetic field along an arbitrary direction in the conducting layers and the field strength from zero to higher fields, where ωτ≫1. We verified the applicability of the formula using the experimental results for an organic quasi-one-dimensional layered metal (TMTSF) 2 ClO 4 . The formula was found to reproduce the experimental results fairly well.

We review the transport phenomena in an organic conductor α-(BEDT-TTF) 2 I 3 . It exhibits various types of transport depending on the circumstance in which it is placed. Under the ambient pressure, it is a charge-ordered insulator below 135 K. When high hydrostatic pressures are applied, it changes to a new type of narrow gap (or zero gap) semiconductor. The conductivity of this system is nearly constant between 300 and 1.5 K. In the same region, however, both the carrier (hole) density and the mobility change by about six orders of magnitude, in a manner so that the effects just cancel out giving rise to the temperature independent conductivity. The temperature ( T ) dependence of the carrier density n obey n ∝ T 2 below 50 K. When it is compressed along the crystallographic a -axis, it changes from the charge ordered insulator to a narrow gap semiconductor. At the boundary between these phases, there appears a superconducting phase. On the other hand, when compressed in the b -axis, the system changes ...

We investigated the magnetoresistance effect on an organic conductor, (ET) 3 Cl(DFBIB), which has a quasi-two-dimensional electron system. The out-of-plane resistivity in transverse magnetic fields...