Temperature independent conductivity of α-(BEDT-TTF)2I3

Transport properties of the organic conductor α-(BEDT-TTF)₂I₃ were studied in the temperature between 300K and 1.5K, in the magnetic field up to 7T and under the pressure up to 20kbar. We found that the electron (hole) density decreases with decreasing temperature, while the mobility of carriers increases with the lowering temperature. It is interesting that those changes just compensate each other and result in the temperature independent conductivity.     

Thin films of the organic superconductor α-(BEDT-TTF)2I3

α-(BEDT-TTF)₂I₃ is a quasi-two-dimensiona1 organic metal with a metal-insulator phase transition at 135 K. Thermal treatment at about 80°C leads to the metallic system α₁-(BEDT-TTF)₂I₃ which becomes superconducting below 8 K. Thin films of the α-phase (between 500 and 3000 A thick) have been evaporated in high vacuum onto several substrates and characterised by means of X-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy (AFM) and a low field microwave absorption technique. Depending on the temperature of the substrate and the evaporation rate, the films exhibit different degrees of microcristallinity, which under certain conditions can be strongly reduced and a completely covering film can be obtained. X-ray diffraction spectra reveal a high orientation with the c-axis perpendicular to the substrate and as well the successful conversion into the α₁- phase by tempering. SEM and AFM investigations prove that the conversion takes place without reducing the mechanical quality of the films. Low-field microwave-absorption experiments show that the α₁-films become superconducting with an onset at 9 K.     

Angular magnetoresistance oscillations and magnetic breakdown in θ-(BEDT-TTF)2I3

We have conducted a systematic investigation of the angular dependent magnetoresistance oscillations (ADMRO) and Shubnikov-de Haas oscillations (SdH) in the quasi-two dimensional organic superconductor θ(BEDT-TTF)₂I₃. The Fermi surface derived from the AMDRO is found to correspond in size to the SdH oscillation orbit β, which is due to magnetic breakdown. The identification of this orbit highlights the sensitivity of the ADMRO. The ADMRO anisotropy is interpreted in terms of magnetic breakdown probability.     

Reexamination of angle dependent magnetoresistance oscillation in θ-(BEDT-TTF)2I3

It is found that the angle-dependent magnetoresistance oscillation in θ-(BEDT-TTF))₂I₃ consists of the short-period oscillation and the long-period one. They are attributed to the magnetic breakdown orbit and the closed orbit at the Brillouin zone boundary, respectively.     

Phase transition of γ-(BEDT-TTF)3(HSO4)2

The structural properties and the metal-insulator (MI) transition of γ-ET₃(HSO₄)₂ (1) are discussed, in comparison with those of γET₃(ClO₄)₂ (2). Transition of 1 is related to a structural change, as in the case of 2. The substitution of hydrogen of the anion with deuterium lengthens the O O distance of intermolecular hydrogen bonds between the two anions, although an MI transition temperature is not changed appreciably. The hydrogen bonds between the two anions make the rotation of the anion molecules strongly hindered and shift their positions close to each other, which enables the puckering motion of the ethylene bridge of ET molecules easily.     

The Shubnikov-de Haas oscillations and a small closed orbit in θ-(BEDT-TTF)2I3

The Shubnikov-de Haas (SdH) experiments on an organic conductor θ-(BEDT-TTF)₂I₃ have been carried out. In addition to the two SdH frequency branches due to the quasi two-dimensional Fermi surfaces, new branches due to small three-dimensional Fermi surfaces are found.     

Electrical anisotropy of α-(BEDT-TTF)2KHg(SCN)4

The transverse magnetoresistance of α-(BEDT-TTF)₂KHg(SCN)₄ was investigated with the magnetic field rotated within a conducting ac-plane. It was found that the magnetic-field-orientation dependence of the magnetoresistance in the weak-field limit, ΔR(B,θ), has the form ΔR(B,θ) = B²(psin²(θ − θ_min) + qcos²(θ − θ_min), where θ is the angle betweeen a-axis and magnetic field direction and (p, q, θ_min) are temperature-dependent parameters. By examining the results based on the classical theory of magnetoresistance, it was concluded that the electrical anisotropy within be-plane is 3.5 4.5 and 2.5 3.0 above and below the phase transition at 10 K, respectively.     

-NMRC-NMR study on α-(BEDT-TTF)2MHg(SCN)4

The low temperature electronic states of the organic conductors, α-(BEDT-TTF)₂MHg(SCN)₄ [M = K, Rb], have been studied by -NMR¹³C-NMR with the use of ¹³C labeled samples. We observed anomalies in the nuclear spin-lattice relaxatin rate, T₁, and shift of the NMR line below 8 K for the K salt and below 12 K for the Rb salt. These indicate some phase transition. However, absence of critical fluctuation near the transition temperature and absense of splitting or broadening of the NMR line is against the static magnetic ordering as found in usual SDW systems such as the TMTSF compounds. The present results imply that the electronic density of states decreases to a half below the transition.     

Superconductivity in reticulate doped polycarbonate films, containing (BEDT-TTF)2I3

Electric properties of conducting reticulate doped polymeric films, containing ET polyiodide crystalline network, have been investigated. In order to improve their conducting properties they were annealed in iodine vapors. It was found, that certain conditions of thermal treatment lead to metal-like state, stable down to helium temperatures. The overall decrease of resistivity R_300K/R_4.2K reaches 45. At some conditions of the thermal treatment an accelerated decrease of resistivity is observed below 7 K, vanishing with application of magnetic field, thus giving the first evidence of the (incomplete) superconducting transition in polymeric organic composite material.     

Low-frequency Raman spectra in κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTF)2Cu[N(CN)2]Br

The ordering of the terminal ethylene groups of the BEDT-TTF molecule, i.e., the staggered or eclipsed conformation, in κ-(BEDT-TTF)₂Cu(NCS)₂ and κ-(BEDT-TTF)₂Cu[N(CN)₂]Br was studied between 18 and 295 K by Raman scattering. The low-frequency spectra of these compounds are similar to each other. Broad peaks at about 55 cm⁻¹ extremely broadened and their intensities became weak with decreasing temperature. The anomalous behaviour was interpreted in terms of critical dynamics of the pseudospin-phonon coupled system, where the spin states represent the conformations of the terminal ethylene groups. It was found that the ordered state is formed around the superconducting critical temperature.     

Specific heat study of α-(BEDT-TTF)2MHg(SCN)4

The low-temperature specific heat of α-phase of (BEDT-TTF)₂MHg(SCN)₄ (M = K, Rb, NH₄), where BEDT-TTF is bis-(etylenedithio) tetrathiafulvalene, is studied. The electronic specific heat coefficient (γ) estimated from the data between 1K and 2.3K amounts to 5.3, 10.3, 28.8mJ/mol K² for M = K, Rb and NH₄, respectively. The K-salt does not have distinct anomally around 8K within the precision of our experiments, implying that the transition occurs with a gradual change of electronic structure.     

Anisotropic superconductivity in κ-(BEDT-TTF)2Cu(NCS)2

To determine the symmetry of the superconducting energy gap, we measured the ac-susceptibility of κ-(BEDT-TTF)₂Cu(NCS)₂. With shielding geomagnetic field, we confirmed the power law temperature dependence of magnetic-field penetration depth. This result indicates an anisotropic superconductivity of gapless nature in this system. The penetration depth extrapolated to 0K for parallel field was found to be 0.22mm of macroscopic size.     

Magnetic field dependence of the cyclotron effective mass in the organic superconductor κ-(BEDT-TTF)2I3

κ-(BEDT-TTF)₂I₃ is an electronically extreme two-dimensional (2d) organic metal with a superconducting transition at around 4K. In magnetic fields above 12T the effective mass, as obtained from the temperature dependence of the amplitudes of Shubnikov-de Haas (SdH) oscillations, is magnetic field dependent as long as the magnetic field is arranged perpendicular to the conducting planes. In contrast to this by turning the magnetic field 27° or even only by 9° the observed effective mass (as obtained from SdH- and de Haas-van Alphen (dHvA)-measurements) is field independent. We suppose that the occurrence of anyons at temperatures below 1K and in fields above 12T is the reason for the observed field dependence of the effective mass.     

High field magnetization measurements on α-(BEDT-TTF)2KHg(SCN)4

We report measurements of the steady and oscillatory part of the magnetization by means of a torquemeter at temperatures down to 0.4K and magnetic fields up to 28T. Below 24T we find the well known splitting in the de Haas van Alphen- (dHvA-) oscillations for T ≤ 1K and Θ ≤ 25°, where Θ is the angle between the magnetic field and the normal to the layers. The phase transition at 24T is indicated by a strong increase of the dHvA amplitude. At low fields the steady torque shows a quadratic dependence on B, whereas in the high field regime a complex dependence on the temperature and the angle θ is observed. For Θ ≥ 60° several field induced anomalies occur up to the maximal field of 28T for temperatures well below 4K. But also for small values of θ our experiments show evidence for a complex magnetic groundstate at 28T for temperatures below 1K.     

Magnetoresistance anisotropy in the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2

We report on systematic studies of the magnetoresistance (MR) anisotropy of κ-(BEDT-TTF)₂Cu(NCS)₂ at ambient pressure and at P = 1.2 kbar. Prominent oscillations of the interplane MR are found under magnetic field rotating in various planes normal to the 2D bc-plane. The oscillations are mainly associated with the existence of the open Fermi surface along the K_aK_c-plane, revealing however somewhat complicated behaviour which is most likely due to the specifics of multiply connected Fermi surface.     

Fermi surface reconstruction in (BEDT-TTF)2TlHg(SCN)4

The Shubnikov-de Haas (SdH) measurements of the two dimensional organic conductor (BEDT-TTF)₂TlHg(SCN)₄ have been performed to investigate the Fermi surface reconstructed by the spin density wave formation. At 0.05 K, ten different SdH oscillations are found. A model of the reconstructed Fermi surface is proposed on the basis of the relation between the observed SdH frequencies.