-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.     

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.     

Magnetic field studies of the peculiar electronic state in the α-(BEDT-TTF)2MHg(SCN)4 family

Magnetoresistance anomalies exhibited by the members of the α-(BEDT-TTF)₂MHg(SCN)₄ family with M = Tl, K and Rb are reviewed in connection with possible modification of their electronic structure at low temperatures. Special attention is paid to angular magnetoresistance oscillations (AMRO). The analysis of the AMRO behavior at various temperatures and magnetic fields, both below and above the phase transition temperature, enables us to get an important information about changes in the electronic band structure at the phase transition. Other peculiarities of both semi-classical magnetoresistance and quantum oscillations are discussed within the proposed model of the low-temperature state of these compounds.     

Magnetic property of the low temperature phase of α-(BEDT-TTF)2KHg(SCN)4

The low temperature phase of the organic conductor, α-(BEDT-TTF)₂KHg(SCN)₄, has been investigated by EPR measurements. The spin susceptibility determined by the EPR intensity is 3 × 10⁻⁴ emu/mol at room temperature, in agreement with the static susceptibility measurement. It decreases slightly with decreasing temperature down to 10 K, below which a rapid drop of the intensity is observed for all directions of the applied field. This temperature is close to the one which the resistivity anomaly was observed at. At temperatures below 20 K, the EPR signal was found to split into two or three lines depending on the external field direction. We discuss the possible origin of the splitting.     

Anisotropic Hc2, Shubnikov de Haas, and angular dependent magnetoresistance in the organic superconductor α-(BEDT-TTF)2MHg(SCN)4

We report measurements on the anisotropic critical field, the angular dependent magnetoresistance, and Shubnikov de Haas oscillations in α-(BEDT-TTF)₂NH₄Hg(SCN)₄, the only ambient pressure superconductor (T_c = 1.2K) in the α-(BEDT-TTF)₂MHg(SCN)₄ organic conductor family. The anisotropic critical field is well described by a 2-D model for layered supercondcutors where the anisotropy ε = Hc\\ / H_c is 25 at low temperatures, and rises to 100 near T_c. Beating behavior is observed in the SdH effect in tilted magnetic fields.     

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.     

Mysterious ground states in the organic conductor α-(BEDT-TTF)2KHg(SCN)4 mixed SDW and CDW?

We present magnetoresistance measurements in static fields up to 30T on the organic conductor α-(BEDT-TTF)₂KHg(SCN)₄ . The current and field direction dependence of the magnetoresistance with respect to the crystallographic axis are described, suggesting a topological change of the Fermi surface. The change results from the phase transition from the normal metal to the mysterious ground state. We discuss a possible ground state of mixed charge-density and spin-density waves from the viewpoint of the orbital and Pauli couplings of the external field to the band electrons.     

Uniaxial stress study of the magnetotransport properties of the quasi-two dimensional organic conductor α-(BEDT-TTF)2KHg(SCN)4

Measurements of the low temperature magnetoresistance of α-(BEDT-TTF)₂KHg(SCN)₄ under uniaxial stress perpendicular to the BEDT-TTF molecular layers indicate that the density wave ground state is suppressed by 1.5 Kbar. The fundamental Shubnikov-de Haas frequency decreases at an approximate rate of −20 tesla/Kbar and a slow, stress dependent oscillation appears for stresses above 0.9 Kbar. These enhanced stress effects are discussed in light of the high sensitivity of the Fermi surface to small changes in the crystal structure.     

Superconductivity ia α(BEDT-TTF)2MHg(SCN)4 (M = K, Rb, Tl, NH4)

The layered organic metals (α-(BEDT-TTF)₂MHg(SCN)₄ (M = K, Rb, Tl) show resistance decrease with the onset at 0.3 K, 0.5 K and 0.1 K, respectively. These decreases are suppressed by a magnetic field less than 0.2 T and considered as an occurence of superconductivity, although zero resistance is not achieved. We discuss this incomplete superconducting transition in terms of the vicinity to a superconductor-insulator transition caused by the elevated sheet resistance due to the antiferromagnetic ordering at 8 10 K.     

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.     

Angular dependence of the high field phase transition in α-(BEDT-TTF)2TlHg(SCN)4

We report results of a systematic study of the anomalous low temperature state (LTS) angular magnetoresistance (MR) in the quasi-two dimensional organic conductor α-(BEDT-TTF)₂TlHg(SCN)₄ in fields up to 30 tesla. The high field suppression of the LTS is found to be independent of angle, while the MR maximum, H_max increases with angle in conjunction with the well studied angular dependent magnetoresistance oscillations (ADMRO). The different magnetic breakdown probabilities above and below H_max are found to be the source of a novel high field change in the ADMRO shape.     

Resistive transition in an extremely Two-Dimensional superconductor, α-(BEDT-TTF)2NH4Hg(SCN)4

In order to investigate the superconductivity in α-(BEDT-TTF)₂NH₄Hg(SCN)₄, resistivity was measured as a function of temperature, magnetic field and exciting current. In addition, simultaneous measurements of I-V characteristics and ac susceptibility were carried out. The results indicate highly two-dimensional character of the superconductivity.     

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.     

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.     

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.     

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.     

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.