New organic superconductor, λ-(BETS)2GaCl4 and metal-insulator transition of BETS conductor with magnetic anions (BETS = bis(ethlenedithio)tetraselenafulvalene)

Crystal structutres and electronic properties of BETS compounds with tetrahedral anions, MX₄ (BETS = bis(ethylenedithio)tetraselenafulvalene; M = Ga, Fe, In; X = Cl, Br) were examined. The salts can be classified into two types (κ, λ). All the compounds with so-called κ-type molecular arrangements retained metallic behavior down to 4 K. λ-(BETS)₂GaCl₄ demonstrated a a superconducting transition around 8 K. Whereas λ-(BETS)₂FeCl₄ with the isomorphic λ-type structure exhibited a sharp metal-insulator transition at 8 K. Low-temperature X-ray studies showed that the structural difference between λ-GaCl₄ and λ-FeCl₄ salts is very small. The structure analysis of κ-(BETS)₂FeCl₄ at 18 K indicated that the FeCl₄ anion has approximately C_3v symmetry. The ESR measurements revealed the high-spin state of Fe³⁺ in λ-FeCl₄ salt and and low-spin state in κ-FeCl₄ salt. ESR intensities of κ- and λ-FeCl₄ salts suggested antiferromagnetic interaction between Fe³⁺ ions.     

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.     

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.     

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.     

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.     

-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.     

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.     

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.     

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 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.     

Thermal expansion studies on Th(MoO4)2, Na2Th(MoO4)3 and Na4Th(MoO4)4

Thermal expansion behavior of Th(MoO₄)₂, Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ was studied under vacuum in the temperature range of 298–1123 K by high temperature X-ray diffractometer. Th(MoO₄)₂ was synthesized by reacting ThO₂ with 2 mol of MoO₃, at 1073 K in air and Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ were prepared by reacting Th(MoO₄)₂ with 1 and 2 mol of Na₂MoO₄, respectively at 873 K in air. The XRD data of Th(MoO₄)₂ was indexed on orthorhombic system where as XRD data of Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ were indexed on tetragonal system. The lattice parameters and cell volume of all the three compounds, fit into polynomial expression with respect to temperature, showed positive thermal expansion (PTE) up to 1123 K. The average value of thermal expansion coefficients for Th(MoO₄)₂, Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ were determined from the high temperature data.     

Relationship between effective mass and superconducting critical temperature in the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2

We report high pressure magnetotransport on the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂. The observation of Shubnikovde Haas and magnetic breakdown oscillations has allowed the pressure dependences of the Fermi surface topology and quasiparticle effective masses to be deduced and compared with simultaneous measurements of the superconducting critical temperature T_c. The data strongly suggest that the enhancement of the effective mass and the superconducting behaviour are directly connected. The results are fitted by calculations of the linearised Eliashberg equations.     

De Haas-van Alphen oscillations near Bc2 in the organic superconductor κ-(ET)2Cu(NCS)2

We report a study of the de Haas-van Alphen effect near the upper critical field in the organic superconductor κ-(ET)₂Cu(NCS)₂. The experiments have been carried out for a range of sample orientations with respect to the magnetic field and of temperatures down to 20 mK. On entering the vortex state, an attenuation of the dHvA signal is observed relative to an extrapolation of the standard Lifshitz-Kosevich formula using band parameters measured in the normal state.     

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.