Y. Tanaka

Magneto-oscillatory behavior and magnetism in quasi-two-dimensional organic conductors

Abstract We have studied the magneto-oscillatory behavior of quasi-two-dimensional (quasi-2D) organic conductors of the type (BEDT-TTF)2XHg(SCN)4, where X = K, Rb, NH4, in order to probe the underlying magnetic behavior of these materials. The K and Rb compounds are metallic down to 50 mK, but are not superconducting. They both display a dual series of magneto-oscillations, a magnetic ordering at low temperatures, below about 10 K, and a sharp drop in the magnetoresistance associated with the destruction of this phase with the application of high magnetic fields at low temperatures. In contrast, the NH4 material is a superconductor at low temperatures (below 1K) up to a pressure of at least 4 kbar. It also displays a complex background magnetoresistance upon which is superposed a single oscillation series. In all three organic materials, there is a strong interaction between the underlying magnetic phases (which influences the open orbits) and the magneto-oscillations which result from the quasi-2D Fermi surface behavior.

Fermi surface and magnetic properties of low-dimensional organic conductors

Abstract We report systematic high-magnetic-field and low-temperature measurements on several members of the α-(BEDT- TTF)2MHg(SCN)4 family of low-dimensional organic conductors. For M = K, transport, high-pressure and magnetic studies show the presence of a magnetic phase which is removed either with high magnetic fields or with high pressure. For M = Rb, transport measurements indicate the existence of a magnetic phase similar to the case for M = K, but with a higher transition temperature and critical field. For both M = K and Rb the split Landau level phenomenon is observed. We also report the pressure dependence of the Fermi surface for M = NH4 and K.

Uniaxial stress study of the transport properties of the quasi-two-dimensional organic superconductor (BEDT-TTF)2Cu(NCS)2

Abstract We report the effects of uniaxial stress applied along the least conducting direction (a-axis) of the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2. We find that the superconducting transition temperature decreases with stress (dTc/dpa = −2 K/kbar). Furthermore, we find an increase in the area of the closed orbits on the Fermi surface with stress. These observations are at variance with predictions based solely on the expected expansion of the conducting plane unit cell by the Poisson effect. We discuss our results in the light of previous stress measurements.

Pressure effects on the low temperature state of the α-(BEDT-TTF)2MHg(SCN)4 organic conductor family

Abstract Systematic magnetotransport measurements as a function of hydrostatic pressure have been made on four members of the α-(BEDT-TTF) 2 MHg(SCN) 4 organic conductor family (M = K, Tl, Rb, NH 4 ). Applied pressure above about 6 Kbar removes the density wave state for M = K, Tl, and Rb. For M = NH 4 the superconducting state is removed with pressure as dT c dP = − 0.25 K/Kbar . In all cases the Shubnikov de Haas oscillation frequency increases with pressure, including the β orbit (which involves the entire Brillouin zone), and new orbits involving very small fractions of the Fermi surface are formed.

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

Abstract Measurements of the low temperature magnetoresistance of α-(BEDT-TTF) 2 KHg(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.

Scanning probe microscopy and spectroscopy study of the organic salt (ET)2KHg(SCN)4

The surfaces of cleaved single crystals of (ET)2KHg(SCN)4 [where ET is bis(ethylenedithio)tetrathiafulvalene] were imaged by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Two distinct surfaces are observed with STM: a complete (ET)+2 layer and a complete KHg(SCN)−4 layer. STM images of the cation layer are independent of scanning tip polarity, which is consistent with its metallic character. By contrast, images of the anion layer are strongly dependent upon the polarity of the tip and thus provide information about occupied and unoccupied molecular orbitals in the KHg(SCN)−4 surface. The AFM images were obtained at ambient conditions and correlate well with STM images of the (ET)+2 layer. Preliminary spectroscopy measurements on cleaved crystals at low temperature indicate that the insulating ground state phase contains a 3 meV gap in the density of states, which has a value of 2Δ/kTc=8.

The phase diagram of α-(ET)2TlHg(SCN)4: An electrodynamic investigation

Abstract We have probed the phase diagram of α-(ET) 2 TlHg(SCN) 4 using a mm-wave technique. A substantially different behavior is seen for the conductivity parallel and perpendicular to the 2D layers as the various phase boundaries are cut. We compare our findings with published data and discuss our results in terms of recent models for the low-temperature phase diagram of this family of organic metals.

Open and closed Fermi surface contributions to the anomalous angular magnetoresistance of α-(BEDT-TTF)2RbHg(SCN)4

Abstract Anomalous angular magnetoresistance (AMR) in the quasi-two dimensional organic conductor α-(BEDT-TTF) 2 RbHg(SCN) 4 is reported. The AMR appears as oscillations with sharp minima below the anitiferromagnetic ordering temperature. The period of these oscillations is anisotropic with respect to the plane of rotation cutting through the conducting layers. Above the ordering temperature, the nature of the AMR changes fundamentally. We propose a model for the AMR that incorporates both open and closed Fermi surfaces, and discuss how temperature and field dependent behaviors of the individual FS contribute to the conductivity.