J.S. Qualls

Coherent versus incoherent interlayer transport in layered metals

The magnetic-field, temperature, and angular dependence of the interlayer magnetoresistance of two different quasi-two-dimensional (2D) organic superconductors is reported. For κ-(BEDT-TTF) 2 I 3 , where BEDT-TTF is bisethylenedithio-tetrathiafulvalene, we find a well-resolved peak in the angle-dependent magnetoresistance at Θ=90° (field parallel to the layers). This clear-cut proof for the coherent nature of the interlayer transport is absent for β-(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 . This and the nonmetallic behavior of the magnetoresistance suggest an incoherent quasiparticle motion for the latter 2D metal.

Argument for charge density wave sub-phases in the ground state of α-(BEDT-TTF)2KHg(SCN)4

Abstract A resistive anomaly at temperature T p in the title compound is associated with a Fermi surface reconstruction from a metallic to a (spin or charge) density wave state. At high magnetic fields a corresponding feature in the magnetoresistance above a field B K indicates the breaking of this state. We argue that T P indicates a second order phase line identical to that measured by specific heat methods and show that it decreases monotonically up to 30T. We find that Pauli (rather than orbital) effects, dominate the reduction in T p . We further argue that B K is a first-order transition between two subphases below T p . We compare the phase diagram with recent theoretical models for CDW and SDW ground states in high magnetic fields.

Field-induced metal-insulator transition in a two-dimensional organic superconductor.

The quasi-two-dimensional organic superconductor beta-(BEDT-TTF)2SF5CH2CF2SO3 (Tc approximately 4.4 K) shows very strong Shubnikov-de Haas (SdH) oscillations which are superimposed on a highly anomalous steady background magnetoresistance, Rb. Comparison with de Haas-van Alphen oscillations allows a reliable estimate of Rb which is crucial for the correct extraction of the SdH signal. At low temperatures and high magnetic fields insulating behavior evolves. The magnetoresistance data violate Kohlers rule, i.e., cannot be described within the framework of semiclassical transport theory, but converge onto a universal curve appropriate for dynamical scaling at a metal-insulator transition.

Low-frequency method for magnetothermopower and Nernst effect measurements on single crystal samples at low temperatures and high magnetic fields

We describe an alternating current method for the measurement of the longitudinal (Sxx) and transverse (Sxy, i.e., Nernst) thermopower of millimeter-size crystal samples at low temperatures (T<1 K) and high magnetic fields (B∼30 T). A low-frequency (33 mHz) heating method is used to increase the resolution and to determine the temperature gradient reliably in high magnetic fields. Samples are mounted between two thermal blocks which are heated by a sinusoidal frequency f0 with a π/2 phase difference. The phase difference between two heater currents gives a temperature gradient at 2f0. The corresponding thermopower and Nernst effect signals are extracted by using a digital signal processing method due to the low frequency of the measurement. An important component of the method involves a superconducting link, YBa2Cu3O7+δ, which is mounted in parallel with sample to remove the background magnetothermopower of the lead wires. The method is demonstrated for the quasi-two-dimensional organic conductor α-(BEDT...

Quantum Melting of the Quasi-Two-Dimensional Vortex Lattice inκ-(ET)2Cu(NCS)2

We report torque magnetization measurements in regions of the mixed state phase diagram ( B approximately mu(o)H(c2) and T(c)/10(3)) of the organic superconductor kappa-(ET)2Cu(NCS)(2), where quantum fluctuations are expected to dominate thermal effects. Over most of the field range below the irreversibility line ( B(irr)), magnetothermal instabilities are observed in the form of flux jumps. The abrupt cessation of these instabilities just below B(irr) indicates a quantum melting transition from a quasi-two-dimensional vortex lattice phase to a quantum liquid phase.

Violation of Kohler's rule by the magnetoresistance of a quasi-two-dimensional organic metal

The interlayer magnetoresistance of the quasi-two-dimensional metal alpha-(BEDT-TTF)(2)KHg(SCN)(4) is considered. In the temperature range from 0.5 to 10 K and for fields up to 10 T the magnetoresistance has a stronger temperature dependence than the zero-field resistance. Consequently Kohlers rule is not obeyed for any range of temperatures or fields. This means that the magnetoresistance cannot be described in terms of semiclassical transport on a single Fermi surface with a single scattering time. Possible explanations for the violations of Kohlers rule are considered, both within the framework of semiclassical transport theory and involving incoherent interlayer transport. The issues considered are similar to those raised by the magnetotransport of the cuprate superconductors. [S0163-1829(98)13219-8].

Josephson plasma resonance in κ − ( BEDT − TTF ) 2 Cu ( NCS ) 2

A cavity perturbation technique is used to study the microwave response of the organic superconductor k-(BEDT-TTF)2Cu(NCS)2. Observation of a Josephson plasma resonance, below Tc (approx. 10 K), enables investigation of the vortex structure within the mixed state of this highly anisotropic, type-II, superconductor. Contrary to previous assumptions, frequency dependent studies (28 - 153 GHz) indicate that the squared plasma frequency depends exponentially on the magnetic field strength. Such behavior has been predicted for a weakly pinned quasi-two-dimensional vortex lattice [Bulaevskii et al. Phys. Rev. Lett. 74, 801 (1995)], but has not so far been observed experimentally. Our data also suggests a transition in the vortex structure near the irreversibility line not previously reported for an organic superconductor using this technique.

Fermiology of the organic superconductor β″-(ET)2SF5CH2CF2SO3

We present a detailed Fermi-surface (FS) investigation of the quasi two-dimensional (2D) organic superconductor (T c = 4.5 K) β-(ET) 2 SF 5 CH 2 CF 2 SO 3 . In line with previous investigations, de Haas-van Alphen measurements in pulsed fields up to 60 T show a single oscillation frequency, F 0 = 200 T, which corresponds to a FS size of about 5% of the first Brillouin zone. Angular dependent magnetoresistance oscillations (AMROs) are utilized for the exact determination of the in-plane FS, which is found to be a strongly elongated ellipsoid with an axes ratio of about 1:9. Transport measurements in static fields up to 33 T show an unusual temperature dependence of the Shubnikov-de Haas (SdH) signal, i.e., a decrease of the SdH amplitude with decreasing temperature.

Magnetothermopower study of the quasi-two-dimensional organic conductor α − ( BEDT − TTF ) 2 KHg ( SCN ) 4

We have used a low-frequency magneto-thermopower (MTEP) method to probe the high-magnetic-field ground-state behavior of

High magnetic field corrections to resistance thermometers at low temperatures

ensuremath{alpha}ensuremath{-}(mathrm{BEDT}ensuremath{-}mathrm{TTF}{)}_{2}mathrm{KHg}(mathrm{SCN}{)}_{4}