S.J. Klepper

Uniaxial stress method for delicate crystals: Application to Shubnikov-de Haas and superconductivity studies in organic conductors

A method to measure the resistance of fragile single crystals under high uniaxial stress is reported. To compensate for its natural brittleness, the crystal is embedded in epoxy, whose shape is optimized to sustain the stress. This method was used to study the magnetoresistance of (BEDT‐TTF)2KHg(SCN)4 at low temperatures and high magnetic fields and the superconducting transition temperature of (BEDT‐TTF)2Cu(NCS)2 under uniaxial stress perpendicular to the planes of BEDT‐TTF molecules. The results obtained indicate a high degree of homogeneity of the stress in the crystal and comparison with previous hydrostatic pressure measurements reveal the true uniaxial nature of the stress applied.

Magnetic breakdown in the organic conductor (BEDT-TTF)2KHg(SCN)4

Abstract We have carried out the Shubnikov-de Haas (SdH) measurements of the quasi-two-dimensional (2D) conductor (BEDT-TTF)2KHg(SCN)4. A new SdH oscillation with the frequency of 4260 T for Hue643b ∗ -axis is observed above 10 T. The oscillation is found to come from the magnetic breakdown (MB) orbit. The observation of the MB indicates that the nesting over the large portion of the 1D Fermi surface is not present under magnetic fields above 10 T. The magnetoresistance maximum behavior and its anisotropy are consistently explained in terms of the MB effect.

Tunnel diode oscillator application to high sensitivity de Haas–van Alphen and superconducting critical field studies of anisotropic organic conductors

We report the first application of a tunnel diode circuit to investigate the de Haas–van Alphen effect and superconducting critical field values in small single crystal organic conductors. The materials of interest are in the family of charge transfer salts (BEDT‐TTF)2X, where X is one of a number of anion structures. Small (≤1 mm3) oriented single crystal samples were placed inside the inductance coil of the LC tank circuit of the oscillator, and measurements were carried out at low temperatures and in high magnetic fields. We observe a marked improvement over previous magnetic methods for the determination of both de Haas–val Alphen signals and for critical field studies. The method is applicable where such information is required for very small samples.

New clues in the mystery of the ground state of the (BEDT-TTF)2MHg(SCN)4 family of organic conductors

Abstract Since the discovery of a kink in the magnetoresistance of (BEDT-TTF)2KHg(SCN)4 by the Tokyo group [Osada et al., Phys. Rev. B41 (1990) 5428] several years ago, this family (where K may be replaced by NH4, Tl, or Rb) has become one of the most exciting areas of organic conductor research. At the heart of the matter is the interplay of the open and closed orbit sections of the Fermi surface, in which subtle details of topology apparently play major roles in the low temperature state (LTS). Recent experiments involving angular dependent magnetoresistance, and determinations of magnetic breakdown orbits provide clues to the mechanisms which act in the LTS. We present a model which addresses the anomalous magnetoresistance phenomena in the LTS. The model involves the connection via magnetic breakdown of otherwise disconnected closed hole and open electron bands.

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.

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

Abstract 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) 2 I 3 . 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.

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.