T. Kinoshita

Fermiology and superconductivity studies on the non-tetrachalcogenafulvalene-structured organic superconductor β-(BDA-TTP)2SbF6

The quantum oscillatory effect and superconductivity in a non-tetrachalcogenafulvalene (TCF) -structure-based organic superconductor β-(BDA-TTP) 2 SbF 6 are studied. Here the Shubnikov-de Haas effect and angular-dependent magnetoresistance oscillations are observed. The oscillation frequency associated with a cylindrical Fermi surface is found to be about 4050 T, which is also verified by a tunnel diode oscillator (TDO) measurement. The upper critical-field (H c 2 ) measurement in a tilted magnetic field and the TDO measurement in the mixed state reveal a highly anisotropic superconducting nature in this material. We compare the physical properties of β-(BDA-TTP) 2 SbF 6 with typical TCF-structure-based quasi-two-dimensional organic conductors. A notable feature of the β-(BDA-TTP) 2 SbF 6 superconductor is a large value of effective cyclotron mass m* c =12.4′1.1m e , which is the largest yet found in an organic superconductor. A possible origin of the enhanced effective mass and its relation to the superconductivity are briefly discussed.

Magnetoresistance and magnetic breakdown in the quasi-two-dimensional conductors (BEDT-TTF)(2)MHg(SCN)(4)[M=K,Rb,Tl]

The magnetic-field dependence of the resistance of~BEDT-TTF! 2MHg~SCN!4@M5K,Rb,Tl# in the densitywave phase is explained in terms of a simple model involving magnetic breakdown and a reconstructed Fermi surface. The theory is compared to measurements in pulsed magnetic fields up to 51 T. The value implied for the scattering time is consistent with independent determinations. The energy gap associated with the densitywave phase is deduced from the magnetic breakdown field. Our results have important implications for the phase diagram. @S0163-1829~96!52036-9#

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.

High-field Shubnikov-de Haas effect in the organic superconductor α-(BEDT-TTF)2NH4Hg(SCN)4

Abstract We report Shubnikov-de Haas (SdH) measurements on the quasi-two dimensional organic superconductor α-(BEDT-TTF)2NH4Hg(SCN)4 in pulsed fields up to 50 T. Analysis of the field and temperature-dependence of the oscillations reveals several unusual features, such as a field-dependent effective mass parameter, non-linear Dingle plots and an anomalously large harmonic ratio. These results suggest departures from the Lifshitz-Kosevich formalism, arising from the highly two-dimensional nature of the material.

Effect of Cooling Speed on Superconductivity of κ-(BEDT-TTF)2Cu[N(CN)2]Br — Isotope Effect Revisited

We studied the effect of cooling rate on superconductivity of both deuterated and undeuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br and observed a significant difference in the effect of cooling speed not only on Tc but also on the superconducting volume fraction. In particular, it was quite unexpected that an extremely slow cooling rate as low as 0.02K/min was found to have still a significant effect on the superconducting state.

Fermi surface reconstruction in (BEDT-TTF)2TlHg(SCN)4

Abstract 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.

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.

Effect of uniaxial strain in organic superconductor κ-(BEDT-TTF)2Cu(NCS)2

Abstract The effect of uniaxial strain on the superconducting transition temperature ( T c ) was studied in organic superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2 . When the stress was applied along the c -axis in the conducting b – c plane, T c increased by application of stress, in contrast to other cases including the uniaxial-stress perpendicular to the conducting plane and the hydrostatic pressure. Detailed experimental studies on the temperature dependence of magnetization under uniaxial strain using SQUID magnetometer are presented.

Applications of pulsed magnetic fields and low temperatures to low-dimensional (organic) conductor physics

Abstract The advent of user-oriented pulsed magnetic fields combined with low temperature capability has opened up a new perspective on low-dimensional, and in particular, organic conductor physics. Opportunities include: the application of Fermiology methods both below and above the magnetic field induced phase transitions in the density wave materials; the quest for the quantum limit in bulk low-dimensional materials; and the search for electron interaction effects in the quasi-one-dimensional limit. This paper will focus on several such applications and directions in the (BEDT—TTF) 2 MHg(SCN) 4 quasi-two-dimensional and (TMTSF) 2 ClO 4 quasi-one-dimensional organic conductors.

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.