C.E. Campos

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.

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.

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.

In-plane uniaxial stress study of a low dimensional organic metal

Abstract We have studied the effects of in-plane uniaxial stress along both principal axes for the quasi-two-dimensional organic conductor (BEDT-TTF) 2 KHg(SCN) 4 in terms of changes in the magnetotransport properties in high magnetic fields. This class of charge transfer organic metal salts is unique due to the co-existence of both closed and open orbit bands at the Fermi level. We find that stress along both the a - and c -axis directions (in the plane of the quasi-two-dimensional conducting planes) destroys the low temperature density wave state, but in differing ways, and increases the extremal areas of the closed orbit bands. For stress along the a -axis, new low area orbits are induced, and for stress along the c -axis, the quantum oscillations in the Shubnikov de Haas effect take on highly two-dimensional character. We compare our results with predictions made from band structure calculations for in-plane uniaxial stress.

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.

Tunneling spectroscopy on the organic salts (TMTSF)2ClO4 and (BEDT-TTF)2KHg(SCN)4 at low temperatures

We performed tunneling spectroscopy using a low temperature STM on the organic conductors (TMTSF)2ClO4 and (BEDT-TTF)2KHg(SCN)4 in the metallic state and the (FI-)SDW state. Going through the phase transition we can see a modification of the tunnel conductance. Furthermore we have indications of an energy gap in the SDW state of (BEDT-TTF)2KHg(SCN)4.

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.

Photoluminescence study of the charge transfer salts (BEDT-TTF)2RbHg(SCN)4 and (BEDT-TTF)2KHg(SCN)4

Abstract We have studied the photoluminescence (PL) spectra of the charge transfer salts (BEDT-TTF)2RbHg(SCN)4 at 1.8 K and (BEDT-TTF)2KHg(SCN)4 in the 1.8–77 K temperature range. Spectra at 1.8 K for both materials display an asymmetric peak at 1.97 eV and a shoulder starting at 1.7 eV. The same features were present in the measured PL spectrum of a neutral-donor BEDT-TTF (ET) molecule crystal, which leads us to associate the peak with a 2 eV intramolecular electronic excitation in the ET and the shoulder with vibronic sidebands. The temperature dependence of the PL spectra of doped and undoped crystals is different, and (ET)2KHg(SCN)4 PL is insensitive to the antiferromagnetic (AFM) phase transition that occurs at 8 K.