Alteration of Density Wave and Superconducting Behavior in Quasi-Two Dimensional Organic Conductors Under Uniaxial Stress.

Superconductivity has been induced with cross -molecular plane (b-axis) uniaxial stress in the density wave organic salt alpha-(BEDT-TTF) _2KHg(SCN)_4. The appearance of superconductivity, together with the associated rise in the carrier's effective mass, cannot be explained by a simple band effect; rather, electron -phonon and/or electron-electron interactions must play a predominant role. A new sample preparation method was devised to allow the measurement of resistance in these fragile crystals at stresses above 4 kbar. alpha-(BEDT-TTF) _2KHg(SCN)_4 belongs to the alpha-(BEDT-TTF)_2 MHg(SCN)_4 (M = K, Rb, Tl, and NH_4) isostructural family of quasi-two dimensional organic salts. Under ambient conditions, the M = NH_4 salt is a superconductor below 1.1 K whereas the other three members have a density wave ground state below ~10 K. Measurements of the low temperature magnetoresistance of the M = K salt indicate that the density wave state is quickly suppressed by 1.5 kbar, the point at which superconductivity (rm T_{c}~1K ) appears. Similar measurements for the M = NH _4 salt showed that T_ {rm c} rises to 3.5 K at 4 kbar. Thus it is shown that uniaxial stress effectively unifies the ground state behavior in this family of organic conductors. Analysis of the Shubnikov-de Haas oscillations revealed that the frequency of the hole orbits decreases linearly with stress in agreement with an expansion of the in-plane unit cell due to Poisson's effect, and there is a correlation between the real space area of the closed hole orbits and T _{rm c} for both materials under uniaxial stress or hydrostatic pressure. Also, for the M = K salt, a slow oscillation appears above ~0.9 kbar, a sign that the nesting conditions of the open orbits changes dramatically with stress. Preliminary investigations of the effects of stress on the quasi-two dimensional superconductor kappa -(BEDT-TTF)_2Cu(NCS) _2 and quasi-one dimensional spin density wave insulator (TMTSF)_2PF _6 were also conducted. For kappa -(BEDT-TTF)_2CU(NCS) _2 stress across the molecular planes (a -axis direction) reduced T_{rm c} rapidly and increased the frequency of the Shubnikov-de Haas oscillations. Both these results contradict the expectations based on a simple in-plane expansion due to Poisson's effect. For (TMTSF) _2PF_6 stress along the c-axis reduced the magnitude of the spin density wave particle excitation gap but did not reduce the metal-insulator transition temperature appreciably, which is inconsistent with the standard mean-field behavior.