# Elastic studies of phase transitions in Bechgaard salts, yttrium barium copper oxygen(7-delta) high critical transition temperature superconductor and fullerene

#### Abstract

Using a vibrating reed technique, we have studied the sound velocity of several materials which undergo electronic and structural phase transitions. The sound velocity in the highly anisotropic (TMTSF)-organic conductors were studied at low temperature and in high magnetic field to investigate the properties associated with the cascade of magnetic field-induced-spin-density-wave (FISDW) transitions. The dramatical response of the elastic constant at the Peierls-like transitions reflects a large stress dependence of the system's critical temperatures and fields. The characteristically different behaviors of the sound velocity in different temperature regions enables us to distinguish a low temperature insulating state from the higher temperature reentrant metallic state. Fast quantum oscillations are observed with a greatly enhanced amplitude accompanied by higher harmonic content and a phase shift when crossing the reentrant phase line, indicating coupling between the FISDW with the fast oscillations. Studies on the high-temperature superconductor YBaCu$\sb3$O$\sb{7-\delta}$ reveals anomalously large softening in the elastic constant at the superconducting transition temperature. Hardening of the lattice below Tc indicates strong dependence of the transition temperature on the second derivative of the stress. The profound attenuation peaks at 135K and 220K suggest subtle structure transformations in the lattice. Probing the flux dynamics in YBaCu$\sb3$O$\sb{7-\delta}$ with the vibrating reed, we observed low level flux dissipation at very low field, contradicting the proposal of a vortex liquid-solid transition in the system. The critical current associated with the flux line pinning is determined to be $0.5\times10\sp6 A/cm\sp2.$ The thermally activated flux creep is characterised by a low energy barrier of 20K. Sound velocity and attenuation studies on C$\sb{60}$ fullerene found a 8% jump in the elastic constant at the first order orientational ordering transition. We discovered a frequency dependent attenuation peak at 160K for $20kHz$ probing frequency. This "glassy" behavior is associated with the relaxation of the C$\sb{60}$ molecules to different orientational states. The energy barrier for the activated relaxation is on the order of $\sim$3000K.

Condensation

#### Recommended Citation

Shi, Xiangdong, "Elastic studies of phase transitions in Bechgaard salts, yttrium barium copper oxygen(7-delta) high critical transition temperature superconductor and fullerene" (1992). Dissertations available from ProQuest. AAI9308661.
https://repository.upenn.edu/dissertations/AAI9308661

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