M. Saint-Paul

Ultrasound study on YBa2Cu3O7−δ and GdBa2Cu3O7−δ single crystals

Abstract Sound velocity results on YBa 2 Cu 3 O 7-δ and GdBa 2 Cu 3 O 7-δ single crystals are reported. Discontinuities in the temperature derivatives of the longitudinal and shear sound velocities are observed at the superconducting transition T c . Hardening in both longitudinal (C 33 ) and shear (C 44 ) modes is observed below T c . Estimations of the second stress derivatives of T c are evaluated using a mean field thermodynamics analysis. A second phase transition is observed around 108 K.

Ultrasound study of YBa2Cu3O7−δ single crystals

Abstract Sound velocity measurements on YBa 2 Cu 3 O 7− δ single crystals are reported. No anomalous behaviour within the experimental resolution (≊5 10 −5 ) was observed in the superconducting state. Present results confirm that the elastic properties of this system can be related to the thermodynamic model which predicts a very small velocity change (≊ 10 −5 ) at the superconducting transition temperature.

Ultrasonic properties of quasi-one dimensional (TaSe4)2I at the Peierls transition

(TaSe4)2I is a quasi-1 D conductor which undergoes a Peierls transition at Tp = 260 K [1]. Phonon softening at 2kF (Fermi momentum) induces lattice modulations associated with an electron charge density wave (CDW). Elastic singularities at the Peierls transition are not yet well understood [2]. The associated singularity in the sound velocity examined theoretically by Nakane [3] varies like |T-Tp|−1/2 around Tp.

Sound velocity in YBa2Cu3O6+x and GdBa2Cu3O6+x single crystals

Abstract Sound velocity measurements on tetragonal YBa 2 Cu 3 O 6+ x and GdBa 2 Cu 3 O 6+ x single crystals are reported. It is shown that the temperature dependence of the relative velocity change Δ v / v in YBa 2 Cu 3 O 6 is fully accounted for by the lattice anharmonicity of an anisotropic continuum. A deviation observed for GdBa 2 Cu 3 O 6 below 100 K is most likely due to residual anelastic relaxation effects.

Monte Carlo simulation of the KCl : OH dipole glass

Abstract Monte Carlo simulation of electric dipole correlations in KCl doped with OH at a concentration of 0.2 at.% are reported. Using a classical model the calculated dielectric susceptibility and specific heat show good agreement with the experimental results obtained around the temperature T F corresponding to the maximum of the dielectric dispersion and characterizing the freezing of the electric dipole system.

Magnetic field dependence of ultrasound velocity in YBaCu3O7 single crystals

Abstract The magnetic field dependence of ultrasound velocity in YBa 2 Cu 3 O 7 single crystal is studied. Below T c ultrasonic dispersion and attenuation are found to be sensitive to the dynamics of the flux-line lattice. The experimental results are compared to the theoretical predictions.

Acoustic anomalies in YBa2CuO3O7−δ thin films

Abstract The attenuation results of surface acoustic waves by epitaxial YBa 2 Cu 3 O 7−δ thin films are reported. The large attenuation observed around 80 K has been attributed to a thermally activated relaxation mechanism characterized by a small activation energy (0.04 eV). The origin of this relaxation mechanism is discussed with the results obtained on bulk samples. No gap structure at T c has been observed in agreement with the small electron-phonon interaction. Magnetic field effects due to the interaction between the surface acoustic wave and the magnetic flux lines are very small.

Sound velocity in Bi2Sr2CaCu2O8 single crystals

Sound velocity results on Bi2Sr2CaCu2O8 single crystals are reported. Softening of the longitudinal mode generated in the basal plane has been observed around the superconducting transition. The elastic anomaly is not easily related to the superconducting transition and it might be connected to structural instabilities.

Ultrasonic properties of one-dimensional (TaSe4)2I

Abstract We report measurements on the quasi-one-dimensional (TaSe) 2 I which undergoes a Peierls transition at T p = 262 K. Softening of the shear elastic constant C 44 has been observed below 40 K. The effective Gruneisen parameter deduced from the sound velocity increases up to 20 when temperature decreases down to 4 K. Such a large increase of Γ eff has been related to the anomalous dispersion curve of the C 44 acoustic branch of this system.