Ultrasonic measurements on polycrystalline YBa2Cu3O7

Temperature dependent ultrasonic attenuation data of polycrystalline YBa₂Cu₃O₇ at 10 MHz show a plateau between 220 and 250 K, and slope changes at 263, 135 and 97 K. The attenuation does not display a sharp decrease at T_c as would be described in the BCS theory. Around 255 K, the sound velocity in the sample increases from its room temperature value by more than 10%.     

Nonlinearity of the current-voltage characteristics for YBa2Cu3O7–x single crystals and the Berezinskii-Kosterlitz-Thouless transition

The nonlinearity of the current-voltage characteristics for YBa₂Cu₃O_{7 ? x} single crystals is investigated in the vicinity of the superconducting transition temperature T _c , and the mechanisms responsible for the nonlinearity of the current-voltage characteristics are considered. The current-voltage characteristics and the amplitudes of the higher voltage harmonics for the YBa₂Cu₃O_{7 ? x} single crystals are directly measured using modulation Fourier analysis. It is shown that the positions of the maxima in the amplitudes of the higher voltage harmonics coincide with the Berezinskii-Kosterlitz-Thouless transition temperature T _BKT. This coincidence can provide a means for determining the transition temperature T _BKT. A number of parameters characterizing the YBa₂Cu₃O_{7 ? x} single crystals in the superconducting state are estimated using the experimental results and the model of a Berezinskii-Kosterlitz-Thouless transition. The parameters thus estimated are in good agreement with the data available in the literature.     

Ac-Josephson effect in YBa2Cu3O7/YBa2Cu3O7 point contact junctions

The ac-Josephson effect could be demonstrated up to dc voltages of 20 mV on the I–V characteristic of adjustable YBa₂Cu₃O₇/YBa₂Cu₃O₇ point contacts at 4.2 K. A detailed analysis of the rf power dependence of the size of microwave-induced constant voltage steps is given for microwave frequencies of 70 and 90 GHz. The results prove that electron pairing is responsible for the superconductivity in YBa₂Cu₃O₇.     

Triangular to square flux lattice phase transition in YBa2Cu3O7

We have used the technique of small-angle neutron scattering to observe magnetic flux lines directly in a YBa2Cu3O7 single crystal at fields higher than previously reported. For field directions close to perpendicular to the CuO2 planes, we find that the flux lattice structure changes smoothly from a distorted triangular coordination to nearly perfectly square as the magnetic induction approaches 11 T. The orientation of the square flux lattice is as expected from recent d-wave theories but is 45 degrees from that recently observed in La(1.83)Sr(0.17)CuO(4+delta).