Yening Wang

Relationship between lattice instability and superconductivity in high Tc oxide superconductors

Abstract There always exists an internal friction peak due to phase-like transition (PLT) at about 10K–30K higher than Tc for YBCO, BSCCO, and TBCCO. X-ray diffraction indicates the PLT is characterized by abrupt change of lattice parameters, but with symmetry unchanged.

Effect of zinc substitution on internal friction in YBCO superconductors

The temperature spectra of internal friction for Zn doped ceramic YBa2(Cu1-xZnx)3O6+ are presented with zinc content x varying from 0 to 0.1. Two thermally activated peaks (P1, P2) near the superconducting transition temperature Tc were found to show different behaviours upon zinc doping: the peak at 110 K (P1) decreases rapidly with increasing of x while another one at 120 K (P2) has no significant change. The relaxation mechanism of P1 is interpreted in terms of jumps of apical oxygen atoms between off-centred positions produced by the Jahn-Teller effect. The effect of zinc substitution on the depression of superconductivity is also discussed.

The mechanical relaxation study of polycrystalline MgC1−xNi3

The mechanical relaxation spectra of a superconducting and a nonsuperconducting MgC-{1-x}Ni-3 samples were measured from liquid nitrogen temperature to room temperature at frequency of kilohertz. There are two internal friction peaks (at 300K labeled as P-1 and 125K as P-2) for the superconducting sample. For the nonsuperconducting one, the position of P-1 shifts to 250K, while P-2 is almost completely depressed. It is found that the peak position of P-2 shifts towards higher temperature under higher measuring frequency. The calculated activation energy is 0.13eV. We propose an explanation relating P-2 to the carbon atom jumping among the off-center positions. And further we expect that the behaviors of carbon atoms maybe correspond to the normal state crossovers around 150K and 50K observed by many other experiments.

Mechanism of mechanical relaxation associated with precipitates

The thermal activated relaxation connected with precipitates in a number of alloys always exhibits a broad and asymmetric peak in T-1 plots, which is very different from the Debye-type relaxation peak. Because solute clustering is present around the precipitates, a strong correlation should exist between the motions of solute atoms. Ngais correlated-state model (1979) is used to simulate the relaxation spectrum. The calculation results not only correlate quite well with the experimental data on the Al-Ag, Ti-H and Ta-H systems but also have profound implications on the mechanism of the internal friction peak.

Internal friction related to carriers in high-Tc superconductors

Abstract For both Y 1− x Pr x Ba 2 Cu 3 O 7− y and Gd 1− x Pr x Ba 2 Cu 3 O 7− y , it is discovered that internal frictions reveal an obvious plateau Q -1 p from 100 to 280 K, and Q -1 p is nearly proportional to the density of Hall carriers in Y(Pr)BaCuO and Gd(Pr)BaCuO samples with various Pr content. These results confirm that Q -1 p is closely related to carriers. Q -1 p can be explained reasonably by using a coupling model for the carriers with a local dynamic distortion for HTSCs.

NQR study on Nd doped La2−xSrxCuO4

Abstract 63 Cu nuclear quadrupole resonance (NQR) experiments were preformed on six Nd-doped La 2− x Sr x CuO 4 samples. The NQR spectra show a systematic evolution with various Nd and/or Sr doping, except for La 1.48 Nd 0.4 Sr 0.12 CuO 4 . A completely different NQR spectrum was observed for the sample, which is similar to that for undoped insulator La 2 CuO 4 . It is speculated that a static stripe phase on the NQR time scale μs may be responsible for the anomalous spectrum.

Internal friction and smearing of superconducting gap structures in high-Tc superconductors

Abstract For both BiSrCaCuO and TlBaCaCuO samples, the internal frictions, Q -1 , in the kHz range reveal a plateau, Q -1 p , above T c and drop rapidly below T c with the turning points located just at T c for various samples with different T c values. This kind o f anomaly cannot be observed for non-superconducting samples. The drop of Q -1 p below T c is found to be closely related to supercondu cting condensation. The existence of Q -1 p for high- T c superconductors can be reasonably explained using the coupling-model among carriers with local dynamic distortion. Further, taking into account the smearing of the superconducting gap structure resulting from the recombination of quasi-particles and by modifying the BCS relative jump rate as S ( E , E ′, Γ )=Re [1- Δ 2 / ( E - iΓ )( E ′- iΓ )], the calculated results of the internal friction below T c are in fair agreement with the experimental data. The superconducting gap, Δ, and the damping rate, Γ, for both BiSrCaCuO and TlBaCaCuO have also been obtained. These values are in accordance with those obtained by tunneling spectrum and NMR methods, etc.