HIGH T-C SUPERCONDUCTIVITY IN YBA2CU3O7-X STUDIED BY PAC AND PAS

High T-c superconductivity has been investigated in YBaCuO by both per turbed angular correlation and positron annihilation spectroscopy tech niques as a function of temperature from 77 to 300 K. An abrupt change has been observed in the positron lifetime and Doppler broadening and the electric field gradient and its asymmetry parameter across T-c, i ndicating a transition of two- to one-dimensional Cu-O-Cu chain struct ure and a charge transfer from CuO layers to CuO chains. An anomaly of the normal state has been demonstrated around 125 K, which is attribu ted to the structural instability.