Pressure effect on the spin gap opening temperature for Ca-doped and oxygen-deficient YBCO superconductors

The spin gap opening temperature in the normal state of bulk Ca-doped and oxygen-deficient YBCO superconductors was investigated under various high hydrostatic pressures. Under the ambient pressure, T* can be expressed in terms of T_c as T* (n) = T *^(OP) +α(1 - T_c (n) /T_c^max)^1/2. Under various high pressures, this expression is still valid, so, T*(n,P) = T*^(OP) +α(P)(1-T_c(n,P)/T_c^max)^1/2. Experiments show that the parameter T*^(OP)is pressure-independent, while α(P) decreases linearly with increasing pressure. This indicates that the structural change considerably affects the spin gap behavior.