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 o xygen-deficient YBCO superconductors was investigated under various high hy drostatic pressures. Under the ambient pressure, T* can be expressed in ter ms of T-c as T* (n) = T *((op)) +alpha (1 - T-c(n)/ T-c(max))(1/2). Under v arious high pressures, this expression is still valid, so, T*(n, P) = T*((o p)) +alpha (P)(1-T-c(n, P)/T-c(max))(1/2). Experiments show that the parame ter T*((op)) is pressure-independent, while a(P) decreases linearly with in creasing pressure. This indicates that the structural change considerably a ffects the spin gap behavior.