Simultaneous ac Susceptibility and Specific Heat Measurements of YBCO Ceramics

Simultaneous ac magnetic susceptibility and specific heat studies of YBCO ceramics permitted the correct correlation of the corresponding critical temperatures. The midpoint of the real component () and the maximum of the imaginary component () of ac susceptibility curves confirmed the T_c = 92 K critical temperature value determined by dc resistance measurements. The specific heat maximum's temperature (T_cc 93.6 K)—-taken as the calorimetric critical one—fell between T_c and T* (the pseudogap onset temperature). We suggest that T_cc signals the appearance of the first localized coherent pairs, while the temperature T_c marks their percolation. The existence of two different critical temperature values confirms the coexistence of partial coherent and coherent electron pairing phenomena in the region below T_cc of the superconducting phase diagram.