Specific heat of YBa₂Cu₃O_7-δ and BaCuO_2+x in magnetic fields up to 6 T

The specific heat C of YBa₂Cu₃O_7-δ (δ=0, 0.69, 0.91), Y_1.1Ba_1.9Cu₃O_7-δ and BaCuO_2+x has been investigated in magnetic fields up to 6 T. In addition to the γ^*T term and the upturn, characteristic behaviors of C/T consisting of the increase of γ^* with magnetic fields, an appearance of a broad hump and a rapid decrease of C/T toward zero at the lowest temperatures are observed in all the YBa₂Cu₃O_7-δ samples investigated. BaCuO_2+x, one of the possible impurity phases included in the Y-Ba-Cu-O system, also shows a similar behavior of C/T as YBa₂Cu₃O_7-δ. Particularly, BaCuO_2+x explains the anomalous phenomenon observed in YBa₂Cu₃O_6.31 at 6 T that the C/T at 1.5 K attains to the γ^* value at 0 T. The γ^* value is attributed to the BaCuO_2+x included in the matrix.