On the phase transition of superconducting cuprates in zero and non-zero magnetic fields

We present the results of measurements of the magnetizationM(T) and the specific heatC _p (T) of YBa₂Cu₃O_6.9 and Bi₂Sr₂CaCu₂O₈ in the vicinity of their respective critical temperature and in magnetic fields between 0 and 5 T. From these data we calculate the temperature- and magnetic field derivatives ofM andC _p . Both these measured and calculated quantities are used for testing theoretical predictions concerning the universality class of the transitions and to verify the influence of fluctuations accompanying the transitions.