Dc and ac magnetic field effects in LaBaCuO/sub x/, YBa2Cu3O7/sub -//sub x/, and other high-T/sub c/ superconductors

Small (<300 Oe) dc magnetic fields are found to significantly lower the superconducting transition temperature T/sub c/ and change the shape of the magnetic flux exclusion M vs T curve in powder compactions of La2/sub -//sub x/Ba/sub x/Cu3O/sub y/, YBa2Cu3O7/sub -//sub x/, and other high-T/sub c/ materials. dT/sub c//dH of 0.02 to 0.08 deg/Oe with H/sub c/1 less than 20 Oe near T/sub c/ are typical. The M vs T curve and the value of H/sub c/1 are sample dependent. The results are interpreted in terms of the effect of particle size and size distribution on the material magnetic penetration depth. Measurements of the real and imaginary parts of the magnetic susceptibility show that the latter is frequency dependent over the range of 0.1--10 kHz when measured with an ac mutual inductance technique. Assuming that the material can be considered as a network of Josephson junctions, the temperature dependence of the imaginary component of the susceptibility can be used to calculate a value for the superconducting energy gap