Absorption at radio frequencies in superconducting Y1Ba2Cu3O/sub y/

The large magnetic-field-dependent ac absorption in superconducting Y1Ba2Cu3O/sub y/ ceramics and powders decreases slowly with increasing frequency in the range 2--16 MHz. The magnetic-field-dependent ac absorption is observed below T/sub c/ in superconducting Y1Ba2Cu3O/sub y/ ceramics, powders and twinned crystals and in (La,Sr)2CuO4 and BaPb/sub 0.75/Bi/sub 0.25/CuO3 powders and appears to contain distinct contributions from trapped flux and/or intragrain tunnel junctions and from ordinary superconductor surface impedance. The contribution from trapped flux and/or intragrain tunnel junctions is important in dc magnetic fields of up to approximately 20 G at all temperatures below T/sub c/. There is an identifiable modulated ac absorption which changes slowly over more than 12 kG at low temperatures and which we propose is primarily caused by the ac surface impedance with possible contributions from the critical state. A twinned crystal of Y1Ba2Cu3O/sub y/ exhibits the same ac absorption except that it has an anisotropic dependence on the applied dc magnetic field