Magnetic-field-induced anomalous microwave response in superconducting YBa[sub 2]Cu[sub 3]O[sub 7-δ] thin films

Rao, X. S.; Rao, X.S.; Ong, C. K.; Ong, C.K.; Feng, Y. P.; Feng, Y.P.
October 2000
Applied Physics Letters;10/30/2000, Vol. 77 Issue 18
Academic Journal
The magnetic-field-induced decrease of microwave surface impedance, Z[sub s]=R[sub s]+jX[sub s], of superconducting YBa[sub 2]Cu[sub 3]O[sub 7-δ] thin films is investigated using a microstrip resonator technique. Through measuring Z[sub s] as the direct current (dc) magnetic field is increased from zero to a certain value H[sub max] and subsequently decreased back to zero, it is found that Z[sub s](H[sub dc]) is totally reversible if H[sub max] does not exceed the threshold field H[sup *], at which R[sub s] shows a minimum, while it is irreversible if H[sub max] is larger than H[sup *]. It is confirmed that the anomalous effect happens in a vortex-free state. The threshold field H[sup *] is identified with the lower critical field for the vortices penetrating into the superconducting thin films. The anomalous effect is believed to be mainly contributed by the imperfect microstructure of the superconducting thin films. © 2000 American Institute of Physics.


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