Abstract
The magnetic-field-induced decrease of microwave surface impedance, Zs=Rs+jXs, of superconducting YBa2Cu3O7−δ thin films is investigated using a microstrip resonator technique. Through measuring Zs as the direct current (dc) magnetic field is increased from zero to a certain value Hmax and subsequently decreased back to zero, it is found that Zs(Hdc) is totally reversible if Hmax does not exceed the threshold field H*, at which Rs shows a minimum, while it is irreversible if Hmax is larger than H*. It is confirmed that the anomalous effect happens in a vortex-free state. The threshold field H* 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.
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