Origin of discrete microwave absorption lines in YBa2Cu3O7-x single crystals

It is shown that the phase of field modulated discrete microwave absorption lines depends on whether flux creeps into or out of the sample, i.e. on the sense of the boundary current. Phase reversals are observed in an YBa2Cu3O7-x single crystal depending on magnetization relaxation and applied field sweep. The present experimental observations provide clues for the modification of the loop model of microwave absorption. The relevant loop is formed by the boundary current of the sample, and flux pinning gives rise to an effective inductance of the loop. (orig.)