Determination of the vortex-glass melting transition in YBa2Cu3O7 by phase-sensitive current-voltage measurements

The transition from the vortex-glass state to a vortex liquid in high-temperature superconductors is probed commonly by the onset of resistance or by a scaling analysis of various current-voltage isotherms. Alternatively, we show that an abrupt change of the phase lag between current and voltage in low-frequency ac-impedance measurements provides a direct indication of the melting transition.