Reduction of the superfluid density in the vortex-liquid phase of Bi2Sr2CaCu2Oy

In-plane complex surface impedance of a Bi2Sr2CaCu2Oy Single crystal was me asured in the mixed state at 40.8 GHz. The surface reactance, which is prop ortional to the real part of thr effective penetration depth, increased rap idly just above the first-order vortex-lattice melting transition field and the: second magnetization peak field. This increase is ascribed to the dec rease in the superfluid density rather than the loss of pinning. This resul t indicates that the vortex melting transition changes the electronic struc ture as well as the vortex structure. [S0031-9007(99)08428-8].