Reduction of the Superfluid Density in the Vortex-Liquid Phase of   Bi2Sr2CaCu2Oy
  In-plane complex surface impedance of a Bi2Sr2CaCu2Oy single crystal was measured in the mixed state at 40.8 GHz.The surface reactance, which is proportional to the real part of the effective penetration depth, increased rapidly just above the first-order vortex-lattice melting transition field and the second magnetization peak field.This increase is ascribed to the decrease in the superfluid density rather than the loss of pinning.This result indicates that the vortex melting transition changes the electronic structure as well as the vortex structure. 
Tetsuo Hanaguri, Takashi Tsuboi, Yoshishige Tsuchiya, Ken-ichi Sasaki   and Atsutaka Maeda