Vanishing of phase coherence in underdoped Bi_2Sr_2CaCu_2O_8+d
  Coherent time-domain spectroscopy is used to measure the screening and dissipation of high-frequency electromagnetic fields in a set of underdoped Bi_2Sr_2CaCu_2O_8+d thin films. The measurements provide direct evidence for a phase-fluctuation driven transition from the superconductor to normal state, with dynamics described well by the Berezinskii-Kosterlitz-Thouless theory of vortex-pair unbinding. 
J. Corson (1), R. Mallozzi (1), J. Orenstein (1), J. N. Eckstein (2),   I. Bozovic (3) ((1) UC Berkeley and Lawrence Berkele...