Role of pair-breaking and phase fluctuations in c-axis tunneling in   underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$
  The Josephson Plasma Resonance is used to study the c-axis supercurrent in the superconducting state of underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ with varying degrees of controlled point-like disorder, introduced by high-energy electron irradiation. As disorder is increased, the Josephson Plasma frequency decreases proportionally to the critical temperature. The temperature dependence of the plasma frequency does not depend on the irradiation dose, and is in quantitative agreement with a model for quantum fluctuations of the superconducting phase in the CuO$_{2}$ layers. 
C.J. van der Beek, P. Spathis, S. Colson, P. Gierlowski, M. Gaifullin,   Yuji Matsuda, P.H. Kes