An explanation is proposed of the huge (over 20) BCS ratio 2Δ(0)/ discovered recently in strongly underdoped . Since no marked shrinking of the Fermi surface was found, our explanation is based on the idea of disruption of the connection between bilayers due to reduction of localized centers providing resonant tunneling between the layers. An increase of phase fluctuations of the order parameter, mostly due to spontaneous vortex formation, leads to a possibility of a drastic reduction of , which practically coincides with , the temperature of the Berezinsky-Kosterlitz-Thouless transition. At the same time, the local energy gap is likely to undergo no major changes. An argument is given why at not too small concentrations of the localized centers the critical temperature can remain rather stable, despite strong variations in the normal c-axis conductivity.
© 1997 The American Physical Society