Superconductivity and the c Axis Spectral Weight of High- T c Superconductors

The temperature dependence of the c axis spectral weight (frequency integral of the interplane conductivity) of high transition temperature (high-T(c)) superconductors is shown to be a probe of thermal and quantal fluctuations of the phase of the superconducting order parameter. The behavior of underdoped cuprates is shown to be a natural consequence of superconducting pairing without long-ranged phase coherence. Very underdoped cuprates are found to have strong phase fluctuations, even for temperatures much less than the transition temperature.