Superconductivity and the c axis spectral weight of high-Tc 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.