Interlayer electrodynamics of high-Tc superconductors: an experimental overview

An astonishing feature of underdoped high-T_c superconductors is that the energy scale associated with the formation of superconducting condensate dramatically exceeds the energy gap and appears to be of the interband caliber [1]. This effect can be interpreted in terms of lowering of the electronic kinetic energy at T < T_c and thus points to a mechanism of superconductivity radically departing from the Bardeen – Cooper – Schrieffer (BCS) theory. This nontrivial superconducting state electrodynamics appears to be directly connected to anomalies of cuprates observed in the normal state, including the pseudogap, and to the lack of well-defined quasiparticles.