Microwave Surface Impedance and Complex Conductivity of High-Tc Single Crystals: Current State and Unsolved Problems

Common and distinctive features of the temperature dependences of microwave surface impedance Z(T) = R(T) + iX(T) and conductivity σ(T) in the ab-plane and along the c-axis of high-Tc single crystals (HTSC) are discussed. The main attention is focused on an evo-lution of these dependences in YBaCuO crystal with the oxygen deficiency. Comparison of YBaCuO with other HTSC crystals reveals a number of peculiarities, namely, the linear de-pendence Rab(T) ∝ T up to Tc/2 in HTSC single crystals with tetragonal lattice and up to Tc/3 in YBaCuO where Rab(T) shows also a broad peak at T ∼ Tc/2; breakdown of normal skin effect in some HTSC crystals; dramatic effect of pseudogap on the superfluid density in the heavily underdoped YBCO; several orders of magnitude higher residual surface resistance in HTSC when compared to conventional superconductors; etc. Possible explanations are discussed in the context of the specific features of HTSC structure and in the framework of recent models of quasiparticles ’ c-transport and pseudogap state in HTSC. KEY WORDS: high-temperature superconductors; surface impedance; microwave conductivity; anisotropy; pseudogap; single crystals