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Abstract: 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-T c single crystals (HTSC) are discussed. The main attention is focused on an evolution 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 dependence Rab(T) ∞ T up to T c/2 in HTSC single crystals with tetragonal lattice and up to T c/3 in YBaCuO where R ab (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.

The c-axis penetration depth λcin Bi2Sr2CaCu2O8+δ(BSCCO) single crystals as a function of temperature has been determined using three high-frequency techniques, namely (1) measurements of the ac-susceptibility at a frequency of 100 kHz for different sample alignments with respect to the ac magnetic field; (2) measurements of the surface impedance in both superconducting and normal states of BSCCO crystals at 9.4 GHz; (3) measurements of the surface barrier field HJ(T) ∝ 1/λc(T) at which Josephson vortices penetrate into the sample. Careful analysis of these measurements, including both numerical solution of the electrodynamic problem of the magnetic field distribution in an anisotropic plate at an arbitrary temperature and influence of defects in the sample has allowed us to estimate λc(0) ≈ 50 μm in BSCCO crystals overdoped with oxygen (Tc≈ 84 K) and λc(0) ≈ 150 μm at the optimal doping level (Tc≈ 90 K). The results obtained by different techniques are in reasonable agreement.