Anisotropy of microwave conductivity of YBaCuO in superconducting and normal states: Crossover 3D-2D

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Imaginary part of the microwave conductivity σ''(T<Tc) and the resistivity ρ(T)=1/σ(T>Tc) along (σab'' and ρab) and perpendicular to (σc'' and ρc) cuprate ab-planes of YBa₂Cu₃O₇₋ₓ crystal were measured in the temperature range 5≤ T ≤ 200 K, having varied the oxygen doping level x in the crystal from 0.07 to 0.47. The superconducting state was found to have the dependences σab''(T)/σab''(0) and σc''(T)/σc''(0) coincide in the optimally doped (x=0.07) crystal. With the increase of x the dependences σc''(T)/σc''(0) have smaller slope at T<Tc/3 with σab''(T)/σab''(0) changing slightly. The ab-plane transport in the normal state of YBa₂Cu₃O₇₋ₓ crystal always remains metallic. However, a crossover from Drude conductivity (at x=0.07) along the c-axis to the hopping one (at x>0.07) takes place. This is proved by both estimating minimum metallic c-conductivity and maximum tunneling c-conductivity and quantitatively comparing the measured dependences ρc(T) with the ones calculated in the polaron model of quasiparticles' c-transport.