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

Abstract

Imaginary part of the microwave conductivity σ&#39;&#39;(T<T_c) and the resistivity ρ(T)=1/σ(T>T_c) along (σ_{ab}&#39;&#39; and ρ_{ab}) and perpendicular to (σ_c&#39;&#39; and ρ_c) cuprate ab-planes of YBa_2Cu_3O_{7-x} crystal were measured in the temperature range 5\leq T \leq 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 σ_{ab}&#39;&#39;(T)/σ_{ab}&#39;&#39;(0) and σ_c&#39;&#39;(T)/σ_c&#39;&#39;(0) coincide in the optimally doped (x=0.07) crystal. With the increase of x the dependences σ_c&#39;&#39;(T)/σ_c&#39;&#39;(0) have smaller slope at T<T_c/3 with σ_{ab}&#39;&#39;(T)/σ_{ab}&#39;&#39;(0) changing slightly. The ab-plane transport in the normal state of YBa_2Cu_3O_{7-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 ρ_c(T) with the ones calculated in the polaron model of quasiparticles&#39; c-transport.