Magnetic field, temperature and frequency dependence of complex conductance of ultra thin YBa₂Cu₃O_7-x films

We have studied the magnetic field, temperature and frequency dependencies of the complex sheet conductance, σ(ω,T), of 1-3 unit cell (UC) thick YBa₂Cu₃O_7-x films sandwiched between semiconducting Pr_0.6Y_0.4Ba₂Cu₃O _7-x layers. Experiments have been carried out at frequencies between 2 MHz -1 GHz, with one-spiral coil technique and at 250-500 GHz, by aid of a quasi optical spectrometer based on backward-wave oscillators. We have found a very large shift of the superconducting transition onset temperature and of the maximum Reσ(T) position to higher temperatures with increasing ω. The dependence of H_c2(T) is exponential close to T_c, with a break in a slope at the Berezinski-Kosterlitz-Thouless (BKT) temperature T_BKT, due to the vortex-antivortex pair unbinding, and linear below TBKT because the Abrikosov vortex formation. We have compared our experimental results with the extended dynamic theory for the BKT transition and found that the research explored the dynamics of bound vortex-antivortex pairs with short separation lengths.