Magnetic field, temperature and frequency dependence of complex conductance of ultra thin YBa2Cu3O7-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 YBa2Cu3O7-x films sandwiched between semiconducting Pr0.6Y0.4Ba2Cu3O7-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 Hc2(T) is exponential close to Tc, with a break in a slope at the Berezinski-Kosterlitz-Thouless (BKT) temperature TBKT, 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.