Observation of Berezinski-Kosterlitz-Thouless transition in PrYBaCuO/YBaCuO/PrYBaCuO trilayers

The temperature dependencies of electromagnetic penetration depth λ_a,b(T) and high-frequency losses Reσ(T) have been investigated using the radio-frequency (1-20 MHz) and microwave (30 GHz) surface impedance methods on 1 unit cell (UC) and 2UC-thick Y₁Ba ₂Cu₃O_7-x epitaxial films sandwiched between Pr_0.6Y_0.4Ba₂Cu₃O_7-x epitaxial layers. It was observed that the λ_a,b ^-2(T) component exhibits a clear break near the critical temperature T_c from a linear BCS-like dependence at low temperatures, and a maximum in Reσ(T) close to the onset point of L_k ^-1(T). The break is destroyed in a very small perpendicular magnetic field (∼1 mT). This feature exhibit saturation as a function of the magnetic field in the range 3-5 mT. At the same time, no influence of the same H on the Reσ(T) dependence at microwave frequencies was detected. A huge increase in T_c (70% and 30% for 1UC and 2UC samples, respectively) as a function offrequency within the frequency range 1 MHz - 30 GHz was discovered. All these features are explained and discussed in the framework of dynamic theories of BKT transition.