Temperature and frequency dependence of complex conductance of ultrathin YBa_2Cu_3O_(7-x) films: Observation of vortex-antivortex pair unbinding

摘要

We have studied the temperature dependencies of the complex sheet conductance, σ(ω, T), of 1–3 unit cell (UC) thick YBa_2Cu_3O_(7-x) films sandwiched between semiconducting Pr_0.6Y_0.4Ba_2Cu_3O_(7−x)layers at high frequencies. Experiments have been carried out in a frequency range between: 2–30 MHz with one-spiral coil technique, in 100 MHz-1 GHz frequency range with a new technique using a single spiral coil cavity, and at 30 GHz by aid of a resonant cavity technique. The real,ReM(T), and imaginary parts of the mutual-inductance M(T,ω), between a coil and a film, were measured and converted to complex conductivity by aid of the inversion procedure. We have found quadratic temperature dependence of the kinetic inductance, L_k(-1)(T), at low temperatures independent of frequency, with a break in slope at T~(dc)_(BKT) , the maximum of real part of conductance and large shift of the onset temperature and the maximum ωσ_1(T) position to higher temperatures with increasing frequency ω. We obtain from these data the universal ratio T~(dc)_(BKT) /L_k(-1)(T~(dc)_(BKT))=25, 25, and 17 nH K for 1, 2 and 3-UC films, respectively in close relation with theoretical prediction of 12 nH K for vortex-antivortex unbinding transition. The activated temperature dependence of the vortex diffusion constant was observed and discussed in the framework of vortex-antivortex pair pinning.