Recent observation of anomalous superconducting behavior of ultrathin YBCO films and single crystals of cluster borides

Анотація

We report the number of deviations from conventional behavior in superconducting properties of ultrathin (1–3 unit-cell (UC)) YBa₂Cu₃O_7–x films sandwiched between semiconducting Pr_0.6Y_0.4Ba₂Cu₃O_7–x layers and for single crystals of cluster superconductor: dodecaboride ZrB₁₂. We have found a quadratic temperature dependence of the kinetic inductance, L_k^-1( T ), at low temperatures independent of frequency, with a break in slope at T_BKT^dc, the maximum of real part of conductance, ws₁(T), and a large shift of the break temperature and the maximum position to higher temperatures with increasing frequency w. We obtain from these data the universal ratio T_BKT^dc /L_k^-1 ( T_BKT^dc ) = 25, 25, and 17 nHK for 1-, 2- and 3-UC films, respectively in close agreement with theoretical prediction for vortex—antivortex unbinding transition. Superfluid density of ZrB₁₂ displays unconventional temperature dependence with pronounced shoulder at T/T_c equal to 0.65. Contrary to conventional theories we found a linear temperature dependence of H_c2(T) from T_c down to 0.35 K. We suggest that both l(T) and H_c2(T) dependencies can be explained by a two band BCS model with different superconducting gap and T_c