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

Abstract

We report the number of deviations from conventional behavior in superconducting properties of ultrathin (1–3 unit-cell (UC)) YBa₀₂Cu₃O₇₋x films sandwiched between semiconducting Pr₀,₆Y₀,₄Ba₂Cu₃O₇₋x layers and for single crystals of cluster superconductor: dodecaboride ZrB₁₂. We have found a quadratic temperature dependence of the kinetic inductance, Lk⁻¹(Т) , at low temperatures independent of frequency, with a break in slope at TBKT dc , the maximum of real part of conductance, ωσ₁(T), and a large shift of the break temperature and the maximum position to higher temperatures with increasing frequency ω. We obtain from these data the universal ratio T /Lk⁻¹ (Tdc ) = 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/Tc equal to 0.65. Contrary to conventional theories we found a linear temperature dependence of Hс₂(Т) from Tc down to 0.35 K. We suggest that both λ(T) and Hс₂(Т) dependencies can be explained by a two band BCS model with different superconducting gap and Tc.