Recent observation of anomalous superconducting behavior of ultrathin YBCO films and single crystals of cluster borides
Institute of Solid State Physics RAS, Chernogolovka 142432, Moscow Region, Russia
Received March 1, 2006, revised April 17, 2006
We report the number of deviations from conventional behavior in superconducting properties
of ultrathin (1–3 unit-cell (UC)) YBa2Cu3O7–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( T ), at low temperatures independent of frequency, with a break in slope at TBKTdc, 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 TBKTdc /Lk-1 ( TBKTdc ) = 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 Hc2(T) from Tc down to 0.35 K. We suggest that both l(T) and Hc2(T) dependencies can be explained by a two band BCS model with different superconducting gap and Tc.PACS: 74.25.Nf - 74.72.Bk - 74.70.Ad - 72.15.Gd -
Key words: YBCO films, dodecaboride ZrB12, inductance, penetration depth, two band superconductions.