Electron transport, penetration depth, and the upper critical magnetic field in ZrB12 and MgB2

We report on the synthesis and measurements of the temperature dependences of the resistivity rho, the penetration depth lambda, and the upper critical magnetic field H-c2, for polycrystalline samples of dodecaboride ZrB12 and diboride MgB2. We conclude that ZrB12 behaves as a simple metal in the normal state with the usual Bloch-Gruneisen temperature dependence of rho(T) and with a rather low resistive Debye temperature T-R = 280 K (to be compared to T-R = 900 K for MgB2). The rho(T) and lambda(T) dependences for these samples reveal a superconducting transition in ZrB12 at T-c = 6.0 K. Although a clear exponential lambda(T) dependence in MgB2 thin films and ceramic pellets was observed at low temperatures, this dependence was almost linear for ZrB12 below T-c/2. These features indicate an delta-wave pairing state in MgB2, whereas a d-wave pairing state is possible in ZrB12. In disagreement with conventional theories, we found a linear temperature dependence, of H-c2(T) for ZrB12 (H-c2(0) = 0.15 T). (c) 2005 Pleiades Publishing, Inc.