ZhETF, Vol. 128,
p. 115 (July 2005)
(English translation - JETP,
Vol. 101, No. 1,
available online at www.springer.com
ELECTRON TRANSPORT, PENETRATION DEPTH, AND THE UPPER CRITICAL MAGNETIC FIELD IN ZrB12 AND MgB2
Gasparov V.A., Sidorov N.S., Zver'kova I.I., Khassanov S.S., Kulakov M.P.
Received: November 26, 2004
PACS: 74.70.Ad, 74.25.Op, 72.15.Gd
We report on the synthesis and measurements of the temperature dependences of the resistivity ρ, the penetration depth λ, and the upper critical magnetic field Hc2 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 ρ(T) and with a rather low resistive Debye temperature TR=280 K (to be compared to TR=900 K for MgB2). The ρ(T) and λ(T) dependences for these samples reveal a superconducting transition in ZrB12 at Tc=6.0 K. Although a clear exponential λ(T) dependence in MgB2 thin films and ceramic pellets was observed at low temperatures, this dependence was almost linear for ZrB12 below Tc/2. These features indicate an s-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 Hc2(T) for ZrB12 (Hc2(0)=0.15 T).