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

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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 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 ρ(T) and with a rather low resistive Debye temperature T_R=280 K (to be compared to T_R=900 K for MgB₂). The ρ(T) and λ(T) dependences for these samples reveal a superconducting transition in ZrB₁₂ at T_c=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 T_c/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 H_c2(T) for ZrB₁₂ (H_c2(0)=0.15 T).