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

TL;DR

This study investigates the superconducting properties of ZrB12 and MgB2, revealing differences in pairing symmetry and critical fields, with implications for understanding their electronic behavior.

Contribution

It provides new measurements of resistivity, penetration depth, and upper critical field for ZrB12 and MgB2, highlighting differences in pairing symmetry and superconducting characteristics.

Findings

ZrB12 shows a possible d-wave pairing state.

MgB2 exhibits an s-wave pairing state.

ZrB12 has a linear Hc2(T) dependence, contrary to conventional theories.

Abstract

We report on the synthesis and measurements of the temperature dependence of resistivity, R(T), the penetration depth, l(T), and upper critical magnetic field, Hc2(T), for polycrystalline samples of dodecaboride ZrB12 and diboride MgB2. We conclude that ZrB12 as well as MgB2 behave like simple metals in the normal state with usual Bloch-Gruneisen temperature dependence of resistivity and with rather low resistive Debye temperature, TR=280 K, for ZrB12 (as compared to MgB2 with TR=900 K). The R(T) and l(T) dependencies of ZrB12 reveal a superconducting transition at Tc=6.0 K. Although a clear exponential l(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 s-wave pairing state in MgB2, whereas a d-wave pairing state is possible in ZrB12. A fit to the data gives a reduced energy gap 2D(0)/kTc=1.6 for MgB2 films and pellets, in good agreement with published data for 3D \pi - sheets of the Fermi surface. Contrary to conventional theories we found a linear temperature dependence of Hc2(T) for ZrB12 (Hc2(0)=0.15 T).