Is ZrB12 two gap superconductor?

TL;DR

This study investigates the superconducting properties of ZrB12, revealing deviations from conventional behavior and suggesting a two-band BCS model with different gaps and critical temperatures as an explanation.

Contribution

It provides experimental evidence of unconventional superconducting behavior in ZrB12 and proposes a two-band BCS model to explain the observed phenomena.

Findings

Quadratic resistivity below 25 K in ZrB12

Linear Hc2(T) dependence from Tc to 0.35 K in ZrB12

Unconventional superfluid density with a shoulder at T/Tc=0.65

Abstract

We report the measurements of the temperature dependence of the resistivity, \rho(T), magnetic penetration depth,\lambda(T) the lower, Hc1(T), and upper, Hc2(T), critical magnetic fields, for single crystals of dodecaboride ZrB12, diboride ZrB2 and thin films of diboride MgB2. We observe a number of deviations from conventional behavior in these materials. Although ZrB12 behaves like a simple metal in the normal state, the resistive Debye temperature, 300 K, is three times smaller relative to that (800-1200 K) calculated from the specific heat, C(T), data. We observe predominantly quadratic temperature behavior of resistivity in ZrB12 below 25 K, and in ZrB2 below 100 K, indicating the possible importance of the electron-electron interaction in these borides. 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) for ZrB12 from Tc down to 0.35 K. We suggest that both \lambda(T) and Hc2(T) dependencies in ZrB12 can be explained by two band BCS model with different superconducting gap and Tc.