Two-gap superconductivity inZrB12: Temperature dependence of critical magnetic fields in single crystals

Abstract

We report the measurements of the temperature dependence of the resistivity, ρ(T), magnetic penetration depth, λ(T) the lower, H c1 (T), and upper, H c2 (T), critical magnetic fields, for single crystals of dodecaboride ZrB 12 , diboride ZrB 2 and thin films of diboride MgB 2 . We observe a number of deviations from conventional behavior in these materials. Although ZrB 12 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 ZrB 12 below 25 K, and in ZrB 2 below 100 K, indicating the possible importance of the electron-electron interaction in these borides. Superfluid density of ZrB 12 displays unconventional temperature dependence with pronounced shoulder at T/T c equal to 0.65. Contrary to conventional theories we found a linear temperature dependence of H c2 (T) for ZrB 12 from T c down to 0.35 K. We suggest that both λ(T) and H c2 (T) dependencies in ZrB 12 can be explained by two band BCS model with different superconducting gap and T c .

Evidence of multiband behavior in the superconducting alloy Zr0.96V0.04B2Renosto1, Consoline2, Santos3 et al. 2013Phys. Rev. B284251View full textAdd to dashboardCite

“…In fact, single crystals of this compound (ZrB 2 ) are not superconducting. 14 The Zr-B phase diagram suggests that contamination with ZrB 12 , a known superconductor with critical temperature 5.94 K. 15 ZrB 2 , also was studied to high pressure (50 GPa) and no superconductivity was observed. 16 In this work we present a systematic study of substitution of Zr for V in Zr 1−x V x B 2 (0 x 1.0) and find that superconductivity emerge in the nonsuperconductor ZrB 2 matrix for a small content of V, an example where an extremely stressed lattice of the AlB 2 prototype yields superconductivity while the matrix compound is nonsuperconducting.…”

The de Haas–van Alphen effect study of the Fermi surface of ZrB12

“…Knowledge of the experimental Fermi surface in this cluster compound is critical for understanding two-gap model suggested in Ref. 6. Until now there have been no direct experimental probes of the FS structure of ZrB 12 .…”

“…Matthias et al [1] suggested that the superconductivity in YB 6 and ZrB 12 was due to the effect of a cluster of light boron atoms. Much smaller isotope effect on T c for boron in comparison with Zr isotopic substitution suggests that the boron in ZrB 12 serves as inert background for the Zr driven superconductivity [2].…”

“…At the same time, it was suggested from the specific heat and thermal expansion measurements [5], that it is a single gap marginal BCS superconductor with 2Δ(0)/k B B T c =3.7. Gasparov et al [6] suggested that unconventional features observed in ZrB 12 can be explained by two band BCS model scenario with different not only superconducting gap but T c as well. [5] Recently the optical study and band structure calculations of ZrB 12 [7] by self-consistent full potential Linear Muffin-Tin Orbital (LMTO) method, have been also reported.…”

“…In this paper, we report a detailed study of the dHvA effect in ZrB 12 single crystals and a comparison with predictions of the band-structure calculations. [8] [6]. The sample dimensions are 0.5×0.5×2 mm and 1×1×0.5 mm , with <110> and <100> axis parallel to its length and normal, respectively.…”