Specific heat, magnetic susceptibility, resistivity and thermal expansion of the superconductor ZrB12 -: art. no. 024547

In an attempt to clarify conflicting published data, we report new measurements of specific heat, resistivity, magnetic susceptibility, and thermal expansivity up to room temperature for the 6 K superconductor ZrB12, using well-characterized single crystals with a residual resistivity ratio > 9. The specific heat gives the bulk result 2 Delta(0)/k(B)T(c)=3.7 for the superconducting gap ratio, and excludes multiple gaps and d-wave symmetry for the Cooper pairs. The Sommerfeld constant gamma(n)=0.34 mJ K-2 gat(-1) and the magnetic susceptibility chi=-2.1x10(-5) indicate a low density of states at the Fermi level. The Debye temperature theta(D) is in the range 1000-1200 K near zero and room temperature, but decreases by a factor of similar to 2 at similar to 35 K. The specific heat and resistivity curves are inverted to yield approximations of the phonon density of states F(omega) and the spectral electron-phonon scattering function alpha F-2(tr)(omega), respectively. Both unveil a 15 meV mode, attributed to Zr vibrations in oversized B cages, which gives rise to electron-phonon coupling. The thermal expansivity further shows that this mode is anharmonic, while the vanishingly small discontinuity at T-c establishes that the cell volume is nearly optimal with respect to T-c.