Tunneling and magnetic characteristics of superconductingZrB12single crystals

Abstract

Bulk and surface properties of high-quality single crystals of zirconium dodecaboride have been studied in the temperature range from $4.5\mathrm{K}$ up to the superconducting transition temperature, which is found to be nearly $6.06\mathrm{K}$. Scanning tunneling spectroscopy data, together with $\mathrm{dc}$ and $\mathrm{ac}$ magnetization measurements, are consistent with the conventional $s$-wave pairing scenario, whereas they disagree in estimates of the electron-phonon coupling strength. We explain the divergence, supposing a great difference between the surface and bulk superconducting characteristics of the compound. This assertion is supported by our findings of a nonlinear magnetic response to an amplitude-modulated alternating magnetic field, testifying to the presence of surface superconductivity in the ${\mathrm{ZrB}}_{12}$ samples at $\mathrm{dc}$ fields exceeding the thermodynamic critical field.