Experimental Evidence for Type-1.5 Superconductivity in ZrB12 Single Crystal

Recent studies proposed the two-band effect and the related type-1.5 superconductivity in ZrB12 single crystal. Here, by combining both macroscopic and microscopic measurements, the superconducting properties and the intricate vortex matter of ZrB12 are studied in detail. The vortex phase diagram is constructed, where the temperature dependence of the upper critical field can be well reproduced using a two-band Werthamer-Helfand-Hohenberg (WHH) model. A pronounced surface superconductivity is also found in the same temperature range, where the semi-Meissner state is observed. Both phenomena are attributed to the weakly coupled two-band effect. The direct visualization of the semi-Meissner state exhibits an inhomogeneous distribution of vortex clusters, vortex chains and large Meissner areas. With the increase of magnetic field, a transition from the semi-Meissner state to the mixed state is revealed and further supported by statistical analysis of the vortex pattern. Our results provide direct experimental evidence for the type-1.5 superconductivity in ZrB12.