Evidence of type-I and type-II superconductivity and their coexistence in ZrB12
Superconductors are categorised as either type-I or type-II depending on their behaviour under a magnetic field. The existence of both types of superconductivity in the same material is rare and of great experimental and theoretical interest. Here we have used the muon spin rotation (muSR) technique to unveil the superconducting phase diagram of the dodecaboride ZrB12. Analysing the local magnetic field distribution obtained from the muSR spectra we identify regions of a Meissner state where the applied magnetic field is entirely expelled from the sample, an intermediate state where normal and Meissner states coexist, and a mixed state where a well-defined flux-line lattice is formed. Neutron diffraction data confirm the presence of a mixed state with a square flux-line lattice at all temperatures and applied magnetic above 175 Oe. While Meissner and intermediate states are the characteristic features of type-I superconductors, the mixed state exists only in type-II superconductors. We also observe regions of the phase diagram where the type-I and type-II states overlap and coexist, which is of relevance in the context of type-1.5 superconductivity. The results strongly suggest that the Ginzburg-Landau parameter of ZrB12 varies as a function of temperature so that pure type-I and type-II, as well as different coexisting states, are realised in this system.
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