Coexistence of type-I and type-II superconductivity signatures in ZrB12 probed by muon spin rotation measurements

Superconductors usually display either type-I or type-II superconductivity and the coexistence of these two types in the same material, for example, at different temperatures, is rare in nature. We employed the muon spin rotation ($\ensuremath{\mu}\mathrm{SR}$) technique to unveil the superconducting phase diagram of the dodecaboride ${\mathrm{ZrB}}_{12}$ and obtained clear evidence of both type-I and type-II characteristics. Most important, we found a region showing unusual behavior where the usually mutually exclusive $\ensuremath{\mu}\mathrm{SR}$ signatures of type-I and type-II superconductivity coexist. We reproduced that behavior in theoretical modeling that required taking into account multiple bands and multiple coherence lengths, which suggests that material has one coherence length larger and another smaller than the magnetic field penetration length (the type-1.5 regime). At stronger fields, a footprint of the type-II mixed state showing square flux-line lattice was also obtained using neutron diffraction.