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

TL;DR

This study reveals the coexistence of type-I and type-II superconductivity signatures in ZrB12 using muon spin rotation, supported by theoretical modeling and neutron diffraction, indicating a complex type-1.5 superconducting regime.

Contribution

It provides the first experimental evidence of simultaneous type-I and type-II superconductivity signatures in ZrB12 and models this behavior considering multiple bands and coherence lengths.

Findings

Evidence of coexistence of type-I and type-II signatures in ZrB12

Identification of a type-1.5 superconducting regime

Observation of square flux-line lattice at higher fields

Abstract

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 the employed muon spin rotation (muSR) technique to unveil the superconducting phase diagram of the dodecaboride ZrB12 and obtained clear evidence of both type-I and type-II characteristics. Most importantly, we found a region showing unusual behavior where the usually mutually exclusive muSR 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.