Observation of a crossover from nodal to gapped superconductivity in LuxZr1−xB12

We have determined the superconducting and magnetic properties of four samples of LuxZr1-xB12 (x=0.04, 0.07, 0.17, and 0.8) using muon spin rotation (μSR) and magnetometry measurements. We observed a strong magnetic signal in both the μSR and magnetometry data in one sample (x=0.07), likely caused by the formation of static moments of size ≈1μB due to a clustering effect of the Lu3+ ions. In all other samples, we find only a small magnetic signal in the μSR data thought to originate from boron nuclei in the B12 cages. The superconductivity is found to evolve with x, with a decrease in x resulting in an increase in critical temperature and a decrease of the penetration depth. Most remarkably, we find evidence for the formation of nodes in the superconducting gap for x≤0.17, providing a potential new example of an s-to-s+d-wave crossover in a superconductor.