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

Abstract

We have determined the superconducting and magnetic properties of four samples of ${\mathrm{Lu}}_{x}{\mathrm{Zr}}_{1\ensuremath{-}x}{\mathrm{B}}_{12}$ ($x=0.04$, 0.07, 0.17, and 0.8) using muon spin rotation ($\ensuremath{\mu}\mathrm{SR}$) and magnetometry measurements. We observed a strong magnetic signal in both the $\ensuremath{\mu}\mathrm{SR}$ and magnetometry data in one sample ($x=0.07$), likely caused by the formation of static moments of size $\ensuremath{\approx}1\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ due to a clustering effect of the ${\mathrm{Lu}}^{3+}$ ions. In all other samples, we find only a small magnetic signal in the $\ensuremath{\mu}\mathrm{SR}$ data thought to originate from boron nuclei in the ${\mathrm{B}}_{12}$ 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 $x\ensuremath{\le}0.17$, providing a potential new example of an $s$-to-$s+d$-wave crossover in a superconductor.