Observation of a crossover from nodal to gapped superconductivity in Lu${}_x$Zr${}_{1-x}$B${}_{12}$

Abstract

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

5 pages, 2 figures