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

TL;DR

This study investigates how superconductivity in Lu$_x$Zr$_{1-x}$B$_{12}$ evolves from nodal to gapped as the Lu concentration changes, revealing an $s$-to-$d$-wave crossover with implications for understanding unconventional superconductivity.

Contribution

It provides the first experimental evidence of an $s$-to-$d$-wave crossover in Lu$_x$Zr$_{1-x}$B$_{12}$, linking composition to superconducting gap symmetry.

Findings

Superconducting gap develops nodes for $x \,\leq\, 0.17$.

Critical temperature increases as Lu content decreases.

Magnetic signals vary with Lu concentration, indicating clustering effects.

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 $\approx 1\,\mu_{\rm B}$ 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 $x \leq 0.17$, providing a new example of an $s$-to-$d$-wave crossover in a superconductor.