Superconducting phase diagrams of LuB$_{12}$ and Lu$_{1-x}$Zr$_x$B$_{12}$ ($x \le 0.45$) down to 50 mK

TL;DR

This study investigates how the superconducting properties of LuB12 evolve with zirconium substitution, revealing a transition from weak to strong coupling and mapping the phase diagram down to 50 mK.

Contribution

It introduces a new susceptometer capable of measuring superconducting properties at ultra-low temperatures and provides detailed phase diagrams for Lu(1-x)Zr(x)B12.

Findings

Tc and Hc increase with zirconium concentration

Superconducting phase diagram shows crossover from weak to strong coupling

New susceptometer enables measurements down to 50 mK

Abstract

Lutetium dodecaboride LuB12 is a simple weak-coupling BCS superconductor with critical temperature Tc = 0.42 K, whilst ZrB12 is a strong-coupling BCS superconductor with the highest critical temperature Tc = 6.0 K among this group of materials. In case of lutetium substitution by zirconium ions in LuB12 the crossover from weak- to strong-coupling superconductor can be studied. We have investigated the evolution of critical temperature Tc and critical field Hc in high-quality single crystalline superconducting samples of Lu(1-x)Zr(x)B12 (0 =< x =< 0.45) by measuring magnetic ac susceptibility between 1 K and 50 mK. To obtain this kind of experimental data, a new susceptometer was designed, constructed and tested, which can work in a wide temperature range of 0.05 K - 3 K in 3He-4He dilution refrigerator. The measurements with this new susceptometer revealed how Tc(x) and Hc(x) increases with increasing concentration of zirconium in Lu(1-x)Zr(x)B12 solid solutions as well as how their superconducting phase diagram develops.