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

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.