  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.
