Superconducting Phase Diagrams of LuB12 and Lu1-xZrxB12 (x≤0.45) down to 50 mK

J. Bačkaia, S. Gabánib, K. Flachbartb, E. Gažob, J. Kušnírc, M. Orendáčb, c, G. Pristášb, N. Sluchankod, e, A. Dukhnenkof, V. Filipovf, N. Shitsevalovaf aFaculty of Electrical Engineering and Informatics, Technical University, Letná str. 9, 04200 Košice, Slovakia bInstitute of Experimental Physics, Slovak Academy of Sciences, Watsonova str. 47, 04001 Košice, Slovakia cInstitute of Physics, Faculty of Science, P.J. Safarik University, Park Angelium str.9, 04154 Košice, Slovakia dProkhorov General Physics Institute, Russian Academy of Sciences, Vavilov str. 38, 119991 Moscow, Russia eMoscow Institute of Physics and Technology, Moscow Region, 141700 Russia fInstitute for Problems of Materials Science, National Academy of Sciences of Ukraine, Krzhyzhanovsky str. 3, 03142 Kiev, Ukraine

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Lutetium dodecaboride LuB12 is a s-wave weak-coupling BCS superconductor with critical temperature Tc~0.42 K. In turn, ZrB12 is 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 one can study the crossover from weak- to strong-coupling superconductor. We have investigated the evolution of critical temperature Tc and critical field Hc in high-quality single crystalline superconducting samples of Lu1-xZrxB12 (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, that can work in a wide temperature range of 0.05-3 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 Lu1-xZrxB12 solid solutions, as well as how their superconducting phase diagram develops.

DOI:10.12693/APhysPolA.137.791 topics: superconductivity, magnetic susceptibility, dodecaborides, low temperatures