The “Unconventional Superconductivity at Microkelvin Temperatures” (UCSMT) project aims at developing novel measurement techniques at ultra-low temperatures and studying superconducting properties of elemental Bismuth and quantum critical YbRh2Si2 single crystals, two unique superconductors. Bismuth is exceptional element with unique electronic properties such as a very small density of states, and exhibits strong quantum effects that have helped in uncovering many interesting phenomena in condensed matter research such as the Seebeck, Nernst, Shubnikov–de Haas, and de Haas–van Alphen (dHvA) effects right from early 1900s. The superconductivity in bulk Bismuth crystals was only recently observed at extremely low temperatures below 530 micro-kelvin using magnetisation measurements making it one of the lowest carrier density superconductor with a upper critical field of 5.2 micro-tesla. The UCSMT project aims at performing first transport measurements on bismuth crystals at micro-kelvin temperatures using current sensing noise thermometry (CSNT) techniques to detect the resistive transition into the superconducting state. The YbRh2Si2, on the other hand is a prototype quantum critical, heavy fermion metal which exhibits anti-ferromagnetic (AFM) order below 70 mK. A small magnetic field of 60 mT, when applied in the basal plane of the tetragonal structure, can suppress the AFM order and drive YbRh2Si2 towards a quantum critical point. Magnetic measurements on high quality single crystal samples at the lowest fields find evidence for superconductivity. Calorimetric measurements suggest the emergence of nuclear spin order around 2 mK. This is suggested to weaken the electronic AFM order so that superconductivity of probably unconventional nature emerges at a QCP shifted to zero field. Analysis of the upper critical field seems to suggest heavy fermion superconductivity. The nuclear Kondo-effect has also been suggested as the possible source of the heavy fermion character of YbRh2Si2. The UCSMT project aims at developing a novel, state-of-the-art magnetically shielded, sensitive magnetization measurement setup to measure the lower critical field and it’s anisotropy in YbRh2Si2, to further enhance the understanding on the superconducting ground.
The development of new measurement techniques in the microkelvin temperature regime is likely to produce new sensors, sensitive thermometers and software which can be used in other fields of cryogenic industry and electronics. Identification of a high-quality single crystal material that can host topological superconductivity is an important step towards topologically protected quantum computing that are expected to play very important role in solving challenging problems in economics, fundamental science and health sectors.
The overall objectives of the UCSMT project are to develop the understanding of superconductivity in Bismuth and YbRh2Si2, and to make significant advances in our ability to study quantum materials into the ultra-low temperature regime, through the development of novel measurement techniques suitable for sub-milli-kelvin temperatures. A parallel goal of this MSCA Individual Fellowship is to foster the development of the individual researcher as well as enable the exchange of knowledge and expertise with the host institute.
