Unconventional Superconductivity at Microkelvin Temperatures

ended

A new frontier is the investigation of quantum materials under the extreme conditions of ultralow temperatures. Its exploration requires the refinement of existing high sensitivity, low dissipation measurement techniques, the development of new ones, and access to ultralow temperature platforms. Quantum materials host a variety of exotic quantum phases, arising from interactions and the effects of strong correlations. An important example is the emergent unconventional superconductivity in heavy fermion systems when tuned by some control parameter to a quantum critical point. This project combines my expertise with the expertise, facilities and instrumentation of the host group. I will investigate two important unconventional superconductors in this regime, YbRh2Si2 and Bismuth, using high quality single crystal samples. YbRh2Si2 is a prototype quantum critical, heavy fermion metal with a field-tuned quantum critical point. Magnetic measurements on high quality single crystal samples at the lowest fields find evidence for superconductivity. I will address the nature of this superconductivity, the role of quantum criticality, the interplay of electro-nuclear magnetism, and the use of strain as a tuning parameter in this system. This will be done through electrical and thermal transport measurements, investigating their crystalline anisotropy, as well as heat capacity studies. In each case I will exploit new methods tailored for this temperature regime. This work will be coupled with studies of the Meissner effect and anisotropy of the critical field. The recent discovery of superconductivity in Bismuth, a system with very low carrier density, below 0.53 mK has provoked significant theoretical interest in the pairing mechanism. The first transport measurements will be performed on this system. The project will advance the understanding of unconventional superconductivity, and contribute to the strategy to study quantum materials into the microkelvin regime.