Spectroscopic imaging of unconventional superconductors at ultra-low temperatures

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Superconductivity is a property that has intrigued the scientific community since it's discovery by Onnes in 1911 [1]. The BCS theory of superconductivity describes the properties of 'conventional' superconductors [2], which describes how phonons mediate an attractive interaction between pairs of electrons - which leads to superconductivity. Many materials, however, fall outside this realm of conventional superconductor, where the attractive electronic interaction is not phonon-mediated. Such materials, for example the high-temperature cuprate superconductors, are known as unconventional superconductors. We can also categorise superconducting materials as spin-singlet or spin-triplet (depending on the spins of the two electrons in each electron pair) - these two categories give rise to different characteristics and potential application. All spin-triplet superconductors would be unconventional and have exciting potential applications in quantum supercomputing, but are yet to be confirmed experimentally [3]. Scanning tunnelling microscopy (STM) can be used to investigate properties such as superconductivity through measuring the density of states and investigating the surfaces of materials [4-6]. This project aims to investigate materials with unclear superconductivity categorisation (e.g., spin-singlet vs. spin-singlet), such as Sr2RuO4, to provide clarity on future potential applications.[1] Van Delft, D. & Kes, P. The discovery of superconductivity. Physics Today 63, 38-43 (Sept.2010).[2] Bardeen, J., Cooper, L. N. & Schrieffer, J. R. Microscopic Theory of Superconductivity. Phys-ical Review 106, 162-164 (Apr. 1957).[3] Devoret, M. H. & Schoelkopf, R. J. Superconducting Circuits for Quantum Information: AnOutlook. Science 339, 1169-1174 (2013).[4] Hoffman, J. E. et al. Imaging Quasiparticle Interference in Bi2Sr2CaCu2O8- . Science 297,1148-1151 (Aug. 2002).[5] Hanaguri, T. et al. Coherence Factors in a High-TC Cuprate Probed by Quasi-Particle Scat-tering Off Vortices. Science 323, 923-926 (Feb. 2009).[6] Kreisel, A. et al. Interpretation of Scanning Tunneling Quasiparticle Interference and ImpurityStates in Cuprates. Physical Review Letters 114, 217022 (May 2015).