Theory of Quantum Materials

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My research is on “quantum materials” where the laws of quantum mechanics conspire with strong interactions to create new and interesting states of matter. These might be new kinds of superconductors, such as high temperature superconductors, or superconductors with complex internal structure such as chiral p-wave. They might be charge, spin or pair density waves, periodic modulations of these properties that coexist in the so-called “pseudogap state” which is the parent state from which high Tc superconductivity arises. Superconductors are made up of Cooper pairs, bound pairs of electrons that condense into a superfluid. A chiral superconductor is one in which the Cooper pairs all spin together clockwise or counter-clockwise, breaking time reversal symmetry. Chiral superconductors have distinctive excitations, Majorana fermions which act like half an electron, and fractional vortices, whose unusual properties can, in principle, be exploited in quantum computing. One system that I study, Sr2RuO4, is thought to be a chiral p-wave superconductor, where p-wave means that the Cooper pairs have angular momentum 1. Such a system should exhibit spontaneous surface supercurrents that have not yet been observed, and part of my research concerns understanding why not or how can they be observed. There may be other chiral superconductors, such as UPt3, and I am working on a theory to describe this system. Another thrust of my research is on high Tc superconductors in high magnetic fields. Physicists were surprised to observe quantum oscillations in high Tc superconductors as a function of magnetic field because it was thought that these oscillations would not survive the strong interactions in these materials. Also the quantum oscillation frequencies that were observed were not as expected; it appeared that the Fermi surface had been “reconstructed” due to static density waves. The question then is are these density waves in the charge or spin or in the Cooper pair wave function itself? My goal is to deepen our understanding of novel pair density wave states and, more generally, of high temperature superconductors. The impact and importance for Canada of such research is best illustrated by an example. In the early 1990's I was studying d-wave superconductivity with an undergraduate student, Geordie Rose, who went on to do a PhD at UBC on a different subject and later to found a company, D-Wave Systems, the world's first quantum computing company, in B.C. Now I am studying other new phenomena, Majorana fermions and pair density wave superconductors. Most of my students will follow conventional paths in basic research or in financial modeling or research utilizing large data sets, where their high-tech skills are in demand by Canadian companies. A few, because they studied things that were completely new, will go on to do something genuinely new and different with the goal of creating new technologies and new companies.