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Colloquium: Dr. Philip M. R. Brydon
March 25, 2016 @ 3:30 pm - 4:30 pm
The Physics department colloquia are usually on Friday afternoons at 3:30 pm in Lapham Hall Room 160. Coffee and cookies are served at 3:15 pm in the same room. Anyone is welcome.
Superconductivity of High-Spin Electrons
Dr. Philip M. R. Brydon Lecturer, Dept. of Physics, University of Otago (New Zealand)
The microscopic building blocks of the superconducting state are two-electron “molecules,” so-called Cooper pairs. How the electrons bind together in a Cooper pair has important implications for the properties of the superconductor, in particular the arrangement of their spins: the spin-1/2 of the electrons constrains them to combine in either singlet (spin 0) or triplet (spin 1) states. However, electrons in a solid can sometimes appear to have a higher spin of 3/2. Although the existence of these “high-spin” electrons is well known, their role in superconductivity is poorly understood. Their apparent higher spin allows for Cooper pairs with exotically high spin, as the electrons can now also combine in quintet (spin 2) or septet (spin 3) states. The discovery of superconductivity in YPtBi makes this issue concrete, as only spin-3/2 electrons are available to form Cooper pairs in this compound. Excitingly, there is strong evidence that the superconductivity is unconventional and has line nodes. We propose that the band structure of YPtBi, in particular the antisymmetric spin-orbit coupling, strongly favours a mixture of s-wave spin singlet and p-wave spin septet Cooper pairs, generically giving a nodal state. Alternatively, a nodal gap can also originate from s-wave spin quintet pairing. Intriguingly, there are strong hints that the pairing of spin-3/2 electrons could be relevant to other unconventional superconductors.