Effective electromagnetic response of topological matter with nodal lines
Aalto Quantum Physics Seminar (Nanotalo). Speaker: Dr. Jaakko Nissinen (ROTA, Aalto University, Finland).
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Recent attention in the topological classification of matter has focused on symmetry protected nodal lines in the fermionic spectrum. That is, the fermionic energy spectrum in a gapless three dimensional system has a stable line of zeroes. In terms of the (co)dimensions of topologically stable Fermi surfaces in three dimensions, such a nodal line of zeroes is in between a topological semi-metal with Weyl/Dirac points and a conventional metallic Fermi surface.
In a nodal semi-metal, the emergent, low energy quasi-relativistic gapless fermions close to the node(s) couple to both (effective) electromagnetic gauge fields and geometric tetrads, giving rise to complicated crossover phenomena in the electromagnetic and geometric response of the system. As a concrete example of such phenomena in topological matter, I will focus on the polar phase of superfluid 3He with a nodal line and the polar-distorted A-phase with Weyl points. In a phase transition through the polar phase, the polar distorted Weyl superfluid is predicted to have a crossover to a non-analytic effective EM response due to the nodal line.
If time permits, aspects of quantum anomalies due to massless fermions, i.e. the breaking of classical symmetries by quantum effects in these systems will be briefly discussed.