The role of electron-electron interactions in the paired-to-nematic transition*

The problem of the ground state of a two-dimensional electron gas at half-filled Landau levels has been a remarkably interesting one. Depending on the number of filled Landau levels, besides the Fermi liquid ground state there are two well-known ordered phases: paired fractional quantum Hall states in the second Landau level and the electronic nematic (or stripe) phases in high Landau levels. Nonetheless, the presence of both of these ordered phases and therefore a transition between them did not seem possible in the absence of an externally applied symmetry breaking field favoring the nematic. This state of affairs has changed recently with the observation a pressure-driven quantum phase transition between the ν=5/2 fractional quantum Hall state, a paired state, and the nematic phase. Here we provide evidence of a similar quantum phase transition at filling factor ν=7/2 and discuss the role of the hydrostatic pressure in driving the transition. Our work highlights universal aspects of the competition of pairing and nematicity present in various condensed matter systems as well as in very clean two-dimensional electron gases. Work done in collaboration with Katherine Schreiber, Nodar Samkharadze, Y. Lyanda-Geller, G. Gardner, M.J. Manfra, L.N. Pfeiffer, and K.W. West.