Quantum Hall bilayer as pseudospin magnet

¹ Division of Physics and Applied Physics, Nanyang Technological University - 637371, Singapore
² Science Institute, University of Iceland - Dunhagi-3, IS-107, Reykjavik, Iceland
³ Niels Bohr Institute, University of Copenhagen - Blegdamsvej 17, DK-2100 Copenhagen, Denmark
⁴ ITMO University - St. Petersburg 197101, Russia

Received: 29 September 2014
Accepted: 16 February 2015

Abstract

We revisit the physics of electron gas bilayers in the quantum Hall regime (MacDonald A. and Eisenstein J., Nature, 432 (2004) 691; Eisenstein J., Science, 305 (2004) 950), where transport and tunneling measurements provided evidence of a superfluid phase being present in the system. Previously, this behavior was explained by the possible formation of a BEC of excitons in the half-filled electron bilayers, where empty states play the role of holes. We discuss the fundamental difficulties with this scenario, and propose an alternative approach based on a treatment of the system as a pseudospin magnet. We show that the experimentally observed tunneling peak can be linked to the XY ferromagnet (FM) to Ising antiferromagnet (AFM) phase transition of the S = 1/2 XXZ pseudospin model, driven by the change in total electron density. This transition is accompanied by a qualitative change in the nature of the low-energy spin wave dispersion from a gapless linear mode in the XY-FM phase to a gapped, quadratic mode in the Ising AFM phase.