CMSA Topological Quantum Matter Seminar: Phase transitions out of quantum Hall states in moire TMD bilayers

SEMINARS, CMSA EVENTS

Speaker:

Xueyang Song - MIT

Motivated by the recent experimental breakthroughs in observing Fractional Quantum Anomalous Hall
(FQAH) states in moir\'e Transition Metal Dichalcogenide (TMD) bilayers, we propose and study various
unconventional phase transitions between quantum Hall phases and Fermi liquids or charge ordered
phases upon tuning the bandwidth. At filling -2/3, we describe a direct transition between the FQAH state
and a Charge Density Wave (CDW) insulator. The critical theory resembles that of the familiar deconfined
quantum critical point (DQCP) but with an additional Chern-Simons term. At filling -1/2, we study the
possibility of a continuous transition between the composite Fermi liquid (CFL) and the Fermi liquid (FL)
building on and refining previous work by Barkeshli and McGreevy. Crucially we show that translation
symmetry alone is enough to enable a second order CFL-FL transition. We argue that there must be critical
CDW fluctuations though neither phase has long range CDW order. A striking signature is a universal jump
of resistivities at the critical point. With disorder, we argue that the CDW order gets pinned and the CFL-FL
evolution happens through an intermediate electrically insulating phase with mobile neutral fermions. A
clean analog of this insulating phase with long range CDW order and a neutral fermi surface can potentially
also exist. We also present a critical theory for the CFL to FL transition at filling -3/4. Our work opens up a
new avenue to realize deconfined criticality and fractionalized phases beyond familiar Landau level physics
in the moire Chern band system.

This seminar will be held in person and on Zoom: https://harvard.zoom.us/j/97514733653?pwd=Q05XN3oxSnYvaXlnS0dsRnVyMXZMUT09

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