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Creators/Authors contains: "Zhang, Ya-Hui"

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  1. Free, publicly-accessible full text available December 13, 2024
  2. Free, publicly-accessible full text available September 1, 2024
  3. We describe the confining instabilities of a proposed quantum spin liquid underlying the pseudogap metal state of the hole-doped cuprates. The spin liquid can be described by a SU(2) gauge theory ofNf= 2 massless Dirac fermions carrying fundamental gauge charges—this is the low-energy theory of a mean-field state of fermionic spinons moving on the square lattice withπ-flux per plaquette in the ℤ2center of SU(2). This theory has an emergent SO(5)fglobal symmetry and is presumed to confine at low energies to the Néel state. At nonzero doping (or smaller Hubbard repulsionUat half-filling), we argue that confinement occurs via the Higgs condensation of bosonic chargons carrying fundamental SU(2) gauge charges also moving inπ2-flux. At half-filling, the low-energy theory of the Higgs sector hasNb= 2 relativistic bosons with a possible emergent SO(5)bglobal symmetry describing rotations between ad-wave superconductor, period-2 charge stripes, and the time-reversal breaking “d-density wave” state. We propose a conformal SU(2) gauge theory withNf= 2 fundamental fermions,Nb= 2 fundamental bosons, and a SO(5)f×SO(5)bglobal symmetry, which describes a deconfined quantum critical point between a confining state which breaks SO(5)fand a confining state which breaks SO(5)b. The pattern of symmetry breaking within both SO(5)s is determined by terms likely irrelevant at the critical point, which can be chosen to obtain a transition between Néel order andd-wave superconductivity. A similar theory applies at nonzero doping and largeU, with longer-range couplings of the chargons leading to charge order with longer periods.

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    Free, publicly-accessible full text available May 23, 2024
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  5. Transition metal dichalcogenide heterostructures show strong interactions and can imprint a moiré potential to a separate layer. 
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  6. Abstract

    Flat band moiré superlattices have recently emerged as unique platforms for investigating the interplay between strong electronic correlations, nontrivial band topology, and multiple isospin ‘flavor’ symmetries. Twisted monolayer-bilayer graphene (tMBG) is an especially rich system owing to its low crystal symmetry and the tunability of its bandwidth and topology with an external electric field. Here, we find that orbital magnetism is abundant within the correlated phase diagram of tMBG, giving rise to the anomalous Hall effect in correlated metallic states nearby most odd integer fillings of the flat conduction band, as well as correlated Chern insulator states stabilized in an external magnetic field. The behavior of the states at zero field appears to be inconsistent with simple spin and valley polarization for the specific range of twist angles we investigate, and instead may plausibly result from an intervalley coherent (IVC) state with an order parameter that breaks time reversal symmetry. The application of a magnetic field further tunes the competition between correlated states, in some cases driving first-order topological phase transitions. Our results underscore the rich interplay between closely competing correlated ground states in tMBG, with possible implications for probing exotic IVC ordering.

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