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Theoretical physicists working at a blackboard collaboration pod in the Beecroft building.
Credit: Jack Hobhouse

Steve Simon

Professorial Research Fellow and Professorial Fellow of Somerville College

Sub department

  • Rudolf Peierls Centre for Theoretical Physics

Research groups

  • Condensed Matter Theory
steven.simon@physics.ox.ac.uk
Telephone: 01865 (2)73954
Rudolf Peierls Centre for Theoretical Physics, room 70.06
  • About
  • Publications

Chern-Simons Modified-RPA Eliashberg theory of the ν = 1/2 + 1/2 quantum Hall bilayer

Physical Review Letters American Physical Society 132 (2024) 176502

Authors:

Tevz Lotric, Steven Simon

Abstract:

The ν = 1/2 + 1/2 quantum Hall bilayer has been previsously modeled using Chern-Simons-RPAEliashberg (CSRPAE) theory to describe pairing between the two layers. However, these approaches are troubled by a number of divergences and ambiguities. By using a “modified” RPA approximation to account for mass renormalization, we can work in a limit where the cyclotron frequency is taken to infinity, effectively projecting to a single Landau level. This, surprisingly, controls the important divergences and removes ambiguities found in prior attempts at CSRPAE. Examining BCS pairing of composite fermions we find that the angular momentum channel l = +1 dominates for all distances d between layers and at all frequency scales. Examining BCS pairing of composite fermion electrons in one layer with composite fermion holes in the opposite layer, we find the l = 0 pairing channel dominates for all d and all frequencies. The strength of the pairing in these two different descriptions of the same phase of matter is found to be almost identical. This agrees well with our understanding that these are two different but dual descriptions of the same phase of matter.

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Double-dome Unconventional Superconductivity in Twisted Trilayer Graphene

(2024)

Authors:

Zekang Zhou, Jin Jiang, Paritosh Karnatak, Ziwei Wang, Glenn Wagner, Kenji Watanabe, Takashi Taniguchi, Christian Schönenberger, SA Parameswaran, Steven H Simon, Mitali Banerjee
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Coulomb-driven band unflattening suppresses K-phonon pairing in moire graphene

Physical Review B American Physical Society 109 (2024) 104504

Authors:

Glenn Wagner, Yves H Kwan, Nick Bultinck, Steven Simon, Siddharth A Parameswaran

Abstract:

It is a matter of current debate whether the gate-tunable superconductivity in twisted bilayer graphene is phonon-mediated or arises from electron-electron interactions. The recent observation of the strong coupling of electrons to so-called K-phonon modes in angle-resolved photoemission spectroscopy experiments has resuscitated early proposals that K-phonons drive superconductivity. We show that the bandwidth-enhancing effect of interactions drastically weakens both the intrinsic susceptibility towards pairing as well as the screening of Coulomb repulsion that is essential for the phonon attraction to dominate at low temperature. This rules out purely K-phonon-mediated superconductivity with the observed transition temperature of ∼1 K. We conclude that the unflattening of bands by Coulomb interactions challenges any purely phonon-driven pairing mechanism, and must be addressed by a successful theory of superconductivity in moiré graphene
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Excitations in the higher lattice gauge theory model for topological phases III: the 3+1d case

Physical Review B American Physical Society 109:3 (2024) 035152

Authors:

Joe Huxford, Steven Simon

Abstract:

In this, the third paper in our series describing the excitations of the higher lattice gauge theory model for topological phases, we will examine the 3+1d case in detail. We will explicitly construct the ribbon and membrane operators which create the topological excitations, and use these creation operators to find the pattern of condensation and confinement. We also use these operators to find the braiding relations of the excitations, and to construct charge measurement operators which project to states of definite topological charge.
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Topological and nontopological degeneracies in generalized string-net models

Physical Review B American Physical Society 109 (2024) 045130

Authors:

Anna Ritz-Zwilling, Jean-Noel Fuchs, Steven Simon, Julien Vidal

Abstract:

Generalized string-net models have been proposed recently in order to enlarge the set of possible topological quantum phases emerging from the original string-net construction. In the present work we do not consider vertex excitations, and we restrict ourselves to plaquette excitations, or fluxons, that satisfy important identities. We explain how to compute the energy-level degeneracies of the generalized string-net Hamiltonian associated with an arbitrary unitary fusion category. In contrast to the degeneracy of the ground state, which is purely topological, the degeneracy of excited energy levels depends not only on the Drinfeld center of the category, but also on internal multiplicities obtained from the tube algebra defined from the category. For a noncommutative category, these internal multiplicities result in extra nontopological degeneracies. Our results are valid for any trivalent graph and any orientable surface. We illustrate our findings with nontrivial examples.
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