Steve Simon

Abstract:

Recent experiments [Banerjee et al, arXiv:1710.00492] have measured thermal conductance of the ν = 5/2 edge in a GaAs electron gas and found it to be quantized as K ≈ 5/2 (in appropriate dimensionless units). This result is unexpected, as prior numerical work predicts that the ν = 5/2 state should be the Anti-Pfaffian phase of matter, which should have quantized K = 3/2. The purpose of this paper is to propose a possible solution to this conflict: if the Majorana edge mode of the Anti-Pfaffian does not thermally equilibrate with the other edge modes, then K = 5/2 is expected. I briefly discuss a possible reason for this nonequilibration, and what should be examined further to determine if this is the case.

Abstract:

We explore the phase diagrams of moiré materials in search of a class of intervalley-coherent correlated insulating state: the Chern texture insulator (CTI). This phase of matter, proposed in a companion paper [Kwan , .], breaks valley <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow> <mi>U</mi> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> </math> symmetry in a nontrivial fashion wherein the valley order parameter is forced to texture in momentum space as a consequence of band topology. Using detailed Hartree-Fock studies, we establish that the CTI emerges as an energetically competitive intermediate-coupling ground state in several moiré systems that lack a twofold rotation symmetry that forbids the single-particle topology essential to the formation of the CTI valley texture. Published by the American Physical Society 2025

Abstract:

We introduce and study interacting topological states that arise in time-reversal symmetric bands with an underlying obstruction to forming localized states. If the U(1) valley symmetry linked to independent charge conservation in each time-reversal sector is spontaneously broken, the corresponding “excitonic” order parameter is forced to form a topologically nontrivial texture across the Brillouin zone. We show that the resulting phase, which we dub a textured exciton insulator, cannot be given a local-moment description because of a form of delicate topology. Using toy models of bands with Chern or Euler obstructions to localization, we construct explicit examples of the Chern or Euler texture insulators (CTIs or ETIs) they support, and demonstrate that these are generically competitive ground states at intermediate coupling. We construct field theories that capture the response properties of these new states. Finally, we identify the incommensurate Kekulé spiral phase observed in magic-angle bi- and trilayer graphene as a concrete realization of an ETI.

Abstract:

The band-projected density operators in a Landau level obey the Girvin-MacDonald-Platzman (GMP) algebra, and a large amount of effort in the study of fractional Chern insulators has been directed toward approximating this algebra in a Chern band. In this Letter, we prove that the GMP algebra, up to form factors, is the only closed algebra that projected density operators can satisfy in two and three dimensions, highlighting the central place it occupies in the study of Chern bands in general. A number of interesting corollaries follow.

Abstract:

We use several techniques to probe the wave functions proposed to describe the A phases by Das, Das, and Mandal [Phys. Rev. Lett. 131, 056202 (2023); Phys. Rev. Lett. 132, 106501 (2024); Phys. Rev. B 110, L121303 (2024).]. As opposed to representing fractional quantum Hall liquids, we find these wave functions to describe states that clearly display strong phase separation. In the process of exploring these wave functions, we have also constructed several new methods for diagnosing phase separation and generating such wave functions numerically. Finally, we uncover a new property of entanglement spectra that can be used as a check for the accuracy of numerics.