Phases of Quasi-One-Dimensional Fractional Quantum (Anomalous) Hall - Superconductor Heterostructures

TL;DR

This paper investigates the complex phases and transitions in heterostructures combining fractional quantum Hall states and superconductors, focusing on $ u=2/3$ states and parafermions, using theoretical and numerical methods.

Contribution

It introduces a comprehensive phase diagram for FQH-superconductor heterostructures, including Mott insulators, Luttinger liquids, and parafermionic edge states, with new numerical and analytical insights.

Findings

Identification of Mott insulating and Luttinger liquid phases.

Observation of BKT and second-order phase transitions.

Insights into the stability of parafermionic edge states.

Abstract

Motivated by recent observations of fractional Chern insulators (FCIs) in the vicinity of superconducting (SC) phases, we study fractional quantum (anomalous) Hall-superconductor heterostructures in the presence of $U(1)$ order-parameter fluctuations and particularly focus on the case of $\nu = 2/3$ quantum Hall states leading to $\mathbb Z_3$ parafermions. We first employ a phenomenological field theory to qualitatively determine the phase diagram. Furthermore, we generalize a previously established alternating pattern of superconductor and tunneling regions, coupled to fractional quantum Hall edge states, to map the problem onto a topological Josephson junction chain involving lattice parafermions. Using density matrix renormalization group simulations, we establish a phase diagram composed of Mott insulating phases and two different Luttinger liquids whose fundamental excitations carry charges 2e and $2e/3$, respectively. In agreement with analytical considerations using conformal field theory, we numerically find transitions of Berezinskii-Kosterlitz-Thouless (BKT) type as well as a continuous $\mathbb Z_3 \times U(1)$ second-order phase transition characterized by central charge c = 9/5. We finally extract information about a possible ground state degeneracy and comment on the stability of parafermionic edge states in the presence of fluctuations. These theoretical foundations can be expected to be of practical importance for gate-defined FCI-SC heterostructures in moir\'e materials, in which broad superconducting transitions indicative of strong order parameter fluctuations were observed.