A cascade of phase transitions in an orbitally mixed half-filled Landau level

TL;DR

This paper investigates the complex phase transitions at filling factor 5/2 in ZnO-based 2D electron systems, revealing multiple distinct phases including gapped, compressible, Fermi-liquid, and nematic-like states driven by Landau level interactions.

Contribution

It demonstrates the rich cascade of phases and the controllability of these states in ZnO, providing new insights into fractional quantum Hall phenomena and Landau level interactions.

Findings

Identification of a gapped phase polarized in N=1 level

Observation of a compressible phase in N=0 level

Discovery of a Fermi-liquid and nematic-like phase near level degeneracy

Abstract

Half-filled Landau levels host an emergent Fermi-liquid which displays an instability towards pairing, culminating in a gapped even-denominator fractional quantum Hall ground state. While this pairing may be probed by tuning the polarization of carriers in competing orbital and spin degrees of freedom, sufficiently high quality platforms offering such tunability remain few. Here we explore the ground states at filling factor $\nu$ = 5/2 in ZnO-based two-dimensional electron systems through a forced intersection of opposing spin branches of Landau levels taking quantum numbers $N$ = 1 and 0. We reveal a cascade of phases with distinct magnetotransport features including a gapped phase polarized in the $N$ = 1 level and a compressible phase in N = 0, along with an unexpected Fermi-liquid, a second gapped, and a strongly anisotropic nematic-like phase at intermediate polarizations when the levels are near degeneracy. The phase diagram is produced by analyzing the proximity of the intersecting levels and highlights the excellent reproducibility and controllability ZnO offers for exploring exotic fractionalized electronic phases.