Landau Quantization and Highly Mobile Fermions in an Insulator

TL;DR

This study reports the first observation of Landau quantization in a 2D insulator, monolayer WTe₂, revealing quantum oscillations that suggest the presence of mobile fermions or neutral Fermi surfaces within an insulating state.

Contribution

It provides experimental evidence of Landau quantization in a monolayer topological insulator, indicating potential fractionalized excitations or neutral Fermi surfaces in an insulator.

Findings

Observation of quantum oscillations in monolayer WTe₂

Oscillations exhibit many periods similar to metals

Landau quantization fully develops above 1.6 Tesla

Abstract

In strongly correlated materials, quasiparticle excitations can carry fractional quantum numbers. An intriguing possibility is the formation of fractionalized, charge-neutral fermions, e.g., spinons and fermionic excitons, that result in neutral Fermi surfaces and Landau quantization in an insulator. While previous experiments in quantum spin liquids, topological Kondo insulators, and quantum Hall systems have hinted at charge-neutral Fermi surfaces, evidence for their existence remains far from conclusive. Here we report experimental observation of Landau quantization in a two dimensional (2D) insulator, i.e., monolayer tungsten ditelluride (WTe$_{2}$), a large gap topological insulator. Using a detection scheme that avoids edge contributions, we uncover strikingly large quantum oscillations in the monolayer insulator's magnetoresistance, with an onset field as small as ~ 0.5 tesla. Despite the huge resistance, the oscillation profile, which exhibits many periods, mimics the Shubnikov-de Haas oscillations in metals. Remarkably, at ultralow temperatures the observed oscillations evolve into discrete peaks near 1.6 tesla, above which the Landau quantized regime is fully developed. Such a low onset field of quantization is comparable to high-mobility conventional two-dimensional electron gases. Our experiments call for further investigation of the highly unusual ground state of the WTe$_{2}$ monolayer. This includes the influence of device components and the possible existence of mobile fermions and charge-neutral Fermi surfaces inside its insulating gap.