Landau quantization and highly mobile fermions in an insulator
In strongly correlated materials, quasiparticle excitations can carry fractional quantum numbers. An intriguing possibility is the formation of fractionalized, charge-neutral fermions—for example, spinons 1 and fermionic excitons 2 , 3 —that result in neutral Fermi surfaces and Landau quantization 4 , 5 in an insulator. Although previous experiments in quantum spin liquids 1 , topological Kondo insulators 6 – 8 and quantum Hall systems 3 , 9 have hinted at charge-neutral Fermi surfaces, evidence for their existence remains inconclusive. Here we report experimental observation of Landau quantization in a two-dimensional insulator, monolayer tungsten ditelluride (WTe 2 ), a large-gap topological insulator 10 – 13 . Using a detection scheme that avoids edge contributions, we find large quantum oscillations in the material’s magnetoresistance, with an onset field as small as about 0.5 tesla. Despite the huge resistance, the oscillation profile, which exhibits many periods, mimics the Shubnikov–de Haas oscillations in metals. 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 the behaviour of high-mobility conventional two-dimensional electron gases. Our experiments call for further investigation of the unusual ground state of the WTe 2 monolayer, including the influence of device components and the possible existence of mobile fermions and charge-neutral Fermi surfaces inside its insulating gap. Pronounced quantum oscillations in magnetoresistance, a phenomenon that was only expected in metals with highly mobile carriers, are observed in the strongly insulating state of two-dimensional WTe 2 .