Landau quantization and highly mobile fermions in an insulator

Abstract

In strongly correlated materials, quasiparticle excitations can carry fractional quantum numbers. An intriguing possibility is the formation of fractionalized, chargeneutral fermions, e.g., spinons 1 and fermionic excitons 2,3 , that result in neutral Fermi surfaces and Landau quantization 4,5 in an insulator. While previous experiments in quantum spin liquids 1 , topological Kondo insulators [6][7][8] , and quantum Hall systems 3,9 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 (WTe2), a large gap topological insulator [10][11][12][13] . 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 WTe2 monolayer. This includes the influence of device components and the possible existence of mobile fermions and charge-neutral Fermi surfaces inside its insulating gap. MainBulk tungsten ditelluride (WTe2) is a compensated semimetal in which an equal number of electrons and holes co-exist 14 . The semimetallic behavior remains when the material is thinned down to the trilayers 11,15 . In bilayers and monolayers, nevertheless, an insulating gap is observed 11 , giving rise to the high-temperature quantum spin Hall effect in monolayers [10][11][12][13] . However, the mechanism for the gap opening remains mysterious 10,11,16,17 . The observation of superconductivity when the monolayer is doped with a low electron density 18,19 highlights the unusual nature of the insulating state.

Evidence for equilibrium exciton condensation in monolayer WTe2

“…normal displacement field or a normal magnetic field of 14 T, confirming the rejection of edge conduction, which is highly sensitive to magnetic field at low temperatures 29 . (A recent paper 30 reports surprising quantum oscillations at low n g , but we have not seen these in any device.) On cooling from room temperature to 100 K, G p versus n p develops a sharp 'V' shape centred close to n g = 0.…”

The universal shear conductivity of Fermi liquids and spinon Fermi surface states and its detection via spin qubit noise magnetometry

“…The suggestion that 1T-TaS 2 might harbor a U(1) spin liquid is more recent [48,49], and more specifically, the case for a spinon FS state has been argued based on heat transport measurements [50]. The possibility that a state like the spinon FS might be realized in monolayer WTe 2 has been highlighted by the recent remarkable observation of clear quantum oscillations of resistivity in a strongly insulating regime [51].…”

Reshaped Weyl fermionic dispersions driven by Coulomb interactions in MoTe2

“…Moreover, quantum oscillation (QO) measured in recent works does not agree well with computations based on a simple approximation of the Coulomb interactions [10][11][12]. Considering recent reports exploring a possible higher-order topological insulating state [20,21] as well as a non-trivial insulating phase [22,23] in MoTe 2 , it is crucial to clarify effects of electron-electrons interactions and their changes with doping or under different screening environments.…”