The interplay between topology and correlations can generate a variety of unusual quantum phases, many of which remain to be explored. Recent advances have identified monolayer WTe2 as a promising material for exploring such interplay in a highly tunable fashion. The ground state of this two-dimensional (2D) crystal can be electrostatically tuned from a quantum spin Hall insulator (QSHI) to a superconductor. However, much remains unknown about the nature of these ground states, including the gap-opening mechanism of the insulating state. Here we report systematic studies of the insulating phase in WTe2 monolayer and uncover evidence supporting that the QSHI is also an excitonic insulator (EI). An EI, arising from the spontaneous formation of electron-hole bound states (excitons), is a largely unexplored quantum phase to date, especially when it is topological. Our experiments on high-quality transport devices reveal the presence of an intrinsic insulating state at the charge neutrality point (CNP) in clean samples. The state exhibits both a strong sensitivity to the electric displacement field and a Hall anomaly that are consistent with the excitonic pairing. We further confirm the correlated nature of this charge-neutral insulator by tunneling spectroscopy. Our results support the existence of an EI phase in the clean limit and rule out alternative scenarios of a band insulator or a localized insulator. These observations lay the foundation for understanding a new class of correlated insulators with nontrivial topology and identify monolayer WTe2 as a promising candidate for exploring quantum phases of ground-state excitons.

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.

Optically detected magnetic resonance of nitrogen vacancy centers in diamond offers novel routes to both DC and AC magnetometry in diamond anvil cells under high pressures (> 3 GPa). However, a serious challenge to realizing experiments has been the insertion of microwave radiation in to the sample space without screening by the gasket material. We utilize designer anvils with lithographically-deposited metallic microchannels on the diamond culet as a microwave antenna. We detected the spin resonance of an ensemble of microdiamonds under pressure, and measure the pressure dependence of the zero field splitting parameters. These experiments enable the possibility for all-optical magnetic resonance experiments on sub-µL sample volumes at high pressures.