Quantum Oscillations Realize Tunable Transitions in Excitonic and Spin Hall Insulators

Quantum Oscillations in WSe2

The pursuit of exotic quantum states of matter has long fascinated scientists seeking breakthroughs in condensed matter physics and materials science. A new study led by Zhongdong Han, Yiyu Xia, and collaborators at Cornell University, together with Kenji Watanabe and Takashi Taniguchi of Japan’s National Institute for Materials Science, reports a striking advance: the ability to oscillate between two distinct insulating phases — quantum spin Hall insulators and excitonic insulators — within twisted bilayer tungsten diselenide (WSe₂).

Tuning Between Quantum States

At the heart of this discovery lies the capacity to control Landau levels — quantized electronic states that emerge under magnetic fields. Fully filled Landau levels generate quantum spin Hall phases with spin-polarized edge currents protected by topology. Half-filled levels, however, favor excitonic insulator states, where electron–hole pairs bind together, opening an energy gap and rendering the system insulating.

The researchers demonstrated that by carefully balancing electron-like and hole-like Landau levels, twisted WSe₂ exhibits periodic oscillations between these phases. At charge neutrality, transitions arise from the competition between cyclotron energy and Coulomb interactions, leading to a rich playground for exploring correlated and topological physics.

Experimental Signatures

Evidence for these phases was obtained through nonlocal resistance measurements, which confirmed spin-momentum-locked edge states in the spin Hall regime, and through temperature-dependent resistance tests revealing the insulating gap of excitonic phases. Up to four pairs of helical edge states were identified, providing robust confirmation of topological protection in this system.

By applying an electric field, the team could also tune the Fermi surface, modifying the stability of the excitonic phase. Interestingly, signs of superconductivity were observed near specific filling factors, hinting that this material could host even richer physics such as fractional spin Hall states or unconventional superconductivity.

Why This Matters

The coexistence and tunability of these states marks an important milestone. Both excitonic insulators and quantum spin Hall insulators have been studied independently, but the ability to induce controlled oscillations between them in the same material system is unprecedented. It highlights the power of moiré superlattices like twisted bilayer WSe₂ as tunable platforms for correlated electron physics.

These results not only deepen our understanding of strongly correlated systems but also open pathways for practical technologies. Potential applications include spintronics, where spin currents replace charge currents for lower-power devices, and quantum information processing, where topologically protected states could form robust qubits.

Future Outlook

This breakthrough feeds into a growing body of research exploring 2D materials and moiré engineering as fertile grounds for emergent quantum phases. With further refinements, researchers may soon realize fractional excitonic insulators or manipulate these transitions dynamically, paving the way toward programmable quantum matter.

You can read the original article here: Quantum Oscillations Realize Tunable Transitions in Excitonic and Spin Hall Insulators .

*This article was prepared with the assistance of AI technologies to support research communication.*

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#QuantumMaterials #SpinHallEffect #ExcitonicInsulator #MoiréSuperlattices #2DMaterials #TwistedBilayer #TopologicalPhysics #QuantumServerNetworks #PWmat #Spintronics #QuantumComputing

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