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Observation of a superfluid-to-insulator transition of bilayer excitons

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Abstract

One of the most remarkable properties associated with Bose–Einstein condensation (BEC) is superfluidity, in which the system exhibits zero viscosity and flows without dissipation. The superfluid phase has been observed in wide-ranging bosonic systems spanning naturally occurring quantum fluids, such as liquid helium, to engineered platforms such as bilayer excitons and cold atom systems1,2,3,4. Theoretical works have proposed that interactions could drive the BEC ground state into another exotic phase that simultaneously exhibits properties of both a crystalline solid and a superfluid—termed a supersolid5,6,7,8. Identifying a material system, however, that hosts the predicted BEC solid phase, driven purely by interactions and without imposing an external lattice potential, has remained unknown9,10,11. Here we report observation of a superfluid-to-insulator transition in the layer-imbalanced regime of bilayer magnetoexcitons. Mapping the transport behaviour of the bilayer condensate as a function of density and temperature suggests that the insulating phase is an ordered state of dilute excitons, stabilized by dipole interactions. The insulator melts into a recovered superfluid on increasing the temperature, which could indicate that the low-temperature solid is also a quantum coherent phase.

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Fig. 1: Transport signatures of the exciton condensate.
Fig. 2: Tuning inter-exciton spacing ℓe with layer imbalance.
Fig. 3: An exciton insulator at large Δν.
Fig. 4: A first-order re-entrant transition between the exciton solid and superfluid phases.

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Acknowledgements

J.I.A.L. and C.R.D. thank S. D. Sarma and K. Yang for their discussions. This research was primarily supported by the US Department of Energy, Office of Science, Basic Energy Sciences, under award no. DE-SC0019481 (transport measurements). Heterostructure and device fabrication were supported by the NSF MRSEC program at Columbia through the Center for Precision-Assembled Quantum Materials (DMR-2011738). Data analysis (Q.S.) was partially supported by the Department of Energy (DE-SC0016703). J.I.A.L. acknowledges support from the Sloan Research Fellowship and NSF DMR-2143384. N.J.Z. acknowledges support from the Air Force Office of Scientific Research. R.Q.N. acknowledges support from the National Science Foundation EPSCoR Program under NSF award OIA-2327206. K.W. and T.T. acknowledge support from the JSPS KAKENHI (grant nos. 20H00354, 21H05233 and 23H02052) and the World Premier International Research Center Initiative (WPI), MEXT, Japan. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation cooperative agreement no. DMR-1157490 and the State of Florida.

Author information

Author notes

  1. These authors contributed equally: Yihang Zeng, Dihao Sun

Authors and Affiliations

  1. Department of Physics, Columbia University, New York, NY, USA

    Yihang Zeng, Dihao Sun, Qianhui Shi, A. Okounkova & C. R. Dean

  2. Department of Physics and Astronomy, Purdue University, West Lafayette, IN, USA

    Yihang Zeng

  3. Department of Physics, Brown University, Providence, RI, USA

    Naiyuan J. Zhang & Ron Q. Nguyen

  4. Research Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, Japan

    K. Watanabe

  5. Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Japan

    T. Taniguchi

  6. Department of Mechanical Engineering, Columbia University, New York, NY, USA

    J. Hone

  7. Department of Physics, University of Texas at Austin, Austin, TX, USA

    J. I. A. Li

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  1. Yihang Zeng
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Contributions

C.R.D. and J.I.A.L. conceived the project. Y.Z., D.S. N.J.Z., R.Q.N., A.O. and J.I.A.L. fabricated the device. Y.Z., D.S., N.J.Z., R.Q.N., Q.S. and J.I.A.L. performed the measurement. K.W. and T.T. provided the material. Y.Z., J.H., C.R.D. and J.I.A.L. wrote the paper.

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Correspondence to C. R. Dean or J. I. A. Li.

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Nature thanks Anindya Das, David Neilson, Changgan Zeng and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Parallel flow and counterflow measurement.

(a) Counterflow conductance (GCF, black trace) and parallel flow conductance (GPF, red trace) as a function of Δν measured at T = 0.3. (b) Counterflow conductance as a function of Δν measured at T = 0.3 K and 1.5 K. The Δν-regime of the exciton superfluid (ES) and exciton insulator (EI) are marked by red and blue shaded background, respectively. The superfluid corresponds to a large counterflow conductance, whereas it is insulating in parallel flow measurement. On the other hand, the exciton insulator displays zero conductance in both parallel flow and counterflow measurements. However, the insulator undergoes a reentrant transition into a superfluid phase with increasing temperature. (c) Counterflow and parallel flow conductance, GCF and GPF (black and red traces), as a function of temperature measured at Δν = −0.57, where the low temperature ground state is an exciton insulator. (d) Counterflow and parallel flow conductance, GCF and GPF (black and red traces), as a function of temperature measured at Δν = −0.44, where the low temperature ground state is an exciton superfluid. All measurements are performed in a Corbino-shaped sample (C1) at ∣νtotal∣ = 1 and B = 29 T. (e) GCF and GPF as a function of Δν near the low-temperature transition induced by Δν. A peak in GPF, marked by the vertical arrow, is observed near the transition. (f) GPF as a function of T measured at Δν = −0.57. Near the EI-to-ES transition with increasing temperature, GPF exhibits a small peak, which is marked by the black vertical arrow.

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Zeng, Y., Sun, D., Zhang, N.J. et al. Observation of a superfluid-to-insulator transition of bilayer excitons. Nature (2026). https://doi.org/10.1038/s41586-025-09986-w

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