Ruishi Qi

All Publications

• An exciton crystal in a moiré excitonic insulator NATURE PHYSICS Qi, R., Li, Q., Kim, H., Nie, J., Zhang, Z., Xia, R., Cui, Z., Xiao, J., Taniguchi, T., Watanabe, K., Crommie, M. F., Wang, F. 2026; 22 (4)

  View details for DOI 10.1038/s41567-026-03184-9

  View details for Web of Science ID 001693660500001

• Competition between excitonic insulators and quantum Hall states in correlated electron-hole bilayers. Nature materials Qi, R., Li, Q., Zhang, Z., Nie, J., Zou, B., Cui, Z., Kim, H., Sanborn, C., Chen, S., Xie, J., Taniguchi, T., Watanabe, K., Crommie, M. F., MacDonald, A. H., Wang, F. 2026; 25 (1): 35-41

  Abstract

  Excitonic insulators represent a unique quantum phase of matter that enables the study of exotic quantum bosonic states. Strongly coupled electron-hole bilayers, which host stable dipolar exciton fluids with an exciton density that can be adjusted electrostatically, offer an ideal platform to investigate correlated excitonic insulators. On the basis of electron-hole bilayers made of MoSe2/hexagonal boron nitride/WSe2 heterostructures, here we study the behaviour of excitonic insulators in a perpendicular magnetic field. We report the observation of excitonic quantum oscillations in both Coulomb drag signals and electrical resistance at low to medium magnetic fields. Under a strong magnetic field, we identify multiple quantum phase transitions between the excitonic insulator phase and the bilayer quantum Hall insulator phase. These findings underscore the interplay between the electron-hole interactions and Landau-level quantization, and enable further exploration of quantum phenomena in composite bosonic insulators.

  View details for DOI 10.1038/s41563-025-02316-5

  View details for PubMedID 40855217

  View details for PubMedCentralID 10719388

• Electrically controlled interlayer trion fluid in electron-hole bilayers. Science (New York, N.Y.) Qi, R., Li, Q., Zhang, Z., Chen, S., Xie, J., Ou, Y., Cui, Z., Dai, D. D., Joe, A. Y., Taniguchi, T., Watanabe, K., Tongay, S., Zettl, A., Fu, L., Wang, F. 2025; 390 (6770): 299-303

  Abstract

  The combination of repulsive and attractive Coulomb interactions in a quantum electron-hole (e-h) fluid can produce correlated phases of multiparticle charge complexes, such as excitons, trions, and biexcitons. We report an experimental realization of an electrically controlled interlayer trion fluid in van der Waals heterostructures. In strongly coupled e-h bilayers, electrons and holes spontaneously form three-particle trion bound states. The interlayer trions can assume 1e-2h and 2e-1h configurations. We show that the two holes in 1e-2h trions form a spin-singlet with a spin gap of approximately one milli-electron volt. By electrostatic gating, the equilibrium state can be continuously tuned into an exciton fluid, a trion fluid, an exciton-trion mixture, or a trion-charge mixture. Our work demonstrates a platform to study correlated phases of tunable Bose-Fermi mixtures.

  View details for DOI 10.1126/science.adn4584

  View details for PubMedID 41100622

• Perfect Coulomb drag and exciton transport in an excitonic insulator. Science (New York, N.Y.) Qi, R., Joe, A. Y., Zhang, Z., Xie, J., Feng, Q., Lu, Z., Wang, Z., Taniguchi, T., Watanabe, K., Tongay, S., Wang, F. 2025; 388 (6744): 278-283

  Abstract

  Strongly coupled electron-hole bilayers can host quantum states of interlayer excitons, such as high-temperature exciton condensates at zero magnetic field. This state is predicted to feature perfect Coulomb drag, where a current in one layer is accompanied by an equal but opposite current in the other. We used an optical technique to probe the electrical transport of correlated electron-hole bilayers based on MoSe2/hBN/WSe2 heterostructures. We observed perfect Coulomb drag in the excitonic insulator phase at low temperatures; the counterflow resistance of interlayer excitons remained finite. These results indicate the formation of an exciton gas that does not condense into a superfluid. Our work demonstrates that dynamic optical spectroscopy provides a powerful tool for probing exciton transport behavior in correlated electron-hole fluids.

  View details for DOI 10.1126/science.adl1839

  View details for PubMedID 40245122

• Thermodynamic behavior of correlated electron-hole fluids in van der Waals heterostructures. Nature communications Qi, R., Joe, A. Y., Zhang, Z., Zeng, Y., Zheng, T., Feng, Q., Xie, J., Regan, E., Lu, Z., Taniguchi, T., Watanabe, K., Tongay, S., Crommie, M. F., MacDonald, A. H., Wang, F. 2023; 14 (1): 8264

  Abstract

  Coupled two-dimensional electron-hole bilayers provide a unique platform to study strongly correlated Bose-Fermi mixtures in condensed matter. Electrons and holes in spatially separated layers can bind to form interlayer excitons, composite Bosons expected to support high-temperature exciton condensates. The interlayer excitons can also interact strongly with excess charge carriers when electron and hole densities are unequal. Here, we use optical spectroscopy to quantitatively probe the local thermodynamic properties of strongly correlated electron-hole fluids in MoSe2/hBN/WSe2 heterostructures. We observe a discontinuity in the electron and hole chemical potentials at matched electron and hole densities, a definitive signature of an excitonic insulator ground state. The excitonic insulator is stable up to a Mott density of ~0.8 × 1012 cm-2 and has a thermal ionization temperature of ~70 K. The density dependence of the electron, hole, and exciton chemical potentials reveals strong correlation effects across the phase diagram. Compared with a non-interacting uniform charge distribution, the correlation effects lead to significant attractive exciton-exciton and exciton-charge interactions in the electron-hole fluid. Our work highlights the unique quantum behavior that can emerge in strongly correlated electron-hole systems.

  View details for DOI 10.1038/s41467-023-43799-7

  View details for PubMedID 38092731

  View details for PubMedCentralID PMC10719388

• Measuring phonon dispersion at an interface. Nature Qi, R., Shi, R., Li, Y., Sun, Y., Wu, M., Li, N., Du, J., Liu, K., Chen, C., Chen, J., Wang, F., Yu, D., Wang, E. G., Gao, P. 2021; 599 (7885): 399-403

  Abstract

  The breakdown of translational symmetry at heterointerfaces leads to the emergence of new phonon modes localized at the interface1. These modes have an essential role in thermal and electrical transport properties in devices, especially in miniature ones wherein the interface may dominate the entire response of the device2. Although related theoretical work began decades ago1,3-5, experimental research is totally absent owing to challenges in achieving the combined spatial, momentum and spectral resolutions required to probe localized modes. Here, using the four-dimensional electron energy-loss spectroscopy technique, we directly measure both the local vibrational spectra and the interface phonon dispersion relation for an epitaxial cubic boron nitride/diamond heterointerface. In addition to bulk phonon modes, we observe modes localized at the interface and modes isolated from the interface. These features appear only within approximately one nanometre around the interface. The localized modes observed here are predicted to substantially affect the interface thermal conductance and electron mobility. Our findings provide insights into lattice dynamics at heterointerfaces, and the demonstrated experimental technique should be useful in thermal management, electrical engineering and topological phononics.

  View details for DOI 10.1038/s41586-021-03971-9

  View details for PubMedID 34789901

  View details for PubMedCentralID 4793224