Bio

Ruishi Qi is a Stanford Science Fellow in the Department of Applied Physics at Stanford University, working with Professor Tony F. Heinz in the Edward L. Ginzton Laboratory. His research explores strongly correlated quantum matter in two-dimensional electron–hole systems. He combines van der Waals heterostructures, low-temperature optical spectroscopy, electronic transport, and thermodynamic measurements to study how excitons, trions, and biexcitons form collective quantum phases, including excitonic insulators, condensates, and crystals.

He received his Ph.D. in Physics from the University of California, Berkeley in 2026, advised by Professor Feng Wang, his M.A. in Physics from UC Berkeley in 2022, and his B.S. in Physics from Peking University in 2020, where he worked with Professor Peng Gao. His earlier research developed electron energy-loss spectroscopy methods for probing phonons and polaritons at the nanoscale. His honors include the Stanford Science Fellowship, the Jackson C. Koo Award in Condensed Matter, and the Alex Zettl Graduate Student Award in Physics.

At UC Berkeley, he taught discussion and laboratory sections for Physics 7A: Physics for Scientists and Engineers in 2020–2021. He also mentored undergraduate researchers through the Berkeley Physics Innovators Initiative (2023–2024) and Berkeley Physics International Education (2022 and 2025).

Honors & Awards

  • Stanford Science Fellowship, Stanford University (2026–2029)
  • Alex Zettl Graduate Student Award in Physics, University of California, Berkeley (2026)
  • Jackson C. Koo Award in Condensed Matter, University of California, Berkeley (2026)
  • Kavli ENSI Graduate Student Fellowship, Kavli Energy NanoScience Institute (2023–2024)
  • Outstanding Graduate, Peking University (2020)
  • Outstanding Undergraduate Research Project, Peking University (2020)
  • Weiming Scholar, Peking University (2020)
  • National Southwest Associated University Grand Scholarship, Peking University (2019)

Professional Education

  • Doctor of Philosophy, University of California Berkeley (2026)
  • Master of Arts, University of California Berkeley (2022)
  • B.S., Peking University, Physics (2020)

Stanford Advisors

Contact

  • Academic
    ruishiqi@stanford.edu
    University - Scholar Department: Applied Physics Position: Postdoctoral Scholar

Additional Info

Links

Current Research and Scholarly Interests

My research aims to establish electrically tunable electron–hole systems as a platform for discovering and controlling strongly correlated quantum matter. In atomically thin van der Waals heterostructures, electrons and holes bind into composite particles—excitons, trions, and biexcitons—that can organize into insulating, crystalline, fluid, and condensate phases.

I combine device fabrication with low-temperature optical spectroscopy, electronic transport, and thermodynamic probes to reveal these phases and their collective response. My work has investigated equilibrium excitonic insulators, perfect Coulomb drag and exciton transport, electrically controlled interlayer trion fluids, exciton crystals, and two-component exciton condensates. By tuning charge density, layer geometry, magnetic fields, and moiré potentials, I explore how the interactions and internal degrees of freedom of composite particles shape their many-body behavior.

Current interests include equilibrium condensates, exciton superfluid transport, crystalline states, and multicomponent quantum fluids, as well as new regimes where strong Coulomb interactions, dimensionality, topology, and optical control meet. A central goal is to create experimental systems in which correlated bosonic and fermionic matter can be measured and controlled with microscopic precision.

My earlier work developed momentum-resolved electron energy-loss spectroscopy to measure local lattice dynamics and phonon dispersion at interfaces and in nanostructures.

All Publications

  • Two-component exciton condensates in an electron-hole bilayer NATURE Qi, R., Li, Q., Nie, J., Xia, R., Kim, H., Lim, H., Xie, J., Taniguchi, T., Watanabe, K., Crommie, M. F., MacDonald, A. H., Wang, F. 2026: 629-634

    Abstract

    Macroscopic quantum coherence emerges when bosons condense into a Bose-Einstein condensate (BEC)1-5. Excitons are a long-sought solid-state route to high-temperature BECs with strong interactions, electrical tunability and potentially multicomponent spinor order, but conclusive evidence for equilibrium condensation has remained elusive. Here we report evidence for two-component exciton BECs in MoSe2/hBN/WSe2 electron-hole bilayers6-9 by probing the spin-valley susceptibility of constituent electrons and holes. This heterostructure hosts equilibrium exciton fluids with four spin-valley flavours. Magneto-optical spectroscopy in a dilution refrigerator reveals three exciton condensate phases with distinct flavour polarizations. At zero magnetic field, the many-body ground state is a coherent superposition of two condensed intravalley exciton flavours. Under a magnetic field, the intravalley exciton condensate first switches to a two-component intervalley condensate through a first-order quantum phase transition at a weak critical field and then turns into a fully polarized single-component condensate at high fields. The condensate signatures form a dome in density-temperature space, persisting up to approximately 1.8 K. Our results establish van der Waals electron-hole bilayers as a versatile platform for strongly interacting, multicomponent exciton BECs.

    View details for DOI 10.1038/s41586-026-10636-y

    View details for Web of Science ID 001788964600001

    View details for PubMedID 42271053

    View details for PubMedCentralID 10719388

  • Orbital-Dependent Coulomb Drag in Electron-Hole Bilayer Graphene Heterostructures PHYSICAL REVIEW LETTERS Zhang, Z., Qi, R., Xie, J., Li, Q., Taniguchi, T., Watanabe, K., Crommie, M. F., Wang, F. 2026; 136 (12): 126303

    Abstract

    We report Coulomb drag studies in an electron-hole bilayer graphene heterostructure in a magnetic field, where the orbital, spin, and valley degrees of freedom are lifted by the combined effects of exchange interaction, Zeeman energy, and a vertical displacement field. Our device enables the application of a large vertical displacement field across both layers. In addition to the well-established strong Coulomb drag between the Landau levels with an orbital quantum number N=0, we observe a Coulomb drag signal between the N=1 Landau levels under a suitable vertical displacement field. As the vertical displacement field increases further, the Coulomb drag signal between N=1 Landau levels weakens, and a Coulomb drag signal emerges between the N=0 and N=1 Landau levels. These findings suggest the important roles of the orbital index and the vertical displacement field in interlayer Coulomb interaction within the quantum Hall regime of coupled bilayer systems.

    View details for DOI 10.1103/4v1f-lkh2

    View details for Web of Science ID 001730388600018

    View details for PubMedID 41965042

  • Nature of Emergent Moire Excitations in MoSe<sub>2</sub>/WS<sub>2</sub> Moire Superlattices NANO LETTERS Lu, Z., Qi, R., Dutta, R., Xie, J., Nie, J., Wang, Z., Feng, Y., Kim, W., Watanabe, K., Taniguchi, T., Raja, A., Naik, M. H., Louie, S. G., Wang, F. 2026; 26 (12): 4096-4102

    Abstract

    Moiré superlattices of semiconducting transition-metal dichalcogenide (TMD) heterobilayers are model systems for investigating strongly correlated electronic and excitonic phenomena. MoSe2/WS2 moiré superlattices, which have a type-I band alignment, host fascinating correlated excitonic states. Although new moiré excitons in MoSe2/WS2 have been widely reported, their microscopic origin has remained unclear. Here, combining large-scale first-principles calculations and microreflection spectroscopy, we identify the nature of these excitons. Our results show that excitonic resonances around MoSe2 A exciton energy are mainly determined by periodic strain patterns arising from atomic reconstruction of the TMD layers rather than interlayer hybridization, which had been the most common explanation previously. Consistently, no interlayer dipole can be observed in the vertical electric-field dependence of the moiré exciton resonance. Our results provide a microscopic explanation for the formation of moiré excitons in MoSe2/WS2 moiré superlattices and highlight the important role of structural reconstructions in general TMD moiré superlattices.

    View details for DOI 10.1021/acs.nanolett.5c06149

    View details for Web of Science ID 001716362600001

    View details for PubMedID 41838843

  • 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

  • Electron Microscopy Measurement of Picometer-Level Distortion Induced Local Phonons at a Defect NANO LETTERS Han, B., Shi, R., Peng, H., Lv, Y., Li, Q., Qi, R., Li, Y., He, P., Gao, X., Lu, J., Zhang, J., Chen, S., Du, J., Yu, P., Gao, P. 2026; 26 (4): 1449-1454

    Abstract

    Defects in oxides usually exhibit exotic properties that may be associated with local lattice dynamics. Here, at atomic spatial resolution, we directly measure phonon modes of an antiphase boundary (APB) in SrTiO3 freestanding membrane and correlate them with the picometer-level structural distortion. We find that the SrTiO3 APB introduces new defect phonon modes that are absent in bulk SrTiO3. These modes are highly sensitive to the subtle structure distortion; i.e., the SrTiO3 APB generates the local electric dipoles, forming an antiferroelectric configuration, which significantly change the frequencies of transverse optical (TO) and longitudinal optical (LO) modes. When the local phonons are correlated with the subtle structural distortion, our findings provide valuable insights into understanding the defect properties in complex oxides and essential information for their applications such as thermoelectric devices.

    View details for DOI 10.1021/acs.nanolett.5c05660

    View details for Web of Science ID 001667340000001

    View details for PubMedID 41558854

  • 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

https://orcid.org/0009-0000-1305-1104