Ruishi Qi

Moiré superlattices formed by transition metal dichalcogenide (TMD) heterobilayers provide a versatile platform for studying strongly correlated electronic, excitonic, and topological phenomena in solids. In particular, angle-aligned MoSe2/WS2 heterobilayers, which have a Type-I band alignment at zero vertical electric field, host rich correlated spin and charge physics. Here, combining large-scale first-principles calculations and optical reflection spectroscopy, we report a thorough study of the emergent moiré excitonic states and interlayer charge-transfer states in angle-aligned electron-doped MoSe2/WS2 moiré superlattices. The moiré excitonic states serve as sensitive optical probes to the localization profile of doped electrons. We observe a series of interlayer charge-transfer transitions from n/n0 = 1 to 4 (where n0 denotes the moiré density) when the vertical electric field switches the heterostructure band alignment from Type-I to Type-II. By tuning the vertical electric field, we can precisely control the interlayer electron localization, realizing a Fermi-Hubbard model with a tunable charge-transfer band on an effective honeycomb lattice. Furthermore, Monte Carlo simulation of the doping dependence of the electric-field susceptibility predicts that multiple correlated charge-ordered states appear at both integer and fractional fillings. Our results provide a holistic understanding of the emergent optical excitations and the correlated charge-transfer states in electron-doped MoSe2/WS2 moiré superlattices.

Macroscopic quantum coherence emerges when bosons condense into a Bose–Einstein condensate (BEC)^1,2,3,4,5. 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 bilayers^6,7,8,9 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.

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.

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.

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.

Phonon polaritons (PhPs) enable subwavelength light control for infrared sensing, imaging, and optoelectronics, but conventional polar materials have narrow Reststrahlen bands, limiting applications. Materials that support PhPs with broad spectral range, strong field confinement, slow group velocity, and high-quality factor are therefore needed. Here, using monochromatic electron energy loss spectroscopy in a scanning transmission electron microscope, we demonstrate that ultrathin SrTiO ₃ membranes have the desired properties. Systematic measurements across varying thicknesses reveal two PhP branches with wide spectral dispersion, strong confinement, and exceptionally slow group velocities spanning from the mid-infrared to terahertz range. Notably, in 3-nanometer-thick membranes, these polaritons exhibit unprecedented confinement factors exceeding 500 and group velocities as low as ~7 × 10 ⁻⁵ c , rivaling the best-performing van der Waals materials. These findings establish perovskite oxide such as SrTiO ₃ as versatile platforms for tailoring light-matter interactions at the nanoscale, providing critical insights for the design of next-generation photonic devices requiring broadband operation and enhanced optical confinement.

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.

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.

Two-dimensional semiconductors and their moiré superlattices have emerged as important platforms for investigating correlated electrons. However, many key properties of these systems, such as the frequency-dependent conductivity, remain experimentally inaccessible because of the mesoscopic sample size. Here we report a technique to directly measure the complex conductivity of electrostatically gated two-dimensional semiconductors in the terahertz frequency range. Applying this technique to a WSe₂ monolayer encapsulated in hBN, we observe a clear Drude-like response between 0.1 and 1 THz, in a density range challenging to access even in DC transport. Our work opens a new avenue for studying tunable van der Waals heterostructures using terahertz spectroscopy.