Cavity-Driven Attractive Interactions in Quantum Materials

Many-body phenomena in quantum materials emerge from the interplay among a broad continuum of electronic states, and controlling these interactions is critical for engineering novel phases. One promising approach exploits fluctuations of the vacuum electromagnetic field confined within optical cavities to tailor electronic properties. Here, we demonstrate that cavity photons can mediate attractive interactions in a tunable van der Waals material and reorganize a continuum of electron-hole transitions into an exciton-like state. We introduce a broadband, sub-wavelength time-domain microscope that integrates exfoliated, dual-gated two-dimensional quantum materials into a terahertz cavity. This approach enables the first-ever measurement of the field-tunable bandgap of bilayer graphene at terahertz frequencies while revealing ultrastrong coupling with a vacuum Rabi frequency exceeding $\Omega_{Rabi}/\omega\approx 40\%$ of the bare photon energy. Crucially, we identify a novel cavity-induced resonance emerging from the interband continuum that resembles Coulomb-bound excitons and remains stable across a broad temperature range. By uniting longstanding theoretical predictions with advanced experimental techniques, our findings open new avenues for designing and probing unique light-matter states and realizing hybrid correlated phases in quantum materials.