Itai Keren, Tatiana A. Webb, Shuai Zhang, Jikai Xu, Dihao Sun, Brian S. Y. Kim, Dongbin Shin, Songtian S. Zhang, Junhe Zhang, Giancarlo Pereira, Juntao Yao, Takuya Okugawa, Marios H. Michael, Emil Viñas Boström, James H. Edgar, Stuart Wolf, Matthew Julian, Rohit P. Prasankumar, Kazuya Miyagawa, Kazushi Kanoda, Genda Gu, Matthew Cothrine, David Mandrus, Michele Buzzi, Andrea Cavalleri, Cory R. Dean, Dante M. Kennes, Andrew J. Millis, Qiang Li, Michael A. Sentef, Angel Rubio, Abhay N. Pasupathy & D. N. Basov

Nature650, 864–868 (2026)

Is it feasible to alter the ground-state properties of a material by engineering its electromagnetic environment? Inspired by theoretical predictions^{1,2,3,4,5,6,7,8,9,10,11,12}, experimental realizations of such cavity-controlled properties without optical excitation are beginning to emerge^{13,14,15,16,17,18,19}. Here we devised and implemented a new platform to realize cavity-altered materials. Single crystals of hyperbolic van der Waals (vdW) compounds provide a resonant electromagnetic environment with enhanced density of photonic states and prominent mode confinement^{20,21,22,23,24}. We interfaced hexagonal boron nitride (hBN) with the molecular superconductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Br (κ-ET). The frequencies of infrared hyperbolic modes (HMs) of hBN (refs. ^25,26) match the infrared-active carbon–carbon (C=C) stretching molecular resonance of κ-ET implicated in superconductivity²⁷. Nano-optical data supported by first-principles molecular Langevin dynamics simulations confirm the presence of resonant coupling between the hBN hyperbolic cavity modes and the C=C stretching mode in κ-ET. Meissner-effect measurements using magnetic force microscopy (MFM) demonstrate a strong suppression of superfluid density near the hBN/κ-ET interface. Non-resonant control heterostructures, including RuCl₃/κ-ET and hBN/Bi₂Sr₂CaCu₂O_8+x (BSCCO), do not show the pronounced superfluid suppression. These observations suggest that hBN/κ-ET realizes a cavity-altered superconducting ground state. Our work highlights the potential of dark cavities devoid of external photons for engineering electronic ground-state properties of complex quantum materials.