Cavity-altered superconductivity

TL;DR

This study demonstrates that embedding a molecular superconductor within a hyperbolic van der Waals cavity can modify its superconducting properties, showing potential for cavity engineering of quantum materials without optical excitation.

Contribution

We developed a novel cavity platform using hyperbolic van der Waals materials to alter the superconducting ground state of a molecular superconductor.

Findings

Resonant coupling between cavity modes and molecular vibrations was observed.

Superfluid density was suppressed near the cavity interface.

Control heterostructures did not show superfluid suppression.

Abstract

Is it feasible to alter the ground state properties of a material by engineering its electromagnetic environment? Inspired by theoretical predictions, experimental realizations of such cavity-controlled properties without optical excitation are beginning to emerge. Here, we devised and implemented a novel 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. We interfaced hexagonal boron nitride (hBN) with the molecular superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br ($\kappa$-ET). The frequencies of infrared (IR) hyperbolic modes of hBN match the IR-active carbon-carbon stretching molecular resonance of ($\kappa$-ET) implicated in superconductivity. 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 carbon-carbon stretching mode in ($\kappa$-ET). Meissner effect measurements via magnetic force microscopy demonstrate a strong suppression of superfluid density near the hBN/($\kappa$-ET) interface. Non-resonant control heterostructures, including RuCl$_3$/($\kappa$-ET) and hBN/$\text{Bi}_2\text{Sr}_2\text{CaCu}_2\text{O}_{8+x}$, do not display the superfluid suppression. These observations suggest that hBN/($\kappa$-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.