Researchers report that placing a few-layer superconductor inside a tuned terahertz cavity can raise its superconducting transition temperature. In a trilayer sample of niobium diselenide (NbSe2) the critical temperature Tc where resistance drops to zero rose from 3.02 K to 3.41 K — an increase of about 10% — when the sample was coupled to a complementary split-ring resonator (CSRR) cavity tuned to 2.04 THz. The cavity mode lies near low-frequency lattice vibrations (phonons) of NbSe2 and the effect is strongest when the cavity and those modes are resonant.

The device stacks a trilayer NbSe2 flake on a SiO2/Si chip, covers it with a roughly 30 nm hexagonal boron nitride (hBN) layer, and places a gold CSRR on top. The hBN keeps the metal from touching the superconductor so the cavity acts through its electromagnetic field, not by direct electrical contact. Simulations show a localized electric field in the CSRR gap of about 0.8 V/m that falls off over a few micrometres. By measuring resistance with multiple voltage probes across the same flake, the authors found a spatial pattern that follows that field: the centre (P1) showed the largest Tc increase (ΔTc = 0.39 K), a nearby spot (P2) showed a smaller increase (ΔTc = 0.27 K), and regions outside the resonator (P3) showed no change.

At a high level, the interpretation is that quantum fluctuations of the electromagnetic field inside the cavity can change how electrons and lattice vibrations interact. Those vacuum field fluctuations are always present, but putting the material into a resonant cavity concentrates and reshapes them. Because the cavity mode sits near NbSe2 phonons, the authors suggest the cavity can modify phonon-related interactions that help pairing between electrons — the microscopic process behind superconductivity. Supporting this picture, the change in Tc depends on frequency: superconductivity was slightly suppressed at lower excitation frequencies, peaked near 2 THz, and disappeared when the frequency was far from the relevant modes.