Historical research guide · 0476 · 2026
Cavity-enhanced superconducting response in an underdoped cuprate
Angela Montanaro · Vadim Plastovets · Nitesh Khatiwada · Jacopo Fiore · Giacomo Jarc · Abdullah Alabbadi · Antonio Mastropasqua · Enrico Maria Rigoni · Shahla Y. Mathengattil · Simone Dal Zilio · Francesca Fassioli Olsen · Fabio Novelli · Stephan Winnerl · Michael A. Sentef · Dante M. Kennes · Andrew J. Millis · Francesco Piazza · Daniele Fausti
In one minute
Put a superconductor close to a mirror and its superconductivity gets stronger. That is the result Angela Montanaro, Daniele Fausti and a European and American collaboration report here. Their sample is a thin film of the copper-oxide superconductor YBCO, deliberately underdoped so that its weak point is not the pairing of electrons but the coherence of the pairs — the pairs form, but their shared quantum phase wobbles and the current-carrying state falls apart. The team faces the film with a semi-transparent gold mirror on piezoelectric actuators, making an adjustable terahertz cavity, and measures the film through it. Below the transition the condensate response is systematically larger inside the cavity than outside, the gain grows as the mirror is brought nearer, and the temperature at which superconductivity switches on moves up by about a kelvin. Their model says why: the mirrors put a gap in the photon spectrum, which shields the phase from electric-field fluctuations and stiffens it.
Connection
Chapter 11 asks whether the macroscopic quantum state of a superconductor can be engineered rather than merely cooled into, and this is that engineering done with nothing but geometry — no doping, no pressure, no field, just a mirror moved closer. Chapter 2 gets the deeper point: what the mirror changes is the vacuum itself, the spectrum of electromagnetic modes the material sits in, and the material answers. Read it beside the companion measurement in two-dimensional NbSe2 at /library/stm-812175a230 and the Strasbourg experiment that first put a superconductor under strong coupling to the vacuum field at /library/stm-21d2922b6e.
From the source
The cavity is formed by the superconducting film itself and a semi-transparent gold mirror on three piezoelectric actuators, and its length is read out in situ from the delay between the directly transmitted terahertz pulse and the first cavity reflection — a calibration of the photon round-trip time that needs no separate probe of the geometry.Figure 1b; Methods, ‘Cavity assembly’ and ‘Terahertz time-domain spectroscopy’
The frequency-averaged imaginary conductivity between 0.4 and 1 THz, which is proportional to the superfluid weight, is systematically larger for the film inside the cavity than for the same film in free space below the critical temperature of 85 K, and the same conclusion follows from two independent analyses — direct time-domain fitting and the phase shift of the transmitted pulse.Figure 2b; Supplementary Information Sec. S3
The enhancement grows as the mirror is brought closer to the film, reaching its largest value at the shortest cavity length of 20 µm, while the real part of the conductivity shows essentially no cavity dependence at any length — so the cavity acts selectively on the inductive, condensate-related channel and leaves the dissipative channel alone.Figures 3a to 3c; Figure 4c
The onset of the superconducting response shifts upward by up to about 1 K inside the cavity, and the shift survives control tests for thermal drift, thermal contact, scan direction and terahertz-induced heating — while it disappears when the gold layer is stripped from the mirror and only its dielectric substrate is brought near the film.Figure 4a; Supplementary Information Secs. S4 and S4.3
The proposed mechanism is cavity-enhanced phase stiffness: the photon gap created by the mirrors reduces the hybridisation of the fluctuating vector potential with the superconductor’s low-energy phase correlations, protecting the phase from electric-field fluctuations, raising the stiffness, and therefore raising the energetic cost of vortex excitations and the Berezinskii-Kosterlitz-Thouless transition temperature — a cavity-coupled XY model reproduces both observed trends.Figure 4b; Supplementary Information Sec. S6.1
Reaching shorter cavity lengths will require depositing cavity-like heterostructures with the superconducting layer embedded, positioned nearer the field antinode; and the sharpest test of the phase-fluctuation reading is to repeat the experiment on more anisotropic cuprates such as Bi2Sr2CaCu2O8+δ, where weaker interlayer coupling makes phase fluctuations more pronounced.Main text, closing paragraphs
The way in
https://arxiv.org/abs/2606.18084How to cite it
Angela Montanaro, Vadim Plastovets, Nitesh Khatiwada, Jacopo Fiore, Giacomo Jarc, Abdullah Alabbadi, Antonio Mastropasqua, Enrico Maria Rigoni, Shahla Y. Mathengattil, Simone Dal Zilio, Francesca Fassioli Olsen, Fabio Novelli, Stephan Winnerl, Michael A. Sentef, Dante M. Kennes, Andrew J. Millis, Francesco Piazza, Daniele Fausti (2026) Cavity-enhanced superconducting response in an underdoped cuprate arXiv:2606.18084