Historical research guide · 0594 · 2019
Exploring Superconductivity under Strong Coupling with the Vacuum Electromagnetic Field
Anoop Thomas · Eloïse Devaux · Kalaivanan Nagarajan · Thibault Chervy · Marcus Seidel · David Hagenmüller · Stefan Schütz · Johannes Schachenmayer · Cyriaque Genet · Guido Pupillo · Thomas W. Ebbesen
In one minute
Thomas Ebbesen’s group in Strasbourg asked whether a superconductor can be changed without shining any light on it at all — by reshaping the empty space around it. Their samples sit on a thin gold film whose surface carries plasmon waves, and those waves squeeze the electromagnetic vacuum into a very small volume. A superconductor’s own lattice vibrations couple to that vacuum field too feebly to matter, so the team added a relay: they mix the superconducting powder into polystyrene, whose molecular vibrations sit at nearly the same frequency and do couple strongly. The polymer dresses the superconductor’s phonons through the shared vacuum field. Measured in a SQUID magnetometer with no laser running, both materials moved. YBCO fell from 92 K to 86 K. Rubidium-doped buckyballs rose from 30 K to 45 K, a fifty per cent increase at ordinary pressure. Polymers whose vibrations do not overlap changed nothing. A simple model traces the rise to a stronger coupling between electrons and phonons.
Connection
Chapter 2 says the vacuum is a real, structured medium whose boundary conditions are engineering parameters, and this is that statement carried into a magnetometer: no drive, no laser, just a plasmonic surface and a resonant molecular relay — and the temperature at which two very different superconductors expel magnetic field moves by degrees. Chapter 11 watches superconductors because they are where vacuum engineering first shows up with a number attached, and here the number is fifteen kelvin on Rb3C60.
From the source
Coupling the phonons of Rb3C60 to surface plasmon polaritons through a strongly coupled polystyrene environment raises the superconducting transition temperature from 30 K to 45 K at normal pressure, an increase of about fifty per cent, with no external laser field applied.Abstract; Results, Rb3C60 section; Figure 3c and 3d
The same treatment applied to YBCO moves its transition the other way, from 92 K down to 86 K, showing that the effect has a sign and is not a generic degradation of the sample.Abstract; Results, YBCO section; Figure 2d
The interaction is with the vacuum electromagnetic field of the plasmon mode rather than with any applied illumination: even in the dark, the Rabi splitting that marks strong coupling has a finite value, and it grows with the number of coupled oscillators in the material.Main text, second paragraph following the abstract
The controls behave as the mechanism requires. Polymers whose vibrational bands do not overlap the superconductor’s phonons, polymethylmethacrylate and polyvinyl acetate, leave the transition temperature unchanged, and repeating the experiment on platinum and silver films instead of gold reproduces the 86 K result, since those metals also support infrared plasmons.Results, YBCO section; Extended Data
The proposed mechanism is cooperative: the polymer, which couples strongly to the surface plasmon, shares its dressing with the superconductor’s phonons through direct dipole-dipole interaction, so the lowest polariton branch carries substantial superconducting phonon weight at wave vectors comparable to the Fermi wave vector and effectively redshifts that phonon, which raises the electron-phonon coupling parameter.Theoretical model section; Figure 4b to 4d
The authors present the result as a proof of principle that opens a toolbox — a way to modify superconducting materials and, at the same time, a way to test which phonons actually matter in a given compound, including the unconventional ones whose microscopic mechanism is still open.Closing paragraph of the main text
The way in
https://arxiv.org/abs/1911.01459How to cite it
Anoop Thomas, Eloïse Devaux, Kalaivanan Nagarajan, Thibault Chervy, Marcus Seidel, David Hagenmüller, Stefan Schütz, Johannes Schachenmayer, Cyriaque Genet, Guido Pupillo, Thomas W. Ebbesen (2019) Exploring Superconductivity under Strong Coupling with the Vacuum Electromagnetic Field doi:10.1063/5.0231202