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Electromagnetic cavity could enhance high-temperature superconductors

20 Apr 2022 Isabelle Dumé
Cavity-magnon interaction
Cavity-magnon interaction. Courtesy: J Curtis

Researchers at Harvard University in the US have put forward a possible new way to enhance superconductivity in cuprate materials. The approach, which involves embedding the materials in an electromagnetic cavity, could pave the way toward realizing room-temperature superconductivity, which the researchers call “a holy grail of modern condensed matter and material science”.

Scientists have long known that shining intense light on a quantum material can alter its properties. More recently, researchers have suggested that similar results could be achieved by resonantly coupling the material to an electromagnetic cavity.

One promising application of this technique would be to control antiferromagnetic correlations in certain copper oxides (cuprates) that conduct electricity without resistance at temperatures above 77 K. Such correlations are thought to underlie many of the exotic and potential useful aspects of these so-called “unconventional” superconductors. Being able to manipulate them could thus have important near-term applications as well as providing a potential route to crafting materials that remain superconducting at still higher temperatures.

Magnon mediation

The conventional theory of superconductivity (known as BCS theory after the initials of its authors) states that below a certain critical temperature Tc, the electrons in a material overcome their mutual repulsion and join up to form so-called Cooper pairs that then travel unimpeded through the material. The formation of these Cooper pairs is mediated by phonons – quasiparticles that arise from vibrations of the material’s crystal lattice.

While this theory holds true for most superconducting materials, it does not apply to the cuprates. For them, Cooper-pair formation is instead thought to be mediated by magnons, which are collective oscillations of the material’s spin magnetic moments. Unlike phonons, these quasiparticles do not need to pair up to travel long distances.

In the new work, a team led by Jonathan Curtis studied an unconventional bilayer superconductor with the chemical formula YBa2Cu3O6 (YBCO) and a Tc of 100 K. The researchers began by trying to understand the various ways in which coupling between phonons and the electromagnetic cavity could affect the magnons in the material. Based on their theoretical results, Curtis and colleagues predict that the strongest effect – that is, the one that could lead to the greatest increase in Tc – occurs when the cavity–phonon–magnon interaction stems from distortions of the crystal lattice of YBCO (and indeed other bilayer cuprates). The team also predicts that other cuprates – including monolayer ones – will experience a weaker effect due to phonons exciting magnons through a relativistic interaction between an electron’s spin and its motion.

Need for experimental verification

The Harvard researchers, who report their work in Physical Review Research, stress that they do not yet know whether it is indeed possible to increase Tc using their technique. However, they believe there are several interesting future directions to take their work. “Of course, the first and most important is for us to carry out real experiments, since after all our study is still only theoretical,” Curtis says. “I think that people are starting to reach the necessary parameter regimes to actually construct these devices and I hope that they will try to realize what we have described in this work.”

Curtis adds that on the theoretical front, the most important thing is to analyse what effects such a resonant device will have on Cooper pairing. “This is a challenging problem in large part because these materials are notoriously difficult to model, but we hope to start analysing these effects and at least determine whether there may be a reasonable effect or not,” he tells Physics World. “At the moment, this is something we are actively working on.”

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