Researchers no longer in the dark on the potential of cavity-altered superconductivity
Even if they had to conduct their experiments in the dark
Superconductors are revolutionizing our world, but to control superconductivity, you need external forces like magnetic fields or lasers. Or do you? This was the question driving a new paper in Nature, "Cavity-altered superconductivity," co-authored by a team of researchers led by Columbia postdoctoral researchers Itai Keren, Tatiana Webb, and Shuai Zhang (now an assistant professor at Fudan University).
The paper demonstrates the potential to leverage an enigmatic phenomenon called quantum fluctuations, which are essentially ripple effects on a subatomic level. When quantum particles are in an agitated state, they release these vibrations, which have the ability to affect electromagnetic fields. Could such fluctuations alone be used to alter the ground state properties of other materials? Materials like superconductors? Sounds impossible, and indeed, that's what co-author and Higgins Professor of Physics, Dmitri Basov, initially thought during a conversation years ago with theoretical physicist, Angel Rubio, from the Max Planck Institute, who believed there was more to explore.
As their discussions evolved, experimental possibilities began to materialize in the form of cavities, which are structures capable of confining electromagnetic waves. If one could remove or keep these charged particles out of cavities, creating so-called dark cavities, the only remaining motion within the structure should be quantum fluctuations. This would make dark cavities ideal for studying the effects of this elusive phenomenon.
Back in Professor Basov's lab, researchers were becoming aware of unusual optic properties in a nanomaterial called hexagonal boron nitride (hBN). Dr. Zhang, using a home-built scanning near-field optical microscope (s-SNOM), observed that when hBN is subjected to ultra-cold temperatures and layered, the agitated particles are capable of modifying nearby crystals. A new question arose: could layering hBN, an insulator, with some type of superconductive material create the dark cavities necessary to study quantum fluctuations? It was a tantalizing prospect. The problem, however, was that s-SNOMs rely on photons, the very particles that would need to be eliminated from the experiment. If the researchers wanted to test quantum fluctuations in hBN cavities, they would need to work in the dark.
As luck would have it, a Columbia Physics colleague, Abhay Pasupathy, had just the technology they needed. His research group specializes in 2D materials and scanning tunnelling microscopy, among other subfields of condensed matter physics, and their lab just so happened to have a cryogenic magnetic force microscope (MFM). Unlike optical microscopy, which depends on light, magnetic force microscopy, as its name implies, depends on magnetic fields. Importantly for this project, MFMs can detect the Meissner effect, which happens in ultra-cold temperatures during the transition to superconducting states, when superconductors spontaneously expel all magnetic fields flowing through them. hBN is nonmagnetic, so if its quantum fluctuations were indeed capable of disrupting superconducting states in other materials, the MFM should detect radical reductions in the Meissner effect.
The team devised a novel experiment in which they would use Professor Pasupathy's MFM to study the effect of hBN on a superconductive material called κ-(BEDT-TTF)2Cu[N(CN)2]Br, or κ-ET. Keren and Webb, with their expertise in operating the MFM, carried out the dexterous experiment by bringing κ-ET into a state of superfluidity and then placing a one-atom-thick flake of hBN on top of it, forming a dark cavity between the two materials. The subsequent MFM measurements were astounding. As both predicted and hoped, the Meissner effect dropped dramatically, and it appeared that the ground state of κ-ET had been altered.
So why did the researchers pair hBN with κ-ET? Because hBN and κ-ET have matching quantum fluctuations, and instinct told them that this resonance between insulator and superconductor could be a critical mechanism in their experiment. This hunch paid off, further supported by their control experiments. In the first, they replaced hBN with a 55-nm-thick crystal of ruthenium(III) chloride (RuCl3), an insulator with similar static permittivity as hBN but a much lower photonic frequency. In other words, the materials had no resonant mode coupling. The results? RuCl3 produced less than 7% of the effect that hBN had demonstrated. In another control, the researchers swapped κ-ET for a microcrystal of the superconducting material bismuth strontium calcium copper oxide (BSCCO), which also has a much lower frequency than hBN and κ-ET. The results were even starker: the readings for BSCCO in the presence of hBN was indistinguishable from the readings of BSCCO alone.
This experiment and its controls strongly suggest that quantum fluctuations are capable of impacting superconductivity and that resonant frequency can play an important role. Still, the researchers acknowledge these findings are not definitive, especially given that κ-ET is an unconventional superconductor. Despite this, their experiment demonstrates a crucial proof of concept that hopefully encourages physicists to continue testing the possibilities of cavity engineering and quantum fluctuations. And who knows? Maybe one day, they can even keep the lights on.