Matching vibrations is all it takes to modify materials

The world is never really at rest. Even in a vacuum near ultracold temperatures where all classical motion should come to a halt, you will find quantum fluctuations. In thin, two-dimensional materials, these include random vibrations that can alter electromagnetic fields – a feature that theorists have long posited could be useful for modifying materials. Angel Rubio, Director of the Theory Department at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg, has been one of the principal architects of this idea. Together with colleagues Rubio developed the theoretical framework predicting that quantum fluctuations inside cavities could reshape the properties of solids – without any external force. Now, that prediction has been confirmed experimentally for the first time. In a new paper published in Nature, an international team of 33 researchers from 17 institutions – including a large MPSD contingent – demonstrates that quantum fluctuations from the vacuum alone inside atom-thin layers of a 2D material can alter the properties of a nearby crystal.

“It’s a holy grail we’ve been searching for decades,” said Dmitri Basov, Higgins Professor of Physics at Columbia University, who led the experimental effort. “We believe we’ve found it.”

The team, led by Columbia postdoctoral fellows Itai Keren, Tatiana Webb, and Shuai Zhang, placed a nanometre-sized flake of hexagonal boron nitride (hBN) on top of the superconducting material κ-(BEDT-TTF)₂Cu[N(CN)₂]Br, known as κ-ET. With no added lasers or other external driving forces, superconductivity came to a halt.

That is not exactly the result those seeking to enhance lossless electrical flow are looking for, but it is an important proof of concept. “Any new knob that people can find for tuning superconductivity is significant,” said Keren.

The quantum fluctuations found between the layers of hBN vibrate at a characteristic resonance that just so happens to match that of κ-ET. “That was our intuition: if the vibrations match, they should interact with each other,” said Keren. As the two interact, the electromagnetic environment in the κ-ET crystal changes in a way that impedes the movement of its electrons, preventing them from reaching a collective, superconducting state. When they tested hBN against a superconductor with a different set of resonances, nothing happened.

The idea first germinated years ago in Central Park. During visits to New York, Rubio explained the potential of quantum fluctuations to a, at the time, sceptical Basov. “I thought his proposal was impossible, but it was so appealing, it was impossible not to try,” Basov recalled. The conversations were part of a broader and ongoing exchange between the MPSD and Columbia through the Max Planck–New York Center for Non-Equilibrium Quantum Phenomena, a joint venture that has become a catalyst for this kind of cross-continental collaboration. Rubio’s theoretical work – including a pioneering 2018 study in Science Advanceswith Sentef and Ruggenthaler showing that cavity vacuum fields could modify electron–phonon coupling in superconductors – provided the conceptual foundation for the experiment.

The question was how, but waiting in Basov’s nano-optics lab was hBN – a solution, waiting for this problem. hBN has become a workhorse in many experimental applications, but typically as an inert, insulating spacer. But beginning in 2014, Basov’s lab began observing interesting optical properties in hBN that, as his conversations with Rubio continued over the years, made it an enticing candidate for a cavity.

A cavity is a structure that confines light and other electromagnetic waves. If no waves are present, it is, in a sense, a vacuum – but that does not mean it is a totally empty void. Cavities still host quantum fluctuations. Conventionally, mirrors have been used to create cavities, but quantum fluctuations strengthen as cavities shrink. Nanoscale-thick sheets of hBN are about as small as it gets.

Using specialised scanning near-field optical microscopes (SNOMs), Zhang, now an assistant professor at Fudan University, and other members of Basov’s lab confirmed over the years that vibrating quasiparticles arising inside layers of hBN can interact with and modify vibrations in other crystals, including superconducting κ-ET. But SNOMs are optical tools that rely on photons – light particles that can also modify materials. To prove what quantum fluctuations by themselves were capable of, Basov needed a way to work in the dark – literally.

Co-author and fellow Columbia physicist Abhay Pasupathy had just the dark probe: a cryogenic magnetic force microscope (MFM). MFMs detect the Meissner effect, which is the repulsive force between a superconductor and a magnet, and Pasupathy’s lab can sense superconductors through covering layers at extremely cold temperatures.

Keren and Webb masterfully executed the MFM experiments, the results of which Rubio thought too good to be true. “Vacuum fluctuations are extremely small, but the effect observed is huge,” he said. Superconductivity was suppressed in the κ-ET to almost half a micrometre – ten times the width of the hBN flake used

Modifying a material’s properties in the past usually involved a shake of some sort, explained Rubio: a mechanical push, some added heat, or a laser pulse, to a short-lived effect. But without the external force, the modifications could be more persistent. He and his fellow theorists on the paper are still working to reconcile a single explanation for the outstanding results. “Even if the theory doesn’t fully explain the results yet, we now have experimental proof of vacuum-mediated interactions in a material system. Long-term, this should be a major milestone,” said Rubio.

hBN’s hyperbolic nature is an important feature. Hyperbolic materials are uniquely structured in a way that enhances any internal vibrations – imagine a “wave” growing from a single person to an entire stadium. “It’s a remarkable effect that you can have with hyperbolic materials,” said Webb, now an assistant professor at Barnard College. “We now have a proof of concept that this is a viable way to modify the electronic properties of materials, and it’s something we could integrate into material designs.”

The vibrations in hBN can be tweaked, for example by changing its thickness. “If we can control these, we can tune our superconductor at will. But we aren’t just talking about superconductors,” said Keren. Different kinds of magnets and ferroelectric materials have specific vibrations associated with their properties; finding a matching cavity could be all it takes to modify them. “We expect to see others hunting for new combinations,” said Keren.

From the vacuum of a quantum cavity, a whole new way to engineer materials is emerging. For the MPSD, the result represents a landmark moment: experimental confirmation of a theoretical vision that has been at the heart of the institute’s Theory Department for over a decade. What began as conversations in Central Park between a theorist and an experimentalist has now produced one of the first demonstrations that the quantum vacuum alone can reshape the properties of matter.

This article is based on this press release by Ellen Neff at Columbia University:  https://quantum.columbia.edu/news/matching-vibrations-all-it-takes-modify-materials