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Ultrafast Microscope Observes Quick-Moving Excitation That Could Be a Long Sought Phase of Matter

An international collaboration of experimental and theoretical physicists writing in the journal Science Advances has found potential evidence for an anomalous phase of matter first predicted to exist in the 1960s. Harnessing its properties could pave the way to new technologies able to share information without energy losses.

The experimentalists from the University of Cambridge who led the study observed the presence of unexpectedly fast waves of energy rippling through a quantum material when they exposed it to short and intense laser pulses. They were able to make these observations by using a microscopic speed camera that can track small and very fast movement on a scale that is challenging when using many other techniques. The new technique probes the material with two light pulses: The first one disturbs it in a small area and creates waves — or oscillations — propagating outward in concentric circles, in the same way as dropping a rock into a pond; the second light pulse takes a snapshot of these waves at various times. Put together, these images allowed them to look at how these waves behave, and to understand their ‘speed limit.’

“At room temperature, these waves move at a hundredth of the speed of light, much faster than we would expect in a normal material. But when we go to higher temperatures, it is as if the pond has frozen,” explains first author Hope Bretscher, who carried out this research at Cambridge’s Cavendish Laboratory. “We don’t see these waves moving away from the rock at all. We spent a long time searching for why such bizarre behavior could occur.”

The only explanation that seemed to fit all the experimental observations was that at room temperature the material hosts an ‘excitonic insulator’ phase of matter, which while theoretically predicted, had eluded detection for decades.

“In an excitonic insulator, the observed waves of energy are supported by charge-neutral particles that can move at electron-like velocities. Importantly, these particles could transport information without being hindered by the dissipation mechanisms that, in most common materials, affect charged particles like electrons,” says Akshay Rao from the Cavendish Laboratory, who led the research. “This property could provide a simpler route toward room-temperature, energy-saving computation than that of superconductivity.”

Theorists with the collaboration developed a model of energy propagation in the potential excitonic insulating phase. “The dissipationless energy transfer challenges our current understanding of transport in quantum materials and opens theorists’ imaginations to new ways for their future manipulation,” says Denis Golež, who led the theoretical component of the study as a research fellow at the Flatiron Institute’s Center for Computational Quantum Physics (CCQ) in New York City. He is now a researcher at the Jožef Stefan Institute and the University of Ljubljana in Slovenia.

“Theorists predicted the existence of this anomalous phase decades ago, but the experimental challenges to see evidence of this has meant that only now we are able to apply previously developed frameworks to provide a better picture of how it behaves in a real material,” says Yuta Murakami from the Tokyo Institute of Technology, who also collaborated on the theoretical component of the study.

Further work will be needed to confirm that the material is entering an excitonic insulating phase, says study co-author Andrew Millis, co-director of the CCQ. “Whether or not that is the ultimate interpretation, the existence of these observed particles and their creation and detection by ultrafast imaging is an important step forward.”

Once confirmed, and with additional research, the excitonic insulator phase could provide a way to carry information through a computer without energy loss. “This work puts us a step closer toward achieving some incredibly energy-efficient applications that can harness this property, including in computers,” Rao says.

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