Exotic electron liquid created at room temperature for the first time

UC Riverside physicists have created the first room-temperature electron liquid opening the way for a potential revolution in optoelectronic devices and basic physics studies.

In conventional electronic devices, electricity requires the movement of electrons (blue spheres) and their positive counterparts, called holes (red spheres), which behave much like the gas molecules in our atmosphere. Although they move rapidly and collide infrequently in the gas phase, electrons and holes can condense into liquid droplets akin to liquid water in devices composed of ultrathin materials ( QMO Lab, UC Riverside)

By bombarding an ultrathin semiconductor sandwich with laser pulses physicists at the University of California have created the first ‘electron liquid’ at room temperature. The breakthrough, published online on February 4th in the journal Nature Photonics, offers a possible avenue to the creation of devices which are capable of detecting light at wavelengths between infrared and microwave.

In addition to diverse applications for these devices in space communication, cancer detection and scanning for concealed weapons — the research could also enable exploration of the basic physics of matter at infinitesimally small scales ushering a new age of quantum metamaterials.

This 3D visualization shows the formation of an unusual state of matter, the electron-hole liquid in MoTe2 (QMO Lab, UC Riverside)

Associate Professor of Physics Nathaniel Gabor who led the research team, says: “Normally, with such semiconductors as silicon, laser excitation creates electrons and their positively charged holes that diffuse and drift around in the material, which is how you define a gas.”

However, in their experiments, the researchers detected evidence of condensation into the equivalent of a liquid. Such a liquid would have properties resembling common liquids such as water, except that it would consist, not of molecules, but of electrons and holes within the semiconductor.

Gabor continues: “We were turning up the amount of energy being dumped into the system, and we saw nothing, nothing, nothing — then suddenly we saw the formation of what we called an ‘anomalous photocurrent ring’ in the material.

“We realized it was a liquid because it grew like a droplet, rather than behaving like a gas.”

“What really surprised us, though, was that it happened at room temperature,” he said. “Previously, researchers who had created such electron-hole liquids had only been able to do so at temperatures colder than even in deep space.”

Terahertz transmitters and receivers could also be used for faster communication systems in outer space. And, the electron-hole liquid could be the basis for quantum computers, which offer the potential to be far smaller than silicon-based circuitry now in use, Gabor said.

More generally, Gabor said, the technology used in his laboratory could be the basis for engineering “quantum metamaterials,” with atom-scale dimensions that enable precise manipulation of electrons to cause them to behave in new ways.

In further studies of the electron-hole ‘nanopuddles’ the scientists will explore their liquid properties such as surface tension.

Gabor says: “Right now, we don’t have any idea how liquidy this liquid is, and it would be important to find out”

Gabor also plans to use the technology to explore basic physical phenomena. For example, cooling the electron-hole liquid to ultra-low temperatures could cause it to transform into a “quantum fluid” with exotic physical properties that could reveal new fundamental principles of matter.

By incorporating advanced imaging techniques with data-intensive strategies developed by UC Riverside students working with NASA’s Jet Propulsion Laboratory, a new type of microscope has been developed that allowed the first observation of an electronic liquid at room temperature ( QMO Lab, UC Riverside)

In their experiments, the researchers used two key technologies. To construct the ultrathin sandwiches of molybdenum ditelluride and carbon graphene, they used a technique called “elastic stamping.” In this method, a sticky polymer film is used to pick up and stack atom-thick layers of graphene and semiconductor.

To both pump energy into the semiconductor sandwich and image the effects, they used “multi-parameter dynamic photoresponse microscopy” developed by Gabor and Arp. In this technique, beams of ultrafast laser pulses are manipulated to scan a sample to optically map the current generated.

Original research: http://dx.doi.org/10.1038/S41566-019-0349-Y