Physicists create exotic electron liquid

Such devices could be used in applications as diverse as communications in outer space, cancer detection, and scanning for concealed weapons.

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. Credit: QMO Lab, UC Riverside
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. Credit: QMO Lab, UC Riverside

By bombarding an ultrathin semiconductor sandwich with powerful laser pulses, physicists at the University of California, Riverside, have made the first “electron liquid” at room temperature. This discovery could pave the way towards the development of practical and efficient devices to generate and detect light at terahertz wavelengths—between infrared light and microwaves.

The examination could likewise empower investigation of the basic physics of matter at infinitesimally small scales and help introduce a time of quantum metamaterials, whose structures are built at atomic dimensions.

During the study, scientists developed an ultrathin sandwich of the semiconductor molybdenum ditelluride between layers of carbon graphene. The layered structure was simply somewhat thicker than the width of a single DNA atom. They at that point assaulted the material with superfast laser heartbeats, estimated in quadrillionths of a second.

Associate Professor of Physics Nathaniel Gabor said, “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.”

In experiments, scientists 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.

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. Credit: QMO Lab, UC Riverside
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. Credit: QMO Lab, UC Riverside

Gabor said, “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. Previously, researchers who had created such electron-hole liquids had only been able to do so at temperatures colder than even in deep space.”

“The electronic properties of such droplets would enable development of optoelectronic devices that operate with unprecedented efficiency in the terahertz region of the spectrum. Terahertz wavelengths are longer than infrared waves but shorter than microwaves, and there has existed a “terahertz gap” in the technology for utilizing such waves.

Terahertz waves could be used to detect skin cancers and dental cavities because of their limited penetration and ability to resolve density differences. Similarly, the waves could be used to detect defects in products such as drug tablets and to discover weapons concealed beneath clothing.

Gabor said, “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.”

“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.

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

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.

The UCR physicists published their findings online Feb. 4 in the journal Nature Photonics. They were led by Associate Professor of Physics Nathaniel Gabor, who directs the UCR Quantum Materials Optoelectronics Lab. Other co-authors were lab members Trevor Arp and Dennis Pleskot, and Associate Professor of Physics and Astronomy Vivek Aji.