New Laser Technique Captures Dynamic Processes of Quantum Materials

A new extreme-ultraviolet (UV) laser source, developed at the University of British Columbia (UBC), enables time-resolved photoemission spectroscopy, a technique for visualizing electron scattering processes at ultrafast timescales.

Photoemission spectroscopy can record, frame-by-frame, how an electron interacts with certain atomic vibrations in a solid, capturing a process that can cause electrical resistance in some materials and macroscopic quantum phenomena such as superconductivity in others. A scattering event between the vibration and the electron, called a phonon, can cause the electron to change both its direction and its energy. Such electron-phonon interactions are the basis of many exotic phases of matter.

Ultrafast pulses of extreme ultraviolet light are created in a gas jet of white plasma and are visible as blue dots on a phosphor screen as well as yellow beams from oxygen fluorescence. Courtesy of Research to Reality.

“The way electrons interact with each other and their microscopic environment determines the properties of all solids,” researcher MengXing Na said. “Once we identify the dominant microscopic interactions that define a material’s properties, we can find ways to ‘turn up’ or ‘down’ the interaction to elicit useful electronic properties.”

Using an ultrashort laser pulse, the researchers excited individual electrons away from their usual equilibrium environment. Using a second laser pulse, in effect as a camera shutter, they captured how the electrons scattered with the surrounding atoms on timescales faster than a trillionth of a second. “Owing to the very high sensitivity of our setup, we were able to measure directly — for the first time — how the excited electrons interacted with a specific atomic vibration, or phonon,” Na said.

The researchers performed the experiment on graphite, a crystalline form of carbon. Using time- and angle-resolved photoemission spectroscopy, they excited electrons in graphite and monitored their decay, which was accompanied by the release of phonons. The time constants of these decay processes provided direct information on electron-phonon couplings occurring in the experimental system. The scattering processes that contribute to electrical resistance could limit the application of carbon-based electronics in nanoelectronics, the researchers said.

Controlling interactions between electrons and atoms could be important for the application of quantum materials, including superconductors, which are used in MRI machines and high-speed magnetic levitation trains, and which could one day be used in energy transport. “By applying these pioneering techniques, we’re now poised to reveal the elusive mystery of high-temperature superconductivity and many other fascinating phenomena of quantum matter,” professor Andrea Damascelli said.

For their research, Na and the team leveraged a laser facility conceived by Damascelli and professor David Jones and developed by researcher Arthur Mills at UBC’s Stewart Blusson Quantum Matter Institute.

The research was published in Science (www.doi.org/10.1126/science.aaw1662).