Lightwave-Driven Gapless Superconductivity

Jigang Wang can break his research goals into just a few words: “To discover and control quantum states of matter.” But, it takes paragraphs, analogies, illus­trations, internet searches and a willingness to decipher talk about non-equi­librium quantum phase discovery via non-thermal ultrafast quench near quantum critical points to get a handle on those eight words. Even though it’s a head-scratcher, Wang’s work could be a big deal to all of us.

Jigang Wang and his research group use quantum terahertz spectroscopy to access, study and control quantum states of matter. (Source: C. Gannon, Iowa State U.)

Harnessing quantum physics could lead to better computing, sensing, communi­cating and data storing tech­nologies. But first researchers such as Wang, a professor of physics and astronomy at Iowa State University, need to provide more answers about the quantum world. In Wang’s case, many of those answers are coming from quantum terahertz spectroscopy that can visua­lize and steer electrons. Wang and his team have announced three disco­veries based on those studies:

The first describes how ultrafast pulses of photons – laser flashes at trillions of cycles per second – can switch on a state of matter hidden by super­conductivity, the flow of electricity without resistance, usually at super-cold tempera­tures. The discovery demons­trates a new tuning knob – a quantum quench – for non-equilibrium materials discovery such as switching on exotic, hidden states without temperature change. The second describes how the terahertz instru­mentation can trace electron pairings in materials, revealing a new, light-induced, long-lived state of matter. And the third describes how the ultrafast flashes of light can be used like a knob to control and acce­lerate supercurrents. The flashes break equilibrium symmetry, thus triggering forbidden quantum oscil­lations that can’t be achieved by any known means.

Wang has several colla­borators who have contributed to the discoveries: the Ilias E. Perakis group at the University of Alabama at Birmingham contri­buted theoretical simu­lations; the Chang-Beom Eom group at the University of Wisconsin-Madison and the Paul Canfield group at Iowa State contributed high-quality super­conducting materials and their charac­terizations.

“Wang’s work is revealing new physics and how we can use light to invoke new properties that are otherwise unavailable,” said Marc Ulrich, physics division chief at the Army Research Office, an element of the U.S. Army Combat Capa­bilities Development Command’s Army Research Labo­ratory. “Light-induced phases may enable tech­nologies such as optical computing, novel sensors or unforeseen ways to control light or electrons.”

The research in Wang’s lab is mostly unexplored territory in condensed matter physics and materials science, Wang said. And so there’s more work ahead to knock down knowledge barriers to help push development of quantum tech­nologies and their high-speed commu­nication capa­bilities. “We’d like to use these tools – these fast flashes and high frequencies – to probe smaller scales, 1 to 10 nanometers,” Wang said. “We’d also like to develop controls using terahertz light for the quantum computation community.”

“I’ve always been fasci­nated by the discovery of new states of matter by developing new tools, especially those states that are difficult or even can’t be accessed by conven­tional means,” Wang said. That means minimum changing of tempera­tures, pressures, chemical compo­sitions or magnetic fields to get to these new states of matter that are typically unstable in equi­librium and often hidden by conven­tional measurement methods but have been stabilized in his experiments, Wang said.

Nor does he focus on acci­dental disco­veries that sometimes happen by just trying something in the lab. Wang wants to develop and apply precise and powerful laboratory tools in a controlled, rational way to find these new states of matter hidden within super­conducting and other complex materials. By doing that, he said he’s learning these intense terahertz flashes produced by his labora­tory instruments really can be a control knob for finding, stabi­lizing, probing and poten­tially controlling these exotic states and their unique pro­perties. “We have established a new approach,” he said, “to access and poten­tially control exotic states of matter.” (Source: Iowa State U.)

Reference: X. Yang et al.: Lightwave-driven gapless superconductivity and forbidden quantum beats by terahertz symmetry breaking, Nat. Phot., online 1 July 2019; DOI: 10.1038/s41566-019-0470-y

Link: Ultrafast Quantum Materials Laboratory, Iowa State University, Ames, USA

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