Riding an Electronic Wave

Ultrafast laser techniques helped MIT physics graduate student Fahad Mahmood and colleagues establish that electrons form charge-density waves in the thin-film superconductive material LSCO cuprate.

"The question is how does this fluctuating charge-density wave compete or not interfere with superconductivity, and what we found is that it actually competes with superconductivity," Mahmood explains. "Electrons for a very short amount of time are in this charge-density wave state, and in another time scale, if you take another snapshot, they'll be in the superconductivity state."

Charge-density waves occur when electron density in a conductor is distributed in a sinusoidal pattern, like ripples on water, instead of the common uniform density.

"It's a fluctuating order that lasts for a very short amount of time and equilibrium probes won't be able to detect it," he says. Using ultrafast spectroscopy, Mahmood and co-authors of a 2013 Nature Materials paper were able to show that for extremely short periods of time — up to about 2 picoseconds — electrons clustered in a density wave that could be measured by its amplitude and phase.

"This was kind of the very first observation in that particular material that [that] kind of charge-density wave exists, and we were able to make some connections to how it's related to superconductivity as well," says Nuh Gedik, the Lawrence C. (1944) and Sarah W. Biedenharn Career Development Associate Professor of Physics. Gedik spoke about his group's research on topological insulators at the Materials Day Symposium, on Oct.14 in MIT Kresge Auditorium.

Mahmood grew up in Pakistan and won a scholarship to Stanford University, where he studied physics and aeronautical engineering. Now one of Gedik's students, he expects to finish his PhD program next June.

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