Freericks Helps Tease Out Complex Electron Interactions In Superconducting Material

December 20, 2016 — Scientists studying high temperature superconductors — materials that carry electric current with no energy loss when cooled below a certain temperature — have been searching for ways to study in detail how these materials work. One challenge is disentangling the many different types of interactions: For example, separating the effects of electrons interacting with themselves from those caused by the interactions of electrons with the atoms of the material.

Now, a team of scientists that includes Georgetown College physics professor Jim Freericks has demonstrated a new laser-driven “strobe-light” technique for studying complex electron interactions under dynamic conditions.

As described in a paper published in the journal Nature Communications, they use one very fast, intense “pump” laser pulse to give electrons a blast of energy, followed by a series of secondary “probe” laser pulses to measure how the electrons lose their extra energy by making the atoms in the material vibrate. These pictures of the electrons are taken at speeds of over one quadrillion frames per second.

The technique — known as time-resolved, angle-resolved photoelectron spectroscopy (tr-ARPES) — combined with complex theoretical simulations and analysis to allow the team to tease out the sequence and energy “signatures” of different types of electron interactions. They were able to pick out distinct signals of interactions among excited electrons (which happen quickly but don’t dissipate much energy), as well as later-stage interactions between electrons and the atoms that make up the crystal lattice (which generate friction and lead to gradual energy loss in the form of heat).

But they also discovered another, unexpected signal — which they say represents a distinct form of extremely efficient energy loss — at a particular energy level and timescale between the other two.

“We see a very strong and peculiar interaction between the excited electrons and the lattice where the electrons are losing most of their energy very rapidly in a coherent, non-random way,” lead author Jonathan Rameau of the Brookhaven National Laboratory said.

At this special energy level, the electrons appear to be interacting with lattice atoms all vibrating at a particular frequency — like a tuning fork emitting a single note.

“When all of the electrons that have the energy required for this unique interaction have given up most of their energy, they start to cool down more slowly by hitting atoms more randomly without striking the ‘resonant’ frequency,” Rameau said.

The frequency of the special lattice interaction “note” is particularly interesting, the scientists say, because it is the same energy as a “kink” feature in the energy signature of the same material studied previously in its superconducting state using a static form of ARPES. At the time, the scientists conducting that research (including some members of the current team) suspected the kink might have something to do with the material’s ability to become a superconductor. They couldn’t detect the same signal above the superconducting temperature.

But the new time-resolved experiments, which were done on the material well above its superconducting temperature, were able to tease out the subtle signal. These new findings indicate that this special condition exists even when the material is too hot to be a superconductor.

“We know now that this interaction doesn’t just switch on when the material becomes a superconductor — it’s actually always there,” Rameau said. 

But the scientists still believe there is something special about the energy level of the unique tuning-fork-like interaction. Other intriguing phenomena have been observed at this same energy level, which Rameau says has been studied in excruciating detail.

It’s possible, he says, that the one-note lattice interaction is essential to superconductivity, but requires some still-to-be-determined additional factor to turn the superconductivity on.

“There is clearly something special about this one note,” Rameau said.

Professor Freericks, the Robert L. and Catherine H. McDevitt Chair and Professor of Physics, contributed to the theoretical and computational analysis of the experiment.

The Georgetown work was supported by the U.S. Department of Energy's Office of Science, Basic Energy Sciences and by the McDevitts. Computational resources were provided by the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility headquartered at Lawrence Berkeley National Laboratory.