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December 20, 2016 鈥� Scientists studying high temperature superconductors 鈥� materials that carry electric current with no energy loss when cooled below a certain temperature 鈥� 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 海角论坛 physics professor聽 has demonstrated a new laser-driven 鈥渟trobe-light鈥� technique for studying complex electron interactions under dynamic conditions.

As described in , they use one very fast, intense 鈥減ump鈥� laser pulse to give electrons a blast of energy, followed by a series of secondary 鈥減robe鈥� 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 鈥� known as time-resolved, angle-resolved photoelectron spectroscopy (tr-ARPES) 鈥� combined with complex theoretical simulations and analysis to allow the team to tease out the sequence and energy 鈥渟ignatures鈥� of different types of electron interactions. They were able to pick out distinct signals of interactions among excited electrons (which happen quickly but don鈥檛 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 鈥� which they say represents a distinct form of extremely efficient energy loss 鈥� at a particular energy level and timescale between the other two.

鈥淲e 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,鈥� lead author 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 鈥� like a tuning fork emitting a single note.

鈥淲hen 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 鈥榬esonant鈥� frequency,鈥� Rameau said.

The frequency of the special lattice interaction 鈥渘ote鈥� is particularly interesting, the scientists say, because it is the same energy as a 鈥渒ink鈥� 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鈥檚 ability to become a superconductor. They couldn鈥檛 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.

鈥淲e know now that this interaction doesn鈥檛 just switch on when the material becomes a superconductor 鈥� it鈥檚 actually always there,鈥� 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鈥檚 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.

鈥淭here is clearly something special about this one note,鈥� Rameau said.

Professor Freericks, the 聽and Professor of Physics,聽contributed to the theoretical and computational analysis of the experiment.

The Georgetown work was supported by the and by the McDevitts. Computational resources were provided by the , a DOE Office of Science User Facility headquartered at Lawrence Berkeley National Laboratory.