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Evidence for Formation of Photoinduced Excitonic State

Evidence for Formation of Photoinduced Excitonic State

Researchers from Ames Laboratory observed nonequilibrium pair breaking in Ba(Fe1−xCox)2As2 superconductors

A team of researchers from the U.S. Department of Energy's Ames Laboratory, in collaboration with University of Alabama Birmingham, identified a remarkably long-lived new state of matter in an iron pnictide (Ba(Fe1−xCox)2As2) superconductor. The new state of matter reveals a laser-induced formation of collective behaviors that compete with superconductivity. Jigang Wang, Ames Laboratory physicist and Iowa State University professor stated that superconductivity is a strange state of matter that contains paired electrons that move faster. The team was focused on determining how different states in a material compete for the pared electrons. The team was also engaged in finding how to balance competition and cooperation in order to increase temperature at which a superconducting state emerge.

In an experiment, the team used laser pulses of less than a trillionth of a second to take a series of snapshots. The technique is known as ultrafast terahertz (THz) pump-probe spectroscopy, which was used to reveal an unusual out-of-equilibrium cooper pair nonlinear dynamics. This technique can be considered as a laser strobe photography in which several quick images reveal the subtle movement of electron pairings inside the materials with the help of long wavelength far-infrared light. The technique also identified a nonequilibrium state driven by femtosecond (fs) photoexcitation of superconductivity (SC) in iron pnictides. The team achieved SC via hot-phonon scattering within a few hundreds of picoseconds. This was followed by observation of SC quench regime prior to any recovery.

Moreover, the team identified a nonlinear pump fluence dependence for such remarkably long prebottleneck dynamics, which are sensitive to both doping and temperature. The team used quantum kinetic modeling to state that the buildup of excitonic interpocket correlation between electron-hole quasiparticles (QP) quenches SC after femtosecond photoexcitation. This in turn leads to long-lived, many-QP excitonic state. Wang stated that the ability to observe real time dynamics and fluctuations aid in better understanding of QP excitonic state. This in turn can offer development of better superconducting electronics and energy-efficient devices. The research was published in the journal Physical Review Letters on December 26, 2018.



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