When a negatively charged electron zips through a lattice of atoms, the nearby positively charged atoms shift closer to the electron. Scientists demonstrated how the movement of electrons deforms the lattice and causes the electron to look as if it is being "dressed" by the deformation. These changes drastically alter the flow of electric current. This work offers strong evidence for the formation of polarons that are important for understanding energy transport in complex materials.
The ultrafast electron diffraction technique used in this study can precisely follow changes of atomic and electron motion in materials. It can track motions that occur in less than a picosecond (a trillionth of a second). This work reveals the vital role of electron-lattice interactions in one material – manganites. This material could be used in data-storage devices with increased data density and reduced power requirements. In addition, scientists can use this method to study such interactions in a wide range of complex materials. Potentially, this method could be used to gain insight into the loss-free conduction of electricity.
When an electron, which is negatively charged, travels through the atomic lattice, just like the attraction between opposite ends of a magnet, the nearby positively charged atoms tend to move closer to the electron. As the atomic lattice is deformed by the attraction to the electron, the electron looks like it is being "dressed" by the lattice deformation. The resultant combined motion of the electron and the lattice deformation, known as a polaron, is a foundation in our understanding of various transport phenomena in solids. This includes the transformation from an insulator to a superconductor (resistance-free flow of charge, in which electrons pair by sharing lattice deformations). However, little was known experimentally about the dynamics of individual phonons (small energy packets or "quantum of energy" associated with, for example, a vibration of a crystal lattice) during polaron formation and motion. Scientists at Brookhaven National Laboratory, with colleagues at Rutgers University and Princeton University, have demonstrated direct experimental evidence of strong coupling between electron motion and deformations in atomic arrangements that affect the flow of current (i.e., the polaron formation) in manganite LaSr2Mn2O7. They achieved these results using a novel ultrafast ultrahigh-energy electron diffraction system developed by the lab to quantify the material's electronic and atomic dynamics upon femtosecond laser illumination. They find that a specific lattice distortion dominates the lattice responses with its atomic constituents showing distinct one-step and two-step relaxation behaviors (scientifically referred to as a Jahn-Teller lattice distortion). This complex lattice relaxation manifests a strong interaction of phonons with the photoexcited electrons, proving that polaron transport is a key mechanism in the manganites' colossal magnetoresistance properties, which could be used in data-storage devices with increased data density and reduced power requirements. The technique can be widely used to understand the dynamics of electrons and atoms to reveal the origins of properties of complex materials.
Brookhaven National Laboratory
U.S. Department of Energy, Office of Science, Basic Energy Sciences
J. Li, W. Yin, L. Wu, P. Zhu, T. Konstantinova, J. Tao, J. Yang, S.W. Cheong, F. Carbone, J.A. Misewich, J.P. Hill, X. Wang, R.J. Cava, and Y. Zhu, "Dichotomy in ultrafast atomic dynamics as direct evidence of polaron formation in manganites." NPJ Quantum Materials 1, 16026 (2016). [DOI: 10.1038/npjquantmats.2016.26]
Brookhaven National Laboratory news release: Ultrafast Imaging Reveals Existence of "Polarons"