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Posted: Dec 19, 2017
Ringing atomic bell probes electrons
(Nanowerk News) Striking a bell with a hammer causes the metal to vibrate. Striking a solid with a burst of light causes the atoms to vibrate. Using this new “ringing” approach, scientists tracked how the atoms oscillated and, in response, how the surrounding electrons quickly re-adjusted. The team clocked the atomic and electronic motions in ultrafast recordings. They found that the coupling of these motions was ten times stronger than predicted (Science, "Femtosecond electron-phonon lock-in by photoemission and x-ray free-electron laser").
An infrared laser beam (orange) triggers atomic vibrations in a thin layer of iron selenide that has unconventional superconductivity. Ultrafast X-ray laser pulses (white) measure the atomic distance as the atoms oscillate in time. The motion of the selenium atoms (red) and iron atoms (blue) changes the energy of the electron orbitals around the atomic nuclei. (Image: SLAC National Accelerator Laboratory)
The positions of atoms are the root of the unique electronic behavior and novel properties of materials. Quantum theory can predict new chemistries and the emergence of truly novel, and potentially exotic, behavior in solids. Knowing where the atoms are and how the electrons respond could let researchers harness materials’ exotic behaviors. This research provides a precise test for the refinement of the quantum theory of solids. The theory is the basis of predicting and designing new materials with traits such as loss-free electricity transmission as well as new magnetic materials.
In this work, the dependence of electronic structure on specific changes in the atomic structure was clearly determined. Researchers used a photon pulse from an ultra-fast infrared laser to excite vibrations in a solid, much like ringing a high-quality bell with a hammer. The material quickly settled into simple resonant vibrations of the atomic structure, again analogous to the clear long-lived tones depending on the size and shape of a bell. Then, the particular vibrational atomic motion that was excited by the laser was tracked over time with scattering from an ultra-fast x-ray laser beam.
The x-ray measurement monitored the small dynamic changes in atomic distances due to the vibrating planes of atoms. It revealed that in the solid, the distance between the iron and selenide atomic planes oscillated at roughly five million cycles in one-millionth of a second.
Subsequently, the laser excited atomic vibration was re-created and monitored with time-resolved ultra-fast electron spectroscopy. Whenever something causes atoms to move, the connecting electronic structure rearranges quickly, tracking changes in atomic positions.
This time, the researchers measured the electron response with an ultra-fast photoemission technique that tracked the shape of the electronic energy levels and the energy gaps between them. The electron energies changed at the same vibrational frequency as the atomic motions measured with x-rays.
The ability to measure both x-ray diffraction and photo-emission spectrum at this ultra-fast timescale makes possible the time correlation of the separate techniques. Both experiments, one sensitive to electron energies and the other to atomic distances, were timed with the same atomic clock of the vibrating atoms.
Matching the atomic lattice distance with electronic shifts gives an unbiased determination of the energy required to deform the lattice and quantifies the coupling strength between lattice distortions and electron response.
In this case, the team determined the coupling was stronger than what is predicted by simple models that treat each electron in an average way. To explain the new data, calculations are required that take the correlated environment of each electron into account.
This new experimental approach will directly measure electron ordering effects and their impact on how the atomic lattice affects electronic structure. The work will provide a stringent test of computational techniques necessary to predict and design electronic properties of new exotic materials.