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Watching an electron digging its own grave in two-dimensions

Recent Research from the group of Professor Xiaoyang Zhu.

Injecting an electron into a crystalline solid of many materials can induce local distortion of the lattice. The result is a polaron, i.e., a localized electron dressed by the much heavier nuclear distortion. Understanding how a polaron forms is critical to the design of materials for charge transport. In the June 15 issue of Physical Review Letters [98, 246801 (2007)], MRSEC postdoc Matthias Muntwiler and Chemistry Professor Xiaoyang Zhu reported a breakthrough discovery: an electron polaron unique to a two-dimensional film, but not a three-dimensional crystal. The researchers applied femtosecond time-resolved photoemission (TR-2PPE) spectroscopy to directly observe the formation of polarons from the localization of conduction band electrons in NaCl thin films of unit cell thickness. Contrary to theoretical prediction for bulk NaCl crystal where an electron polaron does not exist, small polarons readily form in crystalline NaCl thin films on ~100 fs time scales. The increased deformability and the reduced electronic bandwidth of a crystalline lattice in the thin film format are both responsible for the formation of small polarons that are absent in bulk solids. This finding is another manifestation of new physical properties that emerge as physical sizes (in one of the three dimensions in the present case) decrease to the nanometer scale. It has broad implications for research into thin films of electronic materials.

Fig. 1: Panel (a): Two-dimensional pseudo-color representation of 2PPE spectra as a function of pump-probe delay for 3ML NaCl/Cu(111). The energy scale corresponds to intermediate state energy referenced to the Fermi level. Peak positions of the two polarons (B & C) are shown as white circles. Panels (b) & (c): Parallel dispersions (kinetic energy vs. parallel momentum vector) of the electrons en route to localization to form polarons. The effective mass of the electron increases from 0.6-0.8 me at 0 fs to 3.2-3.5 me at 125 fs.

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