1. Electronically Driven Structure Changes of Si Captured by Femtosecond Electron Diffraction Outreach/Collaboration with other research groups, showing impact of IRG1 effort Juan J. De Pablo, University of Wisconsin-Madison, DMR 0520527 This work was supported by the NSF through the University of Wisconsin Materials Research Science and Engineering Center, grant number DMR-0520527. Members of IRG1 worked with a research group at the University of Toronto Canada to provide Si nanomembranes that enabled a unique experiment to be accomplished. The research group of Dwayne Miller, Institute for Optical Sciences and Departments of Physics and Chemistry, University of Toronto has singular capabilities for femtosecond electron diffraction to measure really fast electronic processes in materials. IRG1 has singular capabilities for making the appropriate samples. The outcome so far is a Phys. Rev. Letter in 2008 and a Phys. Rev. paper just published. The diagram below, taken from the Phys. Rev. Letter paper, shows the experimental configuration. The “grid sample” is where the IRG1-produced membranes are placed; this part is supplied by us. The follow-on just published Phys. Rev. paper is summarized by its abstract: Transmission electron diffraction is naturally sensitive to the detection of shear-type deformations in single-crystalline structures due to the effective tilting of the lattice planes characteristic of shear, but in general is insensitive to longitudinal phonon propagation. Here, we report on the generation and detection of both transverse and longitudinal coherent acoustic phonons in 33 nm free-standing (001)-oriented single crystalline Si films using femtosecond electron diffraction (FED) to monitor these laser-induced atomic displacements. The mechanism for excitation of the shear mode that leads to coupling to the longitudinal phonon is attributed to the inhomogeneous lateral profile of the optical-pump pulse and the periodic boundary condition imposed by the supporting grid structure. In this application, the constructive interference in the diffraction process makes FED particularly sensitive to the detection of coherent phonon modes and offers an atomic perspective of the dynamics involving collective motions. Phys Rev. B79, 094301 (2009) The outcome of the experiments is described as follows (given by the abstract of the Phys. Rev Letter): The excitation of a high density of carriers in semiconductors can induce an order-to-disorder phase transition due to changes in the potential- energy landscape of the lattice. We report the first direct resolution of the structural details of this phenomenon in freestanding films of polycrystalline and (001)-oriented crystalline Si, using 200-fs electron pulses. At excitation levels greater than ~6% of the valence electron density, the crystalline structure of the lattice is lost in <500 fs, a time scale indicative of an electronically driven phase transition. We find that the relaxation process along the modified potential is not inertial but rather involves multiple scattering towards the disordered state. Phys. Rev. Letters 100, 155504 (2008)