Wednesday, December 12, 2012

1212.2348 (S. L. Johnson et al.)

Direct observation of structural symmetry-breaking by coherent phonon

S. L. Johnson, P. Beaud, E. Mo öhr-Vorobeva, A. Caviezel, G. Ingold, C. J. Milne
We directly measure by femtosecond time-resolved x-ray diffraction the short-lived symmetry breaking from an E$_g$ symmetry coherent phonon excited in bismuth by strong optical excitation. The magnitude of the E$_g$ mode observed is 0.1 pm, compared against the 2.7 pm initial displacement of the fully-symmetric A$_{1g}$ mode. The much smaller motion of the E$_g$ mode is a consequence of the short lifetime of the electronic states that drive the atomic motion. This result confirms the interpretation of previous experiments on the structural dynamics of bismuth, and in addition offers a unique measurement of the sign and magnitude of the coherent symmetry-breaking not available from previous transient optical reflectivity measurements.
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