In a recent experiment performed at SLAC and reported in the February 2 issue
of Science, David Fritz and his SPPS colleagues have obtained our first
direct view of the motion of atoms inside a crystal. This feat requires
simultaneous Angstrom spatial and femtosecond temporal resolution.
Synchrotrons have been providing subatomic resolution for decades, and
ultrafast lasers have been capable of sub-picosecond timing for more than twenty
years; but SPPS was the first instrument to combine both.
The Science report showed how the atoms in a Bi crystal move following
excitation with an ultrafast laser. The Bismuth crystal structure is almost
perfectly cubic, but has a slightly elongation along the diagonal of the cube,
called a Peierls distortion. Calculating how this distortion arises from the
electronic binding forces is something of a benchmark challenge for Density
Functional Theories. In the work reported in Science, an extremely
short laser pulse excites conduction electrons in the crystal, which suddenly
releases the distortion partially, driving the crystal lattice to oscillate
around a new less distorted position. This kind of rapid excitation leading to
oscillations is much like the motion of a guitar string when plucked. The
change in the crystal distortion with the density of conduction electrons is an
important test of our understanding of nature of solid matter.
The crystal distortion oscillates for a brief time after the laser excites it -
only a few picoseconds, but long enough to be easily resolved by the 160
femtosecond x-rays from SPPS. Fritz and his colleagues used this motion to
measure not only the distortion, but the size and character of the binding
force itself, over a range of distortions. They conclude that the presence of
conduction electrons weakens the crystal bonds in a way that had been predicted
but never before measured directly.
To learn more about this research see the full scientific highlight at:
http://www-ssrl.slac.stanford.edu/research/highlights_archive/SPPS07.html
D. M. Fritz, D. A. Reis, B. Adams, R. A. Akre, J. Arthur, C. Blome, P. H.
Bucksbaum, A. L. Cavalieri, S. Engemann, S. Fahy, R. W. Falcone, P. H. Fuoss,
K. J. Gaffney, M. J. George, J. Hajdu, M. P. Hertlein, P. B. Hillyard, M.
Horn-von Hoegen, M. Kammler, J. Kaspar, R. Kienberger, P. Krejcik, S. H. Lee,
A. M. Lindenberg, B. McFarland, D. Meyer, T. Montagne, É. D. Murray, A. J.
Nelson, M. Nicoul, R. Pahl, J. Rudati, H. Schlarb, D. P. Siddons, K.
Sokolowski-Tinten, Th. Tschentscher, D. von der Linde and J. B. Hastings.
'Ultrafast Bond Softening in Bismuth: Mapping a Solid's Interatomic Potential
with X-rays', Science 315, 633 (2007).
