"Fourier-transform inelastic X-ray scattering from time- and momentum-dependent phonon–phonon correlations"

M. Trigo: M. Fuchs, J. Chen, M. P. Jiang, M. Cammarata, S. Fahy, D. M. Fritz, K. Gaffney, S. Ghimire, A. Higginbotham, S. L. Johnson, M. E. Kozina, J. Larsson, H. Lemke, A. M. Lindenberg, G. Ndabashimiye, F. Quirin, K. Sokolowski-Tinten, C. Uher, G. Wang , J. S. Wark, D. Zhu and D. A. Reis; Nature Physics, 10/27/13.

Additional Authors: M. Fuchs, J. Chen, M. P. Jiang, M. Cammarata, S. Fahy, D. M. Fritz, K. Gaffney, S. Ghimire, A. Higginbotham, S. L. Johnson, M. E. Kozina, J. Larsson, H. Lemke, A. M. Lindenberg, G. Ndabashimiye, F. Quirin, K. Sokolowski-Tinten, C. Uher, G. Wang , J. S. Wark, D. Zhu and D. A. Reis

Abstract:

The macroscopic characteristics of a material are determined
by its elementary excitations, which dictate the response of
the system to external stimuli. The spectrum of excitations
is related to fluctuations in the density–density correlations
and is typically measured through frequency-domain neutron
or X-ray scattering. Time-domain measurements of these
correlations could yield a more direct way to investigate
the excitations of solids and their couplings both near to
and far from equilibrium. Here we show that we can access
large portions of the phonon dispersion of germanium by
measuring the diffuse scattering from femtosecond X-ray free-
electron laser pulses. A femtosecond optical laser pulse slightly
quenches the vibrational frequencies, producing pairs of high-
wavevector phonons with opposite momenta. These phonons
manifest themselves as time-dependent coherences in the
displacement correlations probed by the X-ray scattering. As
the coherences are preferentially created in regions of strong
electron–phonon coupling, the time-resolved approach is a
natural spectroscopic tool for probing low-energy collective
excitations in solids, and their microscopic interactions.