Hamburg 2001 – wissenschaftliches Programm
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DF: Dielektrische Festkörper
DF 6: Poster
DF 6.20: Poster
Mittwoch, 28. März 2001, 14:30–18:00, Foyer S3
The Formation of Satellite Reflections by Ultrasonically Modulated Crystals — •Maik Kaiser1, Klaus-Dieter Liss1, Rainer Hock2, Thierry d’Almeida1, and Andreas Magerl2 — 1European Synchrotron Radiation Facility, B.P.220, F-38043 Grenoble Cedex — 2Lehrstuhl fuer Kristallographie und Strukturphysik, D-91054 Erlangen
The response of a diffraction pattern to an acoustic wave field depends on several parameters like ultrasound amplitude, frequency and wave propagation direction. In this study longitudinally excited pressure waves were induced in the (111)-direction of a 10mm thick perfect silicon crystal acting as an ultrasonic resonator. To have access to the bulk properties of the crystal we used high energy X-rays on a high resolution triple axes diffractometer at the beamline ID15 at the ESRF.
One can distinguish between low frequency ultrasonic waves where the modulation period is larger than the Pendellösung length and high frequencies, where the modulation is shorter. The first case looks to the scattering photons like a gradient crystal, the lattice parameter varying slowly resulting in an overall peak broadening [1]. Time resolved experiments gave full insight into the acoustic field [2].
The second case is distinguished by the evolution of well defined satellite peaks representing the acoustic wavelength, according to the dynamical treatment of X-ray diffraction. The acoustic modulation leads to additional apices around the apex of the Bragg reflection in reciprocal space which were resolved by the high resolution diffractometer.
Varying the photon energy in this case leads to the X-ray acoustic resonance. That means that the acoustic wave vector matches the splitting of the dispersion surfaces in reciprocal space and therefore the intensities of the satellites become strongly enhanced.
References:
[1]K.-D. Liss, A. Magerl, A. Remhof, R. Hock, "Ultrasound induced gradient crystals observed by high energy X-raysNo-dq Acta Crystallographica, (1997). A(53): p. 181-186.
[2]K.-D. Liss, A. Magerl, R. Hock, A. Remhof, B. Waibel, No-dqTowards a new (Q, t) regime by time-resolved X-ray diffraction: ultra-sound excited crystals as an exampleNo-dq Europhysics Letters, (1997). 40(4): p. 369-374.