Dissertation Thomas May (10.7.96)
Anharmonische Effekte in der Gitterdynamik von Metallen
Betreuer: Prof. Dr . Dieter Strauch
Anharmonic phonon-phonon coupling strengths
have been derived from a simple phenomenological
model potential in order to explain the characteristic features
of BCC metals in their high-temperature phase.
The strong quasielastic scattering near
q=2/3 and 1/2  in BCC-Zr and
BCC-Ti is caused by anharmonic broadening,
with the lifetime of these low-frequency
phonons of the order of a vibrational period.
These modes which are connected to the
structural phase transition to the hexagonal or the HCP phase
show the strongest anharmonic effects.
In addition, the calculated anharmonic frequency
shift is able to explain the difference between recent harmonic
ab-initio calculations and experiment.
The interference between one- and
two-phonon neutron scattering processes leads to an unusually
strong intensity variation over adjacent Brillouin zones.
The successful description of both harmonic and
anharmonic lattice dynamics
by a simple central potential indicates that angular
forces may be weak.