Neutron Scattering

1
r x z -- w --Z x E r A -+ L
6
5
_E a
3
F
3
2
00 0.2 0.4 0.6 0.8 0 0.2 0.4 0.4 0.2 0 0.8 0.6 0.4 0.2 0 0.1 0.2 0.3 0.4 0 .5
Reduced wave vector coordinate (ç)
Figure 6 Phonon dispersion of Ne along the three train symmetry directions (from Ref. [2]) .
moments may contribute to the interaction, which have a component perpendicular to
Q .
This yields the possibility to determine not only the size but also the direction of
magnetic moments .
13 .2 Analysis of lattice vibrations
13 .2 .1 Lattice dynamics in simple structures
For a simple crystal lattice one may determine the polarization patterns without detailed
model calculations .
The lattice dynamics of such a system may then be studied
experimentally without particular effort .
In figure 6 we show the phonon dispersion of Ne in the three main symmetry directions,
which according the common use are labeled 0, Y~ and A .
Ne crystallizes in a fcc-lattice
with only one atout in the primitive tell, therefore one expects only three acoustic branches
per q-value .
One observes the three distinct branches only in the [xx0]-direction, whereas
there are only single transverse branches along [x00] and [xxx] .
In the [x00]-direction these
acoustic polarization patterns may be easily understood .
In the longitudinal mode atouts
are vibrating in [100]-direction, i .e . parallel to the wave vector . The two transverse modes
are characterized by displacements in [010] or in [001]-directions, i .e .
perpendicular to the
wave-vector .
Since [100] represents a four-fold axis in the fcc lattice, the latter two modes
cannot be distinguished, they are degenerate . The saute situation is found along the