7.12.2016

Václav Holý (Department of Condensed Matter Physics MFF UK a ÚFKL PřF MU):
X-ray diffraction from random layered systems


Abstract:

X-ray diffraction from crystals consisting of randomly stacked monoatomic or
monomolecular layers have been investigated for several decades. Recently,
these structures are again in focus of attention, since new materials like
topological insulators, multiferroic systems etc. exhibit a random layered
structure. An x-ray diffraction experiment from a random stack of monolayers
can be carried out in two basic arrangements; in a standard set-up the measured
diffracted intensity is collected from a large sample volume, for which the
ergodic hypothesis applies, so that the measured signal is averaged over a
statistical ensemble of all possible monolayer distributions (microstates).
Synchrotron x-ray sources can deliver very narrow and almost coherent x-ray
beams, allowing for another experimental set-up, in which only one microstate
is irradiated. The application of a fully coherent beam makes it possible to
retrieve the phase of the diffracted beam and determine fully the monolayer
sequence under investigation. 

We have used both experimental approaches for the investigation of epitaxial
layers of rhombohedral topological insulators (Bi2Se3), for the investigation
of stacking faults in non-polar GaN layers and for the study of wurtzite and
zincblende segments in III-V nanowires.