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The dynamical properties of solids have recently attracted renewed
interest in connection with the increasing understanding of phase
transitions and re lated phenomena. In particular, soft modes or,
more generally, phonon 'anom alies' seem to play an important role
in structural and electronic phase tran sitions, such as
ferroelectric or superconducting transitions. The understanding of
the mechanisms responsible for the occurrence of unusually low
frequencies in phonon spectra requires a detailed analysis of the
microscopic forces governing the lattice vibrations. Of particular
importance is the influence of the electron lattice interaction in
the adiabatic approximation which in many cases is the origin of
peculiarities in the phonon self-energy. In this work the
vibrational spectra of pure non-metals and of those con taining
point defects are investigated. ' In these materials the
interrelation be tween the pseudo-harmonic forces (determining the
phonon dispersion re lations) and the non-linear anharmonic and
electron-phonon forces (as they act in infrared and Raman spectra)
is most obvious and can be quantitatively analysed in terms of
appropriate models. The main task is to arrive at a physically
correct treatment of electronic degrees of freedom, as for example
in an electronic 'shell' model, which leads to the description of
phonon spectra in terms of long-range polarizabilities and
short-range deformabilities. The pur pose of our review is to
stimulate further investigations which, we hope, will result in
explicit relations between the parameters of the semi-microscopic
models and the matrix elements from the electronic band structure."
The investigation of the properties of nonlinear systems is one of
the fast deve loping areas of physics. In condensed matter physics
this 'terra incognita' is approached from various starting points
such as phase transitions and renormali zation group theory,
nonlinear models, statistical mechanics and others. The study of
the mutual interrelations of these disciplines is important in
developing uni fying methods and models towards a better
understanding of nonlinear systems. The present book collects the
lectures and seminars delivered at the workshop on "Statics and
Dynamics of Nonlinear Systems" held at the Centre for SCientific
Culture "Ettore Majorana." in Erice;. Italy, July 1 to 11, 1983, in
the framework of the International School of Materials Science and
Technology. Experts and young researchers came together to discuss
nonlinear phenomena in condensed matter physics. The book is
divided into five parts, each part containing a few general artic
les introducing the subject, followed by related specialized
papers. The first part deals with basic properties of nonlinear
systems including an introduction to the general theoretical
methods. Contrfbutions to the nonlinear aspects of phase
transitions are collected in the second part. In the third part
properties of incommensurate systems are discussed. Here, competing
interactions lead to charge-density waves, soliton lattices and
other complex structures. Another point of special interest,
illustrated in the fourth part, is the 'chaotic' be havior of
various systems such as Josephson junctions and discrete lattices."
This phonon atlas presents a collection of phonon-dispersion and
density-of states curves of more than a hundred insulating
crystals. It grew out of an appendix to a handbook article on
phonon spectra 2.1J from which it was fin ally separated mainly
because this phonon atlas provides a rather self-con tained tool
for every scientist who is working in the field of dynamical
properties of solids. He often may find it' useful to have a handy
documen tation of the experimental phonon dispersion curves which
have been measured so far, together with information on calculated
dispersion relations and densities of states. The book will be
found to be incomplete by readers who are interested not only in
phonon frequencies of a specific crystal but would also like to
know about related properties such as elastic and dielectric
constants. This is, at the present time, beyond the scope of this
volume, but the authors would welcome all suggestions and criticism
which could be considered for a forth coming edition. Furthermore,
we would be pleased to provide interested readers with information
about phonon spectra which came to our knowledge after completion
of the manuscript. On the other hand, we will be most grateful for
all information about phonon dispersion curves which is missing in
our collection or new data for further editions."
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