These notes were for a course given at the Cavendish Laboratory, Cambridge,in the fall and winter terms of 1961-1962. Nominally, it was for second- and third-year graduate
students who had had a survey course in solid-state physics, and were interested (at least) in theory; but I assumed very litte fonnal theoretical background. I think the nores can be read by anyone who has had a thorough course in quantum mechanics, but the reader who knows something about solids will find them much easier, and will also not be misguided by my rather arbitrary and specialized choice of material.
The idea of the course was to teach a number of central concepts of solid-state physics, trying to choose those - band theory, nearly free electrons, effective Hamilto-nian theory, elementary excitations, broken symmetry - which lay as near as possible to what I consider to be the main stream of development of the subject. Such a choice is necessarily arbitrary - whose fields, su ch as dislocation theory, transport theory and fluctuation-dissipation theorems, magnetic resonance theory in all its forms, and critical fluctuations, which could easily be argued to be quite as important, were ignored, simply because the course was of f/nite length. My choice of examples was even more arbitrary.Forinstance, the choices of the electric field case of effective Hamiltonian theory and of excitons to illustrate collective excitations were made because I thought the students were likely to encounter the more usual examples elsewhere. From time to time, to liven the course up a bit, I introduced original material; the discussions of the limitations of nearly free electron theory, of the philosophies of elementary excitation theory, and of broken symmetry are new, and that of the magnetic state is not widely available.
Thelanguage and presentation are very informal; very few changes were made from my original lecture notes as written. I might add that little effort has been made to bring them up to date. Both Iinutations are ofcourse implicit in the idea of a lecture note volume.