J.T. Devreese

The Advanced Study Institute on "Path Integrals and Their Applications in Quantum, Statistical, and Solid State Physics" was held at the University of Antwerpen (R.U.C.A.), July 17-30, 1977. The Institute was sponsored by NATO. Co-sponsors were: A.C.E.C. (Belgium), Agfa-Gevaert (Belgium), l'Air Li~uide BeIge (Belgium), Be1gonucleaire (Belgium), Bell Telephone Mfg. Co. (Belgium), Boelwerf (Belgium), Generale BankmaatschappiJ (Belgium), I.B.M. (Belgium), Kredietbank (Belgium), National Science Foundation (U.S.A.), Siemens (Belgium). A total of 100 lecturers and partici- pants attended the Institute. The development of path (or functional) integrals in relation to problems of stochastic nature dates back to the early 20's. At that time, Wiener succeeded in obtaining the fundamental solution of the diffusion e~uation using Einstein's joint probability of finding a Brownian particle in a succession of space intervals during a corresponding succession of time intervals. Dirac in the early 30's sowed the seeds of the path integral formulation of ~uantum mecha- nics. However, the major and decisive step in this direction was taken with Feynman's works in ~uantum and statistical physics, and quantum electrodynamicso The applications now extend to areas such as continuous mechanics, and recently functional integration methods have been employed by Edwards for the study of polymerized matter.

The Advanced Study Institute on "Theoretical Aspects and New Developments in Magneto-Optics" was held at the University of Antwerpen (R.U.C.A.), from July 16 to July 28, 1979. The Institute was sponsored by NATO. Co-sponsors were: Agfa-Gevaert (Belgium), A.S.L.K. (Belgium), Bell Telephone Mfg. CO. (Belgium), Esso Belgium, Generale Bankmaatschappij (Belgium), General Motors (Belgium), I.B.M. (Belgium), Kredietbank (Belgium), Metallurgie Hoboken-Over­ pelt (Belgium), National Science Foundation (U.S.A). A total of 60 lecturers and participants attended the Institute. Scope of the Institute The magneto-optic phenomena are due to the change of the polarizability of a substance as a result of the splitting of the quantized energy bands. Most of these phenomena were discovered during the second half of this century. The understanding of the magneto-optical effects of all kinds, however, was brought by the advent of quantum mechanics, and since then important progress has been made in many fields of experimental methods and techniques.

The Advanced Study Institute on 'Elementary Excitations in Solids, Molecules, and Atoms' was held at the University of Antwerp (U.I.A.) from June 18th till June 30th 1973. The In- stitute was sponsored by NATO. Co-sponsors were: Agfa-Gevaert N.V. (Mortsel-Belgium), Bell Telephone Mfg. Co. (Antwerp-Belgium), I.B.M. BelgiumN.V. (Brussels-Belgium), the National Science Founda- tion (Washington D. C. - U. S .A.) and the Uni versi ty of Antwerp (U. I .A.) . A total of 120 lecturers and participants attended the Institute. Over the last few years, substantial progress has been made in the description of the elementary excitations of the elec- tronic and vibrational systems and their interactions. Parallel with this, the experimentalists have obtained outstanding re- sults, partly as a result of availability of coherent light sour- ces from the far infrared through the visible region, and partly because of the availability of synchrotron radiation sources in the soft X-ray region. The results of today will lead to fur- ther progress over the next years. It was the purpose of this NATO Advanced Study Institute to present a state of the art, namely a survey of experiment and theory.

The 1986 Advanced Study Institute on "The Physics of the two-Dimen­ sional Electron Gas" took place at the Conference Centre liTer Helme", close to Oostende (Belgium), from June 2 till 16, 1986. We were motivated to organize this Advanced Study Institute in view of the recent experimental and theoretical progress in the study of the two-dimensional electron gas. An additional motivation was our own theore­ tical interest in cyclotron resonance in two-dimensional electron systems at our institute. It is my pleasure to thank several instances and people who made this Advanced Study Institute possible. First of all, the sponsor of the Advanced Study Institute, the NATO Scientific Committee. Furthermore, the co­ sponsors: Agfa Gevaert, Bell Telephone Mfg. Co. N.V., Burroughs Belgium. Control Data. Digital Equipment Corporation, Esso Belgium. European Research Office (USA). Kredietbank. National Science Foundation (USA). Special thanks are due to the members of the Program Committee and the members of the Organizing Committee. I would also like to thank Mrs. H. Evans for typing assistance.

The 1982 Antwerp Advanced Study Institute on "Physics of Polarons and Excitons in Polar Semiconductors and Ionic Crystals" took place from July 26 till August 5 at the Conference Center Priorij Corsen­ donk, a restored monastery, close to the city of Antwerp. It was the seventh Institute in our series which started in 1971. This Advanced Study Institute, which was held fifty years after Landau introduced the polaron concept, can be considered as the third major international symposium devoted to the physics of pola­ rons. The first such symposium took place in St. Andrews in 1962 under the title "Polarons and Excitons" [I]. The early theoretical developments related to polarons were reviewed in depth at this meeting; the derivation of the polaron hamiltonian by Frohlich, the Frohlich weak coupling theory (and the equivalent weak coupling canonical transformations), the Landau-Pekar and Bogolubov strong coupling theory and the Feynman polaron model formulated with his path integrals. The main emphasis was on the polaron self-energy, effective mass and mobility. From the experimental side the first evidence for polaron effects was provided by the pioneering cyclotron and mobility measurements o~ the silver halides by F. e. Brown and his group. Also the significance of polaron effects for the under­ standing of excitons in ionic crystals was a central topic in St. Andrews. The second Advanced Study Institute concerning polaron physics was organized at the University of Antwerp (R. U. C. A.

This volume contains of the proceedings of the CECAM {Centre d'Etudes de Calcul Atomique et Moleculaire} workshop on "Ab Initio Calculation of Phonon Spectra" which took place at the campus of the University of Antwerpen (Universitaire Instelling Antwerpen) on June 4 and 5, 1981. This workshop was sponsored by the Universitaire Instelling Antwerpen, by the Belgian "Nationaal Fonds voor Weten- schappelijk Onderzoek" and the Belgian Ministry of Education and Cultural Affairs. Although the calculation of phonon spectra in a solid has long been known to solid state physicists, it is only in the last decade that a macroscopic theory which treats the response of the electrons to the vibrations of the ions has been developed in a successful way. In the case of semiconductors numerical calculations of phonon frequencies were not performed until very recently due to the complex- ity of the problem. The correct treatment of the electron in the crystal has also led to an investigation of electron-phonon inter- action, including many body effects. Success has been achieved in the calculation of phonon anomalies and the coupling parameters in transition metals. In the case of surfaces the conductions for the appearance of electronic instabilities and the coupling of the resulting charge density wave to the lattice have been studied.

The Advanced Study Institute on "Theoretical Aspects and New Developments in Magneto-Optics" was held at the University of Antwerpen (R.U.C.A.), from July 16 to July 28, 1979. The Institute was sponsored by NATO. Co-sponsors were: Agfa-Gevaert (Belgium), A.S.L.K. (Belgium), Bell Telephone Mfg. CO. (Belgium), Esso Belgium, Generale Bankmaatschappij (Belgium), General Motors (Belgium), I.B.M. (Belgium), Kredietbank (Belgium), Metallurgie Hoboken-Over­ pelt (Belgium), National Science Foundation (U.S.A). A total of 60 lecturers and participants attended the Institute. Scope of the Institute The magneto-optic phenomena are due to the change of the polarizability of a substance as a result of the splitting of the quantized energy bands. Most of these phenomena were discovered during the second half of this century. The understanding of the magneto-optical effects of all kinds, however, was brought by the advent of quantum mechanics, and since then important progress has been made in many fields of experimental methods and techniques.