Petr A. Braun
(05.04.1943 - 27.12.2020)
Professor

Petr Aleksandrovich Braun was born on April 5, 1943 in Novokashirsk (Moscow region). In 1960 he entered the Physics Department of Leningrad State University and graduated with honors in 1965. In 1966-1968 P.A. Braun studied in the graduate school of Leningrad State University. He got his PhD in 1971 and was admitted to the Quantum Mechanics Division, where he successively held the positions of Assistant Professor, Senior Researcher, Leading Researcher, and finally Professor. In 2005 Petr Aleksandrovich returned to the position of Leading Researcher, since he had to spend much time in Germany, where a fruitful scientific cooperation with the University of Essen was initiated. Finally, P.A. Braun left St. Petersburg State University in 2015, but he always visited Quantum Mechanics Division and participated in scientific seminars whenever he came to St. Petersburg.

The research interests of Petr Aleksandrovich were broad and underwent a certain transformation over the years of his career. His scientific adviser in the graduate school, Professor T.K. Rebane, was a well-known specialist in the theory of molecules and few-body quantum systems. In the graduate school as well as after graduation, Petr Aleksandrovich investigated variational principles for such characteristics as polarizability, excitation probability, and matrix elements of individual transitions. Gradually, he became a recognized specialist in the field of vibrational magnetism of molecules. An important contribution to molecular physics was also created by his theory of the rotational levels of an asymmetric top. In subsequent years, Petr Aleksandrovich paid great attention to the study of Rydberg states of atoms in external electric and magnetic fields of various orientations. His work in this area has received a worldwide recognition. He predicted a number of new effects, which were then experimentally discovered in laboratories in France and in the United States. In the process of this work P.A. Braun created a new theoretical method – the discrete semiclassical approximation. This method allows one to consider in a unified way a wide class of problems with the Hamiltonian represented as a tridiagonal matrix with slowly varying matrix elements. New semiclassical methods for calculating the properties of atoms and molecules in external fields became the topic of P.A. Braun’s Doctor of Science thesis, which he defended in 1983. This extensive and thorough research resulted in a detailed formalism, which made it possible to solve a number of important physical problems. This work was acknowledged by the scientific community, and in 1993 P.A. Braun was invited to publish a review paper in the prestigious Reviews of Modern Physics. In the following years, the research interests of P.A. Braun shifted into the interesting and complicated area of quantum chaos. Many of his works in this area were co-authored by his colleagues from the University of Duisburg-Essen (Germany). These works of P.A. Braun were devoted to studies of the energy spectrum of classically non-integrable systems. Solving these problems, he also used semiclassical methods and obtained important results, in particular, in the theory of spin chains.

Petr Aleksandrovich was a brilliant lecturer. At the Physics Department of St. Petersburg State University he taught a general course in classical mechanics, as well as specialized courses in the theory of molecules and theory of atoms and molecules in external fields for the students of the Quantum Mechanics Division. According to student polls, he was named among the best teachers of the Physics Department. His book "Introduction to the theory of molecular spectra" (1983), co-authored by A.A. Kiselev, served as a teaching aid for several generations of undergraduate and graduate students of the Physics Department.

Being firm in the matters of scientific as well as general human ethics, in communication with the colleagues and students Petr Aleksandrovich was invariably affable and benevolent. No one has ever heard a harsh word from him, even if he did not share someone's point of view during the discussion. Acquaintance with him left the impression of a deeply decent, intelligent and noble person. This is how we will remember Petr Aleksandrovich Braun – an outstanding researcher, a talented teacher and a wonderful person.