Isaac B. Bersuker

Isaac B. Bersuker (Russian: Исаáк Бóрухович (Бори́сович) Берсýкер; born February 12, 1928) is a Soviet-Moldоvan-American theoretical physicist and quantum chemist whose principal research is in chemical physics, solid-state physics, and theoretical chemistry. Known for his "life-long years of experience in theoretical chemistry" working on the electronic structure and properties of coordination compounds, Isaac B. Bersuker is “one of the most widely recognized authorities” in the theory of the Jahn-Teller effect (JTE) and the pseudo-Jahn-Teller effect (PJTE). His accomplishments include explaining the polarization of the atomic core in Rydberg atoms, the effect of tunneling splitting in molecules and solids with a strong JTE, and the discovery of the PJTE origin of ferroelectricity in cubic perovskites. Known as the leading expert in JTE and PJTE, Bersuker is the permanent Chairman of the International Steering Committee of the Jahn-Teller symposia. His present affiliation is with the Oden Institute for Computational Engineering and Science of the Department of Chemistry of the University of Texas at Austin.

Isaac (Izya) Bersuker was born on February 12, 1928, in Chișinău, then part of Greater Romania, to a low-income family of Bessarabian Jewish descent. His father Boruch Bersuker was a carpenter, and his mother Bella Bersuker (Russian: Бéлла Хáймовна Берсýкер, 1896-1981) was a housewife with five kids. As a boy in a family of a modest background, Isaac got his elementary school education in Talmud Torah and ORT.  He was 13 years old when the tragic events of World War II forced his Jewish family to run from the Nazis to an Azerbaijan village. Deprived of the traditional middle and high-school education, he spent four years farming in Azerbaijan kolkhoz. However, he never gave up his dream of getting a higher education and becoming an intellectual. After the war, native Romanian, he barely spoke Russian. Yet, in a self-education way, in а two-year term, he managed to complete a four-year high-school program in a Russian school and enrolled at Chișinău State University. In the best meaning of this expression, Isaac is a self-made man. A fascinating autobiographical section in describes "his scientific ascent, starting from a Jewish childhood in Bessarabia and frequently hampered by antisemitic state directives under the Stalin regime." Dedicated to the study of theoretical physics, in 1952, just six years after being an illiterate boy shepherding sheep, Bersuker graduated from this university with a master's degree in physics. He began his scientific research in atomic spectroscopy as a post-graduate student at Leningrad State University, working under Mikhail G. Veselov at the Division of Quantum Mechanics led by its Chair Vladimir A. Fock. Here, in 1957, Bersuker received his doctorate (Kandidat of science, Russian: Кандидáт наýк) and in 1964 his habilitation degree (Doctor of science, Russian: Дóктор наýк). From 1964 to 1993, back in Chișinău, Bersuker continued his scientific research at the Institute of Chemistry of the Moldavian branch of the USSR Academy of Sciences. Organizationally, Bersuker's success was the creation in 1964, and the leadership of the Laboratory of Quantum Chemistry also dubbed ‘‘the Chișinău school of the Jahn–Teller effect.’’  Elected as a Corresponding Member of this Academy in 1972 and a full Member in 1989, Isaac B. Bersuker moved to the United States In 1993. He became a Senior Research Scientist and Professor of the Department of Chemistry at the University of Texas at Austin. Isaac B. Bersuker served as a doctoral and habilitation supervisor for 31 post-graduate students and post-docs. According to K. Alex Müller, Bersuker was and still is "in full swing at the university, writing books, discussing with great wit, and quick to understand ‒ as I had known him for well over thirty years." In the late eighties of the last century, owing to Bersuker's high motivating role, leadership, and creative ingenuity, Bersuker's school was called "the capital of the Jahn-Teller effect" by some. Bersuker's academic publications have a high impact on the scientific community. According to Google Scholar, since 1993 when he moved to the United States, Bersuker's papers were cited 10428 times, his h-index is 41, and his i10-index is 141.

Isaac B. Bersuker was married in 1951 to Liliya Bersuker (Russian: Ли́лия Бори́совна Берсýкер, 1930-2003), a chemist. He has one son, Gennadi B. Bersuker (b. 1953), a theoretical physicist, and two grandsons, Eugene G. Bersuker (b. 1979) and Kirill G. Bersuker (b. 1985), a molecular biologist.

In his Ph.D. thesis, Bersuker developed the theory of core polarization and its effect on optical transitions in Rydberg atoms. At the time, this was a puzzling problem in absorption spectroscopy. The absorption of light by alkali atoms appeared to violate the electric dipole sum rule. According to Bersuker, the solution to the problem is in the instantaneous polarization of the atomic core by the incident electromagnetic wave creating an additional perturbation to the excitation of the valence electron. Related to this problem, he worked out the adiabatic separation of motion of the valence and the atomic core electrons in electronic structure calculations of atoms. First introduced in 1957, still, decades later, Bersuker's ideas of electron polarization by the incident electromagnetic wave and of the atomic core polarization by the valence electron is used and further explored in atomic spectroscopy.

Bersuker's contributions to the JTE and PJTE theory with applications to physical and chemical phenomena are reflected in his several monographs (some of them written and published with the assistance and involvement of other authors) and major reviews on this subject (see the latest in). First published in 1961-1962, his contributions to the theory of the JTE predicted the tunneling splitting of the vibronic energy levels of the systems with the JTE, later confirmed experimentally. The splitting is due to the tunneling transitions between the equivalent wells on the multiminimum adiabatic potential energy surface produced by this effect. In 1976, "The phenomenon of tunneling splitting of energy levels of polyatomic systems in the state of electronic degeneracy" was qualified as a scientific discovery and registered in the State Register of the USSR (Diploma No. 202). In addition, Bersuker is known for revealing the significance of the PJTE and showing that it may take place at any energy gaps between entangled electronic states. Most important, he proved that the JTE and PJTE are the only sources of structural instability and spontaneous symmetry breaking (SSB) in polyatomic systems. Thus, according to Bersuker, if a polyatomic system has broken symmetry properties, undoubtedly, they are of JTE or PJTE origin. This conclusion elevates the two effects from their assumed earlier rare particular features to general tools for exploring molecular and solid-state properties.

Isaac B. Bersuker wrote 15 books, first in 1962, and more than 400 academic papers. His books on the JTE and PJTE, published in 1984, 1989, and 2006, were most influential. According to Google Scholar, cumulatively, these three monographs were cited more than 3000 times.

Using perovskite crystals as an example, Bersuker showed (first in 1964, published in 1966) that the PJTE produces a spontaneous symmetry breaking resulting under certain conditions in local dipolar instability. It exists in all the crystal phases, and the spontaneous polarization results from the order-disorder interaction between these PJTE-induced local dipolar distortions. Performed in the local octahedral TiO6 center in the BaTiO3 crystal (taken as an example), where vibronic coupling mixes ground A1g and close in energy exited T1u states of opposite parity (but same multiplicity), detailed analysis with calculations proved the PJTE to produce the dipolar distortion. Thus, it shows that Bersuker's PJTE theory of ferroelectricity agrees with the available empirical data and predicts new properties, confirmed experimentally.