Leonid Keldysh Federico Capasso, Paul Corkum, Olga Kocharovskaya, Lev Pitaevskii, and Michael V. Sadovskii Citation: Physics Today 70, 6, 75 (2017); doi: 10.1063/PT.3.3605 View online: https://doi.org/10.1063/PT.3.3605 View Table of Contents: https://physicstoday.scitation.org/toc/pto/70/6 Published by the American Institute of Physics ARTICLES YOU MAY BE INTERESTED IN Spartak Timofeevich Belyaev Physics Today 70, 72 (2017); https://doi.org/10.1063/PT.3.3602 Mildred S. Dresselhaus Physics Today 70, 73 (2017); https://doi.org/10.1063/PT.3.3603 Anthony Philip French Physics Today 70, 74 (2017); https://doi.org/10.1063/PT.3.3604 Hidden worlds of fundamental particles Physics Today 70, 46 (2017); https://doi.org/10.1063/PT.3.3594 The new Moon Physics Today 70, 38 (2017); https://doi.org/10.1063/PT.3.3593 Magnetic monopole search, past and present Physics Today 70, 12 (2017); https://doi.org/10.1063/PT.3.3571 became the chief lecturer and manager Leonid Keldysh Early in his career, in 1957–58, of a course taken by more than 500 Keldysh predicted an electric-field- MIT students. Subsequently, he became eonid Keldysh, a world-prominent induced shift of a semiconductor’s ab- associate head. L Russian theoretical physicist, passed sorption edge, now known as the Franz– The acronym for French’s course be- away from pneumonia on 11 Novem- Keldysh effect. It turned out to be of came notorious. French said, “I called it, ber 2016 in Moscow. He was one of the paramount importance in the develop- blandly, ‘Physics: A New Introductory most imaginative and influential physi- ment of a powerful spectroscopic tech- Course.’ . Within the first week, I cists of the past half century. In addition nique for determining the band structure couldn’t imagine how I could have been to developing the concepts that bear his of semiconductors and phonon-assisted so stupid, as it was known forever after- name—including the Keldysh nonequi- electron tunneling in semiconductors. wards as the PANIC course!” librium diagram technique, the Keldysh Phonon-assisted tunneling was quickly French was a skillful, lucid lecturer. theory of strong-field ionization, and recognized to be the main tunneling Those qualities are also evident in his the Franz–Keldysh effect—he also pre- mechanism in silicon and germanium stylish writing. The four books he wrote dicted the electron–hole liquid phase, the Esaki diodes. while teaching PANIC are still in print. Bose–Einstein condensation of excitons, In 1962 Keldysh proposed that spa- He used demonstrations well and de- phonon-assisted electron tunneling, and tially modulating a crystal lattice creates vised several of his own. He was proud superlattices in semiconductors. Each artificial band structures in semicon - of his prize-winning pressure-of-light prediction opened a new field of research. ductors. In 1970, heterostructure super- experiment and of his low-cost Fabry– Keldysh was born on 7 April 1931 in lattices were studied by Leo Esaki and Perot interferometer made from a micro- Moscow. He belonged to a family of Raphael Tsu and became the basis of scope slide and a cover slip. prominent mathematicians, including many optoelectronic devices. French’s influence in physics educa- his mother, Lyudmila Keldysh, and step- In a 1964 paper that has received 5000 tion reached beyond MIT. From 1975 to father, Petr Novikov, both professors at citations, Keldysh developed a powerful 1981, he was chairman of the Interna- Moscow State University; uncle Mstislav formalism, today called the Keldysh tional Commission on Physics Education Keldysh, a key figure in the Russian nonequilibrium diagram technique, for (ICPE) of the International Union of Pure space program and president of the describing nonequilibrium processes. and Applied Physics. He edited Einstein: Russian Academy of Sciences in 1961–75; Introducing a matrix Dyson equation for A Centenary Volume (Harvard University and brother Sergey Novikov, a Fields the time-ordered Green’s functions, Press, 1979) and coedited Niels Bohr: A Medal recipient. along with what is now known as the Centenary Volume (Harvard University Following graduation from Moscow Keldysh–Schwinger contour, Keldysh Press, 1985). Those efforts on behalf of State University in 1954, Keldysh joined generalized the Feynman diagram tech- the ICPE were recognized with the 1980 the theoretical physics department of nique to nonequilibrium processes. University Medal of Charles University, the P. N. Lebedev Physical Institute, where Many laser-excited condensed-matter Prague, and the 1988 Lawrence Bragg he worked until the end of his life. His sci- systems are described by the Keldysh Medal and Prize of the Institute of entific supervisor was Vitaly Ginzburg, formalism. Now a standard tool in theo- Physics. AAPT also presented him with and his department head was Igor Tamm, retical physics, the approach is fre- awards for his exceptional work: a Dis- both future Nobel Prize recipients. quently applied to femtosecond spec- tinguished Service Citation in 1976 for N. K. FEDOROV, LEBEDEV PHYSICAL INSTITUTE troscopy, condensed-matter physics, US physics education; the Oersted quantum field theory, and quantum Medal, its most prestigious award, in cosmology. 1989 for the teaching of physics; and the Keldysh’s theory of strong-field ioniza- Melba Newell Phillips Medal in 1993 for tion, which was published in 1964 and has services to AAPT. received 5500 citations, laid the founda- French retired from MIT in 1991 tion for the field of intense laser radiation but remained active. In 1993 he directed interaction with atoms, ions, molecules, the creation of the examinations for and solids. The theory introduced optical the XXIV International Physics Olym piad. tunneling and above-threshold ionization, He continued to speak and write about experimentally observed about 15 years how to improve physics instruction, and later. The Keldysh parameter determines until this year he regularly attended the the boundary between multiphoton and physics department’s weekly luncheons. tunneling regimes. Optical tunneling Peter H. Fisher underlies high-harmonic generation and Massachusetts Institute of Technology attosecond science. In 2014 the Journal of Cambridge Physics B celebrated Keldysh’s contri - Charles H. Holbrow bution with a special issue called “Fifty Colgate University Years of Optical Tunneling.” Hamilton, New York Keldysh submitted in 1965 his candi- Massachusetts Institute of Technology date of science thesis on the theory of non - Cambridge Leonid Keldysh equilibrium phenomena. But because of his outstanding level of scientific JUNE 2017 | PHYSICS TODAY 75 OBITUARIES contributions, he was awarded the higher 10 were awarded doctor of science de- Keldysh neither read other people’s doctor of science degree (similar to Ger- grees and became prominent physicists. memoirs nor wrote any himself. He be- many’s habilitation). From 2004 to 2011, Keldysh held a half- lieved that what mattered were not the In 1965 Keldysh and his PhD student time, tenured professorship in the de- personal life events but the contributions Yuri Kopaev introduced the excitonic in- partment of physics and astronomy at a person left. Leonid Keldysh left us a lot. sulator concept now widely used for un- Texas A&M University. He served as an His name will remain in physics forever derstanding the nature of various metal– editorial board member for many phys - through the theories he developed. We semiconductor transitions. In 1968 ics journals and from 2009 to 2016 was will always remember his passion for Keldysh and his PhD student Alexander editor-in-chief of Physics-Uspekhi. physics, benevolence and decency, kind Kozlov predicted the Bose–Einstein con- During the challenging period for smile, and gentle sense of humor. densation of excitons, a hot topic of cur- Russian science following the collapse of Federico Capasso rent experimental research. In the same the Soviet Union, Keldysh served as di- Harvard University year, Keldysh predicted that nonequi - rector of the Lebedev Physical Institute Cambridge, Massachusetts librium excitons in highly excited semi- and head of its theoretical physics de- Paul Corkum conductors would form electron–hole partment in 1989–94 and as head of the University of Ottawa droplets. Keldysh’s idea immediately general physics division of the Russian Ottawa, Ontario, Canada stimulated intense experimental re- Academy of Sciences in 1991–96. His Olga Kocharovskaya search that led to the discovery of that leadership played a crucial role in pre- Texas A&M University new state of matter. In other seminal pa- serving the institute and the high stan- College Station pers, Keldysh studied the concepts of dards of Russian science. He was ad- Lev Pitaevskii deep levels in semiconductors, impact mirably strong and honest. It was ionization, and “phonon wind,” subse- impossible to persuade him to accept University of Trento quently validated experimentally. any decision with which he disagreed. Trento, Italy A professor at Moscow State Univer- Among his many awards, Keldysh Kapitza Institute for Physical Problems sity since 1965, Keldysh established a shared the 1975 Hewlett-Packard Prize of Russian Academy of Sciences world-renowned scientific school on the European Physical Society, the 2011 Moscow nonequilibrium phenomena in con- Eugene Feenberg Memorial Medal, and Michael V. Sadovskii densed-matter physics and nonlinear the 2015 Lomonosov Grand Gold Medal Russian Academy of Sciences, Ural Branch optics. Of his more than 20 PhD students,