Gerasim (Sima) Eliashberg

Condensed-matter theorist Gerasim (Sima) Eliashberg passed away on 8 January 2021 in Chernogolovka, Russia. Eliashberg was a major figure in the development of the microscopic theory of superconductivity. His seminal work on superconducting pairing due to electron–ion interaction in solids has become a cornerstone of modern many-body theory. It provided a description of superconductivity that, strikingly, remained quantitatively accurate and analytically soluble at both weak and strong coupling. With time it has evolved into a broad framework applicable to a wide variety of different interactions, including the strongly interacting regime, endowing theory of superconductivity with an immense predictive power.

Eliashberg was born on 26 July 1930 in Leningrad (presently St. Petersburg). In 1952 he earned an MS in physics from Leningrad State University. That was one of the darkest periods of Stalin’s rule, and Eliashberg’s relatives were among those persecuted. Thus, after graduating with honors, he was assigned to a plant, “Red Chemist,” where he spent the next five years. While there he found ways to keep up with physics. In 1959 six years after Stalin’s death, Eliashberg enrolled in a graduate program at the Leningrad Physical-Technical Institute (LPTI). His work on superconductivity, which laid down what came to be known as Eliashberg theory, was done the same year and published in two 1960 papers.

In 1961 Eliashberg joined LPTI as a junior research scientist. In subsequent years he focused on quantum transport in Fermi liquids. One of the seminal results of that period was his method of analytic continuation, an approach that links the real-time dynamics in quantum systems with the finite-temperature Green’s functions found in thermodynamic equilibrium. That work has quickly become a centerpiece of many-body theory. After receiving his PhD in 1963, Eliashberg left LPTI and moved to Chernogolovka, a Science Center and a small town that was being built at the time some 50 kilometers from Moscow. He spent two years in the theoretical department of the Institute for Chemical Physics and in 1965 joined the newly founded Landau Institute for Theoretical Physics, where he spent the rest of his career.

Eliashberg’s association with the Landau Institute lasted 55 years. His research included contributions to the theory of nonequilibrium superconductivity and mesoscopic physics (with Lev Gor’kov), strongly correlated states in two-dimensional electron systems (with Yuri Bychkov and Sergey Iordanskii), the magnetoelectric phenomena now known as the chiral magnetic effect (with the authors), and, of course, superconductivity. Eliashberg’s work earned him many awards, both in Russia and internationally. His 1994 John Bardeen Prize honored “the development of the pairing theory to account for the thermodynamic and dynamic properties of strong-coupling superconductors.”

As with any big discovery, Eliashberg’s work had two parts to it: an achievement and an impact. Published shortly after the 1957 paper by Bardeen, Cooper, and Schrieffer (BCS) that explained superconductivity by a two-body attraction between electrons, Eliashberg’s paper linked the attraction postulated by BCS to the, by then well-established, Migdal–Froelich theory of electron–phonon interactions. As such, it was instantly recognized as a big achievement. For the first few years of the theory’s existence, it was viewed as an explanation of why the BCS ideas, contentious at the time, were correct. Eliashberg’s theory did for BCS in the 1960s what the new quantum theory by Schrödinger and Dirac did in the 1920s for the old quantum theory by Bohr and Sommerfeld. That impact can be seen very clearly, for instance, in Bardeeen’s Nobel Prize acceptance lecture, a good half of which talks about Eliashberg’s theory and its implications.

During that initial period it was understood that the theory went far beyond BCS and was capable of making predictions about superconductivity in realistic materials. Those predictions, strikingly, remained accurate both at the weak and strong coupling strength. That unique property of Eliashberg‘s theory attracted considerable attention and was confirmed experimentally by Allen and Dynes in 1975 and others.

Next came the impact. As the field grew, with new classes of materials and superconductivity mechanisms being discovered, Eliashberg’s theory was found to be remarkably inclusive. In treating new superconductors, both at strong and weak coupling, it remained as quantitatively precise as at its inception. As a result, the theory evolved into a computational tool with a virtually unlimited predictive power: It offered quantitative predictions and guidance to a countless list of different systems. The long and constantly growing list includes the exotic superfluidity in liquid ³He, the iron-based superconductors (pnictides), the colored superconductivity of quark-gluon systems, and the heavy-fermion superconductivity. The theory provided a validation of the microscopic pairing mechanisms, unique in each case, and connected them to the experimental observations in a precise, quantitatively accurate manner. A recent success of Eliashberg theory is a spectacular prediction of room-temperature superconductivity in superhydrades under high pressure, which was observed in 2020.

For students who had the privilege to have Eliashberg as a mentor, the experience was unforgettable and impacted their lives as much as it did science. At the risk of appearing an outlier, by his work and life, Eliashberg promoted the notion of science as a never-ending dialogue between a researcher and nature—an endeavor that can be painstakingly difficult but can also bring joy beyond anything ordinary, far greater than that of winning a competition. A soft-spoken person, gentle on the surface but upright and high-minded within, Eliashberg was adored by his many colleagues, students, and friends. Treating everyone kindly and in an even-handed manner, he clearly knew something unique and important about life.

Eliashberg’s mentoring at times went well beyond research. On one occasion, a student brought to an appointment no scientific discussion but rather a tangle of complex personal issues: a derailed relationship, no money, and no place to live. Eliashberg listened carefully and with empathy, and said, “Under these circumstances, would you try to work a little more, making small steps towards your PhD goals. I know exactly how you feel, but I wouldn’t like to patronize. There’ve been times when I felt desperate and powerless facing complex life circumstances. I’ve done what I just suggested, and it did help.” That conversation occurred many years ago, but the advice is timeless, offering counsel in the present pandemic crisis and beyond. Sima Eliashberg will be deeply missed.