**Preface to "Nonequilibrium Phenomena in Strongly Correlated Systems"**

Dmitrii Nikolaevich Zubarev was born in November 27, 1917, in Moscow, Russia. On the occasion of his 100th birthday, a honorary colloquium was performed at April 18/19, 2018, at the Bogoliubov Laboratory of Theoretical Physics at the Joint Institute of Nuclear Research, Dubna, Russia. Former coworkers and followers (see Figure 5) contributed with talks:


The organizers decided to collect contributions to prepare this Special Issue. Following the open call, there were, in addition to the contributions from the above speakers, also contributions from colleagues who could not participate in the seminar: ȱ ȱ


**Figure 1.** Photo taken on April 18, 2018, at the honorary colloquium for D. N. Zubarev's 100th birthday. From left to right: Vladimir Morozov (MIREA, Moscow), Heidi Reinholz (Rostock), David Blaschke (Wroclaw and JINR Dubna), Nikolay Plakida (JINR Dubna), Gerd Röpke (Rostock), Yurii Rybakov (RUDN, Moscow). Background: Hermann Wolter (Munich).

We will now outline the biography and scientific legacy of D. N. Zubarev; for details see [1–3].

D. N. Zubarev studied physics at Moscow State University. In 1941, he graduated from the Department of Physics. His academic advisor was N. N. Bogoliubov, who was also the promotor of his PhD thesis work. From 1954 to the end of his life, D. N. Zubarev worked at the V. A. Steklov Institute of Mathematics of the Russian Academy of Sciences. From 1969 to 1971, he was also head of the Statistical Mechanics and Theory of Condensed Matter Group at the Laboratory of Theoretical Physics, JINR, Dubna. He died in Moscow, July 29, 1992, after a traffic accident.

D. N. Zubarev successfully contributed to statistical physics. In 1957, he obtained, together with N.N. Bogoliubov and Yu.A. Tserkovnikov, the asymptotically exact solution of the BCS model Hamiltonian [4], which was an essential contribution to the theory of superconductivity.

**Figure 2.** Photo taken at an excursion to Mzcheta during the Tiflis Conference on LowȬ Temperature Physics in 1959. From left to right: Dmitry Vasil'evich Shirkov (JINR Dubna), Dmitrii Nikolaevich Zubarev (Steklov Inst. Moscow), Anatoly Alekseevich Logunov (JINR Dubna), Yurii Aleksandrovich Tserkovnikov (Steklov Inst. Moscow), Zygmunt Galasiewicz (JINR Dubna & Wroclaw), Albert Nikifirovich Tavkhelidze (JINRDubna).

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**Figure 3.** Photo taken when the group of D. N. Zubarev visited Vladimir in June 1969. From left to right: John Shepherd (UK), KarlȬHartmut Müller (GDR), Celia Shepherd (UK), Wolfgang Götze (FRG), Gerd Röpke (GDR), Dmitrii Nikolaevich Zubarev.

He considered himself as a "father of methods", in particular he published the highly cited review article entitled: "DoubleȬtime Green Functions in Statistical Physics" [5], and wrote the well known monography "Neravnovesnaya statisticheskaya termodinamika" [6], translated from Russian to English by P. J. Shepherd. He made very fundamental contributions to statistical physics published in many scientific articles, based on a sound mathematical approach. His very general approach to nonequilibrium processes with applications in different fields was presented in the Monography [7].

**Figure 4.** D. N. Zubarev at the 6Ȭth Winter School on Theoretical Physics in Karpacz (Poland) 1969. From left to right, back row: Nikolay Maksimilianovich Plakida, Elmar Grigorievich Petrov, Dmitrii Nikolaevich Zubarev, ?, WÙadysÙawa Rybarska (Nawrocka); front row: ?, Tadeusz Paszkiewicz, Jerzy St¿ïlicki, Andrzej P¿kalski, Valery Leonidovich Pokrovsky.

As a leading scientist in statistical physics, D. N. Zubarev had many active collaborations within the former Soviet Union and abroad. Many close relations to colleagues and guests were longȬstanding and are determined by his openȬminded, tolerant and clear scientific position, also his accurate and precise work and discussions. He attracted and educated young scientists, including G. O. Balabamyan, V. P. Kalashnikov, V. G. Morozov, T. Paszkiewicz, N. M. Plakida, L. Pokrovsky, S. Tishchenko, M. V. Tokarchuk and others. He also hosted guests from abroad (see Figure 3), and visited them to give series of lectures, like at the Karpacz Winter School in Poland 1969 (see Figure 4) and at the Miniworkshop on Quantum Statistics in Ahrenshoop (GDR) in 1987 (see Figure 5).ȱ ȱ

**Figure 5.** D. N. Zubarev and M.V. Tokarchuk during a visit at the University of Rostock in November 1987, when Dmitri Nikolaevich was giving a lecture series on the NSO method during a Miniworkshop with the group of Gerd Röpke in Ahrenshoop.

The personality of D. N. Zubarev was formed also by the WWII. On June 25, 1941 he volunteered for duty in the Eighth Division of the People's Militia and participated in the defense of Moscow. At the end of the war D. N. Zubarev was in Berlin with the 47th Army of the First Belorussian Front. He was awarded the Red Star for participation in mine clearing in Berlin. Many of his fellow students died during the war. After war, he worked with G. Hertz who was made head of Institute G, in Agudzery, about 10 km southeast of Sukhumi and a suburb of Gulrip'shi, on separation of isotopes by discussion in a ow of inert gases. After this he worked for several years on important defense problems at the "object," now known as ArzamasȬ16. His association during this period with N. N. Bogoliubov and A. D. Sakharov greatly influenced his scientific career. His wife, Galina Rudolfovna, participated in the Leningrad blockade.

Despite the very hard history, and under the restrictions of the soviet time, he developed an openȬminded and complaisant atmosphere in the contact with physicists around the world. D. N. Zubarev had an unusual talent for social intercourse, which attracted people to him. He exhibited the traits of a true Russian intellectual live interest and openness to anything new in science and in life, honesty and fairness, softness, delicacy, unselfishness and constant readiness to help people, but at the same time he was strict and uncompromising in the search for scientific truth and he unfailingly adhered to strict scientific ethics.

D. Blaschke, A.V. Friesen, V. G. Morozov, N. K. Plakida, G. Röpke. Wroclaw, Dubna, Moscow, Rostock, in July 2020

**Figure 6.** Photo taken at the honorary colloquium for D. N. Zubarev's 100th birthday in the Conference Hall of the Bogoliubov Laboratory of Theoretical Physics at the JINR Dubna. From left to right: D. Blaschke (Wroclaw & JINR Dubna), H. Reinholz (Rostock), V.G. Morozov (MIREA, Moscow), G. Röpke (Rostock), N.M. Plakida (JINR Dubna). Background: Bust of N.N. Bogoliubov.

#### **References**

