"Go to Dubna and meet Plakida"

End of the story. The beginning of the selection of materials
is in the issue №1 of 2023.

In the mid-1970s of the last century, during my senior year at the university, I got the opportunity to choose a scientific centre to do my thesis on the condensed matter theory. I distinctly remember how one of the well-known physicists that conducted a special course on quantum magnetism with us, told me almost literally: "Go to Dubna and meet Plakida." At that time, without hearing it, I was intrigued and involuntarily wondered: "Who is this remarkable scientist with the surname Planida, symbolizing the heavenly body?" It was inappropriate to ask the professor such a question.

I was really lucky to come to Dubna to get involved in the work on the theoretical substantiation of a completely new at that time experimental approach to the research of condensed matter - the method of muon spin relaxation. The method is based on the spatial asymmetry of the weak muon decay, the experiments were carried out at the DLNP synchrocyclotron by the staff members of B.M.Pontecorvo's department. During the first years of my work at JINR, I first got acquainted with V.L.Aksenov that introduced me to N.M.Plakida. Real cooperation with Nikolay Maksimilianovich began after I had been invited to work in his sector at BLTP shortly after the announcement of the discovery of high-temperature superconductivity (HTSC) by I.G.Bednorz and K.A.Muller in late 1986. It should be noted that until that time the HTSC phenomenon had been predicted and expected for almost three decades. Yet, the discovery of this phenomenon in copper-oxide ceramic compounds had a stunning effect on the physics community. Not only HTSC, but also a wide variety of other exotic properties: electronic, magnetic and structural, in new HTSC materials immediately attracted a huge army of specialists, experimentalists and theorists around the world to investigate them. In this diversity, it was important not to get lost and choose the right area. Over time, I appreciated the correctness and prospects of the approaches and models chosen by Plakida for our joint work with him.

In 1989, JINR organized a major international conference on HTSC with the invitation of one of the authors of the discovery, K.A.Muller. In his speech, commenting on the central motive that guided him in his long-term search for a new functional material with the properties of HTSC, Muller noted the need for a "soft phonon mode" in such a material near the structural instability of the crystal lattice. In accordance with the standard phonon mechanism of superconductivity in the framework of the Bardeen-Cooper-Schrieffer (BCS) theory, the occurrence of such a mode can result in a significant increase in the critical temperature Tc of the metal transition to the superconducting state. According to M?ller, two scenarios were the most developed in this area. The first was based on the implementation of the Jahn-Teller effect, the second one assumed the occurrence of strong anharmonicity effects in the phonon spectrum of lattice vibrations. This second scenario, recognized as an anharmonic theory of high-temperature superconductivity, was developed in the early 1980s by N.M.Plakida in collaboration with the young colleagues V.L.Aksenov, Vujicic and S.Stamenkovic. In his speech at the conference, K.A.Muller emphasized the significant influence these papers had had on him. However, by 1989, as a result of two years of extensive research, there was a conviction that not "phonon physics", that is, features of the vibrational states of the lattice, but "Hubbard Physics" - the effects of strong inter-electron local correlations and the resulting features of dynamic magnetic fluctuations of the electronic system should provide the key to the development of the theory of HTSC.

The most instructive and even striking of all that distant history for me was that already in the first months after the announcement of the discovery by I.G.Bednorts and K.A.Muller, N.M.Plakida, being a leading specialist in the theory of structural phase transitions, nevertheless, clearly realized that the key to understanding and describing the physical phenomena in new HTSC materials is in another area, far beyond the standard phonon mechanism of superconductivity of the BCS theory. In the new approach, it was necessary to understand the role of strong electron correlations first of all in the development of the quasiparticle properties of electric current carriers and then of the mechanism of their superconducting pairing. It was assumed that this could be achieved in different versions of the Hubbard electronic model without including the electron-phonon interaction. It was this area of the joint search that Plakida suggested to me in early 1987, although it seemed more comfortable at first to develop the already recognized anharmonic theory of high-temperature superconductivity, generalizing and applying it to new HTSC materials.

One must think that internal criteria of a higher order did not allow N.M.Plakida to follow the already familiar well-worn path, but to choose a more difficult and unknown path to a distant goal. I guessed about such criterion many years later, when Nikolay Maksimilianovich, on quite another occasion, referring to his Moscow colleague, noted that all physicists are classified into "discoverers" and "rammers", the former solve new problems, whilst the latter follow and clarify details in numerous publications.

On the whole, the discovery of HTSCs in copper-oxide compounds has taken the topic of strong electron correlations far beyond the issues concerning superconductivity. In particular, many models of quantum magnetism have been quantitatively substantiated at the quantum physicochemical level and a number of exotic electronic properties of a huge family of practically important metal compounds collectively known as transition metal oxides have been discovered. Along with N.M.Plakida, many researchers have contributed to the development of the magnetic mechanism of superconductivity in strongly correlated electronic systems. Admittedly, despite the unprecedented efforts of researchers over 35 years, a definitive universally accepted theory of HTSC has not yet developed.

Nikolay Maksimilianovich had many events in his life and had a constructive influence on many people in his professional life. In this context, evaluating K.A.Muller's speech at the conference in Dubna in distant 1989, we can acknowledge the following. A well-founded scientific hypothesis in the form of the anharmonic theory of high-temperature superconductivity, developed by N. M. Plakida and co-authors, prompted the outstanding researcher to a long search within the walls of the physicochemical laboratory that culminated in the discovery of the desired physical phenomenon. But at the same time, it turned out that the initial theory does not provide a key to understanding the new phenomenon in all its complexity. Nevertheless, it cannot be ruled out that at the next stage in the development of physics, a young scientist will come to the scientific laboratory, purposefully synthesize new materials and confirm the anharmonic model of superconductivity. "It is not for us to foresee, /How our word will resonate, - /And sympathy is given to us, /As grace is given to us ..." (F.Tyutchev).

Viktor YUSHANKHAI