Professor Ihor Stasyuk, a prominent scientist and lecturer passed away on 16 September 2019. He significantly contributed with remarkable results to the quantum many-body physics and to the theoretical physics of the solid states. In his scientific carrier, he was employed in the Department of Solid State Theory and at the Department of Theoretical Physics of the Ivan Franko State University of Lviv and also the Institute of Applied Problems of Mechanics and Mathematics of the Academy of Sciences of Ukraine. He was a Corresponding Member of the National Academy of Sciences of Ukraine.
Professor Ihor Stasyuk’s achievements are connected with the field of theory of condensed matter physics. He developed mathematical methods of the theory of multilevel systems and fermionic systems with strong short-range correlations and physical properties of crystals undergoing phase transitions. Professor Stasyuk investigated the effect of the Hubbard-type correlations and anharmonicity in the theory of high-temperature superconductivity and is co-author of kinematic mechanisms of superconducting pairing in the Hubbard model. He developed a microscopic theory of the influence of fields like hydrostatic and uniaxial pressure, electric field and single-ion anisotropy on the properties and thermodynamics of ferroelectric and Jahn–Teller crystals. He also worked on the protonic and ionic transport in superionic phases, and on the theory of phase transitions and Bose–Einstein condensation in optical lattices. Professor Stasyuk published over 700 scientific articles and contributions. It is worth noting that he introduced the site elementary excitation operators, predecessors of the well-known Hubbard operators.
His scientific and public activities had been widely acknowledged and awarded. With this obituary, we would like to mention that Professor Stasyuk for many years was the member of the Editorial Board of our journal Phase Transitions. He served as a very thorough reviewer and also acted as guest editors of the special issue on the Fifth Ukrainian-Polish-Lithuanian Meeting on Physics of Ferroelectrics in 2018. His great knowledge and experience were extremely helpful in the case of papers dealing with theoretical physics and submitted to Phase Transitions. With his passing, we lost extraordinary specialist and referee in the field of theoretical physics. We have to say that we were really lucky that we met him and could co-operate with him. It is a great pity that this is the last time we can sincerely thank Professor Stasyuk for helping to shape our journal.