Preface

Preface This issue of the Journal of Physics, Conference Series, reports the main contributions at the 25 th International Conference on the Jahn-Teller Effect held online between 14 th and 18 th May, 2023. The tradition of biannual symposia on the Jahn-Teller effect, beginning from the first one in Bad Honnef in 1976, was interrupted by the COVID pandemic in 2020. Seeing the participants’ health concerns, the International Steering Committee decided to have the 25 th Jahn-Teller conference virtually on Zoom. Hosted by York University in Toronto, Canada, its Department of Chemistry made all the corresponding arrangements. The meeting addressed diverse conceptual, spectroscopic, and structural consequences of the vibronic coupling in polyatomic molecules, nanocrystals, impurity centers in solids, liquids, amorphous solids, and crystals. Exemplified by the Jahn-Teller and pseudo-Jahn-Teller effects, the coupling between electronic states and symmetry-breaking modes influences a variety of molecular and solid-state properties, including spectroscopy, reactivity, photochemistry, local and extended crystal structure, ferroelectricity and multiferroicity, charge and energy transport, superconductivity, molecular magnetism, qubits structure in quantum computers, and other applied fields. Eighty nine participants from fifty research institutes of seventeen countries virtually attended the meeting. Topics raised in thirty nine oral seminars and nineteen poster presentations led to vivid discussions. The conference provided opportunities to less experienced scientists for discussing the most exciting issues with the authorities that shape the current status of the theory of vibronic interactions and its experimental manifestations. All oral presentations were live-recorded and kept available on the conference page: https://jahnteller2023-yorku.ca/index.html. The sixteen papers in this issue fall into four major categories: I. General theory. In a semi-review, Isaac Bersuker explores a novel theoretical concept of the pseudo-Jahn-Teller nature of the energy barriers in chemical transformations, opening uncharted catalytic actions. Ketan Sharma et al. describe the computational perspectives of the two complementary software programs designed to evaluate spin-vibronic wave functions. Maley and Mawhinney developed a computationally robust density functional theory approach to assess the impact of Hartree-Fock exchange on critical parameters of the pseudo-Jahn-Teller effect in molecules with heavy-group carbon analogs. II. Vibronic coupling in polyatomic molecules. In this collection of articles, the Jahn-Teller- and pseudo-Jahn-Teller-induced symmetry breakings in particular molecules are investigated: population transfer from high- to low-symmetry electronic states in benzene radical cation by Hannibal et al., potential barrier for proton transfer in a series of small systems with inversion symmetry by Gorinchoy et al., group-theoretical approach to geometry optimization by finding symmetry descend path for large molecules by Breza, and computation of equilibrium geometry and dipole moment in fullerene cation C 70 + by Nemes. III. Impurity systems. In this section, investigation of the Jahn-Teller effect, including experimental evaluation of vibronic constants and Jahn-Teller stabilization energy, using the temperature dependence of ultrasonic attenuation is reported in two papers of the Averkiev-Gudkov team. There is also a quite unusual study of coupling between electronic states and excited long-wave acoustic phonons as manifested by the zero-phonon line in optical absorption of the Dy atom in liquid 4 He, which is presented by Hyzhnyakov et al. IV. Vibronic coupling in condensed-matter physics. In this section, the first four papers discuss the role of the Jahn-Teller symmetry breaking in magnetic exchange and super-exchange coupling. The last two papers are pretty unusual in the Jahn-Teller community. Archilla et al. discuss the role of vibronic coupling in forming the so-called crowdions and the related phenomenon of hyperconductivity in layered silicates bombarded with alpha particles. Hyzhnyakov considers the zero-phonon type of quantum diffusion in the solid and liquid phases enhanced by vibronic coupling to long-wave acoustic phonons. In the present issue, the scientific community interested in the Jahn-Teller effect and other manifestations of symmetry-breaking effects will find a reference source of essential facts and important information about the current status of this diverse field, which will inspire future studies. We want to thank the members of the International Steering Committee on the Jahn-Teller Effect for their initiative and support, the conference secretary Dr. Ekadashi Pradhan for the perfect administration of the conference website, and the York University, specifically, the Department of Chemistry, the Faculty of Science, and the Office of the Vice-President Research and Innovation, for the hospitality and financial supports of the 25 th International Conference on the Jahn-Teller Effect. Isaac B. Bersuker, The University of Texas at Austin, Austin, USA Victor Polinger, University of Washington, Seattle, USA Tao Zeng, York University, Toronto, Canada List of Committees is available in this pdf.

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