In the present review different effects related to the orbital degrees of freedom are discussed. Leaving aside such aspects as the superexchange mechanism of the cooperative Jahn--Teller distortions and different properties of "Kugel—Khomskii"-like models, we mostly concentrate on other phenomena, which are in the focus of modern condensed matter physics. After a general introduction, we start with the discussion of the concept of effective reduction of dimensionality due to orbital degrees of freedom and consider such phenomena as the orbitally-driven Peierls effect and the formation of small clusters of ions in the vicinity of the Mott transition, which behave like "molecules" embedded in a solid. The second large section is devoted to the orbital-selective effects such as the orbital-selective Mott transition and the suppression of magnetism due to the fact that part of the orbital start to form singlet molecular orbitals. At the end the rapidly growing field of the so-called "spin-orbit-dominated" transition metal compounds is briefly reviewed including such topics as the interplay between the spin-orbit coupling and Jahn—Teller effect, the formation of the spin-orbit driven Mott and Peierls states, the role of orbital degrees of freedom in generation of the Kitaev exchange coupling, and the singlet (excitonic) magnetism in 4d and 5d transition metal compounds.

Keywords: orbital ordering, spin-orbit coupling, transition-metal oxides PACS: 71.20.Be, 71.70.−d, 75.10.−b (all) DOI: 10.3367/UFNe.2017.08.038196 URL: https://ufn.ru/en/articles/2017/11/d/ 000424395100004 2-s2.0-85042164461 2017PhyU...60.1121S Citation: Streltsov S V, Khomskii D I "Orbital physics in transition metal compounds: new trends" Phys. Usp. 60 1121–1146 (2017) BibTexBibNote ® (generic)BibNote ® (RIS)MedlineRefWorks Received: 17th, July 2017, revised: 1st, August 2017, accepted: 16th, August 2017 Оригинал: Стрельцов С В, Хомский Д И «Орбитальная физика в соединениях переходных металлов: новые тенденции» УФН 187 1205–1235 (2017); DOI: 10.3367/UFNr.2017.08.038196