Orbital physics in transition metal compounds: new trends

TL;DR

This review explores recent trends in orbital physics within transition metal compounds, focusing on phenomena like effective dimensionality reduction, orbital-selective effects, and spin-orbit interactions, highlighting their roles in novel quantum states.

Contribution

It provides a comprehensive overview of modern phenomena related to orbital degrees of freedom, excluding traditional models, emphasizing new effects such as orbital-driven Peierls, orbital-selective Mott transitions, and spin-orbit coupling impacts.

Findings

Orbital degrees of freedom can induce effective dimensionality reduction.

Orbital-selective Mott transitions lead to partial localization of electrons.

Spin-orbit coupling influences the emergence of novel quantum states.

Abstract

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 the 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 a 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 orbitals 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 the 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.