Orbital physics in transition metal compounds: new trends

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.

Russian version of Physics-Uspekhi is available; https://ufn.ru/ru/articles/2017/11/d/

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• Lattice dynamics and spontaneous magnetodielectric effect in ilmenite CoTiO${}_3$
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• Trigonal Symmetry Breaking and its Electronic Effects in Two-Dimensional Dihalides and Trihalides
• Highly frustrated magnetism in relativistic ${\mathbf{d}}^4$ Mott insulators: Bosonic analog of Kitaev honeycomb model
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• Nearly itinerant electronic groundstate in the intercalated honeycomb iridate Ag${}_3$LiIr${}_2$O${}_6$
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• Realization of the orbital-selective Mott state at the molecular level in Ba${}_3$LaRu${}_2$O${}_9$
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• Dual Orbital Degeneracy Lifting in a Strongly Correlated Electron System
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• Na9Bi5Os3O24: A Unique Diamagnetic Oxide Featuring a Pronouncedly Jahn-Teller Compressed Octahedral Coordination of Osmium(VI)
• Partial molecular orbitals in face-sharing 3$d$ manganese trimer: Comparative studies on Ba${}_4$TaMn${}_3$O${}_{12}$ and Ba${}_4$NbMn${}_3$O${}_{12}$
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• Raman spectroscopy of the low dimensional antiferromagnet with large Neel temperature SrRu2O6
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• Spin-orbital correlations from complex orbital order in MgV${}_2$O${}_4$
• Can the highly symmetric SU(4) spin-orbital model be realized in alpha-ZrCl3?
• Quantum entanglement, local indicators and effect of external fields in the Kugel-Khomskii model
• Multipeak quasielastic light scattering and high-frequency electronic excitations in honeycomb Li${}_2$RuO${}_3$
• Magnetic ground state of the dimer-based hexagonal perovskite Ba${}_3$ZnRu${}_2$O${}_9$
• Dimerization in honeycomb Na2RuO3 under pressure: a DFT study
• Magnetic oxides
• Spin-orbit coupling induced orbital entanglement in a three-band Hubbard model
• Magnetic phases for two $t_{2g}$ holes with spin-orbit coupling and crystal field
• Jahn-Teller states mixed by spin-orbit coupling in an electromagnetic field
• Inelastic light scattering in the spin cluster Mott insulator Cu${}_2$OSeO${}_3$
• Structural stability of CuAl${}_2$O${}_4$ under pressure
• Unusual ferrimagnetic ground state in rhenium ferrite
• Orthorhombic and triclinic modifications of the arsenate Cu4O(AsO4)2 and isotypic phosphates Cu4O(PO4)2: strongly frustrated antiferromagnetics
• Coupled dynamics of long-range and internal spin cluster order in Cu${}_2$OSeO${}_3$
• Doping effects on the valence bond solid of Li${}_2$RuO${}_3$ with Mn substitution
• Crystal chemistry criteria of the existence of spin liquids on the kagome lattice
• Spin-orbital liquid in Ba${}_3$CuSb${}_2$O${}_9$ stabilized by oxygen holes
• A Novel ground state in a new dimer iridate Ba13Ir6O30 with Ir6+(5d3) Ions
• Breakdown of broken-symmetry approach to exchange interaction
• A primer on Kitaev Model: Basic aspects, material realization, and recent experiments
• Ground-State Properties of the $t$-$J$ Model for the CuO Double-Chain Structure
• Orthorhombic distortion drives orbital ordering in an antiferromagnetic 3$d^1$ Mott insulator
• Mechanism of ferromagnetic ordering of the Mn chains in CaMnGe${}_2$O${}_6$ clinopyroxene
• Dipolar and quadrupolar correlations in the $5d^2$ Re-based double perovskites Ba${}_2$YReO${}_6$ and Ba${}_2$ScReO${}_6$
• Phase separation near the charge neutrality point in FeSe${}_{1-x}$Te${}_x$ crystals with x $<$ 0.15
• Orbital disorder and ordering in NaTiSi2O6: 29Si and 23Na NMR Study
• Ferromagnetic Peierls insulator state in $A$Mg${}_4$Mn${}_6$O${}_{15}$ ($A$ = K, Rb, Cs)
• Theory of unconventional magnetism in a Cu-based kagome metal
• Orbital Mott transition in two dimensional Pyrochlore lattice
• NMR study of the spin correlations in the $S=1$ armchair chain Ni${}_2$NbBO${}_6$
• Quantum magnetic phase transitions in a Kugel-Khomskii model including spin-orbit coupling
• Importance of the many-body effects for structural properties of the novel iron oxide: Fe${}_2$O
• Energy Eigenstates of Electrons, Magnons and Phonons in Fe${}_3$O${}_4$ (magnetite), MnFe${}_2$O${}_4$ (jacobsite), and mixed Mn-Zn ferrites