Orbital ordering in charge transfer insulators

We discuss a new mechanism of orbital ordering, which in charge transfer insulators is more important than the usual exchange interactions and which can make the very type of the ground state of a charge transfer insulator, i.e., its orbital and magnetic ordering, different from that of a Mott-Hubbard insulator. This purely electronic mechanism allows us to explain why orbitals in Jahn-Teller materials typically order at higher temperatures than spins, and to understand the type of orbital ordering in a number of materials, e.g., K2CuF4, without invoking the electron-lattice interaction.     

Isotopically driven transitions in LaPrCaMnO system

The effect of oxygen isotope substitution on transport and magnetic properties is studied for (La_1−yPr_y)_0.7Ca_0.3MnO₃ ceramics and thin films. The temperature dependence of electrical resistivity reveals the metal–insulator transition induced by the ¹⁶O→ ¹⁸O substitution at y=0.75 (ceramics) and y=0.5 (films on LaAlO₃). Neutron powder diffraction at y=0.75 demonstrates a striking difference between magnetic structures in the samples with ¹⁶O and ¹⁸O. Measurements of AC magnetic susceptibility allow us to analyze the magnetic phase diagram and its changes related to the isotope substitution.     

Phase separation, percolation and giant isotope effect in manganites

Phase separation and a tendency to form inhomogeneous structures seems to be a generic property of systems with strongly correlated electrons. After shortly summarising the existing theoretical results in this direction, I concentrate on the phenomena in doped manganites. I discuss general theoretical results on the phase separation at small doping and close to the doping x=0.5. The “global” phase diagram in this region is constructed. These general results are illustrated on the example of the particular system with rich and complicated properties — (LaPr)_1−xCa_xMnO₃, in which there exist a ferromagnetic metallic (FM) phase and a charge ordered (CO) insulating one. The experimental situation in this system is discussed and the interpretation is given in the framework of the model with competition of FM and CO, and the indications of phase separation and percolative nature of this system are given. The giant isotope effect observed in this situation is shortly discussed.     

Phase separation in systems with charge ordering

A simple model of charge ordering is considered. It is explicitly shown that at any deviation from half-filling (n≠1/2), the system is unstable with respect to the phase separation into the charge ordered regions with n=1/2 and the metallic regions with a smaller electron or hole density. A possible structure of this phase-separated state (metallic droplets in a charge ordered matrix) is discussed. The model is extended to account for the strong Hund-rule onsite coupling and the weaker intersite antiferromagnetic exchange. The analysis of this extended model allows us to determine the magnetic structure of the phase-separated state and to reveal the characteristic features of the manganites and other substances with charge ordering.     

Classical dimers and dimerized superstructure in an orbitally degenerate honeycomb antiferromagnet

We discuss the ground state of the spin-orbital model for spin-one ions with partially filled t_{2g} levels on a honeycomb lattice. We find that the orbital degrees of freedom induce a spontaneous dimerization of spins and drive them into nonmagnetic manifold spanned by hard-core dimer (spin-singlet) coverings of the lattice. The cooperative "dimer Jahn-Teller" effect is introduced through a magnetoelastic coupling and is shown to lift the orientational degeneracy of dimers leading to a peculiar valence bond crystal pattern. The present theory provides a theoretical explanation of nonmagnetic dimerized superstructure experimentally seen in Li2RuO3 compound at low temperatures.     

Orbital-assisted metal-insulator transition in VO2

We found direct experimental evidence for an orbital switching in the V 3d states across the metal-insulator transition in VO2. We have used soft-x-ray absorption spectroscopy at the V L2,3 edges as a sensitive local probe and have determined quantitatively the orbital polarizations. These results strongly suggest that, in going from the metallic to the insulating state, the orbital occupation changes in a manner that charge fluctuations and effective bandwidths are reduced, that the system becomes more one dimensional and more susceptible to a Peierls-like transition, and that the required massive orbital switching can only be made if the system is close to a Mott insulating regime.     

Frustrated spin model as a hard-sphere liquid

We show that one-dimensional topological objects (kinks) are natural degrees of freedom for an antiferromagnetic Ising model on a triangular lattice. Its ground states and the coexistence of spin ordering with an extensive zero-temperature entropy can easily be understood in terms of kinks forming a hard-sphere liquid. Using this picture we explain effects of quantum spin dynamics on that frustrated model, which we also study numerically.     

Orbital and magnetic structure of two-dimensional ferromagnets with Jahn-Teller ions

The cooperative ordering of degenerate d-orbitals in two-dimensional ferromagnets of K₂CuF₄ type is considered by employing both the usual Jahn- Teller mechanism and the superexchange one. The orbital structure obtained differs from that usually assumed for K₂CuF₄; the latter is shown to be incompatible with the observed ferromagnetic ordering.