Unusual valence, negative charge-transfer gaps and self-doping in transition-metal compounds

In this paper I discuss the electronic structure and properties of a specific, rather unconventional class of transition metal (TM) compounds, e.g. TM oxides, which formally have unusually high values of the oxidation state, or valence, of TM. In contrast to the typical situation, in this case the charge-transfer gap (excitation energy for the transfer of electrons from the ligands to the TM) is very small and may even become negative. As a result a profound modification of an electronic structure and of all the properties may take place: there appear holes in the oxygen p-bands (``self-doping''); the material may become the metal of a specific type; there may occur insulator--metal transitions of a specific type; magnetic properties may be quite different from the ones expected normally; the character of elementary excitations may change drastically. I give general discussion of such situation and consider several examples of corresponding systems and their specific properties.Comment: 13 pages, 7 figure

Magnetic monopoles and unusual transport effects in magnetoelectrics

It is argued that in magnetoelectrics with diagonal magnetoelectric coupling there should be a monopole-like distribution of magnetization around electric charge. It may lead to nontrivial transport properties of such materials, to understand which the picture of magnetic monopoles attached to electric charges may be very useful.Comment: 6 pages, 4 fugure

Momentum distribution of itinerant electrons in the one-dimensional Falicov-Kimball model

The momentum distribution $n_k$ of itinerant electrons in the one-dimensional Falicov-Kimball model is calculated for various ground-state phases. In particular, we examine the periodic phases with period two, three and four (that are ground-states for all Coulomb interactions) as well as the phase separated states (that are ground states for small Coulomb interactions). For all periodic phases examined the momentum distribution is a smooth function of $k$ with no sign of any discontinuity or singular behavior at the Fermi surface $k=k_F$. An unusual behavior of $n_k$ (a local maximum) is found at $k=3k_F$ for electron concentrations outside half-filling. For the phase separated ground states the momentum distribution $n_k$ exhibits discontinuity at $k=k_0 < k_F$. This behavior is interpreted in terms of a Fermi liquid.Comment: 17 pages, 6 figures, late

Covalent bonds against magnetism in transition metal compounds

Magnetism in transition metal compounds is usually considered starting from a description of isolated ions, as exact as possible, and treating their (exchange) interaction at a later stage. We show that this standard approach may break down in many cases, especially in $4d$ and $5d$ compounds. We argue that there is an important intersite effect -- an orbital-selective formation of covalent metal-metal bonds, which leads to an "exclusion" of corresponding electrons from the magnetic subsystem, and thus strongly affects magnetic properties of the system. This effect is especially prominent for noninteger electron number, when it results in suppression of the famous double exchange, the main mechanism of ferromagnetism in transition metal compounds. We study this novel mechanism analytically and numerically and show that it explains magnetic properties of not only several $4d-5d$ materials, including Nb$_2$O$_2$F$_3$ and Ba$_5$AlIr$_2$O$_{11}$, but can also be operative in $3d$ transition metal oxides, e.g. in CrO$_2$ under pressure. We also discuss the role of spin-orbit coupling on the competition between covalency and magnetism. Our results demonstrate that strong intersite coupling may invalidate the standard single-site starting point for considering magnetism, and can lead to a qualitatively new behaviour

Jahn-Teller distortions and charge, orbital and magnetic orders in NaMn7O12

With the use of the band structure calculations we demonstrate that previously reported [Nat. Materials {\bf 3}, 48 (2004)] experimental crystal and magnetic structures for NaMn$_7$O$_{12}$ are inconsistent with each other. The optimization of the crystal lattice allows us to predict a new crystal structure for the low temperature phase, which is qualitatively different from the one presented before. The AFM-CE type of the magnetic order stabilizes the structure with the elongated, not compressed Mn$^{3+}_B$O$_6$ octahedra, striking NaMn$_7$O$_{12}$ out of the list of the anomalous Jahn-Teller systems. The orbital correlations were shown to exist even in the cubic phase, while the charge order appears only in the low temperature distorted phase.Comment: 5 page

Orbital ordering in the ferromagnetic insulator Cs2_2AgF4_4 from first principles

We found, using density-functional theory calculations within the generalized gradient approximation, that Cs$_2$AgF$_4$ is stabilized in the insulating orthorhombic phase rather than in the metallic tetragonal phase. The lattice distortion present in the orthorhombic phase corresponds to the $x^2-z^2$/$y^2-z^2$ hole-orbital ordering of the Ag$^{2+}$ $4d^9$ ions, and this orbital ordering leads to the observed ferromagnetism, as confirmed by the present total-energy calculations. This picture holds in the presence of moderate 4d-electron correlation. The results are compared with the picture of ferromagnetism based on the metallic tetragonal phase.Comment: 5 pages, 4 figures, 1 table; a few energy/moment entries in Table I are corrected due to a proper treatment of the Ag 4s semicore stat