Role Of Orbitals Research Articles

Role of Orbitals in Valence Transitions

Role of Orbitals in Valence Transitions

Effects beyond the [formula omitted]-model in the low-energy physics of cuprates

The role of orbitals beyond the Cu 3d O 2p σ set employed in the widely used extended Hubbard model is considered with respect to low-energy charge excitations in cuprates. We focus on the description of the EELS and optical conductivity data of Sr 2CuO 3 and Sr 2CuO 2Cl 2 with respect to O 2 p π states. Consequences for the orbital character of doped holes are discussed. The achieved novel description of the EELS data in transverse chain response is robust with respect to the addition of Cu 3 d xy and 4s orbitals.

Role of Orbitals in the Physics of Correlated Electron Systems

Rich properties of systems with strongly correlated electrons, such as transition metal (TM) oxides, is largely connected with an interplay of different degrees of freedom in them: charge, spin, orbital ones as well as crystal lattice. Specific and often very important role is played by orbital degrees of freedom. They can lead to a formation of different superstructures (an orbital ordering) which are associated with particular types of structural phase transitions — one of very few examples where the microscopic origin of these transitions is really known; they largely determine the character of magnetic exchange and the type of magnetic ordering; they can also strongly influence many other important phenomena such as insulator-metal transitions (IMT), etc.In this comment I will try to shortly summarize the main concepts and discuss some of the well-known manifestations of orbital degrees of freedom, but will mostly concentrate on a more recent development in this field. More traditional material is covered in several review articles [Kugel K I and Khomskii D I 1982 Sov. Phys. Usp. 25 231, Tokura Y and Nagaosa N 2000 Science 288 462, van den Brink J et al 2002 Colossal Magnetoresistive Manganites (Kluwer)]. Although I tried to cover the main new development in this area, the choice of topics of course is influenced by my own interests; other people probably would have stressed other parts of this big field.

Role of orbitals in manganese oxides—ordering and fluctuation

We study the manganese oxides from the viewpoint of the strongly correlated doped Mott insulator. The magnetic ordering and the charge transport are governed by the orbital degrees of freedom, and their dimensionality is controlled by the anisotropic transfer integrals between the e g -orbitals. As x increases, the magnetic structure is predicted to change as A→F→A→C→G (F, ferromagnet; A, layered antiferromagnet; C, rod-type antiferromagnet; G, usual antiferromagnet), in agreement with experiments. Especially the orbital is aligned as d x 2− y 2 in the metallic A state, which explains the quasi 2D transport and no canting of the spin observed experimentally. Next we discuss the ferromagnetic state without the orbital ordering due to the quantum fluctuation. Here the interplay between the electron repulsion U and the Jahn–Teller electron–phonon interaction E LR is studied with a large d model. In addition to this strong correlation, we propose that the dynamical phase separation could explain the specific heat as well as the various anomalous physical properties, e.g. resistivity, photo-emission, etc.

On the special role of symmetric periodic orbits in a chaotic system

After early work of Hénon it has become folk knowledge that symmetric periodic orbits are of particular importance. We reinforce this belief by additional studies and we further find that invariant closed symplectic submanifolds caused by discrete symmetries prove to be an important complement to the long known role of the orbits. The latter have particular importance in semi-classics. Based on the structural stability of hyperbolic horseshoes we give an argument that opens an avenue to the understanding of these facts.