U. Schwingenschlögl

The vanadium Magnéli phases VnO2n‐1

To compare the metal-insulator transitions (MITs) of VO2 and V2O3 we analyze the relations between the structural and electronic properties of the vanadium Magneli phases. These materials set up the homologous series VnO2n-1 (3 ≤ n ≤ 9) and have crystal structures comprising typical dioxide-like and sesquioxide-like regions. As the MITs of the vanadium Magneli phases are accompanied by structural transformations, we are able to discuss the effects of characteristic changes in the local atomic environments. The systematic investigation of the transport properties is based on a new and unifying description of the crystal structures of the Magneli phases including VO2 and V2O3. Our results lead to a comprehensive understanding of the MITs in the Magneli class and shed new light on the role of particular electronic states for the MIT of V2O3.

Spectroscopic signatures of spin-charge separation in the quasi-one-dimensional organic conductor TTF-TCNQ.

The electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ is studied by angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant discrepancies to band theory. We demonstrate that the measured dispersions can be consistently mapped onto the one-dimensional Hubbard model at finite doping. This interpretation is further supported by a remarkable transfer of spectral weight as a function of temperature. The ARPES data thus show spectroscopic signatures of spin-charge separation on an energy scale of the conduction bandwidth.

Interface relaxation and electrostatic charge depletion in the oxide heterostructure LaAlO3/SrTiO3

Performing an analysis within density functional theory, we develop insight into the structural and electronic properties of the oxide heterostructure LaAlO3/SrTiO3. Electrostatic surface effects are decomposed from the internal lattice distortion in order to clarify their interplay. We first study the interface relaxation by a multi-layer system without surface, and the surface effects, separately, by a substrate-film system. While elongation of the TiO6 octahedra at the interface enhances the metallicity, reduction of the film thickness has the opposite effect due to a growing charge depletion. The interplay of these two effects, as reflected by the full lattice relaxation in the substrate-film system, however, strongly depends on the film thickness. An inversion of the TiO6 distortion pattern for films thinner than four LaAlO3 layers results in an insulating state.

Surface effects on oxide heterostructures

We report on surface effects on the electronic properties of interfaces in epitaxial LaAlO3/SrTiO3 heterostructures. Our results are based on first-principles electronic structure calculations for well-relaxed multilayer configurations, terminated by an ultrathin LaAlO3 surface layer. On varying the thickness of this layer, we find that the interface conduction states are subject to almost rigid band shifts due to a modified Fermi energy. Confirming experimental data, the electronic properties of heterointerfaces therefore can be tuned systematically by alterating the surface-interface distance. We expect that this mechanism is very general and applies to most oxide heterostructures.

Intrinsic doping at YBCO-metal interfaces: Quantitative results

Charge redistribution in high-Tc superconductors due to structural defects or interfaces is known to be crucial for electronic applications as the band structure is modified on a local scale. In order to investigate these effects in more detail, we address the normal-state properties of YBa2Cu3O7 (YBCO) in the vicinity of YBCO-metal interfaces by electronic structure calculations for well relaxed interface configurations. Our findings can be interpreted in terms of a band-bending mechanism complemented by local screening effects. We derive quantitative results for the intrinsic doping of the superconducting CuO2 planes due to the metal interface. In particular, the net charge transfer amounts to 0.13 electrons in favour of each intraplane copper site, which appears to be a typical value for interfaces of high-Tc superconductors, thus opening great possibilities for a systematic optimization of wires and tapes from high-Tc materials.

Seat Biases in Proportional Representation Systems with Thresholds

In proportional representation systems, apportionment methods are used to convert the number of votes of a party into the number of seats allocated to this party. An interesting characteristic of any such method are the seat biases, that is, the expected differences between the actual seat allocation and the ideal share of seats, separately for each party, when parties are ordered from largest to smallest. For electoral systems with a threshold, that is, with a minimum percentage of votes that parties must reach in order to be eligible to participate in the apportionment process, we show that seat biases decrease from their maximum to zero, as the threshold increases from zero to its maximum, and that all seat biases decrease linearly.

Exponential decay of relaxation effects at LaAlO3/SrTiO3 heterointerfaces

Abstract We study the decay of interface induced structural and electronic relaxation effects in epitaxial LaAlO 3 /SrTiO 3 heterostructures. The results are based on first-principles band structure calculations for a multilayer configuration with an ultrathin LaAlO 3 layer sandwiched between bulk-like SrTiO 3 layers. We carry out the structure optimization for the heterointerface and investigate the electronic states of the conducting interface layer, which is found to extend over two SrTiO 3 unit cells. The decay of atomic displacements is analyzed as a function of the distance to the interface, and the resulting exponential law is evaluated quantitatively.

Covalent bonding and hybridization effects in the corundum-type transition-metal oxides V2O3 and Ti2O3

The electronic structure of the corundum-type transition-metal oxides V2O3 and Ti2O3 is studied by means of the augmented spherical wave method, based on density-functional theory and the local density approximation. Comparing the results for the vanadate and the titanate allows us to understand the peculiar shape of the metal 3d a1g density of states, which is present in both compounds. The a1g states are subject to pronounced bonding-antibonding splitting due to metal-metal overlap along the c-axis of the corundum structure. However, the corresponding partial density of states is strongly asymmetric with considerably more weight on the high-energy branch. We argue that this asymmetry is due to an unexpected broadening of the bonding a1g states, which is caused by hybridization with the egπ bands. In contrast, the antibonding a1g states display no such hybridization and form a sharp peak. Our results shed new light on the role of the a1g orbitals for the metal-insulator transitions of V2O3. In particular, due to a1g-egπ hybridization, an interpretation in terms of molecular orbital singlet states on the metal-metal pairs along the c-axis is not an adequate description.

One-electron singular branch lines of the Hubbard chain

The momentum and energy dependence of the weight distribution in the vicinity of the one-electron spectral-function singular branch lines of the 1D Hubbard model is studied for all values of the electronic density and on-site repulsion U. To achieve this goal, we use the recently introduced pseudofermion dynamical theory. Our predictions agree quantitatively for the whole momentum and energy bandwidth with the peak dispersions observed by angle-resolved photoelectron spectroscopy in the quasi-1D organic conductor TTF-TCNQ.

Electronic structure of Li-inserted V6O13 battery cathodes: Rigid band behavior and effects of hybridization

Resonant soft x-ray emission (SXE) spectroscopy was used to study the electronic structure of LixV6O13 battery cathodes. We observe that the V 3d-bands of V6O13 exhibit a rather rigid behavior. Upon lithiation, electrons enter the top of the valence band and add intensity to the corresponding part of the V L-emission spectrum without significantly distorting the lower lying bands. We perform ab initio calculations which are in good agreement with the experimental results. Moreover, we find that lithiation leads to an overall decrease of the V 3d–O 2p hybridization. In contrast to x-ray diffraction, it is possible to study charge transfer effects in Li-batteries with SXE spectroscopy over the entire lithiation range.