Magnetic ground state of coupled edge-sharing CuO_2 spin-chains

By means of density functional theory, we investigate the magnetic ground state of edge-sharing CuO_2 spin-chains, as found in the (La,Ca,Sr)_14Cu_24O_41system, for instance. Our data rely on spin-polarized electronic structure calculations including onsite interaction (LDA+U) and an effective model for the interchain coupling. Strong doping dependence of the magnetic order is characteristic for edge-sharing CuO_2 spin-chains. We determine the ground state magnetic structure as function of the spin-chain filling and quantify the competing exchange interactions.Comment: 5 pages, 2 figures, 3 tables, accepted by Phys. Rev. Let

Ferromagnetic insulating state in tensile-strained LaCoO3_3 thin films

With local density approximation + Hubbard $U$ (LDA+$U$) calculations, we show that the ferromagnetic (FM) insulating state observed in tensile-strained LaCoO$_3$ epitaxial thin films is most likely a mixture of low-spin (LS) and high-spin (HS) Co, namely, a HS/LS mixture state. Compared with other FM states, including the intermediate-spin (IS) state (\textit{metallic} within LDA+$U$), which consists of IS Co only, and the insulating IS/LS mixture state, the HS/LS state is the most favorable one. The FM order in HS/LS state is stabilized via the superexchange interactions between adjacent LS and HS Co. We also show that Co spin state can be identified by measuring the electric field gradient (EFG) at Co nucleus via nuclear magnetic resonance (NMR) spectroscopy

Self-assembled Pt nanowires on Ge(001): Relaxation effects

Absorption of Pt on the Ge(001) surface results in stable self-organized Pt nanowires, extending over some hundred nanometers. Based on band structure calculations within density functional theory and the generalized gradient approximation, the structural relaxation of the Ge--Pt surface is investigated. The surface reconstruction pattern obtained agrees well with findings from scanning tunneling microscopy. In particular, strong Pt--Pt dimerization is characteristical for the nanowires. The surface electronic structure is significantly perturbed due to Ge--Pt interaction, which induces remarkable shifts of Ge states towards the Fermi energy. As a consequence, the topmost Ge layers are subject to a metal-insulator transition.Comment: 4 pages, 2 figures, final version accepted by Europhys. Lett., minor modifications of the tex

Prediction of axial-flow instabilities in a turbojet engine by use of a multistage compressor simulation on the digital computer

A method of estimating the undistorted stall line for an axial-flow compressor by using the digital computer is presented. The method involves linearization of nonlinear dynamic equations about an operating point on a speed line, and then application of the first method of Lyapunov to determine the stability of the nonlinear system from the stability of the linear system. The method is applied to a simulation of the J85 compressor, which utilizes stage stacking and lumped volume techniques for the interstage regions to simulate steady-state and dynamic compressor performance. The stability boundary predicted by the digital simulation compares quite well with the stall line predicted by a dynamic simulation of the J85 compressor programmed on the analog computer. Since previous studies have shown that the analog-predicted stall line agrees well with the stall line of the compressor, the digital method presented is also a good means of estimating the stall line

Dynamic analysis of a photovoltaic power system with battery storage capability

A photovolataic power system with a battery storage capability is analyzed. A dual battery current control concept is proposed, which enables the battery to either supply or accept power depending upon system environment and load conditions. A simulation of the power system, including the battery current control, is developed and evaluated. The evaulation demonstrate the visbility of the battery control concept of switch the battery from a charge to discharge mode and back as required by load and environmental conditions. An acceptable system operation is demonstrated over the entire insolation range. Additionally, system sensitivity, bandwidth, and damping characteristics of the battery control are shown to be acceptable for a projected hardware implementation

Implementation of screened hybrid functionals based on the Yukawa potential within the LAPW basis set

The implementation of screened hybrid functionals into the WIEN2k code, which is based on the LAPW basis set, is reported. The Hartree-Fock exchange energy and potential are screened by means of the Yukawa potential as proposed by Bylander and Kleinman [Phys. Rev. B 41, 7868 (1990)] for the calculation of the electronic structure of solids with the screened-exchange local density approximation. Details of the formalism, which is based on the method of Massidda, Posternak, and Baldereschi [Phys. Rev. B 48, 5058 (1993)] for the unscreened Hartree-Fock exchange are given. The results for the transition-energy and structural properties of several test cases are presented. Results of calculations of the Cu electric-field gradient in Cu2O are also presented, and it is shown that the hybrid functionals are much more accurate than the standard local-density or generalized gradient approximations

SECOR observations in the Pacific

Geometric adjustment technique for Pacific SECOR observations based on least squares metho

Electronic structure and magnetic properties of epitaxial FeRh(001) ultra-thin films on W(100)

Epitaxial FeRh(100) films (CsCl structure, $\sim 10\ ML\$ thick), prepared {\it in-situ} on a W(100) single crystal substrate, have been investigated via valence band and core level photoemission. The presence of the temperature-induced, first-order, antiferromagnetic to ferromagnetic (AF$\rightarrow$ FM) transition in these films has been verified via linear dichroism in photoemission from the Fe 3$p$ levels. Core level spectra indicate a large moment on the Fe atom, practically unchanged in the FM and AF phases. Judging from the valence band spectra, the metamagnetic transition takes place without substantial modification of the electronic structure. In the FM phase, the spin-resolved spectra compare satisfactorily to the calculated spin-polarized bulk band structure.Comment: 7 pages, 5 figure

Are the renormalized band widths in TTF-TCNQ of structural or electronic origin? - An angular dependent NEXAFS study

We have performed angle-dependent near-edge x-ray absorption fine structure measurements in the Auger electron yield mode on the correlated quasi-one-dimensional organic conductor TTF-TCNQ in order to determine the orientation of the molecules in the topmost surface layer. We find that the tilt angles of the molecules with respect to the one-dimensional axis are essentially the same as in the bulk. Thus we can rule out surface relaxation as the origin of the renormalized band widths which were inferred from the analysis of photoemission data within the one-dimensional Hubbard model. Thereby recent theoretical results are corroborated which invoke long-range Coulomb repulsion as alternative explanation to understand the spectral dispersions of TTF-TCNQ quantitatively within an extended Hubbard model.Comment: 6 pages, 5 figure