Electronic structure and magnetism of Fe3−x_{3-x}Vx_{x}X (X = Si, Ga and Al) alloys by the KKR-CPA method

We present first principles charge- and spin-selfconsistent electronic structure computations on the Heusler-type disordered alloys Fe$_{3-x}$V$_{x}$X for three different metalloids X=(Si, Ga and Al). In these calculations we use the methodology based on the Korringa-Kohn- Rostoker formalism and the coherent-potential approximation (KKR-CPA), generalized to treat disorder in multi-component complex alloys.Comment: RevTeX, 17 pages, 15 figures, to appear in Phys. Rev. B on Nov. 15 199

Spin moment over 10-300 K and delocalization of magnetic electrons above the Verwey transition in magnetite

In order to probe the magnetic ground state, we have carried out temperature dependent magnetic Compton scattering experiments on an oriented single crystal of magnetite (Fe$_3$O$_4$), together with the corresponding first-principles band theory computations to gain insight into the measurements. An accurate value of the magnetic moment $\mu_S$ associated with unpaired spins is obtained directly over the temperature range of 10-300K. $\mu_S$ is found to be non-integral and to display an anomalous behavior with the direction of the external magnetic field near the Verwey transition. These results reveal how the magnetic properties enter the Verwey energy scale via spin-orbit coupling and the geometrical frustration of the spinel structure, even though the Curie temperature of magnetite is in excess of 800 K. The anisotropy of the magnetic Compton profiles increases through the Verwey temperature $T_v$ and indicates that magnetic electrons in the ground state of magnetite become delocalized on Fe B-sites above $T_v$.Comment: 5 pages, 5 figures, to appear in Journal of Physics and Chemistry of Solid

Detection and imaging of the oxygen deficiency in single crystalline YBa2_{\text{2}}Cu3_{\text{3}}O7−δ_{\text{7}-\delta} thin films using a positron beam

Single crystalline YBa$_{\text{2}}$Cu$_{\text{3}}$O$_{\text{7}-\delta}$ (YBCO) thin films were grown by pulsed laser deposition (PLD) in order to probe the oxygen deficiency $\delta$ using a mono-energetic positron beam. The sample set covered a large range of $\delta$ (0.191<$\delta$<0.791) yielding a variation of the critical temperature $T_{\text{c}}$ between 25 and 90\,K. We found a linear correlation between the Doppler broadening of the positron electron annihilation line and $\delta$ determined by X-ray diffraction (XRD). Both, the origin of the found correlation and the influence of metallic vacancies, were examined with the aid of ab-initio calculations that allowed us (i) to exclude the presence of Y vacancies and (ii) to ensure that positrons still probe $\delta$ despite the potential presence of Ba or Cu vacancies. In addition, by scanning with the positron beam the spatial variation of $\delta$ could be analyzed. It was found to fluctuate with a standard deviation of up to $0.079(5)$ within a single YBCO film

Temperature dependent orbital degree of freedom in a bilayer manganite by magnetic Compton scattering

We have measured temperature-dependent magnetic Compton profiles (MCPs) from a single crystal of La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$. The MCPs, which involved the scattering of circularly polarized x-rays, are in general related to the momentum density of all the unpaired spins in the system. Nevertheless, we show that when the x-ray scattering vector lies along the [110] direction, the number of magnetic electrons of a specific symmetry, i.e. $d$-electrons of $x^2-y^2$ symmetry, yield a distinct signature in the MCP, allowing us to monitor substantial changes in the occupancy of the $d_{x^2-y^2}$ states over the investigated temperature range of 5-200K. This study indicates that magnetic Compton scattering can provide a powerful window on the properties of specific magnetic electrons in complex materials.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let

Lindhard and RPA susceptibility computations in extended momentum space in electron doped cuprates

We present an approximation for efficient calculation of the Lindhard susceptibility $\chi^{L}(q,\omega)$ in a periodic system through the use of simple products of real space functions and the fast Fourier transform (FFT). The method is illustrated by providing $\chi^{L}(q,\omega)$ results for the electron doped cuprate Nd$_{2-x}$Ce$_{x}$CuO$_{4}$ extended over several Brillouin zones. These results are relevant for interpreting inelastic X-ray scattering spectra from cuprates.Comment: 6 pages, 6 figures, accepted in Physical Review

Direct observation of localization in the minority-spin-band electrons of magnetite below the Verwey temperature

Two-dimensional spin-uncompensated momentum density distributions, $\rho_{\rm s}^{2D}({\bf p})$s, were reconstructed in magnetite at 12K and 300K from several measured directional magnetic Compton profiles. Mechanical de-twinning was used to overcome severe twinning in the single crystal sample below the Verwey transition. The reconstructed $\rho_{\rm s}^{2D}({\bf p})$ in the first Brillouin zone changes from being negative at 300 K to positive at 12 K. This result provides the first clear evidence that electrons with low momenta in the minority spin bands in magnetite are localized below the Verwey transition temperature.Comment: 13 pages, 4 figures, accepted in Physical Review

A High-Resolution Compton Scattering Study of the Electron Momentum Density in Al

We report high-resolution Compton profiles (CP's) of Al along the three principal symmetry directions at a photon energy of 59.38 keV, together with corresponding highly accurate theoretical profiles obtained within the local-density approximation (LDA) based band-theory framework. A good accord between theory and experiment is found with respect to the overall shapes of the CP's, their first and second derivatives, as well as the anisotropies in the CP's defined as differences between pairs of various CP's. There are however discrepancies in that, in comparison to the LDA predictions, the measured profiles are lower at low momenta, show a Fermi cutoff which is broader, and display a tail which is higher at momenta above the Fermi momentum. A number of simple model calculations are carried out in order to gain insight into the nature of the underlying 3D momentum density in Al, and the role of the Fermi surface in inducing fine structure in the CP's. The present results when compared with those on Li show clearly that the size of discrepancies between theoretical and experimental CP's is markedly smaller in Al than in Li. This indicates that, with increasing electron density, the conventional picture of the electron gas becomes more representative of the momentum density and that shortcomings of the LDA framework in describing the electron correlation effects become less important.Comment: 7 pages, 6 figures, regular articl

Proposal to determine the Fermi-surface topology of a doped iron-based superconductor using bulk-sensitive Fourier-transform Compton scattering

We have carried out first-principles calculations of the Compton scattering spectra to demonstrate that the filling of the hole Fermi surface in LaO$_{1-x}$F$_{x}$FeAs produces a distinct signature in the Fourier transformed Compton spectrum when the momentum transfer vector lies along the [100] direction. We thus show how the critical concentration $x_c$, where hole Fermi surface pieces are filled up and the superconductivity mediated by antiferromagnetic spin fluctuations is expected to be suppressed, can be obtained in a bulk-sensitive manner.Comment: 4 pages, 6 figures, accepted in Physical Review

Sarcopenia: a rising geriatric giant:health and economic outcomes of community-dwelling older adults with sarcopenia

Future government policy assumes that elderly people will live at home independently for as long as possible instead of relying on care services. This requires a proactive policy to prevent vulnerability. Elderly people often suffer from poor muscle mass and function, known as sarcopenia, which can lead to vulnerability and an increased fall risk. This dissertation examines the frequency and consequences of sarcopenia in the elderly. Sarcopenia was present in 12% of elderly people who live at home alone, 42% of elderly people who receive home care and 59% of nursing home residents. Elderly patients with sarcopenia experienced greater limitations in their daily activities, lower quality of life and higher healthcare costs. These results confirm the link between sarcopenia and negative consequences for health and healthcare costs. Detecting sarcopenia at an early stage is therefore important for initiating targeted diet and exercise measures

Role of Oxygen Electrons in the Metal-Insulator Transition in the Magnetoresistive Oxide La2−2x_{2-2x}Sr1+2x_{1+2x}Mn2_2O7_7 Probed by Compton Scattering

We have studied the [100]-[110] anisotropy of the Compton profile in the bilayer manganite. Quantitative agreement is found between theory and experiment with respect to the anisotropy in the two metallic phases (i.e. the low temperature ferromagnetic and the colossal magnetoresistant phase under a magnetic field of 7 T). Robust signatures of the metal-insulator transition are identified in the momentum density for the paramagnetic phase above the Curie temperature. We interpret our results as providing direct evidence for the transition from the metallic-like to the admixed ionic-covalent bonding accompanying the magnetic transition. The number of electrons involved in this phase transition is estimated from the area enclosed by the Compton profile anisotropy differences. Our study demonstrates the sensitivity of the Compton scattering technique for identifying the number and type of electrons involved in the metal-insulator transition.Comment: 4 pages, 4 figures, accepted for publication in Physical Review Letter