arXiv subjects

Christoph Hugenschmidt

Publications and source records attributed to Christoph Hugenschmidt.

24 records · Page 2Linked to original sources

arXiv2015

Spin-resolved Fermi surface of the localized ferromagnetic Heusler compound Cu$_2$MnAl measured with spin-polarized positron annihilation

We determined the bulk electronic structure in the prototypical Heusler compound Cu$_2$MnAl by measuring the Angular Correlation of Annihilation Radiation (2D-ACAR) using spin-polarized positrons. To this end, a new algorithm for reconstructing 3D densities from projections is introduced that allows us to corroborate the excellent agreement between our electronic structure calculations and the experimental data. The contribution of each individual Fermi surface sheet to the magnetization was identified, and summed to a total spin magnetic moment of $3.6\,\pm\,0.5\,μ_B/\mathrm{f.u.}$.

cond-mat.str-el↗

arXiv2015

Detection and imaging of the oxygen deficiency in single crystalline YBa$_{\text{2}}$Cu$_{\text{3}}$O$_{\text{7}-δ}$ thin films using a positron beam

Single crystalline YBa$_{\text{2}}$Cu$_{\text{3}}$O$_{\text{7}-δ}$ (YBCO) thin films were grown by pulsed laser deposition (PLD) in order to probe the oxygen deficiency $δ$ using a mono-energetic positron beam. The sample set covered a large range of $δ$ (0.191<$δ$<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 $δ$ 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 $δ$ despite the potential presence of Ba or Cu vacancies. In addition, by scanning with the positron beam the spatial variation of $δ$ could be analyzed. It was found to fluctuate with a standard deviation of up to $0.079(5)$ within a single YBCO film.

cond-mat.mtrl-sci↗

arXiv2015

Electron-electron interaction strength in ferromagnetic nickel determined by spin-polarized positron annihilation

The two-photon momentum distribution of annihilating electron-positron pairs in ferromagnetic nickel (Ni) was determined by measuring the spin-polarized two-dimensional angular correlation of annihilation radiation (ACAR). The spectra were compared with theoretical results obtained within LDA+DMFT, a combination of the local density approximation (LDA) and the many-body dynamical mean-field theory (DMFT). The self-energy describing the electronic correlations in Ni is found to make important anisotropic contributions to the momentum distribution which are not present in LDA. Based on a detailed comparison of the theoretical and experimental results the strength of the local electronic interaction U in ferromagnetic Ni is determined as 2.0 +- 0.1 eV.

cond-mat.str-el↗

arXiv2013

First 2D-ACAR Measurements on Cu with the new Spectrometer at TUM

The two-dimensional measurement of the angular correlation of the positron annihilation radiation (2D-ACAR) is a powerful tool to investigate the electronic structure of materials. Here we report on the first results obtained with the new 2D-ACAR spectrometer at the Technische Universität München (TUM). To get experience in processing and interpreting 2D-ACAR data, first measurements were made on copper. The obtained data are treated with standard procedures and compared to theoretical calculations. It is shown that the measurements are in good agreement with the calculations and that the Fermi surface can be entirely reconstructed using three projections only.

cond-mat.str-el↗

arXiv2013

First measurements with the Munich 2D-ACAR spectrometer on Cr

The Munich 2D-ACAR spectrometer at the Maier-Leibnitz accelerator laboratory in Garching has recently become operational. In the present implementation a 2D-ACAR spectrometer is set up, with a baseline of 16.5m, a conventional 22Na positron source and two Anger-type gamma-cameras. The positrons are guided onto the sample by a magnetic field generated by a normal conducting electromagnet. The sample can be either cooled by a standard closed-cycle-cryostat to low temperatures or heated by a resistive filament to temperatures up to 500K. We present the key features of this new 2D-ACAR spectrometer and, in addition, discuss first measurements on the pure metal system Cr. The 2D-ACAR measurements have been performed on Cr at different temperatures: at 5K and at room temperature in the anti-ferromagnetic phase and at 318K slightly above the paramagnetic phase transition.

cond-mat.str-el↗

arXiv2012

Performance of the new 2D ACAR spectrometer in Munich

Angular Correlation of Annihilation Radiation (ACAR) is a well established technique for the investigation of the electric structure. A major limitation of ACAR studies is the available positron flux at a small spot on the sample. Fore this reason, the focus of this work is put on the discussion of a newly developed source-sample stage which uses an optimized static magnetic field configuration to guide the positrons onto the sample. The achieved spot size is $d_{\mathrm{FWHM}}=5.4\,$mm, with a high efficiency over the whole energy spectrum of the $^{22}$Na positron source. The implications of the performance of the source-sample stage are discussed with regard to 2D-ACAR measurements of single crystalline $α$-quartz, which serves as a model system for the determination of the total resolution.

physics.ins-det↗