arXiv subjects

Christoph Hugenschmidt

Publications and source records attributed to Christoph Hugenschmidt.

At least 19 recordsLinked to original sources

arXiv2025

A model for positron annihilation in multi-layer systems by solving the diffusion equation using different positron affinities

We present a method for solving the positron diffusion equation in multi-layer systems. Our approach incorporates material-specific implantation profiles, diffusion parameters, and positron affinities. It utilizes a Markov chain approach to model annihilation probabilities and provides fitting capabilities for experimental S (lineshape) parameter data. We have implemented this algorithm in Python and made it available for free under the name LIMPID. To demonstrate its performance, we analyze depth-resolved Doppler-Broadening Spectroscopy measurements of a Cu layer on a Si substrate, achieving excellent agreement with the experimental profiles. The LIMPID tool enhances the reproducibility and comparability of positron defect characterization measurements across different research groups.

cond-mat.mtrl-sci↗

arXiv2024

High-efficiency position resolved gamma ray detectors for 2D-measurements of the angular correlation of annihilation radiation

The measurement of the 2D-Angular Correlation of Electron Positron Annihilation Radiation (ACAR) provides unique information about the bulk electronic structure of single crystals. We set up a new prototype for 2D-ACAR measurements using two 24 x 24 (26.8 mm x 26.8 mm) pixelated LYSO scintillation crystals in combination with a glass light guide and 8 x 8 (24 mm x 24 mm) Multi Pixel Photon Counters (MPPCs). Compared to conventional Anger-cameras, typically comprising large NaI(Tl) scintillators read out with photomultiplier arrays a larger implementation of our prototype would drastically improve resolution and count rate by taking advantage of the small pixel size of the scintillator, its much higher attenuation coefficient for 511 keV {\gamma}-quanta and faster digital readout. With our prototype we achieved a detection efficiency of 45%, i.e. five times higher compared to NaI(Tl) used in our Anger cameras, leading to a 25 (!) times higher coincidence count rate in ACAR measurements. A spatial resolution of 1 mm was obtained, which is limited by the pixel size of the scintillator. We demonstrate the high performance of the setup by (i) imaging the local distribution of 22Na in a proton-irradiated aluminum target and (ii) determining the Fermi energy of Cu from 2D-ACAR spectra recorded for a polycrystalline copper sample.

physics.ins-det↗

arXiv2024

The Origin of Enhanced Conductivity and Structure Change in Defective Li4Ti5O12 or Blue-LTO : a study combined theoretical and experimental perspectives

The spinel Li4Ti5O12 (LTO) has emerged as a promising anode material for the next generation of all-solid-state Li-ion batteries (ASSB), primarily due to its characteristic "zero strain" charge/discharge behavior and exceptional cycling stability, which significantly prolongs battery lifespan. Pristine LTO, however, is hindered by poor ionic and electronic conductivity. By employing tailored sintering protocols that create oxygen vacancies, a high-performing, blue LTO material is achieved. It has been proposed that the increased electronic conductivity could stem from vacancy-induced polarons. Yet, detailed insights into polaron stability, distribution, and dynamics within both the LTO bulk and surface have remained elusive due to limited information on structural changes. Utilizing Positron Annihilation Lifetime Spectroscopy (PALS) and Coincidence Doppler Broadening Spectroscopy (CDBS), in conjunction with Two Component Density Functional Theory (TCDFT) with the on-site Hubbard U correction, enables us to probe the depth profile of defect species introduced by sintering in a reductive environment. Our research provides direct evidence of oxygen vacancy formation within the subsurface region, an inference drawn from the observation of \ch{Ti^{3+}}. Our investigation into Li16d vacancy formation within the bulk region uncovers the interactions between mobile species, namely Li-ions and polarons. Furthermore, we delve into the polaron stability on the LTO surface, offering an explanation for the superior performance of the (100) facet exposed LTO nanoparticle, as compared to its (111) exposed counterpart.

cond-mat.mtrl-sci↗

arXiv2024

Surface and Near-Surface Positron Annihilation Spectroscopy at Very Low Positron Energy

We present a monoenergetic positron beam specifically tailored to the needs of (near-) surface positron annihilation spectroscopy. The Setup for LOw-energy Positron Experiments (SLOPE) comprises a high-activity 22Na source, a tungsten moderator, electrostatic extraction and acceleration, magnetic beam guidance, as well as an analysis chamber with a movable sample holder and a γ-ray detection system. The tungsten moderator foil, biased between 0 and 30 V, in combination with the HV-biasable sample holder, enables positron implantation energies between 3 eV and 40 keV. At low energies (<20 eV), the count rate typically amounts to 4400 counts per second, and the beam diameter is smaller than 12 +/- 3 mm. We conduct phase space simulations of the positron beam using COMSOL Multiphysics (c) to characterize the beam properties and compare the findings with the experimentally determined energy-dependent beam diameter. To showcase the capabilities of SLOPE, we perform studies of positronium (Ps) formation on boehmite and depth-resolved coincidence Doppler-broadening spectroscopy (CDBS) of copper. In particular, the Ps formation at the hydrogen-terminated surface of boehmite is found to be maximum at a positron implantation energy of 10 eV. The range of positron energies for which we observe Ps formation agrees with the hydrogen ionization energy.

physics.ins-det↗

arXiv2024

Proton Beam Based Production of Positron Emitters by Exploiting the 27Al(p,x)22Na Reaction

Positron annihilation experiments on an laboratory scale depend on the supply and the availability of $β^+$ emitters. Here we present the production of positron sources based on the $^{27}$Al(p,x)$^{22}$Na reaction by irradiation of Al with a 68\,MeV proton beam. We simulated the energy loss, range and radial scattering of the protons in Al in order to design a simple target consisting of a stack of Al discs. Our approach allows (i) the direct use of the Al discs as positron emitters that inherently avoids wet chemical processes as usually applied in commercial production of carrier-free $^{22}$Na, (ii) the production of multiple positron sources at once, and (iii) the simple measurement of the depth and lateral distribution of $^{22}$Na. We precisely determined the cross section of the $^{27}$Al(p,x)$^{22}$Na reaction which was found to differ from literature values particularly for proton energies between 27 and 40\,MeV. The activity of all nuclides produced (apart from $^{22}$Na) was shown to be negligible 15 days after irradiation. The production of radionuclides such as $^{48}$Sc, $^{54}$Mn and $^{56}$Co can be prevented by using Al of a higher purity. The concept presented here can easily be adapted for the production of stronger $^{22}$Na sources by increasing the proton current or/and the irradiation time.

nucl-ex↗

arXiv2024

Spatially-resolved charge detectors for particle beam optimization with femtoampere resolution achieved by in-vacuum signal preamplification

We present the design of a Faraday cup-like charged particle detector in a four quadrant configuration aimed at facilitating the alignment of low-intensity beams of exotic particles. The device is capable of assessing the current on the electrodes with a resolution of 33fA within 15ms or a maximal resolution of 1.8fA with a measurement time of 12.4s. This performance is achieved by minimizing the noise through a preamplification circuit installed in vacuum, as close as possible to the electrodes. We tested the detector with the positron beam of ELBE, achieving the nominal maximum resolution with high reproducibility. We then exploited the capabilities of the detector to resolve the two-dimensional shape of the beam, and revealed the presence of a weak electron beam being transported alongside the positrons. Characterization of the detector performance showed that in a variety of scenarios it can be used to quickly center positron beams thus allowing for the prompt optimization of beam transport.

physics.ins-det↗

arXiv2023

Determination of the Spacing Between Hydrogen-Intercalated Quasi-Free-Standing Monolayer Graphene and 6H-SiC(0001) Using Total-Reflection High-Energy Positron Diffraction

We have investigated the structure of hydrogen-intercalated quasi-free-standing monolayer graphene (QFMLG) grown on 6H-SiC(0001) by employing total-reflection high-energy positron diffraction (TRHEPD). At least nine diffraction spots of the zeroth order Laue zone were resolved along <11-20> and three along <1-100>, which are assigned to graphene, SiC and higher order spots from multiple diffraction on both lattices. We further performed rocking curve analysis based on the full dynamical diffraction theory to precisely determine the spacing between QFMLG and the SiC substrate. Our study yields a spacing of d = 4.18(6)Å that is in excellent agreement with the results from density-functional theory (DFT) calculations published previously.

cond-mat.mtrl-sci↗

arXiv2023

Novel Data Analysis Tool for the Evaluation of Coincidence Doppler Broadening Spectra of the Positron-Electron Annihilation Line

Coincidence Doppler Broadening Spectroscopy (CDBS) of the 511 keV annihilation line reveals the elemental signature at the annihilation site in matter. For this reason, CDBS enables the analysis of foreign atoms in the host matrix, vacancy-solute complexes and precipitates in solids. Due to the lack of a comprehensive program for the analysis of CDBS data we developed a modular python-based open source software solution for the reliable extraction of the annihilation line to provide so-called ratio curves highlighting the elemental signature in the spectra. In particular, the new program allows the readout and spectra averaging of multiple detector pairs. Moreover, we implemented an improved algorithm to further reduce the background by about one order of magnitude at high Doppler shifts.

physics.ins-det↗

arXiv2021

Total-Reflection High-Energy Positron Diffractometer at NEPOMUC -- Instrumentation, Simulation and First Measurements

We report the instrumentation of a new positron diffractometer that is connected to the high-intensity positron source NEPOMUC. Crucial elements for the adaption of the positron beam are presented, which include the magnetic field termination, the optional transmission-type remoderator for brightness enhancement and the electrostatic system for acceleration and beam optics. The positron trajectories of the remoderated and the twofold remoderated beam have been simulated to optimize the system, i.e. to obtain a coherent beam of small diameter. Within a first beamtime we tuned the system and characterized the direct beam. For the twofold remoderated beam of 10keV energy, we experimentally observe a beam diameter of d < 1.3mm, which agrees well with the simulation.

physics.ins-det↗

arXiv2021

Fermi-Surface Modeling of Light-Rare-Earth Hexaborides with 2D-ACAR Spectroscopy

Two dimensional angular correlation of the positron annihilation radiation (2D-ACAR) spectra are measured for $\mathrm{LaB}_6$ along high symmetry directions and compared with first principle calculations based on density functional theory (DFT). This allows the modeling of the Fermi surface in terms of ellipsoid electron pockets centered at $X$-points elongated along the $Σ$ axis (${Γ-M}$ direction). The obtained structure is in agreement with quantum oscillation measurements and previous band structure calculations. For the isostructural topologically not-trivial $\mathrm{SmB}_6$ the similar ellipsoids are connected through necks that have significantly smaller radii in the case of $\mathrm{LaB}_6$. A theoretical analysis of the 2D-ACAR spectra is also performed for $\mathrm{CeB}_6$ including the on-site repulsion $U$ correction to the local-density approximation (LDA+$U$) of the DFT. The similarities of 2D-ACAR spectra and the Fermi-surface projections of these two compounds allow to infer that both $\mathrm{LaB}_6$ and $\mathrm{CeB}_6$ are topologically trivial correlated metals.

cond-mat.mtrl-sci↗

arXiv2020

Absolute fraction of emitted Ps determined by GEANT4 supported analysis of gamma spectra

The fraction of positronium (Ps) emitted from a surface of a germanium single crystal at high temperature is usually assumed to approach unity at zero positron implantation energy. In the experiment, however, the determination of the absolute Ps fraction is not straight forward since recording a reference spectrum with $100\,\%$ Ps formation remains demanding. We use GEANT4-simulated detector responses to $2γ$ and $3γ$ radiation sources mimicking positron and Ps annihilation inside the (coincidence) Doppler-broadening spectrometer at NEPOMUC, FRM II, in order to derive a reliable value for the Ps fraction reemitted from a Ge(100) target heated close to its melting point. Analysis of the measured spectra by fitting the simulated spectra shows an absolute value of $(72\,\pm{4})\,\%$ maximum Ps formation, contradicting the $100\,\%$ assumption.

physics.atom-ph↗

arXiv2020

Breakdown of the Arrhenius Law of the Temperature Dependent Vacancy Concentration in fcc-Lanthanum

We measured the temperature dependent equilibrium vacancy concentration using in-situ positron annihilation spectroscopy in order to determine the enthalpy $H_\text{f}$ and entropy $S_\text{f}$ of vacancy formation in elementary fcc-La. The Arrhenius law applied for the data analysis, however, is shown to fail in explaining the unexpected high values for both $S_\text{f}$ and $H_\text{f}$: in particular $S_\text{f}=17(2)~k_\text{B}$ is one order of magnitude larger compared to other elemental metals, and the experimental value of $H_\text{f}$ is found to be more than three standard deviations off the theoretical one $H_\text{f}=1.46~\text{eV}$ (our \acs{dft} calculation for La at $T=0~\text{K}$). A consistent explanation is given beyond the classical Arrhenius approach in terms of a temperature dependence of the vacancy formation entropy with $S_\text{f}^\prime=-0.0120(14)~k_\text{B}/\text{K}$ accounting for the anharmonic potential introduced by vacancies.

cond-mat.mtrl-sci↗

arXiv2018

Nature of the positron state in CdSe quantum dots

Previous studies have shown that positron-annihilation spectroscopy is a highly sensitive probe of the electronic structure and surface composition of ligand-capped semiconductor Quantum Dots (QDs) embedded in thin films. Nature of the associated positron state, however, whether the positron is confined inside the QDs or localized at their surfaces, has so far remained unresolved. Our positron-annihilation lifetime spectroscopy (PALS) studies of CdSe QDs reveal the presence of a strong lifetime component in the narrow range of 358-371 ps, indicating abundant trapping and annihilation of positrons at the surfaces of the QDs. Furthermore, our ab-initio calculations of the positron wave function and lifetime employing a recent formulation of the Weighted Density Approximation (WDA) demonstrate the presence of a positron surface state and predict positron lifetimes close to experimental values. Our study thus resolves the longstanding question regarding the nature of the positron state in semiconductor QDs, and opens the way to extract quantitative information on surface composition and ligand-surface interactions of colloidal semiconductor QDs through highly sensitive positron-annihilation techniques.

cond-mat.mes-hall↗

arXiv2017

Defect Imaging and Detection of Precipitates Using a New Scanning Positron Microbeam

We report on a newly developed scanning positron microbeam based on threefold moderation of positrons provided by the high intensity positron source NEPOMUC. For brightness enhancement a remoderation unit with a $100\,$nm thin Ni(100) foil and 9.6% efficiency is applied to reduce the area of the beam spot by a factor of 60. In this way, defect spectroscopy is enabled with a lateral resolution of $33\,μ$m over a large scanning range of $19\times19\,$mm$^{2}$. Moreover, 2D defect imaging using Doppler broadening spectroscopy (DBS) is demonstrated to be performed within exceptional short measurement times of less than two minutes for an area of $1\times1$mm$^{2}$ ($100\times100\,μ$m$^{2}$) with a resolution of $250\,μ$m ($50\,μ$m). We studied the defect structure in laser beam welds of the high-strength agehardened Al alloy (AlCu6Mn, EN AW-2219 T87) by applying (coincident) DBS with unprecedented spatial resolution. The visualization of the defect distribution revealed a sharp transition between the raw material and the welded zone as well as a very small heat affected zone. Vacancy-like defects and Cu rich precipitates are detected in the as-received material and, to a lesser extent, in the transition zone of the weld. Most notably, in the center of the weld vacancies without forming Cu-vacancy complexes, and the dissolution of the Cu atoms in the crystal lattice, i.e. formation of a supersaturated solution, could be clearly identified.

cond-mat.mtrl-sci↗

arXiv2017

In-situ observation of temperature-dependent segregation of Ni adatoms on oriented Pd surfaces

We report the direct observation of the in-situ temperature-dependent surface segregation of Ni adatoms on single crystalline Pd surfaces using Positron annihilation induced Auger Electron Spectroscopy (PAES). For this study, a single atomic layer of Ni was grown on Pd with the crystallographic orientations Pd(111), Pd(110) and Pd(100). The sample temperature was increased from room temperature to 350$^\circ$C and the intensity of the Ni and Pd signal was evaluated from the recorded PAES spectra. Due to the outstanding surface sensitivity of PAES a clear tendency for Pd segregation was observed for all samples. Moreover the activation temperature T$_0$ for surface segregation was found to depend strongly on the surface orientation: We determined T$_0$ to 172$\pm$4$^\circ$C, 261$\pm$12$^\circ$C and 326$\pm$11$^\circ$C for Pd(111), Pd(100) and Pd(110), respectively.

cond-mat.mtrl-sci↗

arXiv2016

Electronic Correlations in Vanadium Revealed by Electron-Positron Annihilation Measurements

The electronic structure of vanadium measured by Angular Correlation of electron-positron Annihilation Radiation (ACAR) is compared with the predictions of the combined Density Functional and Dynamical Mean-Field Theory (DMFT). Reconstructing the momentum density from five 2D projections we were able to determine the full Fermi surface and found excellent agreement with the DMFT calculations. In particular, we show that the local, dynamic self-energy corrections contribute to the anisotropy of the momentum density and need to be included to explain the experimental results.

cond-mat.str-el↗

arXiv2016

Positrons in Surface Physics

Within the last decade powerful methods have been developed to study surfaces using bright low-energy positron beams. These novel analysis tools exploit the unique properties of positron interaction with surfaces, which comprise the absence of exchange interaction, repulsive crystal potential and positron trapping in delocalized surface states at low energies. By applying reflection high-energy positron diffraction (RHEPD) one can benefit from the phenomenon of total reflection below a critical angle that is not present in electron surface diffraction. Therefore, RHEPD allows the determination of the atom positions of (reconstructed) surfaces with outstanding accuracy. The main advantages of positron annihilation induced Auger-electron spectroscopy (PAES) are the missing secondary electron background in the energy region of Auger-transitions and its topmost layer sensitivity for elemental analysis. In order to enable the investigation of the electron polarization at surfaces low-energy spin-polarized positrons are used to probe the outermost electrons of the surface. Furthermore, in fundamental research the preparation of well defined surfaces tailored for the production of bound leptonic systems plays an outstanding role. In this report, it is envisaged to cover both, the fundamental aspects of positron surface interaction and the present status of surface studies using modern positron beam techniques.

cond-mat.mtrl-sci↗

arXiv2016

Positron spectroscopy of point defects in the skyrmion-lattice compound MnSi

Outstanding crystalline perfection is a key requirement for the formation of new forms of electronic order in a vast number of widely different materials. Whereas excellent sample quality represents a standard claim in the literature, there are, quite generally, no reliable microscopic probes to establish the nature and concentration of lattice defects such as voids, dislocations and different species of point defects on the level relevant to the length and energy scales inherent to these new forms of order. Here we report an experimental study of the archetypical skyrmion-lattice compound MnSi, where we relate the characteristic types of point defects and their concentration to the magnetic properties by combining different types of positron spectroscopy with ab-initio calculations and bulk measurements. We find that Mn antisite disorder broadens the magnetic phase transitions and lowers their critical temperatures, whereas the skyrmion lattice phase forms for all samples studied underlining the robustness of this topologically non-trivial state. Taken together, this demonstrates the unprecedented sensitivity of positron spectroscopy in studies of new forms of electronic order.

cond-mat.str-el↗