arXiv subjects

C. Baehtz

Publications and source records attributed to C. Baehtz.

9 recordsLinked to original sources

arXiv2026

Volume Collapse Without a Structural Transition in Shock-Compressed FeO

We report x-ray diffraction and emission spectroscopy of FeO under laser-driven shock compression between 31-199 GPa. FeO retains the B1 (rocksalt) structure along the Hugoniot to the melt boundary at 191 GPa. While the phase and volume are broadly consistent with results from static compression, we observe an anomalous 7-10% volume collapse around 60 GPa absent in static experiments. We identify this as an isostructural high-spin to low-spin metallic transition in FeO. The low-spin state is directly evidenced by x-ray emission spectroscopy at 180 GPa.

cond-mat.mtrl-sci↗

arXiv2025

X-ray thermal diffuse scattering as a texture-robust temperature diagnostic for dynamically compressed solids

We present a model of x-ray thermal diffuse scattering (TDS) from a cubic polycrystal with an arbitrary crystallographic texture, based on the classic approach of Warren. We compare the predictions of our model with femtosecond x-ray diffraction patterns obtained from ambient and dynamically compressed rolled copper foils obtained at the High Energy Density (HED) instrument of the European X-Ray Free-Electron Laser (EuXFEL), and find that the texture-aware TDS model yields more accurate results than does the conventional powder model owed to Warren. Nevertheless, we further show that: with sufficient angular detector coverage, the TDS signal is largely unchanged by sample orientation and in all cases strongly resembles the signal from a perfectly random powder; shot-to-shot fluctuations in the TDS signal resulting from grain-sampling statistics are at the percent level, in stark contrast to the fluctuations in the Bragg-peak intensities (which are over an order of magnitude greater); and TDS is largely unchanged even following texture evolution caused by compression-induced plastic deformation. We conclude that TDS is robust against texture variation, making it a flexible temperature diagnostic applicable just as well to off-the-shelf commercial foils as to ideal powders.

physics.app-ph↗

arXiv2025

Shake-off in XFEL heated solid density plasma

In atoms undergoing ionisation, an abrupt re-arrangement of free and bound electrons can lead to the ejection of another bound electron (shake-off). The spectroscopic signatures of shake-off have been predicted and observed in atoms and solids. Here, we present the first observation of this process in a solid-density plasma heated by an x-ray free electron laser. The results show that shake-off of L-shell electrons persists up to temperatures of 10 eV at solid density, and follow the probability predicted for solids. This work shows that shake-off should be included in plasma models for the correct interpretation of emission spectra.

physics.plasm-ph↗

arXiv2025

Femtosecond temperature measurements of laser-shocked copper deduced from the intensity of the x-ray thermal diffuse scattering

We present 50-fs, single-shot measurements of the x-ray thermal diffuse scattering (TDS) from copper foils that have been shocked via nanosecond laser-ablation up to pressures above 135~GPa. We hence deduce the x-ray Debye-Waller (DW) factor, providing a temperature measurement. The targets were laser-shocked with the DiPOLE 100-X laser at the High Energy Density (HED) endstation of the European X-ray Free-Electron Laser (EuXFEL). Single x-ray pulses, with a photon energy of 18 keV, were scattered from the samples and recorded on Varex detectors. Despite the targets being highly textured (as evinced by large variations in the elastic scattering), and with such texture changing upon compression, the absolute intensity of the azimuthally averaged inelastic TDS between the Bragg peaks is largely insensitive to these changes, and, allowing for both Compton scattering and the low-level scattering from a sacrificial ablator layer, provides a reliable measurement of $T/\Theta_D^2$, where $\Theta_D$ is the Debye temperature. We compare our results with the predictions of the SESAME 3336 and LEOS 290 equations of state for copper, and find good agreement within experimental errors. We thus demonstrate that single-shot temperature measurements of dynamically compressed materials can be made via thermal diffuse scattering of XFEL radation.

cond-mat.mtrl-sci↗

arXiv2024

Plasma screening in mid-charged ions observed by K-shell line emission

Dense plasma environment affects the electronic structure of ions via variations of the microscopic electrical fields, also known as plasma screening. This effect can be either estimated by simplified analytical models, or by computationally expensive and to date unverified numerical calculations. We have experimentally quantified plasma screening from the energy shifts of the bound-bound transitions in matter driven by the x-ray free electron laser (XFEL). This was enabled by identification of detailed electronic configurations of the observed K{\alpha}, K\b{eta} and K{\gamma} lines. This work paves the way for improving plasma screening models including connected effects like ionization potential depression and continuum lowering, which will advance the understanding of atomic physics in Warm Dense Matter regime.

physics.plasm-ph↗

arXiv2015

High Curie temperature and perpendicular magnetic anisotropy in homoepitaxial InMnAs films

We have prepared the dilute magnetic semiconductor (DMS) InMnAs with different Mn concentrations by ion implantation and pulsed laser melting. The Curie temperature of the In1-xMnxAs epilayer depends on the Mn concentration x, reaching 82 K for x=0.105. The substitution of Mn ions at the Indium sites induces a compressive strain perpendicular to the InMnAs layer and a tensile strain along the in-plane direction. This gives rise to a large perpendicular magnetic anisotropy, which is often needed for the demonstration of electrical control of magnetization and for spin-transfer-torque induced magnetization reversal.

cond-mat.mtrl-sci↗

arXiv2015

Hyperdoping silicon with selenium: solid vs. liquid phase epitaxy

Chalcogen-hyperdoped silicon shows potential applications in silicon-based infrared photodetectors and intermediate band solar cells. Due to the low solid solubility limits of chalcogen elements in silicon, these materials were previously realized by femtosecond or nanosecond laser annealing of implanted silicon or bare silicon in certain background gases. The high energy density deposited on the silicon surface leads to a liquid phase and the fast recrystallization velocity allows trapping of chalcogen into the silicon matrix. However, this method encounters the problem of surface segregation. In this paper, we propose a solid phase processing by flash-lamp annealing in the millisecond range, which is in between the conventional rapid thermal annealing and pulsed laser annealing. Flash lamp annealed selenium-implanted silicon shows a substitutional fraction of around 70% with an implanted concentration up to 2.3%. The resistivity is lower and the carrier mobility is higher than those of nanosecond pulsed laser annealed samples. Our results show that flash-lamp annealing is superior to laser annealing in preventing surface segregation and in allowing scalability.

cond-mat.mtrl-sci↗

arXiv2015

A comprehensive study of the magnetic, structural and transport properties of the III-V ferromagnetic semiconductor InMnP

The manganese induced magnetic, electrical and structural modification in InMnP epilayers, prepared by Mn ion implantation and pulsed laser annealing, are investigated in the following work. All samples exhibit clear hysteresis loops and strong spin polarization at the Fermi level. The degree of magnetization, the Curie temperature and the spin polarization depend on the Mn concentration. The bright-field transmission electron micrographs show that InP samples become almost amorphous after Mn implantation but recrystallize after pulsed laser annealing. We did not observe an insulator-metal transition in InMnP up to a Mn concentration of 5 at./%. Instead all InMnP samples show insulating characteristics up to the lowest measured temperature. Magneotresistance results obtained at low temperatures support the hopping conduction mechanism in InMnP. We find that the Mn impurity band remains detached from the valence band in InMnP up to 5 at./% Mn doping. Our findings indicate that the local environment of Mn ions in InP is similar to GaMnAs, GaMnP and InMnAs, however, the electrical properties of these Mn implanted III-V compounds are different. This is one of the consequences of the different Mn binding energy in these compounds.

cond-mat.mtrl-sci↗

arXiv2011

Evidence for charge and orbital order in the doped titanates RE_(1-x)Ca_xTiO_3 (RE=Y, Er, Lu)

Combining macroscopic and diffraction methods we have studied the electric, magnetic and struc- tural properties of RE_(1-x)Ca_xTiO_3 (RE=Y, Er, Lu) focusing on the concentration range near the metal-insulator transition. The insulating phase, which is stabilized by a smaller rare-earth ionic ra- dius, exhibits charge order with a predominant occupation of the dxy orbital. The charge and orbital ordering explains the broad stability range of the insulating state in RE_(1-x)Ca_xTiO_3 with smaller rare-earth ions. The strong modulation of the Ti-O bond distances indicates sizeable modulation of the electric charge.

cond-mat.str-el↗