Ralf Röhlsberger

With the development of X-ray free-electron lasers (XFELs), producing pulses of femtosecond durations comparable with the coherence times of X-ray fluorescence, it has become possible to observe intensity–intensity correlations due to the interference of emission from independent atoms. This has been used to compare durations of X-ray pulses and to measure the size of a focusedX-ray beam, for example. Here it is shown that it is also possible to observe the interference of fluorescence photons through the measurement of the speckle contrast of angle-resolved fluorescence patterns. Speckle contrast is often used as a measure of the degree of coherence of the incident beam or the fluctuations of the illuminated sample as determined from X-ray diffraction patterns formed by elastic scattering, rather than from fluorescence patterns as addressed here. Commonly used approaches to estimate speckle contrast were found to suffer when applied to XFEL-generated fluorescence patterns due to low photon counts and a significant variation of the excitation pulse energy from shot to shot. A new method to reliably estimate speckle contrast under such conditions, using a weighting scheme, is introduced. The method is demonstrated by comparing the speckle contrast of fluorescence observed with pulses of 3 fs to 15 fs duration.

High-brilliance synchrotron radiation sources have opened new avenues for X-ray polarization analysis that go far beyond conventional polarimetry in the optical domain. With linear X-ray polarizers in a crossed setting polarization extinction ratios down to 10$^{-10}$ can be achieved. This renders the method sensitive to probe tiniest optical anisotropies that would occur, for example, in strong-field QED due to vacuum birefringence and dichroism. Here we show that high-purity polarimetry can be employed to reveal electronic anisotropies in condensed matter systems with utmost sensitivity and spectral resolution. Taking CuO and La$_2$CuO$_4$ as benchmark systems, we present a full characterization of the polarization changes across the Cu K-absorption edge and their separation into dichroic and birefringent contributions. At diffraction-limited synchrotron radiation sources and X-ray lasers, where polarization extinction ratios of 10$^{-12}$ can be achieved, our method has the potential to assess birefringence and dichroism of the quantum vacuum in extreme electromagnetic fields.

Dichroism is one of the most important optical effects in both the visible and the X-ray range. Besides absorption, scattering can also contribute to dichroism. This paper demonstrates that, based on the example of polyimide, materials can show tiny dichroism even far from electronic resonances due to scattering. Although the effect is small, it can lead to a measurable polarization change and might have influence on highly sensitive polarimetric experiments.

This work presents the improvements in the design and testing of polarimeters based on channel-cut crystals for nuclear resonant scattering experiments at the 14.4 keV resonance of ⁵⁷Fe. By using four asymmetric reflections at asymmetry angles of α₁ = −28°, α₂ = 28°, α₃ = −28° and α₄ = 28°, the degree of polarization purity could be improved to 2.2 × 10⁻⁹. For users, an advanced polarimeter without beam offset is now available at beamline P01 of the storage ring PETRA III.

Zusammenfassung: In der Materialforschung bricht ein neues Zeitalter an. Gekennzeichnet ist es durch die Suche nach einem tieferen Verständnis von komplexen, hierarchisch strukturierten oder ungeordneten Materialien. Solche Materialien spielen in der Natur, etwa in lebender Materie, eine zentrale Rolle, zunehmend aber auch in vielfältigen technischen Anwendungen. Ihre Erforschung erfordert abbildende Methoden, die alle Größenskalen von der Makro‐ bis zur Mikrowelt der Atome erfassen können. Ideal hierfür sind wenige Mikrometer feine Röntgenstrahlen mit einem hohen Anteil an kohärentem Licht. Dieses Röntgenlicht sollen zukünftige rein beugungsbegrenzte Synchrotron‐Strahlungsquellen liefern. Mit dem Umbau des PETRA III‐Speicherrings zu PETRA IV soll 2027 die weltweit leistungsfähigste Strahlungsquelle dieser Art in Betrieb gehen. Sie soll auch hochaufgelöste Einblicke in dynamische Prozesse, etwa chemische Reaktionen oder Phasenumwandlungen, ermöglichen.

The high brilliance of the modern synchrotron radiation sources facilitates experiments with high energy x-rays. In this Letter we report on Nuclear Resonance Scattering at the 73 keV nuclear level in 193Ir. The transitions between the hyperfine split levels show an exceptionally large E2/M1 multi-polarity mixing ratio combined with an increased sensitivity to certain changes in the hyperfine field direction compared to non-mixing transitions. The method opens a new way for probing local magnetic and electronic properties of correlated materials containing iridium and provides novel insights into their anisotropic magnetism. In particular, unexpected out-of-plane components of magnetic hyperfine fields and non-zero electric field gradients in Sr2IrO4 have been detected and attributed to the presence of strong spin-orbit interaction. Due to the high, 62% natural abundance of the 193Ir isotope, no isotopic enrichment of the samples is required, qualifying the method for a broad range of applications.

Magnetoelectronic multilayer devices are widely used in today's information and sensor technology. Their functionality, however, is limited by the inherent properties of magnetic exchange or dipolar coupling which constrain possible spin configurations to collinear or perpendicular alignments of adjacent layers. Here, a deposition procedure is introduced that allows for a new class of layered materials in which complex spin structures can be accurately designed to result in a multitude of new and precisely adjustable spintronic and magnetoresistive properties. The magnetization direction and coercivity of each individual layer are determined by the deposition process in oblique incidence geometry and can be completely decoupled from neighboring layers. This applies for layers of any ferromagnetic material down to layer thicknesses of a few nm and lateral dimensions of a few 100 nm, enabling the design of efficient and compact magnetoelectronic devices, encompassing precision magnetoresistive sensors as well as layer systems with multiple addressable remanent states for magnetic memory applications.