arXiv subjects

Sven Velten

Publications and source records attributed to Sven Velten.

6 recordsLinked to original sources

arXiv2026

Customized spin spirals in ferromagnetic thin films

The advancement of spintronic nanoscale devices hinges on the ability to flexibly engineer magnetic spin structures in thin-film stacks with precision and control. Meeting this demand remains a challenge for stable non-collinear spin configurations and, more specifically, vertical spin spirals in thin films. Innovative methods are required for their fabrication, stabilization and control. Here, we use oblique-incidence deposition to design and stabilize vertical spin spirals at room temperature and without an external field in magnetic thin films. We induce two crossed uniaxial magnetic anisotropies at the thin film boundaries. These anisotropies are tunable in direction and strength, thus providing control over the angular range and depth profile of the resulting spin spiral. The combination of polarized neutron reflectometry and nuclear resonant scattering enables precise and direct determination of the depth-dependent spin configurations. Our results establish a single-film design approach, in which the surface anisotropies independently serve as controllable design parameters for tailoring the vertical spin-spiral profile. Potential applications include nanoscale energy-storage devices, magnetic sensors, and ferromagnetic-resonance filters, advancing all-spin-based device engineering in general.

cond-mat.mtrl-sci↗

arXiv2026

Phonons and magnetism of kagome FeGe probed by nuclear resonant scattering

Kagome FeGe hosts a $2\times2\times2$ charge-density wave (CDW) that strongly interplays with antiferromagnetic order. Here, we report $^{57}$Fe nuclear resonant scattering measurement to study FeGe across its long-range CDW and incommensurate magnetic transitions. Upon entering the CDW state, hardening of acoustic phonons and optical phonons around 22~meV, 27~meV, and 31~meV are observed in the Fe partial phonon density of states, which can be qualitatively captured by first-principle calculations. Upon entering the incommensurate magnetic phase, neither the phonon density of states nor the hyperfine interaction parameters change significantly, although a subtle feature associated with the incommensurate magnetic order or slow fluctuations is detected in the time-domain M\"{o}ssbauer spectra. These findings show that the CDW in kagome FeGe significantly modifies its lattice dynamics and magnetism, evidencing an intertwined nature of the spin, charge, and lattice degrees of freedom.

cond-mat.str-el↗

arXiv2025

Single-shot sorting of M\"ossbauer time-domain data at X-ray free-electron lasers

M\"ossbauer spectroscopy is widely used to study structure and dynamics of matter with remarkably high energy resolution, provided by the narrow nuclear resonance line widths. However, the narrow width implies low count rates, such that experiments commonly average over extended measurement times or many x-ray pulses (``shots''). This averaging impedes the study of non-equilibrium phenomena. It has been suggested that X-ray free-electron lasers (XFELs) could enable M\"ossbauer single-shot measurements without averaging, and a proof-of-principle demonstration has been reported. However, so far, only a tiny fraction of all shots resulted in signal-photon numbers which are sufficiently high for a single-shot analysis. Here, we demonstrate coherent nuclear-forward-scattering of self-seeded XFEL radiation, with up to 900 signal-photons per shot. We develop a sorting approach which allows us to include all data on a single-shot level, independent of the signal content of the individual shots. It utilizes the presence of different dynamics classes, i.e. different nuclear evolutions after each excitation. Each shot is assigned to one of the classes, which can then be analyzed separately. Our approach determines the classes from the data without requiring theory modeling nor prior knowledge on the dynamics, making it also applicable to unknown phenomena. We envision that our approach opens up new grounds for M\"ossbauer science, enabling the study of out-of-equilibrium transient dynamics of the nuclei or their environment.

quant-ph↗

arXiv2025

Energy Time Ptychography for one-dimensional phase retrieval

Phase retrieval is at the heart of adaptive optics and modern high-resolution imaging. Without phase information, optical systems are limited to intensity-only measurements, hindering full reconstruction of object structures and wavefront dynamics essential for advanced applications. Here, we address a one-dimensional phase problem linking energy and time, which arises in X-ray scattering from ultrasharp nuclear resonances. We leverage the M\"ossbauer effect, where nuclei scatter radiation without energy loss to the lattice, and are sensitive to their magneto-chemical environments. Rather than using traditional spectroscopy with radioactive gamma-ray sources, we measure nuclear forward scattering of synchrotron X-ray pulses in the time domain, providing superior sensitivity and faster data acquisition. Extracting spectral information from a single measurement is challenging due to the missing phase information, typically requiring extensive modeling. Instead, we use multiple energetically overlapping measurements to retrieve both the transmission spectrum and the phase of the scattering response, similar to ptychographic phase retrieval in imaging. Our robust approach can overcome the bandwidth limitations of gamma-ray sources, opening new research directions with modern X-ray sources and M\"ossbauer isotopes.

physics.optics↗

arXiv2024

TEMPUS, a Timepix4-based system for event-based X-rays detection

A readout system based on the Timepix4 ASIC, is being developed for photon science. The TEMPUS detector can be operated in two distinct modes: a photon counting mode, which allows for conventional full-frame readout at rates up to 40 kfps; and an event-driven time-stamping mode, which allows excellent time resolution in the nanosecond regime in measurements with moderate X-ray flux. In this paper, we introduce the initial prototype, a single-chip system, and present the first results obtained at PETRA III and ESRF.

physics.ins-det↗

arXiv2024

Collective nuclear excitation and pulse propagation in single-mode x-ray waveguides

Waveguides offer a means to controllably couple atomic ensembles to the electromagnetic field therein. Here, we demonstrate x-ray propagation in planar thin-film waveguides coupled to M\"ossbauer nuclei under collective resonant excitation by short pulses of synchrotron radiation. We record x-ray photons that have been emitted into resonant modes of the waveguide. Depending on the geometry and mode of excitation, two fundamentally different signatures of the collective emission are observed, for which we present a unifying theoretical model. Our results form a new platform for waveguide quantum electrodynamics in the hard x-ray regime with the potential to provide a coherent narrowband source of x-rays on the nanometer scale.

quant-ph↗