arXiv subjects

Yves Acremann

Publications and source records attributed to Yves Acremann.

7 recordsLinked to original sources

arXiv2022

Tracking the surface atomic motion in a coherent phonon oscillation

X-ray photoelectron diffraction is a powerful tool for determining the structure of clean and adsorbate-covered surfaces. Extending the technique into the ultrafast time domain will open the door to studies as diverse as the direct determination of the electron-phonon coupling strength in solids and the mapping of atomic motion in surface chemical reactions. Here we demonstrate time-resolved photoelectron diffraction using ultrashort soft X-ray pulses from the free electron laser FLASH. We collect Se 3d photoelectron diffraction patterns over a wide angular range from optically excited Bi$_2$Se$_3$ with a time resolution of 140 fs. Combining these with multiple scattering simulations allows us to track the motion of near-surface atoms within the first 3 ps after triggering a coherent vibration of the A$_{1g}$ optical phonons. Using a fluence of 4.2 mJ/cm$^2$ from a 1.55 eV pump laser, we find the resulting coherent vibrational amplitude in the first two interlayer spacings to be on the order of 1 pm.

cond-mat.mtrl-sci↗

arXiv2021

Ultrafast electronic line width broadening in the C 1s core level of graphene

Core level binding energies and absorption edges are at the heart of many experimental techniques concerned with element-specific structure, electronic structure, chemical reactivity, elementary excitations and magnetism. X-ray photoemission spectroscopy (XPS) in particular, can provide information about the electronic and vibrational many-body interactions in a solid as these are reflected in the detailed energy distribution of the photoelectrons. Ultrafast pump-probe techniques add a new dimension to such studies, introducing the ability to probe a transient state of the many-body system. Here we use a free electron laser to investigate the effect of a transiently excited electron gas on the core level spectrum of graphene, showing that it leads to a large broadening of the C 1s peak. Confirming a decade-old prediction, the broadening is found to be caused by an exchange of energy and momentum between the photoemitted core electron and the hot electron system, rather than by vibrational excitations. This interpretation is supported by a line shape analysis that accounts for the presence of the excited electrons. Fitting the spectra to this model directly yields the electronic temperature of the system, in agreement with electronic temperature values obtained from valence band data. Furthermore, making use of time- and momentum-resolved C 1s spectra, we illustrate how the momentum change of the outgoing core electrons leads to a small but detectable change in the time-resolved photoelectron diffraction pattern and to a nearly complete elimination of the core level binding energy variation associated with the narrow $σ$-band in the C 1s state. The results demonstrate that the XPS line shape can be used as an element-specific and local probe of the excited electron system and that X-ray photoelectron diffraction investigations remain feasible at very high electronic temperatures.

cond-mat.str-el↗

arXiv2020

An open-source, end-to-end workflow for multidimensional photoemission spectroscopy

Characterization of the electronic band structure of solid state materials is routinely performed using photoemission spectroscopy. Recent advancements in short-wavelength light sources and electron detectors give rise to multidimensional photoemission spectroscopy, allowing parallel measurements of the electron spectral function simultaneously in energy, two momentum components and additional physical parameters with single-event detection capability. Efficient processing of the photoelectron event streams at a rate of up to tens of megabytes per second will enable rapid band mapping for materials characterization. We describe an open-source workflow that allows user interaction with billion-count single-electron events in photoemission band mapping experiments, compatible with beamlines at $3^{\text{rd}}$ and $4^{\text{th}}$ generation light sources and table-top laser-based setups. The workflow offers an end-to-end recipe from distributed operations on single-event data to structured formats for downstream scientific tasks and storage to materials science database integration. Both the workflow and processed data can be archived for reuse, providing the infrastructure for documenting the provenance and lineage of photoemission data for future high-throughput experiments.

physics.data-an↗

arXiv2020

Observation of an excitonic Mott transition through ultrafast core-$\textit{cum}$-conduction photoemission spectroscopy

Time-resolved soft-X-ray photoemission spectroscopy is used to simultaneously measure the ultrafast dynamics of core-level spectral functions and excited states upon excitation of excitons in WSe$_2$. We present a many-body approximation for the Green's function, which excellently describes the transient core-hole spectral function. The relative dynamics of excited-state signal and core levels reveals a delayed core-hole renormalization due to screening by excited quasi-free carriers, revealing an excitonic Mott transition. These findings establish time-resolved core-level photoelectron spectroscopy as a sensitive probe of subtle electronic many-body interactions and an ultrafast electronic phase transition.

cond-mat.mes-hall↗

arXiv2019

Time- and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser

Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of non-equilibrium electronic processes, transient states in chemical reactions or the interplay of electronic and structural dynamics offers fascinating opportunities for future research. Combining valence-band and core-level spectroscopy with photoelectron diffraction for electronic, chemical and structural analysis requires few 10 fs soft X-ray pulses with some 10 meV spectral resolution, which are currently available at high repetition rate free-electron lasers. The PG2 beamline at FLASH (DESY, Hamburg) provides a high pulse rate of 5000 pulses/s, 60 fs pulse duration and 40 meV bandwidth in an energy range of 25-830 eV with a photon beam size down to 50 microns in diameter. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines FEL capabilities together with a multidimensional recording scheme for photoemission studies. We use a full-field imaging momentum microscope with time-of-flight energy recording as the detector for mapping of 3D band structures in ($k_x$, $k_y$, $E$) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 Å $^{-1}$ diameter in a binding-energy range of several eV, resolving about $2.5\times10^5$ data voxels. As an example, we present results for the ultrafast excited state dynamics in the model van der Waals semiconductor WSe$_2$.

cond-mat.mtrl-sci↗

arXiv2018

The Ultrafast Einstein-De Haas Effect

The original observation of the Einstein-de Haas effect was a landmark experiment in the early history of modern physics that illustrates the relationship between magnetism and angular momentum. Today the effect is still discussed in elementary physics courses to demonstrate that the angular momentum associated with the aligned electron spins in a ferromagnet can be converted to mechanical angular momentum by reversing the direction of magnetisation using an external magnetic field. In recent times, a related problem in magnetism concerns the time-scale over which this angular momentum transfer can occur. It is known experimentally for several metallic ferromagnets that intense photoexcitation leads to a drop in the magnetisation on a time scale shorter than 100 fs, a phenomenon called ultrafast demagnetisation. The microscopic mechanism for this process has been hotly debated, with one key question still unanswered: where does the angular momentum go on these sub-picosecond time scales? Here we show using femtosecond time-resolved x-ray diffraction that a large fraction of the angular momentum lost from the spin system on the laserinduced demagnetisation of ferromagnetic iron is transferred to the lattice on sub-picosecond timescales, manifesting as a transverse strain wave that propagates from the surface into the bulk. By fitting a simple model of the x-ray data to simulations and optical data, we roughly estimate that the angular momentum occurs on a time scale of 200 fs and corresponds to 80% of the angular momentum lost from the spin system. Our results show that interaction with the lattice plays an essential role in the process of ultrafast demagnetisation in this system.

cond-mat.str-el↗

arXiv2017

Time- and spatially-resolved magnetization dynamics driven by spin-orbit torques

Current-induced spin-orbit torques (SOTs) represent one of the most effective ways to manipulate the magnetization in spintronic devices. The orthogonal torque-magnetization geometry, the strong damping, and the large domain wall velocities inherent to materials with strong spin-orbit coupling make SOTs especially appealing for fast switching applications in nonvolatile memory and logic units. So far, however, the timescale and evolution of the magnetization during the switching process have remained undetected. Here, we report the direct observation of SOT-driven magnetization dynamics in Pt/Co/AlO$_x$ dots during current pulse injection. Time-resolved x-ray images with 25 nm spatial and 100 ps temporal resolution reveal that switching is achieved within the duration of a sub-ns current pulse by the fast nucleation of an inverted domain at the edge of the dot and propagation of a tilted domain wall across the dot. The nucleation point is deterministic and alternates between the four dot quadrants depending on the sign of the magnetization, current, and external field. Our measurements reveal how the magnetic symmetry is broken by the concerted action of both damping-like and field-like SOT and show that reproducible switching events can be obtained for over $10^{12}$ reversal cycles.

cond-mat.mtrl-sci↗