228 research outputs found
Electronic origin of x-ray absorption peak shifts
Encoded in the transient x-ray absorption (XAS) and magnetic circular (MCD) response functions resides a wealth of information of the microscopic processes of ultrafast demagnetization. Employing state-of-the-art first-principles dynamical simulations we show that the experimentally observed energy shift of the L3 XAS peak in Ni, and the absence of a corresponding shift in the dichroic MCD response, can be explained in terms of laser-induced changes in band occupation. Strikingly, we predict that for the same ultrashort pump pulse applied to Co the opposite effect will occur: a substantial shift upward in energy of the MCD peaks will be accompanied by very small change in the position of XAS peaks, a fact we relate to the reduced d-band filling of Co that allows a greater energetic range above the Fermi energy into which charge can be excited. We also carefully elucidate the dependence of this effect on pump pulse parameters. These findings (i) establish an electronic origin for early-time peak shifts in transient XAS and MCD spectroscopy and (ii) illustrate the rich information that may be extracted from transient response functions of the underlying dynamical system
Light Scattering and Electron Microscopy Study of the Surface Morphology of GaAs Films Grown by Molecular Beam Epitaxy
The surface morphology of thermally quenched GaAs films grown by molecular beam epitaxy on GaAs substrates has been studied by elastic light scattering, by scanning electron microscopy and by scanning tunneling microscopy (STM) in air. STM shows that the oxide-desorbed surface of GaAs is pitted, but smooths after deposition of a few hundred nanometers of material. Light scattering shows that, after the surface has smoothed, the power spectral density of the surface approaches a q-2 dependence on spatial frequency over the spatial frequency range 0.2 μm-1 \u3c q \u3c 20 μm-1 that is accessible to the light scattering measurements at 488 nm. This result is in agreement with the predictions of dynamical scaling theory in the case where the time evolution of the surface morphology is described by an Edwards-Wilkinson type equation
Photoelectron Soft X-Ray Fluorescence Coincidence Spectroscopy on Free Molecules
A technique for measuring core-level photoemission from free molecules in coincidence with the soft x-ray fluorescence decay is presented. Zero-kinetic-energy photoelectrons are detected in a time-of-flight electron spectrometer, and photons are collected in a large solid angle by a detector situated close to the interaction region. The coincidence spectrum of N2 shows an adiabatic 1s line, free from electron-electron postcollision interaction effects. The results open up new aspects on core-hole excitation-emission dynamics
Mapping the energy-time landscape of spins with helical X-rays
Unveiling the key mechanisms that determine optically driven spin dynamics is
essential both to probe the fundamental nature of ultrafast light-matter
interactions, but also to drive future technologies of smaller, faster, and
more energy efficient devices. Essential to this task is the ability to use
experimental spectroscopic tools to evidence the underlying energy- and
spin-resolved dynamics of non-equilibrium electron occupations. In this joint
theory and experimental work, we demonstrate that ultrafast helicity-dependent
soft X-ray absorption spectroscopy (HXAS) allows access to spin-, time- and
energy specific state occupation after optical excitation. We apply this method
to the prototype transition metal ferromagnet cobalt and find convincing
agreement between theory and experiment. The richly structured energy-resolved
spin dynamics unveil the subtle interplay and characteristic time scales of
optical excitation and spin-orbit induced spin-flip transitions in this
material: the spin moment integrated in an energy window below the Fermi level
first exhibits an ultrafast increase as minority carriers are excited by the
laser pulse, before it is reduced as spin-flip process in highly localized, low
energy states start to dominate. The results of this study demonstrate the
power of element specific transient HXAS, placing it as a potential new tool
for identifying and determining the role of fundamental processes in optically
driven spin dynamics in magnetic materials
Optical inter-site spin transfer probed by energy and spin-resolved transient absorption spectroscopy
Optically driven spin transport is the fastest and most efficient process to manipulate macroscopic magnetization as it does not rely on secondary mechanisms to dissipate angular momentum. In the present work, we show that such an optical inter-site spin transfer (OISTR) from Pt to Co emerges as a dominant mechanism governing the ultrafast magnetization dynamics of a CoPt alloy. To demonstrate this, we perform a joint theoretical and experimental investigation to determine the transient changes of the helicity dependent absorption in the extreme ultraviolet spectral range. We show that the helicity dependent absorption is directly related to changes of the transient spin-split density of states, allowing us to link the origin of OISTR to the available minority states above the Fermi level. This makes OISTR a general phenomenon in optical manipulation of multi-component magnetic systems
Photon correlation spectroscopy with heterodyne mixing based on soft-x-ray magnetic circular dichroism
Many magnetic equilibrium states and phase transitions are characterized by
fluctuations. Such magnetic fluctuation can in principle be detected with
scattering-based x-ray photon correlation spectroscopy (XPCS). However, in the
established approach of XPCS, the magnetic scattering signal is quadratic in
the magnetic scattering cross section, which results not only in often
prohibitively small signals but also in a fundamental inability to detect
negative correlations (anticorrelations). Here, we propose to exploit the
possibility of heterodyne mixing of the magnetic signal with static charge
scattering to reconstruct the first-order (linear) magnetic correlation
function. We show that the first-order magnetic scattering signal reconstructed
from heterodyne scattering now directly represents the underlying magnetization
texture. Moreover, we suggest a practical implementation based on an absorption
mask rigidly connected to the sample, which not only produces a static charge
scattering signal but also eliminates the problem of drift-induced artificial
decay of the correlation functions. Our method thereby significantly broadens
the range of scientific questions accessible by magnetic x-ray photon
correlation spectroscopy
Monolithic focused reference beam x-ray holography
Fourier transform holography is a highly efficient and robust imaging method, suitable for single-shot imaging at coherent X-ray sources. In its common implementation, the image contrast is limited by the reference signal generated by a small pinhole aperture. Increased pinhole diameters improve the signal, whereas the resolution is diminished. Here we report a new concept to decouple the spatial resolution from the image contrast by employing a Fresnel zone plate to provide the reference beam. Superimposed on-axis images of distinct foci are separated with a novel algorithm. Our method is insensitive to mechanical drift or vibrations and allows for long integration times common at low-flux facilities like high harmonic generation sources. The application of monolithic focused reference beams improves the efficiency of high-resolution X-ray Fourier transform holography beyond all present approaches and paves the path towards sub-10 nm single-shot X-ray imaging
Field-free deterministic ultra fast creation of skyrmions by spin orbit torques
Magnetic skyrmions are currently the most promising option to realize
current-driven magnetic shift registers. A variety of concepts to create
skyrmions were proposed and demonstrated. However, none of the reported
experiments show controlled creation of single skyrmions using integrated
designs. Here, we demonstrate that skyrmions can be generated deterministically
on subnanosecond timescales in magnetic racetracks at artificial or natural
defects using spin orbit torque (SOT) pulses. The mechanism is largely similar
to SOT-induced switching of uniformly magnetized elements, but due to the
effect of the Dzyaloshinskii-Moriya interaction (DMI), external fields are not
required. Our observations provide a simple and reliable means for skyrmion
writing that can be readily integrated into racetrack devices
XUV digital in-line holography using high-order harmonics
A step towards a successful implementation of timeresolved digital in-line
holography with extreme ultraviolet radiation is presented. Ultrashort XUV
pulses are produced as high-order harmonics of a femtosecond laser and a
Schwarzschild objective is used to focus harmonic radiation at 38 nm and to
produce a strongly divergent reference beam for holographic recording.
Experimental holograms of thin wires are recorded and the objects
reconstructed. Descriptions of the simulation and reconstruction theory and
algorithms are also given. Spatial resolution of few hundreds of nm is
potentially achievable, and micrometer resolution range is demonstrated.Comment: 8 pages, 8 figure
- …