arXiv subjects

Jasmin Jarecki

Publications and source records attributed to Jasmin Jarecki.

4 recordsLinked to original sources

arXiv2025

Experimental evidence of dominant ultrafast diffusive energy transport by hot electrons in Cu

When the dimensions of structures shrink to the order of the inelastic mean free path of the energy-carrying quasi-particles, the character of energy transport changes from diffusive to ballistic. However, the point of transition remains a matter of debate. Here, we determine the dominant channel of energy transport through a nanoscale Cu layer as a function of its thickness. The energy rapidly transferred across Cu via hot electrons from a photo-excited Pt layer into a buried Ni detection layer translates into a rapid expansion of the Ni layer probed via ultrafast x-ray diffraction. The non-linear dependence of the Ni strain amplitude on the absorbed laser fluence indicates that the transport through Cu becomes more efficient with increasing fluence. This fluence-dependent transport efficiency is reproduced by a diffusive energy transport model and serves as a generally applicable experimental approach to distinguish diffusion from ballistic transport. Following this approach, we identify diffusive electronic energy transport to govern the spatial energy distribution for Cu layer thicknesses larger than twice the electronic inelastic mean free path.

cond-mat.mtrl-sci↗

arXiv2024

Ultrafast energy-dispersive soft-x-ray diffraction in the water window with a laser-driven source

Time-resolved soft-x-ray-diffraction experiments give access to microscopic processes in a broad range of solid-state materials by probing ultrafast dynamics of ordering phenomena. While laboratory-based high-harmonic generation (HHG) light sources provide the required photon energies, their limited photon flux is distributed over a wide spectral range, rendering typical monochromatic diffraction schemes challenging. Here, we present a scheme for energy-dispersive soft-x-ray diffraction with femtosecond temporal resolution and photon energies across the water window from 200 to 600 eV. The experiment utilizes the broadband nature of the HHG emission to efficiently probe large slices in reciprocal space. As a proof-of-concept, we study the laser-induced structural dynamics of a Mo/Si superlattice in an ultrafast, non-resonant soft-x-ray diffraction experiment. We extract the underlying strain dynamics from the measured shift of its first order superlattice Bragg peak in reciprocal space at photon energies around 500 eV via soft-x-ray scattering simulations.

physics.optics↗

arXiv2024

Controlling effective field contributions to laser-induced magnetization precession by heterostructure design

Nanoscale heterostructure design can control laser-induced heat dissipation and strain propagation as well as their efficiency for driving magnetization precession. We use insulating MgO layers incorporated into metallic Pt-Cu-Ni heterostructures to block the propagation of hot electrons. Ultrafast x-ray diffraction (UXRD) experiments quantify how this enables controlling the spatio-temporal shape of the transient heat and strain, which drive the magnetization dynamics in the Ni layer. The frequency of the magnetization precession observed by the time-resolved magneto-optical Kerr effect (MOKE) in polar geometry is systematically tuned by the magnetic field orientation. The combined experimental analysis (UXRD and MOKE) and modeling of transient strain, heat and magnetization uniquely highlights the importance of quasi-static strain as a driver of precession, when the magnetic material is rapidly heated via electrons. The concomitant effective field change originating from demagnetization partially compensates the change induced by quasi-static strain. Tailored strain pulses shaped via the nanoscale heterostructure design provide an equally efficient, phase-matched driver of precession, paving the way for opto-magneto-acoustic devices with low heat energy deposited in the magnetic layer.

cond-mat.mtrl-sci↗

arXiv2024

Speed limits of the laser-induced phase transition in FeRh

We use ultrafast x-ray diffraction (UXRD) and the polar time-resolved magneto-optical Kerr effect (tr-MOKE) to study the laser-induced metamagnetic phase transition in two FeRh films with thicknesses below and above the optical penetration depth. In the thin film, we identify an intrinsic timescale for the light-induced nucleation of ferromagnetic (FM) domains in the antiferromagnetic material of $8\,\text{ps}$ that is substantially slower than the speed of sound. For the inhomogeneously excited thicker film, only the optically excited near-surface part transforms within $8\,\text{ps}$. For strong excitations we observe an additional slow rise of the FM phase, which we experimentally relate to a growth of the FM phase into the depth of the layer by comparing the transient magnetization in front- and backside excitation geometry. In the lower lying parts of the film, which are only excited via near-equilibrium heat transport, the FM phase emerges significantly slower than $8\,\text{ps}$ after heating above the transition temperature.

cond-mat.mtrl-sci↗