arXiv subjects

Marc Herzog

Publications and source records attributed to Marc Herzog.

11 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

Unveiling the nanomorphology of HfN thin films by ultrafast reciprocal space mapping

Hafnium Nitride (HfN) is a promising and very robust alternative to gold for applications of nanoscale metals. Details of the nanomorphology related to variations in strain states and optical properties can be crucial for applications in nanophotonics and plasmon-assisted chemistry. We use ultrafast reciprocal space mapping (URSM) with hard x-rays to unveil the nanomorphology of thin HfN films. Static high-resolution x-ray diffraction reveals a twofold composition of the thin films being separated into regions with identical lattice constant and similar out-of-plane but hugely different in-plane coherence lengths. URSM upon femtosecond laser excitation reveals different transient strain dynamics for the two respective Bragg peak components. This unambiguously locates the longer in-plane coherence length in the first 15\,nm of the thin film adjacent to the substrate. The transient shift of the broad diffraction peak displays the strain dynamics of the entire film, implying that the near-substrate region hosts nanocrystallites with small and large coherence length, whereas the upper part of the film grows in small columnar grains. Our results illustrate that URSM is a suitable technique for non-destructive investigations of the depth-resolved nanomorphology of nanostructures.

cond-mat.mtrl-sci↗

arXiv2023

Concepts and use cases for picosecond ultrasonics with x-rays

This review discusses picosecond ultrasonics experiments using ultrashort hard x-ray probe pulses to extract the transient strain response of laser-excited nanoscopic structures from Bragg-peak shifts. This method provides direct, layer-specific, and quantitative information on the picosecond strain response for structures down to few-nm thickness. We model the transient strain using the elastic wave equation and express the driving stress using Grüneisen parameters stating that the laser-induced stress is proportional to energy density changes in the microscopic subsystems of the solid, i.e., electrons, phonons and spins. The laser-driven strain response can thus serve as an ultrafast proxy for local energy-density and temperature changes, but we emphasize the importance of the nanoscale morphology for an accurate interpretation due to the Poisson effect. The presented experimental use cases encompass ultrathin and opaque metal heterostructures, continuous and granular nanolayers as well as negative thermal expansion materials, that each pose a challenge to established all-optical techniques.

cond-mat.mtrl-sci↗

arXiv2022

Ultrafast charge carrier and exciton dynamics in an excitonic insulator probed by time-resolved photoemission spectroscopy

An excitonic insulator phase is expected to arise from the spontaneous formation of electron-hole pairs (excitons) in semiconductors where the exciton binding energy exceeds the size of the electronic band gap. At low temperature, these ground state excitons stabilize a new phase by condensing at lower energy than the electrons at the valence band top, thereby widening the electronic band gap. The envisioned opportunity to explore many-boson phenomena in an excitonic insulator system is triggering a very active debate on how ground state excitons can be experimentally evidenced. Here, we employ a nonequilibrium approach to spectrally disentangle the photoinduced dynamics of an exciton condensate from the entwined signature of the valence band electrons. By means of time-and angle-resolved photoemission spectroscopy of the occupied and unoccupied electronic states, we follow the complementary dynamics of conduction and valence band electrons in the photoexcited low-temperature phase of Ta2 NiSe5 , the hitherto most promising single-crystal candidate to undergo a semiconductor-to-excitonic-insulator phase transition. The photoexcited conduction electrons are found to relax within less than 1 ps. Their relaxation time is inversely proportional to their excess energy, a dependence that we attribute to the reduced screening of Coulomb interaction and the low dimensionality of Ta2NiSe5 . Long after (> 10 ps) the conduction band has emptied, the photoemission intensity below the Fermi energy has not fully recovered the equilibrium value. Notably, this seeming carrier imbalance cannot be rationalized simply by the relaxation of photoexcited electrons and holes across the semiconducting band gap. [...]

cond-mat.str-el↗

arXiv2018

Inhibition of the photoinduced structural phase transition in the excitonic insulator Ta$_2$NiSe$_5$

Femtosecond time-resolved mid-infrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator Ta$_2$NiSe$_5$ below the critical temperature of its structural phase transition. We find that the phonon dynamics show a strong coupling to the excitation of free carriers at the Γ point of the Brillouin zone. The optical response saturates at a critical excitation fluence $F_C = 0.30~\pm~0.08$~mJ/cm$^2$ due to optical absorption saturation. This limits the optical excitation density in Ta$_2$NiSe$_5$ so that the system cannot be pumped sufficiently strongly to undergo the structural change to the high-temperature phase. We thereby demonstrate that Ta$_2$NiSe$_5$ exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition.

cond-mat.str-el↗

arXiv2017

Quantitative disentanglement of coherent and incoherent laser-induced surface deformations by time-resolved x-ray reflectivity

We present time-resolved x-ray reflectivity measurements on laser excited coherent and incoherent surface deformations of thin metallic films. Based on a kinematical diffraction model, we derive the surface amplitude from the diffracted x-ray intensity and resolve transient surface excursions with sub-Angstrom spatial precision and 70 ps temporal resolution. The novel analysis allows for decomposition of the surface amplitude into multiple coherent acoustic modes and a substantial contribution from incoherent phonons which constitute the sample heating.

cond-mat.mtrl-sci↗

arXiv2017

Ultrafast Electronic Band Gap Control in an Excitonic Insulator

We report on the nonequilibrium dynamics of the electronic structure of the layered semiconductor Ta$_2$NiSe$_5$ investigated by time- and angle-resolved photoelectron spectroscopy. We show that below the critical excitation density of $F_{C} = 0.2$ mJ cm$^{-2}$, the band gap $narrows$ transiently, while it is $enhanced$ above $F_{C}$. Hartree-Fock calculations reveal that this effect can be explained by the presence of the low-temperature excitonic insulator phase of Ta$_2$NiSe$_5$, whose order parameter is connected to the gap size. This work demonstrates the ability to manipulate the band gap of Ta$_2$NiSe$_5$ with light on the femtosecond time scale.

cond-mat.str-el↗

arXiv2017

Spatio-temporal coherent control of thermal excitations in solids

X-ray reflectivity (XRR) measurements of femtosecond laser-induced transient gratings are applied to demonstrate the spatio-temporal coherent control of thermally induced surface deformations on ultrafast timescales. Using gracing incidence X-ray diffraction we unambiguously measure the amplitude of transient surface deformations with sub-Å resolution. Understanding the dynamics of femtosecond TG excitations in terms of superposition of acoustic and thermal gratings makes it possible to develop new ways of coherent control in X-ray diffraction experiments. Being the dominant source of TG signal, the long-living thermal grating with spatial period $Λ$ can be canceled by a second, time-delayed TG excitation shifted by $Λ/2$. The ultimate speed limits of such an ultrafast X-ray shutter are inferred from the detailed analysis of thermal and acoustic dynamics in TG experiments.

cond-mat.mtrl-sci↗

arXiv2014

Instantaneous band gap collapse in photoexcited monoclinic VO$_2$ due to photocarrier doping

Using femtosecond time-resolved photoelectron spectroscopy we demonstrate that photoexcitation transforms monoclinic VO$_2$ quasi-instantaneously into a metal. Thereby, we exclude an 80 femtosecond structural bottleneck for the photoinduced electronic phase transition of VO$_2$. First-principles many-body perturbation theory calculations reveal a high sensitivity of the VO$_2$ bandgap to variations of the dynamically screened Coulomb interaction, supporting a fully electronically driven isostructral insulator-to-metal transition. We thus conclude that the ultrafast band structure renormalization is caused by photoexcitation of carriers from localized V 3d valence states, strongly changing the screening \emph{before} significant hot-carrier relaxation or ionic motion has occurred.

cond-mat.str-el↗

arXiv2013

Direct time domain sampling of sub-THz coherent acoustic phonon spectra in SrTiO$_3$ using ultrafast X-ray diffraction

We synthesize sub-THz longitudinal quasi-monochromatic acoustic phonons in a SrTiO$_3$ single crystal using a SrRuO$_3$/SrTiO$_3$ superlattice as an optical-acoustic transducer. The generated acoustic phonon spectrum is determined using ultrafast X-ray diffraction. The analysis of the generated phonon spectrum in the time domain reveals a k-vector dependent phonon lifetime. It is observed that even at sub-THz frequencies the phonon lifetime agrees with the 1/$ω^2$ power law known from Akhiezer's model for hyper sound attenuation. The observed shift of the synthesized spectrum to the higher $q$ is discussed in the framework of non-linear effects appearing due to the high amplitude of the synthesized phonons.

cond-mat.mtrl-sci↗

arXiv2013

Ultrafast x-ray diffraction studies of photoexcited coherent phonons in SrRuO$_3$ thin films

We present ultrafast x-ray diffraction experiments on thin films of metallic SrRuO$_3$ (SRO) after their excitation with ultrashort intense laser pulses. Depending on the layer thickness, the data exhibit a transient splitting of the (002) SRO Bragg peak evidencing the generation and propagation of sharp acoustic strain waves. These distinct structural dynamics are due to the exceptionally fast electron-phonon relaxation that gives rise to a quasi-instantaneous thermal stress in SRO. The interpretation is corroborated by numerical simulations which show excellent agreement with the experimental findings. Despite the qualitatively different lattice dynamics for different SRO layer thicknesses, we identify a universal evolution of the transient average layer strain. The inferred discrepancy of the thermal stress profile from the excitation profile may hint toward a temperature-dependent effective Grüneisen parameter of SRO.

cond-mat.mes-hall↗