arXiv subjects

Peter Gaal

Publications and source records attributed to Peter Gaal.

10 recordsLinked to original sources

arXiv2026

Nonlinear Elasticity at the Damage Threshold of Semiconductor Nanocrystals

The nonlinear photoacoustic response of indium phosphide nanocrystals on silicon nanotip arrays is investigated using time-resolved optical pump-probe spectroscopy and synchrotron-based X-ray diffraction. Femtosecond laser excitation triggers low-frequency and high-frequency radial breathing modes of the nanocrystals at 8 GHz and 10.3 GHz, respectively. At excitation fluences above 3 mJ/cm^2, nonlinear frequency mixing occurs, including sum- and difference-frequency generation, indicative of strain-induced nonlinear elasticity. A higher-order extension of Hooke's law models the fluence-dependent spectral response and yields a physically valid elastic energy potential. Ex-situ energy-dispersive X-ray spectroscopy reveals a correlation between nanocrystal oxidation and the emergence of nonlinear acoustic modes. Time-resolved X-ray diffraction confirms the nanocrystals as the origin of the low-frequency modes and supports the hypothesis of acoustic decoupling from the substrate. These findings provide insight into the mechanical limits of semiconductor nanostructures under intense optical excitation and suggest new pathways for material characterization and optomechanical control at the nanoscale. The results advance the understanding of nonlinear phonon dynamics in nanocrystals and highlight their potential for integration into next-generation photonic and quantum devices.

cond-mat.mtrl-sci↗

arXiv2025

Sub-nanosecond structural dynamics of the martensitic transformation in Ni-Mn-Ga

Martensitic transformations drive a multitude of emerging applications, which range from high stroke actuation and, mechanocaloric refrigeration, to thermoelastic energy harvesting. All these applications benefit from faster transformations, as a high cycle frequency is essential for achieving high power density. However, systematic investigations of the fast dynamics and fundamental speed limits of martensitic transformations are scarce. Especially for ultrashort time transformations, the temperature evolution throughout the transformation is not measured, which is a substantial shortcoming as temperature is the intrinsic force driving the transformation. Here, we present a synchrotron-based time-resolved X-ray diffraction study of a 270 fs laser-induced martensitic transformation in a Ni-Mn-Ga-based epitaxial thin film. We observe the transformation from martensite to austenite within about 100 ps, just limited by the synchrotron probe pulse duration. Furthermore, a full transformation cycle from martensite to austenite and back to martensite can almost be finished within 5 ns, which is the fastest martensitic transformation reported so far. Measurements and calculations of the temperature evolution allow us to analyse the influence of temperature on transformation time. By time-resolved strain measurements we demonstrate that in addition to temperature, thermal film stress must be considered as a competing influence on the martensitic transformation. Our experimental findings are supported by molecular dynamics simulations with machine learned force fields adapted to density functional theory calculations. These reveal that the huge distortion during a martensitic transformation requires the collective movement of many atoms within the microstructure, which delays the transformation.

cond-mat.mtrl-sci↗

arXiv2025

Demonstration of Advanced Timing Schemes in Time-Resolved X-ray Diffraction Measurements

We present time-resolved X-ray diffraction measurements using advanced timing schemes that provide high temporal resolution while also maintaining a high flux in the X-ray probe beam. The method employs patterned probe pulse sequences that are generated with the WaveGate solid-state pulse picker. We demonstrate the feasibility of our method at two different beamlines on millisecond and microsecond timescales.

cond-mat.mtrl-sci↗

arXiv2024

Laser-initiated electron and heat transport in gold-skutterudite CoSb$_3$ bilayers resolved by pulsed x-ray scattering

Electron and lattice heat transport have been investigated in bilayer thin films of gold and CoSb$_3$ after photo-excitation of the nanometric top gold layer through picosecond x-ray scattering in a pump-probe setup. The unconventional observation of a larger portion of the deposited heat being detected first in the underlying CoSb$_3$ layer supports the picture of ballistic transport of the photo-excited electrons from gold to the underlying layer. The lattice expansion recorded by x-ray scattering allows accounting for the energy deposition and heat transport.

cond-mat.mtrl-sci↗

arXiv2022

What is the speed limit of martensitic transformations?

Structural martensitic transformations enable various applications, which range from high stroke actuation and sensing to energy efficient magnetocaloric refrigeration and thermomagnetic energy harvesting. All these emerging applications benefit from a fast transformation, but up to now the speed limit of martensitic transformations has not been explored. Here, we demonstrate that a martensite to austenite transformation can be completed in under ten nanoseconds. We heat an epitaxial Ni-Mn-Ga film with a laser pulse and use synchrotron diffraction to probe the influence of initial sample temperature and overheating on transformation rate and ratio. We demonstrate that an increase of thermal energy drives this transformation faster. Though the observed speed limit of 2.5 x 10^{27} (Js)^{-1} per unit cell leaves plenty of room for a further acceleration of applications, our analysis reveals that the practical limit will be the energy required for switching. Our experiments unveil that martensitic transformations obey similar speed limits as in microelectronics, which are expressed by the Margolus-Levitin theorem.

cond-mat.mtrl-sci↗

arXiv2020

Transition regime in the ultrafast laser heating of solids

Based on the phenomenological theory of heat diffusion, we show that the generated peak temperature $T_{\text{max}}$ after absorption of a laser pulse strongly depends on the pulse duration. We identify three different heat conduction regimes which can be identified via a simple parameter that depends only on the pulse duration and on material constants. The phenomenological approach is supported by numerical simulations of heat diffusion and measurements of the thermal surface expansion after transient grating excitation with 1 ps and 10 ns optical pulses.

physics.app-ph↗

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

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↗

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↗