arXiv subjects

Mariana Brede

Publications and source records attributed to Mariana Brede.

3 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↗

arXiv2023

Verification of ultrafast spin transfer effects in FeNi alloys

The optical intersite spin transfer (OISTR) effect was recently verified in Fe$_{50}$Ni$_{50}$ using magneto-optical Kerr measurements in the extreme ultraviolet range. However, one of the main experimental signatures analyzed in this work, namely a magnetic moment increase at a specific energy in Ni, was subsequently found also in pure Ni, where no transfer from one element to another is possible. Hence, it is a much-discussed issue whether OISTR in FeNi alloys is real and whether it can be verified experimentally or not. Here, we present a comparative study of spin transfer in Fe$_{50}$Ni$_{50}$, Fe$_{19}$Ni$_{81}$ and pure Ni. We conclusively show that an increase in the magneto-optical signal is indeed insufficient to verify OISTR. However, we also show how an extended data analysis overcomes this problem and allows to unambiguously identify spin transfer effects. Concomitantly, our work solves the long-standing riddle about the origin of delayed demagnetization behavior of Ni in FeNi alloys.

cond-mat.mtrl-sci↗