Picosecond pump pulses probe the relevance of hot electrons for the laser-induced phase transition in FeRh

Recent ultrafast photoemission experiments showed signatures of an ultrafast modification of the electronic band structure in FeRh indicative of a ferromagnetic (FM) state that is initiated by a non-equilibrium occupation of the electronic states upon femtosecond laser excitation. We use ultrafast x-ray diffraction to examine the impact of hot electrons on the antiferromagnetic (AFM) to FM phase transition. By increasing the pump-pulse duration up to $10.5\,\text{ps}$, we eliminate hot electrons and see that the nucleation of FM domains still proceeds at the intrinsic timescale of $8\,\text{ps}$, which starts when the deposited energy surpasses the threshold energy. For long pulses, the phase transition proceeds considerably faster than predicted by a convolution of the dynamics observed for ultrafast excitation with the long pump pulse duration. We predict that quite generally, slow photoexcitation can result in a fast response, if the non-linear threshold behavior of a first-order phase transition is involved

The Nachtlichter app: a citizen science tool for documenting outdoor light sources in public space

The relationship between satellite based measurements of city radiance at night and the numbers and types of physical lights installed on the ground is not well understood. Here we present the "Nachtlichter app", which was developed to enable citizen scientists to classify and count light sources along street segments over large spatial scales. The project and app were co-designed: citizen scientists played key roles in the app development, testing, and recruitment, as well as in analysis of the data. In addition to describing the app itself and the data format, we provide a general overview of the project, including training materials, data cleaning, and the result of some basic data consistency checks

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

Comprehensive ultrahigh resolution whole brain in vivo MRI dataset as a human phantom

Here, we present an extension to our previously published structural ultrahigh resolution T1-weighted magnetic resonance imaging (MRI) dataset with an isotropic resolution of 250 µm, consisting of multiple additional ultrahigh resolution contrasts. Included are up to 150 µm Time-of-Flight angiography, an updated 250 µm structural T1-weighted reconstruction, 330 µm quantitative susceptibility mapping, up to 450 µm structural T2-weighted imaging, 700 µm T1-weighted back-to-back scans, 800 µm diffusion tensor imaging, one hour continuous resting-state functional MRI with an isotropic spatial resolution of 1.8 mm as well as more than 120 other structural T1-weighted volumes together with multiple corresponding proton density weighted acquisitions collected over ten years. All data are from the same participant and were acquired on the same 7 T scanner. The repository contains the unprocessed data as well as (pre-)processing results. The data were acquired in multiple studies with individual goals. This is a unique and comprehensive collection comprising a "human phantom" dataset. Therefore, we compiled, processed, and structured the data, making them publicly available for further investigation