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We present a multi‐tau two‐time correlation (MT‐2TC) analysis for X‐ray photon correlation spectroscopy that enables efficient analysis of large‐scale, high‐frame‐rate datasets collected at modern synchrotron beamlines. The method combines the advantages of the multi‐tau autocorrelation and two‐time correlation approaches, maintaining sensitivity to non‐stationary dynamics while reducing computational complexity and memory requirements from quadratic to linear scaling with the number of frames. This formulation not only improves processing efficiency but also extends the accessible correlation time range, allowing analysis over much broader time scales than conventional techniques. The algorithm is implemented in both dense and sparse data representations, allowing direct processing of sparsified photon‐counting data from ultra‐fast detectors. We demonstrate the performance of the MT‐2TC scheme using experiments on a bulk GeO2 glass and on a v‐Ta2O5 thin film conducted at the ESRF ID10 beamline with a 1 kHz Eiger 4M detector. The results show perfect agreement with standard correlation analyses while extending the temporal correlation range by nearly two decades and improving signal‐to‐noise ratios at long lag times. The MT‐2TC approach thus provides a scalable and flexible tool for XPCS at fourth‐generation synchrotrons, where enhanced coherent flux and detector speed demand correlation strategies capable of spanning microsecond‐to‐day timescales. A scalable multi‐tau two‐time correlation framework enables efficient, time‐resolved XPCS analysis of long duration high‐frame‐rate modern synchrotron data while preserving sensitivity to non‐stationary dynamics.

In this article, we present the experimental protocol and data‐processing framework for megahertz X‐ray Photon Correlation Spectroscopy (MHz‐XPCS) experiments on soft matter samples implemented at the Materials Imaging and Dynamics (MID) instrument of the European X‐ray Free‐Electron Laser (EuXFEL). Due to the introduction of a standard configuration and the implementation of a highly automated data‐processing pipeline, MHz‐XPCS measurements can now be conducted and analyzed with minimal user intervention. A key challenge lies in managing the extremely large data volumes generated by the Adaptive Gain Integrating Pixel Detector (AGIPD) – often reaching several petabytes within a single experiment. We describe the technical implementation, discuss the hardware requirements related to effective parallel data processing and propose strategies to enhance data quality, in particular related to data reduction strategies and an improvement of the signal‐to‐noise ratio. Finally, we address strategies for making the processed data FAIR (Findable, Accessible, Interoperable, Reusable), in alignment with the goals of the DAPHNE4NFDI project. We present a standardized and highly automated pipeline for megahertz X‐ray photon correlation spectroscopy (MHz‐XPCS) on soft‐matter samples at the MID instrument of the European XFEL. The workflow addresses AGIPD petabyte‐scale data handling, detector‐artifact correction, correlation analysis and FAIR data output, enabling routine MHz‐XPCS with minimal user intervention.

X‐ray photon correlation spectroscopy (XPCS) holds strong promise for observing atomic‐scale dynamics in materials, both at equilibrium and during non‐equilibrium transitions. Here an in situ XPCS study of the relaxor ferroelectric PbMg1/3Nb2/3O3 (PMN) is reported. A weak applied AC electric field generates strong response in the speckle of the diffuse scattering from the polar nanodomains, which is captured using the two‐time correlation function. Correlated motions of the Bragg peak are also observed, which indicate dynamic tilting of the illuminated volume. This tilting quantitatively accounts for the observed two‐time speckle correlations. The magnitude of the tilting would not be expected solely from the modest applied field, since PMN is an electrostrictive material with no linear strain response to the field. A model is developed based on non‐uniform static charging of the illuminated surface spot by the incident micrometre‐scale X‐ray beam and the electrostrictive material response to the combination of static and dynamic fields. The model qualitatively explains the direction and magnitude of the observed tilting, and predicts that X‐ray‐induced piezoresponse could be an important factor in correctly interpreting results from XPCS and nanodiffraction studies of other insulating materials under applied AC field or varying X‐ray illumination. X‐ray illumination induces surface charging and gives rise to substantially enhanced piezoresponse in a relaxor ferroelectric captured by X‐ray photon correlation spectroscopy.

pyXPCSviewer, a Python‐based graphical user interface that is deployed at beamline 8‐ID‐I of the Advanced Photon Source for interactive visualization of XPCS results, is introduced. pyXPCSviewer parses rich X‐ray photon correlation spectroscopy (XPCS) results into independent PyQt widgets that are both interactive and easy to maintain. pyXPCSviewer is open‐source and is open to customization by the XPCS community for ingestion of diversified data structures and inclusion of novel XPCS techniques, both of which are growing demands particularly with the dawn of near‐diffraction‐limited synchrotron sources and their dedicated XPCS beamlines. The Python‐based graphical user interface pyXPCSviewer that is deployed at beamline 8‐ID‐I of the Advanced Photon Source for interactive visualization of X‐ray photon correlation spectroscopy results is introduced.

Dynamics and kinetics in soft matter physics, biology, and nanoscience frequently occur on fast (sub)microsecond but not ultrafast timescales which are difficult to probe experimentally. The European X-ray Free-Electron Laser (European XFEL), a megahertz hard X-ray Free-Electron Laser source, enables such experiments via taking series of diffraction patterns at repetition rates of up to 4.5 MHz. Here, we demonstrate X-ray photon correlation spectroscopy (XPCS) with submicrosecond time resolution of soft matter samples at the European XFEL.We show that the XFEL driven by a superconducting accelerator provides unprecedented beam stability within a pulse train. We performed microsecond sequential XPCS experiments probing equilibrium and nonequilibrium diffusion dynamics in water. We find nonlinear heating on microsecond timescales with dynamics beyond hot Brownian motion and superheated water states persisting up to 100 μs at high fluences. At short times up to 20 μs we observe that the dynamics do not obey the Stokes–Einstein predictions.

We probe the microstructural yielding dynamics of a concentrated colloidal system by performing creep/recovery tests with simultaneous collection of coherent scattering data via X-ray Photon Correlation Spectroscopy (XPCS). This combination of rheology and scattering allows for time-resolved observations of the microstructural dynamics as yielding occurs, which can be linked back to the applied rheological deformation to form structure–property relations. Under sufficiently small applied creep stresses, examination of the correlation in the flow direction reveals that the scattering response recorrelates with its predeformed state, indicating nearly complete microstructural recovery, and the dynamics of the system under these conditions slows considerably. Conversely, larger creep stresses increase the speed of the dynamics under both applied creep and recovery. The data show a strong connection between the microstructural dynamics and the acquisition of unrecoverable strain. By comparing this relationship to that predicted from homogeneous, affine shearing, we find that the yielding transition in concentrated colloidal systems is highly heterogeneous on the microstructural level.

X-ray photon correlation spectroscopy (XPCS) is a powerful technique for evaluating microscopic dynamics using coherent X-rays. Detecting fast or small-scale dynamics typically requires strong illumination and wide-angle scattering detection; however, such conditions can cause non-negligible sample damage. This study presents a non-uniform pulse-interval XPCS approach that enables quantitative dynamical analysis with substantially reduced X-ray exposure. In conventional XPCS, continuous acquisition at uniform time intervals leads to long cumulative exposure, which can introduce radiation-induced artefacts. In this study, only 11 scattering images were recorded at non-uniform intervals, providing a broad range of delay times from a single measurement and enabling dense temporal sampling without increasing the exposure dose. The resulting dataset was analyzed using both XPCS- and X-ray speckle visibility spectroscopy (XSVS)-based schemes, and the results demonstrated that these two independent analyses yield consistent relaxation behaviors. The proposed approach offers an efficient framework for probing complex or non-Brownian dynamics in radiation-sensitive materials and expands the applicability of XPCS to soft and biological systems.

Coefficients for translational and rotational diffusion characterize the Brownian motion of particles. Emerging X-ray photon correlation spectroscopy (XPCS) experiments probe a broad range of length scales and time scales and are well-suited for investigation of Brownian motion. While methods for estimating the translational diffusion coefficients from XPCS are well-developed, there are no algorithms for measuring the rotational diffusion coefficients based on XPCS, even though the required raw data are accessible from such experiments. In this paper, we propose angular-temporal cross-correlation analysis of XPCS data and show that this information can be used to design a numerical algorithm (Multi-Tiered Estimation for Correlation Spectroscopy [MTECS]) for predicting the rotational diffusion coefficient utilizing the cross-correlation: This approach is applicable to other wavelengths beyond this regime. We verify the accuracy of this algorithmic approach across a range of simulated data.

Synchrotron-radiation-based techniques are a powerful tool for the investigation of materials. In particular, the availability of highly brilliant sources has opened the possibility to develop techniques sensitive to dynamics at the atomic scale such as X-ray photon correlation spectroscopy (XPCS). XPCS is particularly relevant in the study of glasses, which have been often investigated at the macroscopic scale by, for example, differential scanning calorimetry. Here, we show how to adapt a Flash calorimeter to combine XPCS and calorimetric scans. This setup paves the way to novel experiments requiring dynamical and thermodynamic information, ranging from the study of the crystallization kinetics to the study of the glass transition in systems that can be vitrified thanks to the high cooling rates reachable with an ultrafast calorimeter.

A new experimental setup combining X-ray photon correlation spectroscopy (XPCS) in the hard X-ray regime and a high-pressure sample environment has been developed to monitor the pressure dependence of the internal motion of complex systems down to the atomic scale in the multi-gigapascal range, from room temperature to 600 K. The high flux of coherent high-energy X-rays at fourth-generation synchrotron sources solves the problems caused by the absorption of diamond anvil cells used to generate high pressure, enabling the measurement of the intermediate scattering function over six orders of magnitude in time, from 10 −3  s to 10 3  s. The constraints posed by the high-pressure generation such as the preservation of X-ray coherence, as well as the sample, pressure and temperature stability, are discussed, and the feasibility of high-pressure XPCS is demonstrated through results obtained on metallic glasses.