2020, Bagschik, Kai, Schneider, Michael, Wagner, Jochen, Buss, Ralph, Riepp, Matthias, Philippi-Kobs, Andre, Müller, Leonard, Roseker, Wojciech, Trinter, Florian, Hoesch, Moritz, Viefhaus, Jens, Eisebitt, Stefan, Grübel, Gerhard, Oepen, Hans Peter, Frömter, Robert

Direct 2D spatial-coherence measurements are increasingly gaining importance at synchrotron beamlines, especially due to present and future upgrades of synchrotron facilities to diffraction-limited storage rings. We present a method to determine the 2D spatial coherence of synchrotron radiation in a direct and particularly simple way by using the Fourier-analysis method in conjunction with curved gratings. Direct photon-beam monitoring provided by a curved grating circumvents the otherwise necessary separate determination of the illuminating intensity distribution required for the Fourier-analysis method. Hence, combining these two methods allows for time-resolved spatial-coherence measurements. As a consequence, spatial-coherence degradation effects caused by beamline optics vibrations, which is one of the key issues of state-of-the-art X-ray imaging and scattering beamlines, can be identified and analyzed. © 2020 Optical Society of America.

2021, Ricci, Alessandro, Poccia, Nicola, Campi, Gaetano, Mishra, Shrawan, Müller, Leonard, Joseph, Boby, Shi, Bo, Zozulya, Alexey, Buchholz, Marcel, Trabant, Christoph, Lee, James C. T., Viefhaus, Jens, Goedkoop, Jeroen B., Nugroho, Agustinus Agung, Braden, Markus, Roy, Sujoy, Sprung, Michael, Schüßler-Langeheine, Christian

We study the temporal stability of stripe-type spin order in a layered nickelate with x-ray photon correlation spectroscopy and observe fluctuations on timescales of tens of minutes over a wide temperature range. These fluctuations show an anomalous temperature dependence: they slow down at intermediate temperatures and speed up on both heating and cooling. This behavior appears to be directly connected with spatial correlations: stripes fluctuate slowly when stripe correlation lengths are large and become faster when spatial correlations decrease. A low-temperature decay of nickelate stripe correlations, reminiscent of what occurs in cuprates as a result of a competition between stripes and superconductivity, hence occurs via loss of both spatial and temporal correlations.