Ultrafast Electronic Domain Fluctuations Detected in a Nonequilibrium X-Ray Speckle Visibility Experiment

N. Hua,1 Y. Sun,2 P. Rao,3 N. Z. Hagström,3 B. Stoychev,4 E. Lamb,4 Meera,3
S. T. Botu,3 S. Jeppson,3 S.-W. Huang,5 V. Esposito,2 D. Zhu,2 T. Sato,2 S. Song,2
E. E. Fullerton,6 O. G. Shpyrko,4 R. Kukreja,3 S. Gerber1

1Laboratory for X-Ray Nanoscience and Technologies, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

2Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA

3Department of Materials Science Engineering, University of California-Davis, Davis, California 95616, USA

4Department of Physics, University of California, San Diego, La Jolla, California 92093, USA

5Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

6Center for Memory and Recording Research, University of California, San Diego, La Jolla, California, 92093, USA

Since the advent of X-ray free electron lasers, a standard method to study metal-insulator transitions is in a nonequilibrium pump-probe diffraction experiment to disentangle how different order parameters evolve at ultrafast timescales. However, this technique is blind to domain fluctuations of the order parameter that may play a critical role in driving these nonequilibrium transitions. To directly couple to these domain fluctuations at ultrafast timescales requires coherent X-ray probes following a laser excitation. Here we employed a novel coherent X-ray technique that uses a split-and-delay line in a pump-probe-probe experimental scheme to measure ultrafast domain fluctuations for the first time. This experiment was carried out at the X-ray Correlation Spectroscopy (XCS) beamline of the Linac Coherent Light Source (LCLS) where we accessed the speckle pattern of a resonant charge order peak in Fe3O4 to quantify domain fluctuations with 1-picosecond temporal resolution. A complementary, standard X-ray pump-probe experiment on the same charge order peak was carried out at the Bernina end station of SwissFEL to reveal ultrafast melting of the electronic order parameter. Together these two experiments reveal the nonequilibrium evolution of the charge order at picosecond and nanometer length scales.