Research of the Coherent X-Ray Scattering Group

Structure and Dynamics of Colloidal Systems

X-ray Cross Correlation Analysis (XCCA)

Ultrafast magnetization dynamics

X-Ray Delay Unit & X-Ray Beam Splitting

Hard X-Ray Delay Line

Complex Fluids and Glasses

In this research area we study structure and dynamics of amorphous materials with special attention on colloidal fluids and glasses. Colloidal systems consist of nanoparticles with a typical size between 50 and 500 nm that are suspended in a solvent. In general, these colloidal suspensions can be divided into two different systems, specifiable on the basis of their stabilization which both are synthezised and characterized in our group: hard sphere systems consist of sterically stabilized particles and soft sphere systems consist of charge stabilized particles. Depending on various parameters such as particle concentration and charge, such colloidal systems form fluid, crystal or glassy states.
 
We use Small Angle X-ray Scattering (SAXS) to determine both the structure of a single particle (intraparticle correlations) and the averaged structure of the whole suspension (interparticle correlations).
 
By using coherent x-rays as probe, the exact arrangement of the colloidal particles in the suspension is encoded in the diffraction pattern, the so called speckle pattern. Studying changes of the speckle patterns with time allows to reveal the dynamic behavior of the sample. This method is known as X-Ray Photon Correlation Spectroscopy (XPCS). We use XPCS to study the dynamics of colloidal systems over the whole phase diagram with special attention on glass transition dynamics.
 
In contrast to crystalline matter, liquids and glasses do not show a long range structural order. The analysis of speckle patterns allows to overcome the averaging present in typical (incoherent) scattering experiments giving rise to the orientational averaged structure factor only. Our recently developed X-ray Cross Correlation Analysis (XCCA) scheme offers the possibility to probe the orientational local order of disordered samples. We use XCCA to reveal the structure of various colloidal systems ranging from fast-relaxing liquids to slow glassy samples. In particular, the structure of molecular and atomic liquids is investigated by making use of ultrashort FEL pulses.
 

References

G. Grübel and F. Zontone. Correlation spectroscopy with coherent x-rays. Journal of Alloys and Compounds 362, 3 (2004).

P. Wochner, C. Gutt, T. Autenrieth, T. Demmer, V. Bugaev, A. Diaz Ortiz, A. Duri, F. Zontone, G. Grübel, and H. Dosch. X-ray Cross Correlation Analysis Uncovers Hidden Local Symmetries in Disordered Matter. Proc. Natl. Acad. Sci. 106, 11511 (2009).

Ultrafast magnetization dynamics

Understanding magnetization dynamics on the femtosecond time and nanometer length scale is a challenging problem in modern magnetism. Ultrashort pulses of XUV radiation delivered by FEL sources combined with a suitable pump (e.g. IR femtosecond laser, THz radiation...) allow for the first time to address both scales at the same time giving new insights in the physics of demagnetization of magnetic domain systems. Furthermore, real space images can be taken with a single FEL pulse employing the lensless Fourier Transform Holography (FTH) technique. We aim at combining pump-probe techniques with FTH imaging further deepen our understanding of the demagnetization process.

References

C. Gutt et al. Single-pulse resonant magnetic diffraction using a soft x-ray free electron laser. Phys. Rev. B 81, 100401 (2010).

L. Müller et al. Breakdown of the X-ray Resonant Magnetic Scattering Signal during Intense Pulses of Extreme Ultraviolet Free-Electron Laser Radiation. Phys. Rev. Lett. 110, 234801 (2013). Ph

Hard X-ray Delay Line

Probing a matter on a time scale of femto to nano seconds will be possible with the future LINAC driven X-ray lasers. With 3rd generation synchrotron light sources one can try to perform time resolved experiments utilizing delay techniques. The most crucial activity is then to build a device capable to split an X-ray pulse into two adjustable fractions, delay one of them and recombine both pulses.

References 

W. Roseker, S. Lee, M. Walther, H. Schulte-Schrepping, H. Franz, A. Gray, M. Sikorski, P.H. Fuoss, G.B. Stephenson, A. Robert, and G. Grübel. Hard x-ray delay line for x-ray photon correlation spectroscopy and jitter-free pump-probe experiments at LCLS. SPIE Conference Proceedings 8504, 85040I (2012).