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Higher-order correlations in glass- and crystal-forming complex liquids
  Molecular Physics seminar

Monday 20 February 2017
from 10:00 to 11:00
at FD41
Speaker : Felix Lehmkühler (DESY (Hamburg))
Abstract : Although glasses are very common in everyday life, the glass transition is one of the mysteries in condensed matter physics. Upon approaching this transition, the sample dynamics slows down by orders of magnitude while the static structure remains almost unchanged. In addition, relaxation phenomena become non-exponential pointing to a broad distribution of relaxation times and the existence of dynamical heterogeneities, suggested to be closely connected to spatial heterogeneities. In this talk I will discuss the role of higher-order correlation functions for the glass transition of complex liquids from coherent X-ray scattering experiments. First, I will present a microrheology study on the dynamics of polypropylene-glycol by means of X-ray photon correlation spectroscopy (XPCS) at temperatures ranging from room temperature to the glass transition temperature Tg. Beside a cross-over from Brownian motion to hyperdiffusive and ballistic dynamics, dynamical heterogeneities increase dramatically upon cooling. This leads to two effects: (a) increasing spatial heterogeneity and (b) correlated motion at temperatures close to and below 1.12 Tg. Second, I will discuss structural higher-order correlations in colloidal systems by means of X-ray cross correlation analysis (XCCA). Examples will cover studies on thin films, colloidal crystals and various 3D-bulk systems. Special attention is set on hard sphere systems, where the impact of such correlations on accessing the sample structure beyond pair-correlations is presented. In the vicinity of the hard-sphere glass transition, one observes (1) appearance of ordered clusters prior to the glass transition and crystallisation and (2) increasing medium-range order in colloidal glasses. Both observations are accompanied by a slowing down of the relaxation time, proving experimentally the relation of order formation and dynamics in a soft matter system.

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