Resonant and high resolution scattering for structural studies of chiral liquid crystals

P. Barois, C. Ybert, P. Cluzeau, E. Grelet, F. Nallet, P. Fernandes, L. Navailles, B. Pansu, H.T. NGuyen, P. Gisse, V. Ravaine, A.-M. Levelut, C.C. Huang, J.S. Micha, F. Rieutord

The scientists of Centre de recherche Paul Pascal in Bordeaux (France) have several times carried out experiments on GMT whose common points are studies of liquid crystals (LC). BM32 and the GMT instrument offer in this context many assets: high flux to observe the emergence of weak peaks but not too high to avoid damaging the fragile samples or disturbing their physical state very sensitive to the temperature, high photon energy easily available (>12keV) and tunable (excellent stability of the optical parameter of the beam over more than 100 eV), detection and incidence angles of high precision.

The goal of the first experiment (SC-586) was to look for resonant scattering from a chiral smectic C LC at energy close to the absorbing K-edge of Bromine, used as resonant element in specially synthetized LCs. A special oven designed for free standing films (10mK accuracy, helium flushed flight path, low scattering windows, video monitoring of optical textures) was then brought over and installed successfully on the beamline. The choice of elements with high resonant energy such as Bromine (Br) or Selenium (Se) (i.e. in the 12-13 keV range) was critical to apply the technique in device geometries, under strong anchoring conditions and external electric field. Such experimental conditions were impossible to achieve with soft x-rays used previously. The experiments showed that a good resonant signal was observed at Br edge (fig. xx and ref 1). The resonant signal was later found to be much more intense at Se K-edge on a selenophene LC.

resonantsignal This suggested that a selenophene material might be incorporated as a molecular probe in host molecules, hence extending the capabilities of the technique to LCs without resonant element. This possibility was confirmed recently on BM32: the fine structure of two ferrielectric phases of the reference material MHPOBC were evidenced for the first time by mixing this historical liquid crystal with a few percent of a selenophene probe [2].
Fig 1: Resonant x-ray scattering effect on peaks intensity of a brominated chiral liquid crystal material exhibiting antiferro, ferri and ferroelectric smectic C phases [1].  

X-ray resonant scattering
Resonant scattering reveal structural peaks that conventional crystallography (far from absorbing edge) forbids since they correspond to gliding planes or screw axes. Close to absorbing (resonant) edge, the tensorial nature of the form factor (rank 2) is more pronounced (non negligible anti-diagonal terms). In that condition the structure factor - sum of the scattered amplitude from element of the crystallographic cell - is no more systematically nul with respect to electric field direction and the relative position vectors of the scattering elements. For liquid crystals presenting layered, helical and twisted structure, resonant scattering is a powerfull tool for investigating their structural and physical properties.

A second set of experiments was devoted to the study of the beautiful analogy pointed out by de Gennes between smectic liquid crystals and superconductors [3]. High-resolution x-ray scattering data provided structural information on the twisted LC phase that corresponds, in the type II superconductor analogy, to the liquid of magnetic vortices resulting from the fusion of the Abrikosov flux lattice. The evolution of the smectic correlation length with temperature shows a sudden change that agrees well with this picture. The value of the experimental parameters is also consistent with theoretical models of the liquid of screw dislocations, LC analogue of the liquid of vortices. Beyond the elegance of the analogy, evidenced here, these experiments on liquid crystals provide a direct mean to measure the correlation function of the order parameter: a possibility that may trigger new interactions with the superconductor community [4].

 

 

References

 

[1] P. Cluzeau, P. Gisse, V. Ravaine, A.-M. Levelut, P. Barois, C.C. Huang, F. Rieutord and H.T. NGuyen, Ferroelectrics, 244 (2000) 301, "Resonant X-ray diffraction study of a new brominated chiral SmCA* liquid crystal"
[2] P. Fernandes, E. Grelet, F. Nallet, J.S. Micha and P. Barois, «Extension of the resonant scattering technique to liquid crystalline materials without resonant element» in preparation
[3] De Gennes P.-G., solid state Commun., 10 (1972) 753
[4] C. Ybert, L. Navailles, B. Pansu, F. Rieutord, H.T. NGuyen and P. Barois, Europhys. Lett. 63 (2003) 840

 

More informations:

 

Liquid Crystals group in Centre de Recherche Paul Pascal, Bordeaux, France