title: High-resolution x-ray study of the nematic-smectic-A and smectic-A-smectic-C transitions in liquid-crystal-aerosil gels.
abstract: The effects of dispersed aerosil nanoparticles on two of the phase transitions of the thermotropic liquid-crystal material 4-n-pentylphenylthiol-4(')-n-octyloxybenzoate (8;S5) have been studied using high-resolution x-ray diffraction techniques. The aerosils hydrogen bond together to form a gel which imposes a weak quenched disorder on the liquid crystal. The smectic-A fluctuations are well characterized by a two-component line shape representing thermal and random-field contributions. An elaboration on this line shape is required to describe the fluctuations in the smectic-C phase; specifically the effect of the tilt on the wave-vector dependence of the thermal fluctuations must be explicitly taken into account. Both the magnitude and the temperature dependence of the smectic-C tilt order parameter are observed to be unaffected by the disorder. This may be a consequence of the large bare smectic correlation length in the direction of modulation for this transition. These results show that the understanding developed for the nematic to smectic-A transition for octylcyanobiphenyl and octyloxycyanobiphenyl liquid crystals with quenched disorder can be extended to quite different materials and transitions.
authors.affiliation: Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7.
authors.forename: P S
authors.initials: PS
authors.lastname: Clegg
authors.forename: R J
authors.initials: RJ
authors.lastname: Birgeneau
authors.forename: S
authors.initials: S
authors.lastname: Park
authors.forename: C W
authors.initials: CW
authors.lastname: Garland
authors.forename: G S
authors.initials: GS
authors.lastname: Iannacchione
authors.forename: R L
authors.initials: RL
authors.lastname: Leheny
authors.forename: M E
authors.initials: ME
authors.lastname: Neubert