title: Frank's constant in the hexatic phase.
abstract: Using videomicroscopy data of a two-dimensional colloidal system the bond-order correlation function G{6} is calculated and used to determine both the orientational correlation length xi{6} in the liquid phase and the modulus of orientational stiffness, Frank's constant F{A}, in the hexatic phase. The latter is an anisotropic fluid phase between the crystalline and the isotropic liquid phase. F{A} is found to be finite within the hexatic phase, takes the value 72/pi at the hexatic<-->isotropic liquid phase transition, and diverges at the hexatic<-->crystal transition as predicted by the Kosterlitz-Thouless-Halperin-Nelson-Young theory. This is a quantitative test of the mechanism of breaking the orientational symmetry by disclination unbinding.
authors.affiliation: Universität Graz, 8010 Graz, Austria.
authors.forename: P
authors.initials: P
authors.lastname: Keim
authors.forename: G
authors.initials: G
authors.lastname: Maret
authors.forename: H H
authors.initials: HH
authors.lastname: von Grünberg