  Even though chiral nematic phases were the first liquid crystals
experimentally observed more than a century ago, the origin of the
thermodynamic stability of cholesteric states is still unclear. In this Letter
we address the problem by means of a novel density functional theory for the
equilibrium pitch of chiral particles. When applied to right-handed hard
helices, our theory predicts an entropy-driven cholesteric phase, which can be
either right- or left-handed, depending not only on the particle shape but also
on the thermodynamic state. We explain the origin of the chiral ordering as an
interplay between local nematic alignment and excluded-volume differences
between left- and right-handed particle pairs.
