  We report the experimental determination of the structure and response to
applied electric field of the lower-temperature nematic phase of the previously
reported calamitic compound
4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its
electro-optics to visualize the appearance, in the absence of applied field, of
a permanent electric polarization density, manifested as a spontaneously broken
symmetry in distinct domains of opposite polar orientation. Polarization
reversal is mediated by field-induced domain wall movement, making this phase
ferroelectric, a 3D uniaxial nematic having a spontaneous, reorientable,
polarization locally parallel to the director. This polarization density
saturates at a low temperature value of ~ 6 microcoulombs/cm-sqd, the largest
ever measured for an organic material or for any fluid. This polarization is
comparable to that of solid state ferroelectrics, and is close to the average
value obtained by assuming perfect, polar alignment of molecular long axes in
the nematic. We find a host of spectacular optical and hydrodynamic effects
driven by ultra-low applied field (E~1V/cm), produced by the coupling of the
large polarization to nematic birefringence and flow. Electrostatic
self-interaction of the polarization charge renders the transition from the
nematic phase mean-field-like and weakly first-order, and controls the director
field structure of the ferroelectric phase. Atomistic molecular dynamics
simulation reveals short-range polar molecular interactions that favor
ferroelectric ordering, including a tendency for head-to-tail association into
polar, chain-like assemblies having polar lateral correlations. These results
indicate a significant potential for transformative new nematic science and
technology based on the enhanced understanding, development, and exploitation
of molecular electrostatic interaction.
