Paraelectric and ferroelectric order in two-state dipolar fluids

Monte Carlo simulations are used to examine the cooperative creation of a polar state in fluids of two-state particles with nonzero dipole in the excited state. With lowering temperature such systems undergo a second-order transition from nonpolar to polar, paraelectric phase. The transition is accompanied by a dielectric anomaly of polarization susceptibility increasing by three orders of magnitude. The paraelectric phase is then followed by a formation of a nematic ferroelectric which further freezes into a fcc ferroelectric crystal by a first-order transition. A mean-field model of phase transitions is discussed.