Orientational Order/Disorder Phase Transitions
The problem of spontaneous order in dipolar systems goes back to Debye who suggested a possibility of spontaneously ordered ferroelectric phase in dipolar liquids. The idea was dismissed by Onsager and Kirkwood, but the effect of spontaneous ordering has been discussed by several mean-field theories of polar liquids and magnetic colloids. Recent
computer simulations have indicated that ferroelectric phase is possible for polar fluids at a non-zero temperature. Most of the discussion of spontaneous order in dipolar systems has been focused on systems with permanent dipoles. Real systems, either of molecular or
nano-scale dimension, are composed of polarizable particles. We have therefore decided to look for ferroelectric phase in fluids composed of polarizable two-state molecules. The ultimate goal is to explore the possibility of cooperativity of charge transfer in systems with dense arrangement of charge-transfer molecules (thin films on the surface, Lamgmuir-Blogett films, etc.).
The model fluid of two-state molecules (Phys. Rev, Lett., submitted) shows a complex and rich phase diagram including the transition from a nonpolar to polar, paraelectric phase ( in the Figure) followed by the spontaneous creation of the ferroelectric phase at The system shows some unique physical properties potentially useful for a range of applications including molecular electronics, molecular switching, and solar energy conversion. The non-polar/paraelectric phase transition is characterized by the change
in the dipolar susceptibility amounting 3 orders of magnitude. The dielectric constant in the ferroelectric phase is several thousands in our simulations and its magnitude is limited only by he size of the ferroelectric domain. We are currently pursuing this avenue by
exploring the interfacial phenomena where charge-transfer cooperativity can be modulated electrochemically or photochemically.
|