Polarization transport in ferroelectrics

The duality between electric and magnetic dipoles in electromagnetism only partly applies to condensed matter. In particular, the elementary excitations of the magnetic and ferroelectric orders, namely magnons and ferrons, respectively, have received asymmetric attention from the condensed matter community in the past. In this Perspective, we introduce and summarize the current state of the budding field of “ferronics.” We argue that the introduction of dipole-carrying elementary excitations allows the modeling of many observables and potentially leads to applications in thermal, information, and communication technologies.

Figure 1

The green arrows indicate symmetry-restoring ferron excitations in selected ferroelectric materials. (a) Sketch of the soft-phonon excitation in the free energy landscape of a displacive ferroelectric with a temperature-dependent barrier (blue arrow). (b) In typical order-disorder ferroelectrics, the symmetry is restored by the hopping of protons in hydrogen bonds. (c) Thermally activated rotations destroy the order of permanent molecular dipoles. (d) The rigid relative shift of the bilayers reverses the electric polarization in a double-well potential. (e) In electronic ferroelectrics, the symmetry is restored by the motion of electrons, not the ions.

Figure 2

Sketch of (a) a “Goldstone” transverse and (b) a “Higgs” longitudinal ferron mode. In (a) the modulus of the electric dipole $P_0$ is conserved during transverse oscillations, as in stable molecular dipoles. The projection on the equilibrium polarization $|\overline{P}|<|P_0|$. In (b) the length of the dipole fluctuates in an anharmonic potential $V(z)$ that also reduces the average value $|\overline{P}|<|P_0|$.

Figure 3

Electric field dependence of the thermal conductivity of the ferroelectric PZT (adapted from Ref. [42]).