Phonon thermal conductivity in ferroelectric materials: the case for “polarization caloritronics”

What is the nature of the thermal fluctuations of polarization in ferroelectric materials, the equivalent of magnons in ferromagnetic materials? In displacement ferroelectrics they are phonons involving the atoms that carry Born effective charges; these describe the electrical polarization induced by the displacement of individual atomic sublattices. These phonons are highly anharmonic and can be acoustic as well as optical phonons. A recent theoretical paper 1 called them “ferrons” for short. We show experimentally how the lattice thermal conductivity, the thermal diffusivity and the sound velocity of lead zirconium titanate (PZT) are affected by an external electric field (a 2% effect at 2MV/m). We develop an elementary but predictive theory based on the piezoelectric coefficients and the Grüneisen parameters that describes these effects quantitatively and without adjustable parameters. 2 We the apply the theory to the relaxor ferroelectric lead magnesium niobate-lead titanate (PMN-PT, 33%) where a much larger (6% at 0.4MV/m) effect of the opposite sign is predicted and indeed observed. Since these results show that propagating phonons are affected by the electric field, it is reasonable to wonder whether a flux of polarization could accompany the flux of heat, like a spin flux accompanies a heat flux in the spin-Seebeck effect. This allowed Bauer and co-workers to write Onsager relations 1 for mixed polarization/heat conduction, thus defining “polarization caloritronic (Seebeck, Peltier)” coefficients. We will discuss future experiments that might put these in evidence, because if a polarization flux were to exist, it could open new applications 3 in heat control, energy conversion, THz generation and phonon-based logic.

If time allows, we will also describe how magnetic fields affect the lattice thermal conductivity of diamagnetic InSb, again by influencing phonon anharmonicity. 4 This effects can exceed 10% and is explained quantitatively without adjustable parameters.

Speaker: Joseph Heremans, Ohio State University