Polarization caloritronics and “ferron” excitations in ferroelectrics

Ferromagnets and ferroelectrics are “ferroic” materials that exhibit spontaneous order of magnetic and electric dipoles below a critical temperature, respectively. The quasi-particle excitations of magnetic order are magnons that carry energy as well as elementary magnetic dipoles or spins. The transport of spin and heat by magnons has been extensively studied in the past decade in the field of spin caloritronics which features the spin Seebeck and Peltier effects. However, to our best knowledge, the quasi-particles and associated transport in ferroelectrics–an important material class with many technological applications–have so far remained unexplored. We previously addressed the elementary excitations and the associated polarization and heat transport in ferroelectrics by a phenomenological diffusion equation [1] and a simple ball-spring model within the assumption of local electric dipoles with fixed modulus [2]. The latter was inspired by the magnons that preserve the magnetization magnitude and thus holds for the small class of Ising-type ferroelectrics that are formed by the ordering of stable molecular dipoles. However, most ferroelectrics are “displacive”, i.e., formed by the condensation of a particular soft phonon with a flexible dipole moment (or are of mixed type) and cannot be described by our previous model. Very recently, we have formulated the bosonic excitations, termed “ferrons”, in displacive ferroelectrics that carry elementary electric dipoles besides energy from the phenomenological Landau-Ginzburg-Devonshire theory [3]. These ferrons emerge from the concerted action of anharmonicity and broken inversion symmetry. In contrast to magnons, the transverse excitations of the magnetic order, the ferrons in displacive ferroelectrics are longitudinal with respect to the ferroelectric order. Based on the ferron spectrum, we predict temperature dependent pyroelectric and electrocaloric properties, electric-field-tunable heat and polarization transport, and ferron-photon hybridization.

[1] G. E. W. Bauer, R. Iguchi, and K. Uchida, Physical Review Letters 126, 187603 (2021)

[2] P. Tang, R. Iguchi, K. Uchida, and G. E. W. Bauer, Physical Review Letters 128, 047601(2022).

[3] P. Tang, R. Iguchi, K. Uchida, and G. E. W. Bauer, Physical Review B (Letter) 106, L081105 (2022).