Ferroelectric domain wall phonon polarizer

Abstract

Modulating the polarization of a beam of quantum particles is a powerful method to tailor the macroscopic properties of the ensuing energy flux as it directly influences the way in which its quantum constituents interact with other particles, waves or continuum media. Practical polarizers, being well developed for electric and electromagnetic energy, have not been proposed to date for heat fluxes carried by phonons. Here we report on atomistic phonon transport calculations demonstrating that ferroelectric domain walls can operate as phonon polarizers when a heat flux pierces them. Our simulations for representative ferroelectric perovskite PbTiO3 show that the structural inhomogeneity associated to the domain walls strongly suppresses transverse phonons, while longitudinally polarized modes can travel through multiple walls in series largely ignoring their presence.Controlling heat conduction has always been a challenging task, to the point that our ability to manipulate thermal fluxes lags far behind our long-standing knowhow in manipulating electric and electromagnetic fluxes. In solids, the principal complication in manipulating heat stems from the a priori impossibility to use electrical signals or electromagnetic fields for that purpose. This is because phonons, the dominant heat carriers in insulating materials, do not possess a bare charge.Heat conduction can be instead modulated following basically two fundamentally different approaches. In the first approach, the most extended one, structural inhomogeneities such as atomic-scale defects, materials interfaces or surfaces, are incorporated in a physical material to scatter phonons diffusively and reduce the thermal conductivity. [1,2] In addition to this approach, which exploits the corpuscular nature of phonons, waveinterference phenomena have been recently demonstrated to effectively block the propagation of phonons traveling through periodic superlattices or phononic crystals.[3-5] Interestingly, both approaches offer the possibility to focus on a fixed frequency window, i.e., to perform a frequency filtering effect, by choosing the size of the structural inhomogeneities or their periodicity.However, candidate systems to effectively filter phonons of a certain polarization, the so-called phonon polarizers, are yet to come forth. The only previous hints of a mode-dependent phonon scattering have been observed by means of ballistic phonon imaging techniques at very low temperatures (< 3 K) in highly dislocated LiF crystals [6] and in an exotic ferroelectric as KH 2 PO 4 .[7] Nevertheless, in these systems a clear polarizer effect capable to discern between, at least, longitudinal and transverse phonons, was not observed. It has also been suggested that material interfaces enclosing a molecular selfassembled monolayer should exhibit a phonon-polarizing effect due to the anisotropy in the bond strength across the length and breadth of the monolayer, [8] but such a conjecture has not been experimentally nor numerically proved. The polarization of phonons inf...

Reduction of thermal conductivity in ferroelectric SrTiO3 thin films

“…The laser spot in our FDTR setup has a 1/e 2 radius of 2.85 μm, and therefore each measurement senses several domains and DW configurations. In any case, purely ferroelectric 180 • DWs have been also proposed to filter transverse phonons, resulting in important reductions of κ [11].…”

“…The presence of domains with electrical polarization pointing along different directions will riddle the material with polarization and strain gradients extending along ferroelectric domain walls (FEDWs), which in turn are proposed to be effective phonon scatterers [10][11][12][13][14][15][16]. In this regard, strained STO offers a unique model system to quantify the effect of FEDWs on the thermal conductivity κ (T ).…”

Domain wall-localized phonons in BiFeO3: spectrum and selection rules

“…This explains why the GHz microwave conductivity at DWs in hexagonal manganites does not show a narrow peak in the frequency domain, but rather rises monotonically towards higher frequencies. The proximity of DW phonons to acoustic branches also leads to phonon scattering and affects thermal conductivity 27 . The low frequency of sliding modes is also behind the giant enhancement of the static dielectric permittivity 19 .…”

Infrared nano-spectroscopy of ferroelastic domain walls in hybrid improper ferroelectric Ca3Ti2O7

“…This research opens the door to broadband imaging of heterogeneity in ferroics and represents a first step to revealing the rich dynamics of domain walls in these systems. At the same time, it provides crucial information for the development of ultra low-power devices, switches, polarizers, and computing architectures based upon domain walls [35,36]. Loss mechanisms involving phonons are also key to controlling decoherence in domain wall-based computing architectures [28,37].…”