Ferroelectric materials are characterized by a spontaneous polar distortion. The behavior of such distortions in the presence of free charge is the key to the physics of metallized ferroelectrics in particular, and of structurally-polar metals more generally. Using first-principles simulations, here we show that a polar distortion resists metallization and the attendant suppression of longrange dipolar interactions in the vast majority of a sample of 11 representative ferroelectrics. We identify a meta-screening effect, occurring in the doped compounds as a consequence of the charge rearrangements associated to electrostatic screening, as the main factor determining the survival of a non-centrosymmetric phase. Our findings advance greatly our understanding of the essentials of structurally-polar metals, and offer guidelines on the behavior of ferroelectrics upon field-effect charge injection or proximity to conductive device elements.

The ease with which domain walls (DWs) in ferroelectric materials can be written and erased provides a versatile way to dynamically modulate heat fluxes. In this work we evaluate the thermal boundary resistance (TBR) of 180$^{\circ}$ DWs in prototype ferroelectric perovskite PbTiO$_3$ within the numerical formalisms of nonequilibrium molecular dynamics and nonequilibrium Green's functions. An excellent agreement is obtained for the TBR of an isolated DW derived from both approaches, which reveals the harmonic character of the phonon-DW scattering mechanism. The thermal resistance of the ferroelectric material is shown to increase up to around 20%, in the system sizes here considered, due to the presence of a single DW, and larger resistances can be attained by incorporation of more DWs along the path of thermal flux. These results, obtained at device operation temperatures, prove the viability of an electrically actuated phononic switch based on ferroelectric DWs

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...

Our second-principles results illustrate a strategy for obtaining topological electric textures akin to magnetic skyrmions.

Our first-principles results show that geometric frustration can be induced in thin films of multiferroic BiFeO(3). We find that competing magnetic interactions occur in the so-called supertetragonal phase of this material, which can be grown on strongly compressive substrates. We show that the frustration level can be tuned by appropriately choosing the substrate; in fact, the phase diagram of the films presents a critical line at which the three-dimensional spin order gets annihilated. We argue that these effects are not exclusive to BiFeO(3) and predict that they also occur in BiCoO(3).