We used first-principles calculations to investigate the existence and origin of the ferroelectric instability in the ABF3 fluoroperovskites. While the ground states of most ABF3 compounds are paraelectric (Pnma phase), we find that many fluoroperovskites have a ferroelectric instability in their high-symmetry cubic structure that is of similar amplitude to that commonly found in oxide perovskites. In contrast to the oxides, however, the fluorides have nominal Born effective charges, indicating a different mechanism for the instability. We show that the instability originates from ionic size effects, and is therefore in most cases largely insensitive to pressure and strain, again in contrast to the oxide perovskites. An exception is NaMnF3, where coherent epitaxial strain matching to a substrate with equal in-plane lattice constants destabilizes the bulk Pnma structure, leading to a ferroelectric, and indeed multiferroic, ground state with an unusual polarization/strain response.

Geometric ferroelectricity in fluoroperovskites, A. C. Garcia Castro, N. A. Spaldin, A. H. Romero and E. Bousquet in Physical Review B 89 p. 104107 (2014).

The response of oxide thin films to polar discontinuities at interfaces and surfaces has generated enormous activity due to the variety of interesting effects that it gives rise to. A case in point is the discovery of the electron gas at the interface between LaAlO3 and SrTiO3, which has since been shown to be quasi-two-dimensional, switchable, magnetic and/or superconducting. Despite these findings, the origin of the two-dimensional electron gas is highly debated and several possible mechanisms remain. Here we review the main proposed mechanisms and attempt to model expected effects in a quantitative way with the ambition of better constraining what effects can/cannot explain the observed phenomenology.

The origin of two-dimensional electron gases at oxide interfaces: insights from theory, N. C. Bristowe, P. Ghosez, P. B. Littlewood and E. Artacho, topical review in J. Phys.: Condens. Matter 26 p. 143201 (2014).

First-principles techniques are used to investigate the behavior of BiFeO3/LaFeO3 perovskite oxide superlattices epitaxially grown on a (001)-SrTiO3 substrate. The calculations show that 1/1 superlattices exhibit a Pmc21 ground state combining a trilinear coupling of one polar and two oxygen rotational lattice modes, and weak ferromagnetism. The microscopic mechanism allowing one to manipulate the magnetization with an electric field in such systems is presented and its dependence on strain and chemical substitution is discussed. BiFeO3/LaFeO3 artificial superlattices appear to be good candidates to achieve electric switching of magnetization at room temperature.

Electric control of the magnetization in BiFeO3/LaFeO3 superlattices. Zeila Zanolli, Jacek C. Wojdeł, Jorge Iniguez, and Philippe Ghosez, Phys. Rev. B 88, 060102(R) (2013).

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