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Hybrid improper ferroelectricity, where an electrical polarization can be induced via a trilinear coupling to two nonpolar structural distortions of different symmetries, recently was demonstrated experimentally in the n=2 Ruddlesden-Popper compound Ca3Ti2O7. In this paper we use group theoretic methods and first-principles calculations to identify possible ferroelectric switching pathways in Ca3Ti2O7. We identify low-energy paths that reverse the polarization direction by switching via an orthorhombic twin domain or via an antipolar structure. We also introduce a chemically intuitive set of local order parameters to give insight into how these paths are relevant to ferroelectric switching nucleated at domain walls. Our findings suggest that switching may proceed via more than one mechanism in this material. [ABSTRACT FROM AUTHOR]

In the cuprates, carrier doping of the Mott insulating parent state is necessary to realize superconductivity as well as a number of other exotic states involving charge or spin density waves. Cation substitution is the primary method for doping carriers into these compounds, and is the only known method for electron doping in these materials. Here, we report electron doping without cation substitution in epitaxially stabilized thin films of La2CuO4 grown via molecular-beam epitaxy. We use angle-resolved photoemission spectroscopy to directly measure their electronic structure and conclusively determine that these compounds are electron doped with a carrier concentration of 0.09±0.02 e-/Cu. We propose that intrinsic defects, most likely oxygen vacancies, are the sources of doped electrons in these materials. Our results suggest a new approach to electron doping in the cuprates, one which could lead to a more detailed experimental understanding of their properties. [ABSTRACT FROM AUTHOR]

In the cuprates, carrier doping of the Mott insulating parent state is necessary to realize superconductivity as well as a number of other exotic states involving charge or spin density waves. Cation substitution is the primary method for doping carriers into these compounds, and is the only known method for electron doping in these materials. Here, we report electron doping without cation substitution in epitaxially stabilized thin films of La_{2}CuO_{4} grown via molecular-beam epitaxy. We use angle-resolved photoemission spectroscopy to directly measure their electronic structure and conclusively determine that these compounds are electron doped with a carrier concentration of 0.09±0.02 e^{-}/Cu. We propose that intrinsic defects, most likely oxygen vacancies, are the sources of doped electrons in these materials. Our results suggest a new approach to electron doping in the cuprates, one which could lead to a more detailed experimental understanding of their properties.