218 research outputs found
Prediction of thickness limits of ideal polar ultrathin films
Competition between electronic and atomic reconstruction is a constantly
recurring theme in transition-metal oxides. We use density functional theory
calculations to study this competition for a model system consisting of a thin
film of the polar, infinite-layer structure ACuO2 (A=Ca, Sr, Ba) grown on a
nonpolar, perovskite SrTiO3 substrate. A transition from the bulk planar
structure to a chain-type thin film accompanied by substantial changes to the
electronic structure is predicted for a SrCuO2 film fewer than five unit cells
thick. An analytical model explains why atomic reconstruction becomes more
favorable than electronic reconstruction as the film becomes thinner, and
suggests that similar considerations should be valid for other polar films
Mixture of easy trials enables transient and sustained perceptual improvements through priming and perceptual learning.
The sense of vision allows us to discriminate fine details across a wide range of tasks. How to improve this perceptual skill, particularly within a short training session, is of substantial interest. Emerging evidence suggests that mixing easy trials can quickly improve performance in hard trials, but it is equivocal whether the improvement is short-lived or long-lasting, and additionally what accounts for this improvement. Here, by tracking objective performance (accuracy) and subjective experience (ratings of target visibility and choice confidence) over time and in a large sample of participants, we demonstrate the coexistence of transient and sustained effects of mixing easy trials, which differ markedly in their timescales, in their effects on subjective awareness, and in individual differences. In particular, whereas the transient effect was found to be ubiquitous and manifested similarly across objective and subjective measures, the sustained effect was limited to a subset of participants with weak convergence from objective and subjective measures. These results indicate that mixture of easy trials enables two distinct, co-existing forms of rapid perceptual improvements in hard trials, as mediated by robust priming and fragile learning. Placing constraints on theory of brain plasticity, this finding may also have implications for alleviating visual deficits
Polarity-induced oxygen vacancies at LaAlO3|SrTiO3 interfaces
Using first-principles density functional theory calculations, we find a
strong position and thickness dependence of the formation energy of oxygen
vacancies in LaAlO3|SrTiO3 (LAO|STO) multilayers and interpret this with an
analytical capacitor model. Oxygen vacancies are preferentially formed at
p-type SrO|AlO2 rather than at n-type LaO|TiO2 interfaces; the excess electrons
introduced by the oxygen vacancies reduce their energy by moving to the n-type
interface. This asymmetric behavior makes an important contribution to the
conducting (insulating) nature of n-type (p-type) interfaces while providing a
natural explanation for the failure to detect evidence for the polar
catastrophe in the form of core level shifts
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