Abstract.First principles studies of multiferroic materials, such as bismuth ferrite (BFO), require methods that extend beyond standard density functional theory (DFT). The DFT+U method is one such extension that is widely used in the study of BFO. We present a systematic study of the effects of the U parameter on the structural, ferroelectric and electronic properties of BFO. We find that the structural and ferroelectric properties change negligibly in the range of U typically considered for BFO (3-5 eV). In contrast, the electronic structure varies significantly with U. In particular, we see large changes to the character and curvature of the valence band maximum and conduction band minimum, in addition to the expected increase in band gap, as U increases. Most significantly, we find that the t 2g /e g ordering at the conduction band minimum inverts for U values larger than 4 eV. We therefore recommend a U value of at most 4 eV to be applied to the Fe d orbitals in BFO. More generally, this study emphasises the need for systematic investigations of the effects of the U parameter not merely on band gaps but on the electronic structure as a whole, especially for strongly correlated materials.arXiv:1706.04369v2 [cond-mat.mtrl-sci] 9 Apr 2018 2

We report a comprehensive muon spin rotation (µSR) study of the prototypical magnetoelectric antiferromagnet Cr2O3. We find the positively charged muon (µ + ) occupies several distinct interstitial sites, and displays a rich dynamic behavior involving local hopping, thermally activated site transitions and the formation of a charge-neutral complex composed of a muon and an electron polaron. The discovery of such a complex has implications for the interpretation of µSR spectra in a wide range of magnetic oxides, and opens a route to study the dopant characteristics of interstitial hydrogen impurities in such materials. We address implications arising from implanting a µ + into a linear magnetoelectric, and discuss the challenges of observing a local magnetoelectric effect generated by the charge of the muon.

Ferroelectric-based photovoltaics have shown great promise as a source of renewable energy, thanks to their in-built charge separation capability, yet their efficiency is often limited by low charge carrier mobilities. In this work, we compare the photovoltaic prospects of various phases of the multiferroic material BiFeO 3 by evaluating their charge carrier effective masses from first-principles simulations. We identify a tetragonal phase with the promising combination of a large spontaneous polarisation and relatively light charge carriers. From a systematic investigation of the octahedral distortions present in BiFeO 3 , we clarify the relationship between structure and effective masses. This relationship is explained in terms of changes to the orbital character and overlap at the band edges that result from changes in the geometry. Our findings suggest some design principles for how to tune effective masses in BiFeO 3 and similar materials through the manipulation of their crystal structures in experimentally accessible ways. arXiv:1804.02898v2 [cond-mat.mtrl-sci]

We perform detailed muon spin rotation (µSR) measurements in the classic antiferromagnet Fe2O3 and explain the spectra by considering dynamic population and dissociation of charge-neutral muonpolaron complexes. We show that charge-neutral muon states in Fe2O3, despite lacking the signatures typical of charge-neutral muonium centers in non-magnetic materials, have a significant impact on the measured µSR frequencies and relaxation rates. Our identification of such polaronic muon centers in Fe2O3 suggests that isolated hydrogen (H) impurities form analogous complexes, and that H interstitials may be a source of charge carrier density in Fe2O3.

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