Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells

We report a model describing the molecular orientation disorder in CH3NH3PbI3, solving a classical Hamiltonian parametrised with electronic structure calculations, with the nature of the motions informed by ab-initio molecular dynamics. We investigate the temperature and static electric field dependence of the equilibrium ferroelectric (molecular) domain structure and resulting polarisability. A rich domain structure of twinned molecular dipoles is observed, strongly varying as a function of temperature and applied electric field. We propose that the internal electrical fields associated with microscopic polarisation domains contribute to hysteretic anomalies in the current--voltage response of hybrid organic-inorganic perovskite solar cells due to variations in electron-hole recombination in the bulk.Comment: 10 pages; 4 figures, 2 SI figure

Coulomb interaction and ferroelectric instability of BaTiO3

Using first-principles calculations, the phonon frequencies at the $\Gamma$ point and the dielectric tensor are determined and analysed for the cubic and rhombohedral phases of BaTiO$_{3}$. The dipole-dipole interaction is then separated \`a la Cochran from the remaining short-range forces, in order to investigate their respective influence on lattice dynamics. This analysis highlights the delicate balance of forces leading to an unstable phonon in the cubic phase and demonstrates the extreme sensitivity of this close compensation to minute effective charge changes. Within our decomposition, the stabilization of the unstable mode in the rhombohedral phase or under isotropic pressure has a different origin.Comment: 9 pages, 4 tables, 1 figur

Reduction spheroids from the Upper Carboniferous Hopewell Group, Dorchester Cape, New Brunswick: notes on geochemistry, mineralogy and genesis

The bajada-playa sequence of terrestrial Upper Carboniferous redbeds of the Hopewell Group at Dorchester Cape, New Brunswick, hosts innumerable reduction spheroids in fine- to coarse-grained clastic sedimentary rocks, paleosols and caliche beds. The spheroids are grey-green, typically 2 to 3 cm in diameter, and may contain a dark, mineralized central core and less commonly one or more mineralized rings, concentric about the core. They decrease progressively in average diameter from 2.75 cm at the base to 0.85 cm at the top of an overall upward-fining 250 m thick measured stratigraphic section. Conditions controlling the genesis of the spheroids were established shortly after sedimentation. Development was subtly controlled by groundwater flow patterns and by various sedimentary structures. The enclosing redbeds provide an adequate source for the metals contained in the mineralized spheroids. Low-temperature chloride complexes originating from evaporative fluids concentrated during redbed formation are believed to have been responsible for transport of the metals to reduction sites. Precipitation probably occurred as a result of a change in redox potential governed by electrical self-potentials of various detrital and early diagenetic grains, particularly pyritc and/or Fe-Ti oxides. Electron microprobc analyses reveal Cu, Ag, Fe, Ti, U, V and REE enrichments in reduction spheroid cores. Mineralized rings arc less enriched in these elements but contain slightly greater concentrations of Ti-oxide minerals. Minerals observed other than bulk redbed constituents include: chatcocite, covellitc, cuprite, pyritc, il-menite, rutile, mottramite, roscoelite, xenotime, monazite, native copper, azurite and native silver with trace amounts of alloyed Hg. RÉSUMÉ La séquence de bajadas-playas des formations rouges terrestres du Carbonifere supérieur du groupe de Hopewell à Dorchester Cape, Nouveau-Brunswick, abrite d'innombrables sphéroides de réduction dans des couches de caliche, dc paléosol et de sédiments clastiques à grain fin à gros. Les sphéroides sont de couleur gris-vert, its ont généralement 2 à 3 cm de diamètre et ils peuvent renfermer un noyau central minéralisé de teinte foncée de même que, moins couramment, un ou plusieurs anneaux minéralisés concentriques au noyau. Leur diamètre moyen diminuc graduellement, passant de 2,75 cm à la base a 0,85 cm au sommet d'une section stratigraphique à affinement généralement ascendant d'une épaisseur mesurée de 250 m. Les conditions ay ant régi l'origine des sphéroldes ont été établies peu après la sédimentation. Leur développement a été subtilement gouverné par les configurations d'écoulement des eaux souterraines ct par diverses structures sédimentaires. Les formations rouges encaissantes constituent une source adéquate des métaux que ren ferment les sphéroldes minéraliseés. On croit que les complexes chlorurés de basse température provenant des fluides volatils concentrés pendant la constitution des formations rouges ont causé le transport des métaux à des points de réduction. Des précipitations sont probablement survenucs par suite d'un changement du potentiel d'oxydor&Juction gouverné par les polarisations spontanées éiectriques de divers grains diagénétiques détritiques ct anciens, en particulier de la pyrite ou des oxydes de fer-titane. Les analyses par microsonde éiectronique révèlent des enrichissements de Cu, d'Ag, de Fe, de Ti, d'U, de V et d'éléments des terres rares dans les noyaux des sphéroldes de réduction. Les anneaux minéralisés renferment des proportions moindres de certains de ces éléments, mais des concentrations légèrement plus fortes d'oxyde dc titane. Outre les composants bruts des couches rouges, les minéraux observes comprennent de la chalcocyte, de la covellite, de la cuprite, de la pyritc, de l'ilménite, du rutile, de la mottramite, de la roscoélite, du xénotime, du monazite, du cuivrc natif, de l’azurite et de l’argent natif avec des quantités infimes de mcrcure allie. [Traduit par la rédaction

Conduction of topologically-protected charged ferroelectric domain walls

We report on the observation of nanoscale conduction at ferroelectric domain walls in hexagonal HoMnO3 protected by the topology of multiferroic vortices using in situ conductive atomic force microscopy, piezoresponse force microscopy, and kelvin-probe force microscopy at low temperatures. In addition to previously observed Schottky-like rectification at low bias [Phys. Rev. Lett., 104, 217601 (2010)], conductance spectra reveal that negatively charged tail-to-tail walls exhibit enhanced conduction at high forward bias, while positively charged head-to-head walls exhibit suppressed conduction at high reverse bias. Our results pave the way for understanding the semiconducting properties of the domains and domain walls in small-gap ferroelectrics.Comment: 8 pages, 4 figure

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Double polarization hysteresis loop induced by the domain pinning by defect dipoles in HoMnO3 epitaxial thin films

We report on antiferroelectriclike double polarization hysteresis loops in multiferroic HoMnO3 thin films below the ferroelectric Curie temperature. This intriguing phenomenon is attributed to the domain pinning by defect dipoles which were introduced unintentionally during film growth process. Electron paramagnetic resonance suggests the existence of Fe1+ defects in thin films and first principles calculations reveal that the defect dipoles would be composed of oxygen vacancy and Fe1+ defect. We discuss migration of charged point defects during film growth process and formation of defect dipoles along ferroelectric polarization direction, based on the site preference of point defects. Due to a high-temperature low-symmetry structure of HoMnO3, aging is not required to form the defect dipoles in contrast to other ferroelectrics (e.g., BaTiO3).Comment: 4 figure

First-principles investigation of 180-degree domain walls in BaTiO_3

We present a first-principles study of 180-degree ferroelectric domain walls in tetragonal barium titanate. The theory is based on an effective Hamiltonian that has previously been determined from first-principles ultrasoft-pseudopotential calculations. Statistical properties are investigated using Monte Carlo simulations. We compute the domain-wall energy, free energy, and thickness, analyze the behavior of the ferroelectric order parameter in the interior of the domain wall, and study its spatial fluctuations. An abrupt reversal of the polarization is found, unlike the gradual rotation typical of the ferromagnetic case.Comment: Revtex (preprint style, 13 pages) + 3 postscript figures. A version in two-column article style with embedded figures is available at http://electron.rutgers.edu/~dhv/preprints/index.html#pad_wal

A Multiferroic Ceramic with Perovskite Structure: La0.5Bi0.5Mn0.5Fe0.5O3.09

ABO3 perovskite multiferroic La0.5Bi0.5Mn0.5Fe0.5O3.09 where the B-site cations is responsible for the magnetic properties and the A-site cation with lone pair electron is responsible for the ferroelectric properties was synthesized at normal conditions. This oxide exhibits a ferromagnetic transition around 240 K with a well defined hysteresis loop, and a significant reversible remnant polarization below 67K similar to ferroelectric behavior. The magnetic interaction is interpreted by the ferromagnetic Fe3+-O-Mn3+ and antiferromagnetic Fe3+(Mn3+)-O-Fe3+(Mn3+) interactions competed each other, whereas the ferroelectricity is predominantly due to the polar nature introduced by the 6s2 lone pair of Bi3+ cationsComment: Submitted to Applied Physics Letters, 7 pages, 3 figure

First-principles study of stability and vibrational properties of tetragonal PbTiO_3

A first-principles study of the vibrational modes of PbTiO_3 in the ferroelectric tetragonal phase has been performed at all the main symmetry points of the Brillouin zone (BZ). The calculations use the local-density approximation and ultrasoft pseudopotentials with a plane-wave basis, and reproduce well the available experimental information on the modes at the Gamma point, including the LO-TO splittings. The work was motivated in part by a previously reported transition to an orthorhombic phase at low temperatures [(J. Kobayashi, Y. Uesu, and Y. Sakemi, Phys. Rev. B {\bf 28}, 3866 (1983)]. We show that a linear coupling of orthorhombic strain to one of the modes at Gamma plays a role in the discussion of the possibility of this phase transition. However, no mechanical instabilities (soft modes) are found, either at Gamma or at any of the other high-symmetry points of the BZ.Comment: 8 pages, two-column style with 3 postscript figures embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/index.html#ag_pbt

Structural, electronic and magnetic properties of SrRuO3_3 under epitaxial strain

Using density functional theory within the local spin density approximation, structural, electronic and magnetic properties of SRO are investigated. We examine the magnitude of the orthorhombic distortion in the ground state and also the effects of applying epitaxial constraints, whereby the influence of large (in the range of $\pm 4$) in-plane strain resulting from coherent epitaxy, for both [001] and [110] oriented films, have been isolated and investigated. The overall pattern of the structural relaxations reveal coherent distortions of the oxygen octahedra network, which determine stability of the magnetic moment on the Ru ion. The structural and magnetic parameters exhibit substantial changes allowing us to discuss the role of symmetry and possibilities of magneto-structural tuning of \SRO-based thin film structures.Comment: 11 page