Spin-flip effects on the current-in-plane magnetotransport in magnetic multilayers with arbitrary magnetization alignments

An extended Boltzmann equation approach, with nondiagonal components of the electron distribution, function taken into account, is proposed to study spin-flip effects on the magnetoresistance (MR) in magnetic inhomogeneous systems with arbitrary magnetization alignments. The presence of spin-flip scattering is found to reduce the MR and to decrease deviation of the MR from linear dependence on sin 2(θ/2) where θ is the angle between the magnetizations of successive magnetic films.published_or_final_versio

Theory of electric-field-induced metal-insulator transition in doped manganites

The insulator to metal transition (IMT) induced by the application of an electric field in doped manganites is investigated theoretically. Starting from the double-exchange mechanism with the long-range Coulomb interaction included, we find that the electric field may suppress the charge ordering and drive the system from the antiferromagnetic and charge-ordered state with an energy gap at the Fermi level to the ferromagnetic and gapless state, resulting in the IMT. A numerical simulation is performed for manganite films with intrinsic inhomogeneities, and an important impact of the inhomogeneities on this electric-field-induced transition is obtained. Our results can naturally account for the recently observed electric-filed-induced IMT phenomenon in manganites.published_or_final_versio

Spin and orbital excitations in undoped manganites

We develop a theory for spin and orbital excitations in undoped manganites to account for the spin and orbital orderings observed experimentally. It is found that the anisotropy of the magnetic structure is closely related to the orbital ordering, and the Jahn-Teller effect stabilizes the orbital ordering. The phase diagram and the low-energy excitation spectra for both spin and orbital orderings are obtained. The calculated critical temperatures can be quantitatively comparable to the experimental data. © 2000 American Institute of Physics.published_or_final_versio

Phase diagram of an extended Kondo lattice model for manganites: The Schwinger-boson mean-field approach

We investigate the phase diagram of an extended Kondo lattice model for doped manganese oxides in the presence of strong but finite Hund's coupling and on-site Coulomb interaction. By means of the Schwinger-boson mean-field approach, it is found that, besides magnetic ordering, there will be nonuniform charge distributions, such as charge ordering and phase separation, if the interaction between electrons prevails over the hybridization. Which of the charge ordering and phase separation appears is determined by a competition between effective repulsive and attractive interactions due to virtual processes of electron hopping. Calculated results show that strong electron correlations caused by the on-site Coulomb interaction as well as the finite Hund's coupling play an important role in the magnetic ordering and charge distribution at low temperatures. ©2000 The American Physical Society.published_or_final_versio

Orbital ordering and two ferromagnetic phases in low-doped La 1-xSr xMnO 3

We present a theory for the transition between two ferromagnetic phases observed experimentally in lightly doped La 1-xSr xMnO 3. Starting from an electronic model, the instabilities to various types of orbital orderings are studied within the random-phase approximation. In most cases, the instabilities occur in the region of strong correlations. A phase diagram is calculated in the case of strong correlation by means of the projected perturbation technique and the Schwinger boson technique. A phase transition between two types of orbital ordering occurs at a low doping, which may be closely relevant to recent experimental observations.published_or_final_versio

Macroscopic theory of giant magnetoresistance in magnetic granular metals

A macroscopic theory of giant magnetoresistance in granular magnetic materials is developed to improve on that of Rubinstein [Phys. Rev. B 50, 3830 (1994)]. By using a self-consistent method and introducing a useful parametrization, we show the magnetotransport in granular systems to be between those for currents in the plane of layers and currents perpendicular to the plane of the layers in multilayers. The theoretical result in the local limit is found to be in agreement with the observed singular dependence of the giant magnetoresistance on annealing temperature.published_or_final_versio

Enhanced electronic transport in Fe3+-doped TiO2 for high efficiency perovskite solar cells

Oxygen vacancies in non-stoichiometric TiO2 electron transport layers can capture injected electrons and act as recombination centers. In this study, the compact TiO2 electron transport layers of perovskite solar cells (PSCs) are doped with different molar ratios of Fe3+ in order to passivate such defects and improve their electron transport properties. The electrical conductivity, absorption, crystal structure, and the performance of the PSCs are systematically studied. It shows that Fe3+-doping improves the conductivity of TiO2 compact layers compared with the pristine TiO2, boosting the photovoltaic performance of PSCs. The reduced trap-filled limit voltage (VTFL) of the Fe3+-doped TiO2 compact layers suggests that trap density in the Fe3+-TiO2 films is much lower than that of a pristine TiO2 film. With the optimized doping concentration (1 mol%) of Fe3+, the best power conversion efficiency of PSCs is improved from 16.02% to 18.60%

Stitching triple cation perovskite by a mixed anti-solvent process for high performance perovskite solar cells

With the rapid development of organic-inorganic lead halide perovskite photovoltaics, increasingly more attentions are paid to explore the growth mechanism and precisely control the quality of perovskite films. In this study, we propose a “stitching effect” to fabricate high quality perovskite films by using chlorobenzene (CB) as an anti-solvent and isopropyl alcohol (IPA) as an additive into this anti-solvent. Because of the existence of IPA, CB can be efficiently released from the gaps of perovskite precursors and the perovskite film formation can be slightly modified in a controlled manner. More homogeneous surface morphology and larger grain size of perovskite films were achieved via this process. The reduced grain boundaries ensure low surface defect density and good carrier transport in the perovskite layer. Meanwhile, we also performed the Fourier transform infrared (FTIR) spectroscopy to investigate the film growth mechanism of unannealed and annealed perovskite films. Solar cells fabricated by using the “stitching effect” exhibited a best efficiency of 19.2%. Our results show that solvent and solvent additives dramatically influenced the formation and crystallization processes for perovskite materials due to their different coordination and extraction capabilities. This method presents a new path towards controlling the growth and morphology of perovskite films

Theoretical lifetime extraction and experimental demonstration of stable cesium-containing tri-cation perovskite solar cells with high efficiency

Despite the high power conversion efficiency, the severe performance degradation of organic-inorganic lead halide perovskite solar cells caused by moisture and thermal phase transition is an obstacle to commercialization of the perovskite solar cells. We propose the theoretical lifetime extraction of perovskite solar cells with a mixed-cations lead halide perovskite absorber containing CH3NH3+, CH3(NH2)2+ and Cs+. The estimated mean time to failure (MTTF) of the triple cation perovskite solar cells is up to 180 days in ambient. Compared with the perovskite solar cells based on CH3NH3PbI3, the triple-cation perovskite solar cells, whose power conversion efficiency reaches 18.2% in this study, have a much better performance in terms of thermal stability and humidity stability. Improvements of both performance and stability pave the way for commercialization of perovskite solar cells

Comparison of sequencing-based methods to profile DNA methylation and identification of monoallelic epigenetic modifications.

Analysis of DNA methylation patterns relies increasingly on sequencing-based profiling methods. The four most frequently used sequencing-based technologies are the bisulfite-based methods MethylC-seq and reduced representation bisulfite sequencing (RRBS), and the enrichment-based techniques methylated DNA immunoprecipitation sequencing (MeDIP-seq) and methylated DNA binding domain sequencing (MBD-seq). We applied all four methods to biological replicates of human embryonic stem cells to assess their genome-wide CpG coverage, resolution, cost, concordance and the influence of CpG density and genomic context. The methylation levels assessed by the two bisulfite methods were concordant (their difference did not exceed a given threshold) for 82% for CpGs and 99% of the non-CpG cytosines. Using binary methylation calls, the two enrichment methods were 99% concordant and regions assessed by all four methods were 97% concordant. We combined MeDIP-seq with methylation-sensitive restriction enzyme (MRE-seq) sequencing for comprehensive methylome coverage at lower cost. This, along with RNA-seq and ChIP-seq of the ES cells enabled us to detect regions with allele-specific epigenetic states, identifying most known imprinted regions and new loci with monoallelic epigenetic marks and monoallelic expression