Angular dependent vortex pinning mechanisms in YBCO coated conductors and thin films

We present a comparative study of the angular dependent critical current density in YBa2Cu3O7 films deposited on IBAD MgO and on single crystal MgO and SrTiO3 substrates. We identify three angular regimes where pinning is dominated by different types of correlated and uncorrelated defects. We show that those regimes are present in all cases, indicating that the pinning mechanisms are the same, but their extension and characteristics are sample dependent, reflecting the quantitative differences in texture and defect density. In particular, the more defective nature of the films on IBAD turns into an advantage as it results in stronger vortex pinning, demonstrating that the critical current density of the films on single crystals is not an upper limit for the performance of the IBAD coated conductors.Comment: 14 pages, 3 figures. Submitted to AP

Induced magnetization in La0.7_{0.7}Sr0.3_{0.3}MnO3_3/BiFeO3_3 superlattices

Using polarized neutron reflectometry (PNR), we observe an induced magnetization of 75$\pm$ 25 kA/m at 10 K in a La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO)/BiFeO$_3$ superlattice extending from the interface through several atomic layers of the BiFeO$_3$ (BFO). The induced magnetization in BFO is explained by density functional theory, where the size of bandgap of BFO plays an important role. Considering a classical exchange field between the LSMO and BFO layers, we further show that magnetization is expected to extend throughout the BFO, which provides a theoretical explanation for the results of the neutron scattering experiment.Comment: 5 pages, 4 figures, with Supplemental Materials. To appear in Physical Review Letter

Size-Dependent Photon Emission from Organometal Halide Perovskite Nanocrystals Embedded in an Organic Matrix

10.1021/jz502615eJournal of Physical Chemistry Letters63446-45

Colossal terahertz magnetoresistance at room temperature in epitaxial La0.7Sr0.3MnO3 nanocomposites and single-phase thin films

Colossal magnetoresistance (CMR) is demonstrated at terahertz (THz) frequencies by using terahertz time-domain magnetospectroscopy to examine vertically-aligned nanocomposites (VANs) and planar thin films of La_0.7Sr_0.3MnO_3. At the Curie temperature (room temperature) the THz conductivity of the VAN was dramatically enhanced by over 2 orders of magnitude under the application of a magnetic field, with a non-Drude THz conductivity that increased with frequency. The dc CMR of the VAN is controlled by extrinsic magnetotransport mechanisms such as spin-polarized tunneling between nano-grains. In contrast, we find that THz CMR is dominated by intrinsic, intragrain transport: the mean free path was smaller than the nanocolumn size, and the planar thin-film exhibited similar THz CMR to the VAN. Surprisingly, the observed colossal THz magnetoresistance suggests that the magnetoresistance can be large for ac motion on nanometre length scales, even when the magnetoresistance is negligible on the macroscopic length scales probed by dc transport. This suggests that colossal magnetoresistance at THz frequencies may find use in nanoelectronics and in THz optical components controlled by magnetic fields. The VAN can be scaled in thickness while retaining a high structural quality, and offers a larger THz CMR at room temperature than the planar film

Interface-Coupled BiFeO<inf>3</inf>/BiMnO<inf>3</inf> Superlattices with Magnetic Transition Temperature up to 410 K

This research was funded by the Engineering and Physical Sciences Research Council, (EP/P50385X/1), the European Research Council (ERC-2009-AdG 247276 NOVOX). The work at Texas A&M was funded by the U.S. National Science Foundation (DMR-1401266). The work at Los Alamos was supported by the U.S. Department of Energy through the LANL/LDRD program and was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/admi.20150059

Strongly Enhanced Current Densities in Superconducting Coated Conductors of YBa2Cu3O7-x + BaZrO3

There are numerous potential applications for superconducting tapes, based on YBa2Cu3O7-x (YBCO) films coated onto metallic substrates. A long established goal of more than 15 years has been to understand the magnetic flux pinning mechanisms which allow films to maintain high current densities out to high magnetic fields. In fact, films carry 1-2 orders of magnitude higher current densities than any other form of the material. For this reason, the idea of further improving pinning has received little attention. Now that commercialisation of conductors is much closer, for both better performance and lower fabrication costs, an important goal is to achieve enhanced pinning in a practical way. In this work, we demonstrate a simple and industrially scaleable route which yields a 1.5 to 5-fold improvement in the in-field current densities of already-high-quality conductors

Size-Dependent Photon Emission from Organometal Halide Perovskite Nanocrystals Embedded in an Organic Matrix.

In recent years, organometal halide perovskite materials have attracted significant research interest in the field of optoelectronics. Here, we introduce a simple and low-temperature route for the formation of self-assembled perovskite nanocrystals in a solid organic matrix. We demonstrate that the size and photoluminescence peak of the perovskite nanocrystals can be tuned by varying the concentration of perovskite in the matrix material. The physical origin of the blue shift of the perovskite nanocrystals’ emission compared to its bulk phase is also discussed.D.D. acknowledges the Department of Physics, University of Cambridge and the KACST-Cambridge University Joint Centre of Excellence for financial support. G.L. thanks the Gates Cambridge Trust for support. Q.S. acknowledges the Imperial College Junior Research Fellowship. J.L.M.D. acknowledges ERC Advanced Investigator Grant, Novox, ERC-2009-adG247276. This work was supported by the Engineering and Physical Sciences Research Council, UK.This is the final published version. It first appeared at http://pubs.acs.org/doi/abs/10.1021/jz502615e

Engineering Schottky contacts in open-air fabricated heterojunction solar cells to enable high performance and ohmic charge transport.

The efficiencies of open-air processed Cu2O/Zn(1-x)Mg(x)O heterojunction solar cells are doubled by reducing the effect of the Schottky barrier between Zn(1-x)Mg(x)O and the indium tin oxide (ITO) top contact. By depositing Zn(1-x)Mg(x)O with a long band-tail, charge flows through the Zn(1-x)Mg(x)O/ITO Schottky barrier without rectification by hopping between the sub-bandgap states. High current densities are obtained by controlling the Zn(1-x)Mg(x)O thickness to ensure that the Schottky barrier is spatially removed from the p-n junction, allowing the full built-in potential to form, in addition to taking advantage of the increased electrical conductivity of the Zn(1-x)Mg(x)O films with increasing thickness. This work therefore shows that the Zn(1-x)Mg(x)O window layer sub-bandgap state density and thickness are critical parameters that can be engineered to minimize the effect of Schottky barriers on device performance. More generally, these findings show how to improve the performance of other photovoltaic system reliant on transparent top contacts, e.g., CZTS and CIGS.This work was supported by EPSRC of the UK (award number RG3717)This is the accepted manuscript. The final version is available from ACS at http://pubs.acs.org/doi/abs/10.1021/am5058663

The changing role of china in the global illegal cigarette trade

This study explores the history of the illegal production, distribution, and smuggling of cigarettes in mainland China. Data were obtained from a content analysis of 931 media reports retrieved from LexisNexis for the time period 1975 until 2010, and from other open sources. The illegal cigarette trade first emerged in the form of violations of state tobacco monopoly regulations. In the course of the restructuring of the legal tobacco sector, which occurred under external political pressure to open the Chinese market to foreign competition, an illegal cigarette industry emerged which at first primarily produced fake Chinese brand cigarettes for the domestic black market. At the same time, China became a destination country for smuggled genuine Western brand cigarettes. It was only after effective crackdowns against cigarette smuggling and domestic distribution channels in the late 1990s that the Chinese illegal cigarette industry shifted to exporting large numbers of counterfeit Western brand cigarettes to black markets abroad. China’s current role as a leading supplier of counterfeit cigarettes is a result of the contradictions of the economic reform process and of external licit and illicit forces that worked toward opening up the Chinese tobacco sector to the outside world