73 research outputs found

    Glancing angle deposition of sculptured thin metal films at room temperature

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    Metallic thin films consisting of separated nanostructures are fabricated by evaporative glancing angle deposition at room temperature. The columnar microstructure of the Ti and Cr columns is investigated by high resolution transmission electron microscopy and selective area electron diffraction. The morphology of the sculptured metallic films is studied by scanning electron microscopy. It is found that tilted Ti and Cr columns grow with a single crystalline morphology, while upright Cr columns are polycrystalline. Further, the influence of continuous substrate rotation on the shaping of Al, Ti, Cr and Mo nanostructures is studied with view to surface diffusion and the shadowing effect. It is observed that sculptured metallic thin films deposited without substrate rotation grow faster compared to those grown with continuous substrate rotation. A theoretical model is provided to describe this effec

    Antiferromagnetic order and dielectric gap within the vortex core of antiferromagnetic superconductor

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    The structure of a superconducting vortex has been studied theoretically for a dirty antiferromagnetic superconductor (AFSC), modelling an AFSC as a doped semi-metal with s-wave superconducting pairing and antiferromagnetic (dielectric) interaction between electrons (holes). It is also supposed that the quasiparticles dispersion law possesses the property of nesting. The distribution of the superconducting and magnetic order parameters near the vortex core is calculated. It is shown that the antiferromagnetic order, been suppressed at large distances, is restored around the superconducting flux and the vortex core is in fact insulating and antiferromagnetic, in stark contrast to the normal metal cores of traditional superconductors. Moreover, our model calculations predict that as the temperature decreases the flux region of the superconductivity and antiferromagnetism coexistence increases.Comment: 9 pages, 3 Postscript figures,NATO Advanced Research Workshop on "Vortex dynamics in superconductors and other complex systems" Yalta, Crimea, Ukraine, 13-17 September 200

    Pseudogap in 1d revisited

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    Two decades ago, Sadovskii found an exact solution of a model describing a pseudogap in electron energy spectrum (first introduced by Lee, Rice and Anderson). The discovery of a pseudogap in high-Tc superconductors has revived the interest to his exact solution. I review the model with the emphasis on physical content, point out an error in the original Sadovskii's solution and explain which problem he actually solved. A recent incorporation of Sadovskii's ideas into a description of "hot spots" on the Fermi surface in cuprate superconductors (Schmalian, Pines and Stojkovic) is briefly discussed.Comment: Final version to appear in PR

    Sliding charge density wave in manganites

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    The so-called stripe phase of the manganites is an important example of the complex behaviour of metal oxides, and has long been interpreted as the localisation of charge at atomic sites. Here, we demonstrate via resistance measurements on La_{0.50}Ca_{0.50}MnO_3 that this state is in fact a prototypical charge density wave (CDW) which undergoes collective transport. Dramatic resistance hysteresis effects and broadband noise properties are observed, both of which are typical of sliding CDW systems. Moreover, the high levels of disorder typical of manganites result in behaviour similar to that of well-known disordered CDW materials. Our discovery that the manganite superstructure is a CDW shows that unusual transport and structural properties do not require exotic physics, but can emerge when a well-understood phase (the CDW) coexists with disorder.Comment: 13 pages; 4 figure

    The {\eta}'-carbon potential at low meson momenta

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    The production of η\eta^\prime mesons in coincidence with forward-going protons has been studied in photon-induced reactions on 12^{12}C and on a liquid hydrogen (LH2_2) target for incoming photon energies of 1.3-2.6 GeV at the electron accelerator ELSA. The η\eta^\prime mesons have been identified via the ηπ0π0η6γ\eta^\prime\rightarrow \pi^0 \pi^0\eta \rightarrow 6 \gamma decay registered with the CBELSA/TAPS detector system. Coincident protons have been identified in the MiniTAPS BaF2_2 array at polar angles of 2θp112^{\circ} \le \theta _{p} \le 11^{\circ}. Under these kinematic constraints the η\eta^\prime mesons are produced with relatively low kinetic energy (\approx 150 MeV) since the coincident protons take over most of the momentum of the incident-photon beam. For the C-target this allows the determination of the real part of the η\eta^\prime-carbon potential at low meson momenta by comparing with collision model calculations of the η\eta^\prime kinetic energy distribution and excitation function. Fitting the latter data for η\eta^\prime mesons going backwards in the center-of-mass system yields a potential depth of V = -(44 ±\pm 16(stat)±\pm15(syst)) MeV, consistent with earlier determinations of the potential depth in inclusive measurements for average η\eta^\prime momenta of \approx 1.1 GeV/cc. Within the experimental uncertainties, there is no indication of a momentum dependence of the η\eta^\prime-carbon potential. The LH2_2 data, taken as a reference to check the data analysis and the model calculations, provide differential and integral cross sections in good agreement with previous results for η\eta^\prime photoproduction off the free proton.Comment: 9 pages, 13 figures. arXiv admin note: text overlap with arXiv:1608.0607

    Experimental constraints on the ω\omega-nucleus real potential

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    In a search for ω\omega mesic states, the production of ω\omega-mesons in coincidence with forward going protons has been studied in photon induced reactions on 12^{12}C for incident photon energies of 1250 - 3100 MeV. The π0γ\pi^0 \gamma pairs from decays of bound or quasi-free ω\omega-mesons have been measured with the CBELSA/TAPS detector system in coincidence with protons registered in the MiniTAPS forward array. Structures in the total energy distribution of the π0γ\pi^0 \gamma pairs, which would indicate the population and decay of bound ω 11\omega~^{11}B states, are not observed. The π0γ\pi^0 \gamma cross section of 0.3 nb/MeV/sr observed in the bound state energy regime between -100 and 0 MeV may be accounted for by yield leaking into the bound state regime because of the large in-medium width of the ω\omega-meson. A comparison of the measured total energy distribution with calculations suggests the real part V0V_0 of the ω 11\omega~^{11}B potential to be small and only weakly attractive with V0(ρ=ρ0)=15±V_0(\rho=\rho_0) = -15\pm 35(stat) ±\pm20(syst) MeV in contrast to some theoretical predictions of attractive potentials with a depth of 100 - 150 MeV.Comment: 13 pages, 8 figure

    Systematic Cu-63 NQR studies of the stripe phase in La(1.6-x)Nd(0.4)Sr(x)CuO(4) for 0.07 <= x <= 0.25

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    We demonstrate that the integrated intensity of Cu-63 nuclear quadrupole resonance (NQR) in La(1.6-x)Nd(0.4)Sr(x)CuO(4) decreases dramatically below the charge-stripe ordering temperature T(charge). Comparison with neutron and X-ray scattering indicates that the wipeout fraction F(T) (i.e. the missing fraction of the integrated intensity of the NQR signal) represents the charge-stripe order parameter. The systematic study reveals bulk charge-stripe order throughout the superconducting region 0.07 <= x <= 0.25. As a function of the reduced temperature t = T/T(charge), the temperature dependence of F(t) is sharpest for the hole concentration x=1/8, indicating that x=1/8 is the optimum concentration for stripe formation.Comment: 10 pages of text and captions, 11 figures in postscript. Final version, with new data in Fig.

    The polarization observables T, P, and H and their impact on γppπ0\gamma p \to p\pi^0 multipoles

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    Data on the polarization observables T, P, and H for the reaction γppπ0\gamma p\to p\pi^0 are reported. Compared to earlier data from other experiments, our data are more precise and extend the covered range in energy and angle substantially. The results were extracted from azimuthal asymmetries measured using a transversely polarized target and linearly polarized photons. The data were taken at the Bonn electron stretcher accelerator ELSA with the CBELSA/TAPS detector. Within the Bonn-Gatchina partial wave analysis, the new polarization data lead to a significant narrowing of the error band for the multipoles for neutral-pion photoproduction

    First measurement of the helicity asymmetry for γppπ0\gamma p\rightarrow p\pi^0 in the resonance region

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    The first measurement of the helicity dependence of the photoproduction cross section of single neutral pions off protons is reported for photon energies from 600 to 2300\,MeV, covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, and BnGa partial wave analyses. Strikingly large differences between data and predictions are observed which are traced to differences in the helicity amplitudes of well known and established resonances. Precise values for the helicity amplitudes of several resonances are reported
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