689 research outputs found

    Superconducting gap and vortex lattice of the heavy fermion compound CeCu_2Si_2

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    The order parameter and pairing mechanism for superconductivity in heavy fermion compounds are still poorly understood. Scanning tunneling microscopy and spectroscopy at ultra-low temperatures can yield important information about the superconducting order parameter and the gap structure. Here, we study the first heavy fermion superconductor, CeCu_2Si_2. Our data show the superconducting gap which is not fully formed and exhibits features that point to a multi-gap order parameter. Spatial mapping of the zero bias conductance in magnetic field reveals the vortex lattice, which allows us to unequivocally link the observed conductance gap to superconductivity in CeCu_2Si_2. The vortex lattice is found to be predominantly triangular with distortions at fields close to \sim 0.7 H_{c2}.Comment: 6 pages, 4 figures, revised version accepted for publication in PR

    Tailoring exciton diffusion and domain size in photovoltaic small molecules by processing

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    Funding: European Research Council (grant 321305); UK EPSRC for equipment grant (EP/L017008/1) and (EP/M508214/1) (OB).Exciton diffusion is an important part of light harvesting in organic photovoltaics (OPVs) because it enables excitons to reach the interface betweeen donor and acceptor and contribute to the photocurrent. Here we used simple and cost-effective techniques of thermal annealing and solvent vapour annealing to increase the exciton diffusion coefficient and exciton diffusion length in two liquid crystalline electron donor materials BQR and BTR. We found that the three-dimensional exciton diffusion length increased to ~40 nm upon annealing in both materials. Grazing-incidence wide angle X-ray scattering (GIWAXS) measurements show an increase of crystallite size to ~37 nm in both materials after thermal annealing. We determined an average domain size of these materials in the blends with PC71BM using diffusion-limited fluorescence quenching and found that it increased to 31 nm in BTR PC71BM blends and to 60 nm in BQR PC71BM blends. Our results provide understanding of how annealing improves device efficiency.PostprintPeer reviewe

    Control of superconductivity with a single ferromagnetic layer in niobium/erbium bilayers

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    Superconducting spintronics in hybrid superconductor{ferromagnet (S{F) heterostructures provides an exciting potential new class of device. The prototypical super-spintronic device is the superconducting spin-valve, where the critical temperature, Tc, of the S-layer can be controlledby the relative orientation of two (or more) F-layers. Here, we show that such control is also possible in a simple S/F bilayer. Using eld history to set the remanent magnetic state of a thin Er layer, we demonstrate for a Nb/Er bilayer a high level of control of both Tc and the shape of the resistive transition, R(T), to zero resistance. We are able to model the origin of the remanent magnetization, treating it as an increase in the e ective exchange eld of the ferromagnet and link this, using conventional S{F theory, to the suppression of Tc. We observe stepped features in the R(T) which we argue is due to a fundamental interaction of superconductivity with inhomogeneous ferromagnetism, a phenomena currently lacking theoretical description

    Strong coupling in a microcavity containing β-carotene

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    © 2018 Optical Society of America. Abstract: We have fabricated an open-cavity microcavity structure containing a thin film of the biologically-derived molecule β-carotene. We show that the β-carotene absorption can be described in terms of a series of Lorentzian functions that approximate the 0-0, 0-1, 0-2, 0-3 and 0-4 electronic and vibronic transitions. On placing this molecular material into a microcavity, we obtain anti-crossing between the cavity mode and the 0-1 vibronic transition, however other electronic and vibronic transitions remain in the intermediate or weak-coupling regime due to their lower oscillator strength and broader linewidth. We discuss the consequences of strong-coupling for the possible modification of photosynthetic processes, or a re-ordering of allowed and optically-forbidden states
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