91 research outputs found
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Pulsed laser deposition of thick BaHfO3-doped YBa 2Cu307-ÎŽ films on highly alloyed textured Ni-W tapes
YBa2Cu3O7-ÎŽ (YBCO) films with a thickness of up to 3 ÎŒm containing nano-sized BaHfO3 (BHO) have been grown on Y2O3/Y-stabilized ZrO2/CeO 2 buffered Ni-9at% W tapes by pulsed laser deposition (PLD). Structural characterization by means of X-ray diffraction confirmed that the YBCO layer grew epitaxial. A superconducting transition temperature T c of about 89 K with a transition width of 1 K was determined, decreasing with increasing BHO content. Critical current density in self-field and at 0.3 T increased with increasing dopant level
Anisotropy of strong pinning in multi-band superconductors
The field-angular dependence and anisotropy of the critical current density
in iron-based superconductors is evaluated using a phenomenological approach
featuring distinct anisotropy factors for the penetration depth and the
coherence length. Both the weak collective pinning limit, and the strong
pinning limit relevant for iron-based superconductors at low magnetic fields
are considered. It is found that in the more anisotropic materials, such as
SmFeAsO and NdFeAsO, the field-angular dependence is completely dominated by
the coherence-length (upper-critical field) anisotropy, thereby explaining
recent results on the critical current in these materials. In less anisotropic
superconductors, strong pinning can lead to an apparent inversion of the
anisotropy. Finally, it is shown that, under all circumstances, the ratio of
c-axis and ab-plane critical current densities for magnetic field along the
ab-plane directly yields the coherence length anisotropy factor
{\epsilon}{\xi}.Comment: 21 pages, 2012 Special Issue on Iron Based Superconductors in
Superconducting Science and Technology (accepted); glitches removed on May
31st, 2012; further typos removed Sept. 15th, 201
High field superconducting properties of Ba(FeâââCoâ)âAsâ thin films
In general, the critical current density, Jc, of type II superconductors and its anisotropy with respect to magnetic field orientation is determined by intrinsic and extrinsic properties. The Fe-based superconductors of the â122â family with their moderate electronic anisotropies and high yet accessible critical fields (Hc2 and Hirr) are a good model system to study this interplay. In this paper, we explore the vortex matter of optimally Co-doped BaFe2As2 thin films with extended planar and c-axis correlated defects. The temperature and angular dependence of the upper critical field is well explained by a two-band model in the clean limit. The dirty band scenario, however, cannot be ruled out completely. Above the irreversibility field, the flux motion is thermally activated, where the activation energy U0 is going to zero at the extrapolated zero-kelvin Hirr value. The anisotropy of the critical current density Jc is both influenced by the Hc2 anisotropy (and therefore by multi-band effects) as well as the extended planar and columnar defects present in the sample
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Intrinsic and extrinsic pinning in NdFeAs(O,F): Vortex trapping and lock-in by the layered structure
Fe-based superconductors (FBS) present a large variety of compounds whose properties are affected to different extents by their crystal structures. Amongst them, the REFeAs(O,F) (RE1111, RE being a rare-earth element) is the family with the highest critical temperature Tc but also with a large anisotropy and Josephson vortices as demonstrated in the flux-flow regime in Sm1111 (TcââŒâ55âK). Here we focus on the pinning properties of the lower-Tc Nd1111 in the flux-creep regime. We demonstrate that for H//c critical current density Jc at high temperatures is dominated by point-defect pinning centres, whereas at low temperatures surface pinning by planar defects parallel to the c-axis and vortex shearing prevail. When the field approaches the ab-planes, two different regimes are observed at low temperatures as a consequence of the transition between 3D Abrikosov and 2D Josephson vortices: one is determined by the formation of a vortex-staircase structure and one by lock-in of vortices parallel to the layers. This is the first study on FBS showing this behaviour in the full temperature, field, and angular range and demonstrating that, despite the lower Tc and anisotropy of Nd1111 with respect to Sm1111, this compound is substantially affected by intrinsic pinning generating a strong ab-peak in Jc
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Tailor-made nanostructures bridging chaos and order for highly efficient white organic light-emitting diodes
Organic light-emitting diodes (OLEDs) suffer from notorious light trapping, resulting in only moderate external quantum efficiencies. Here, we report a facile, scalable, lithography-free method to generate controllable nanostructures with directional randomness and dimensional order, significantly boosting the efficiency of white OLEDs. Mechanical deformations form on the surface of poly(dimethylsiloxane) in response to compressive stress release, initialized by reactive ions etching with periodicity and depth distribution ranging from dozens of nanometers to micrometers. We demonstrate the possibility of independently tuning the average depth and the dominant periodicity. Integrating these nanostructures into a two-unit tandem white organic light-emitting diode, a maximum external quantum efficiency of 76.3% and a luminous efficacy of 95.7 lm Wâ1 are achieved with extracted substrate modes. The enhancement factor of 1.53 ± 0.12 at 10,000 cd mâ2 is obtained. An optical model is built by considering the dipole orientation, emitting wavelength, and the dipole position on the sinusoidal nanotexture
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Analysis of the Annealing Budget of Metal Oxide Thin-Film Transistors Prepared by an Aqueous Blade-Coating Process
Metal oxide (MO) semiconductors are widely used in electronic devices due to their high optical transmittance and promising electrical performance. This work describes the advancement toward an eco-friendly, streamlined method for preparing thin-film transistors (TFTs) via a pure water-solution blade-coating process with focus on a low thermal budget. Low temperature and rapid annealing of triple-coated indium oxide thin-film transistors (3C-TFTs) and indium oxide/zinc oxide/indium oxide thin-film transistors (IZI-TFTs) on a 300 nm SiO2 gate dielectric at 300 °C for only 60 s yields devices with an average field effect mobility of 10.7 and 13.8 cm2 Vâ1 sâ1, respectively. The devices show an excellent on/off ratio (>106), and a threshold voltage close to 0 V when measured in air. Flexible MO-TFTs on polyimide substrates with AlOx dielectrics fabricated by rapid annealing treatment can achieve a remarkable mobility of over 10 cm2 Vâ1 sâ1 at low operating voltage. When using a longer post-coating annealing period of 20 min, high-performance 3C-TFTs (over 18 cm2 Vâ1 sâ1) and IZI-TFTs (over 38 cm2 Vâ1 sâ1) using MO semiconductor layers annealed at 300 °C are achieved
Selective mass enhancement close to the quantum critical point in BaFeâ(Asâââ Pâ )â
A quantum critical point (QCP) is currently being conjectured for the BaFeâ(Asâââ Pâ )â system at the
critical value xc â 0.3. In the proximity of a QCP, all thermodynamic and transport properties are
expected to scale with a single characteristic energy, given by the quantum fluctuations. Such a
universal behavior has not, however, been found in the superconducting upper critical field Hc2. Here
we report Hc2 data for epitaxial thin films extracted from the electrical resistance measured in very
high magnetic fields up to 67 Tesla. Using a multi-band analysis we find that Hcâ is sensitive to the QCP,
implying a significant charge carrier effective mass enhancement at the doping-induced QCP that is
essentially band-dependent. Our results point to two qualitatively different groups of electrons in
BaFeâ(Asâââ Pâ )â . The first one (possibly associated to hot spots or whole Fermi sheets) has a strong mass
enhancement at the QCP, and the second one is insensitive to the QCP. The observed duality could also
be present in many other quantum critical systems
Non-thermal response of YBCO thin films to picosecond THz pulses
The photoresponse of YBa2Cu3O7-d thin film microbridges with thicknesses
between 15 and 50 nm was studied in the optical and terahertz frequency range.
The voltage transients in response to short radiation pulses were recorded in
real time with a resolution of a few tens of picoseconds. The bridges were
excited by either femtosecond pulses at a wavelength of 0.8 \mu m or broadband
(0.1 - 1.5 THz) picosecond pulses of coherent synchrotron radiation. The
transients in response to optical radiation are qualitatively well explained in
the framework of the two-temperature model with a fast component in the
picosecond range and a bolometric nanosecond component whose decay time depends
on the film thickness. The transients in the THz regime showed no bolometric
component and had amplitudes up to three orders of magnitude larger than the
two-temperature model predicts. Additionally THz-field dependent transients in
the absence of DC bias were observed. We attribute the response in the THz
regime to a rearrangement of vortices caused by high-frequency currents
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