110 research outputs found
Epitaxial LaFeAsOF thin films grown by pulsed laser deposition
Superconducting and epitaxially grown LaFeAsOF thin films were successfully
prepared on (001)-oriented LaAlO3 substrates using pulsed laser deposition. The
prepared thin films show exclusively a single in-plane orientation with
epitaxial relation (001)[100] parallel to (001)[100] and a FWHM value of 1deg.
Furthermore, resistive measurement of the superconducting transition
temperature revealed a Tc90 of 25K with a high residual resistive ratio of 6.8.
The applied preparation technique, standard thin film pulsed laser deposition
at room temperature in combination with a subsequent post annealing process, is
suitable for fabrication of high quality LaFeAsO1-xFx thin films. A high upper
critical field of 76.2 T was evaluated for magnetic fields applied
perpendicular to the c-axis and the anisotropy was calculated to be 3.3
assuming single band superconductivity.Comment: 6 pages, 4 Figure
Highly anisotropic energy gap in superconducting Ba(FeCo)As from optical conductivity measurements
We have measured the complex dynamical conductivity, , of superconducting Ba(FeCo)As ( K) at terahertz frequencies and temperatures 2 - 30 K. In the frequency
dependence of below , we observe clear signatures of the
superconducting energy gap opening. The temperature dependence of
demonstrates a pronounced coherence peak at frequencies below 15 cm (1.8
meV). The temperature dependence of the penetration depth, calculated from
, shows power-law behavior at the lowest temperatures. Analysis of
the conductivity data with a two-gap model, gives the smaller isotropic s-wave
gap of meV, while the larger gap is highly anisotropic with
possible nodes and its rms amplitude is meV. Overall, our
results are consistent with a two-band superconductor with an gap
symmetry.Comment: 6 pages, 4 figures, discussion on pair-barking scattering and
possible lifting of the nodes is adde
Bulk high-Tc superconductors with drilled holes: how to arrange the holes to maximize the trapped magnetic flux ?
Drilling holes in a bulk high-Tc superconductor enhances the oxygen annealing
and the heat exchange with the cooling liquid. However, drilling holes also
reduces the amount of magnetic flux that can be trapped in the sample. In this
paper, we use the Bean model to study the magnetization and the current line
distribution in drilled samples, as a function of the hole positions. A single
hole perturbs the critical current flow over an extended region that is bounded
by a discontinuity line, where the direction of the current density changes
abruptly. We demonstrate that the trapped magnetic flux is maximized if the
center of each hole is positioned on one of the discontinuity lines produced by
the neighbouring holes. For a cylindrical sample, we construct a polar
triangular hole pattern that exploits this principle; in such a lattice, the
trapped field is ~20% higher than in a squared lattice, for which the holes do
not lie on discontinuity lines. This result indicates that one can
simultaneously enhance the oxygen annealing, the heat transfer, and maximize
the trapped field
Point-contact study of ReFeAs(1-x)Fx (Re=La, Sm) superconducting films
Point-contact (PC) Andreev-reflection (AR) measurements of the
superconducting gap in iron-oxipnictide ReFeAsO_{1-x}F_x (Re=La, Sm) films have
been carried out. The value of the gap is distributed in the range 2\Delta
\simeq 5-10 meV (for Re=Sm) with a maximum in the distribution around 6 meV.
Temperature dependence of the gap \Delta(T) can be fitted well by BCS curve
giving reduced gap ratio 2\Delta /kT_c^*\simeq 3.5 (here T_c^* is the critical
temperature from the BCS fit). At the same time, an expected second larger gap
feature was difficult to resolve distinctly on the AR spectra making
determination reliability of the second gap detection questionable. Possible
reasons for this and the origin of other features like clear-cut asymmetry in
the AR spectra and current regime in PCs are discussed.Comment: 6 two-column pages, 6 figs., 26 Refs., to be published in
Superconductor Science and Technolog
Symmetry and disorder of the vitreous vortex lattice in an overdoped BaFe_{2-x}Co_xAs_2 superconductor: Indication for strong single-vortex pinning
The disordered flux line lattice in single crystals of the slightly overdoped
aFe_{2-x}Co_xAs_2 (x = 0.19, Tc = 23 K) superconductor is studied by
magnetization measurements, small-angle neutron scattering (SANS), and magnetic
force microscopy (MFM). In the whole range of magnetic fields up to 9 T, vortex
pinning precludes the formation of an ordered Abrikosov lattice. Instead, a
vitreous vortex phase (vortex glass) with a short-range hexagonal order is
observed. Statistical processing of MFM datasets lets us directly measure its
radial and angular distribution functions and extract the radial correlation
length \zeta. In contrast to predictions of the collective pinning model, no
increase in the correlated volume with the applied field is observed. Instead,
we find that \zeta decreases as 1.3*R1 ~ H^(-1/2) over four decades of the
applied magnetic field, where R1 is the radius of the first coordination shell
of the vortex lattice. Such universal scaling of \zeta implies that the vortex
pinning in iron arsenides remains strong even in the absence of static
magnetism. This result is consistent with all the real- and reciprocal-space
vortex-lattice measurements in overdoped as-grown aFe_{2-x}Co_xAs_2 published
to date and is thus sample-independent. The failure of the collective pinning
model suggests that the vortices remain in the single-vortex pinning limit even
in high magnetic fields up to 9 T.Comment: 11 pages, 6 figure
Symmetry and disorder of the vitreous vortex lattice in an overdoped BaFe_{2-x}Co_xAs_2 superconductor: Indication for strong single-vortex pinning
The disordered flux line lattice in single crystals of the slightly overdoped
aFe_{2-x}Co_xAs_2 (x = 0.19, Tc = 23 K) superconductor is studied by
magnetization measurements, small-angle neutron scattering (SANS), and magnetic
force microscopy (MFM). In the whole range of magnetic fields up to 9 T, vortex
pinning precludes the formation of an ordered Abrikosov lattice. Instead, a
vitreous vortex phase (vortex glass) with a short-range hexagonal order is
observed. Statistical processing of MFM datasets lets us directly measure its
radial and angular distribution functions and extract the radial correlation
length \zeta. In contrast to predictions of the collective pinning model, no
increase in the correlated volume with the applied field is observed. Instead,
we find that \zeta decreases as 1.3*R1 ~ H^(-1/2) over four decades of the
applied magnetic field, where R1 is the radius of the first coordination shell
of the vortex lattice. Such universal scaling of \zeta implies that the vortex
pinning in iron arsenides remains strong even in the absence of static
magnetism. This result is consistent with all the real- and reciprocal-space
vortex-lattice measurements in overdoped as-grown aFe_{2-x}Co_xAs_2 published
to date and is thus sample-independent. The failure of the collective pinning
model suggests that the vortices remain in the single-vortex pinning limit even
in high magnetic fields up to 9 T.Comment: 11 pages, 6 figure
Pulsed-field magnetization of drilled bulk high-temperature superconductors: flux front propagation in the volume and on the surface
We present a method for characterizing the propagation of the magnetic flux
in an artificially drilled bulk high-temperature superconductor (HTS) during a
pulsed-field magnetization. As the magnetic pulse penetrates the cylindrical
sample, the magnetic flux density is measured simultaneously in 16 holes by
means of microcoils that are placed across the median plane, i.e. at an equal
distance from the top and bottom surfaces, and close to the surface of the
sample. We discuss the time evolution of the magnetic flux density in the holes
during a pulse and measure the time taken by the external magnetic flux to
reach each hole. Our data show that the flux front moves faster in the median
plane than on the surface when penetrating the sample edge; it then proceeds
faster along the surface than in the bulk as it penetrates the sample further.
Once the pulse is over, the trapped flux density inside the central hole is
found to be about twice as large in the median plane than on the surface. This
ratio is confirmed by modelling
DC superconducting quantum interference devices fabricated using bicrystal grain boundary junctions in Co-doped BaFe2As2 epitaxial films
DC superconducting quantum interference devices (dc-SQUIDs) were fabricated
in Co-doped BaFe2As2 epitaxial films on (La, Sr)(Al, Ta)O3 bicrystal substrates
with 30deg misorientation angles. The 18 x 8 micro-meter^2 SQUID loop with an
estimated inductance of 13 pH contained two 3 micro-meter wide grain boundary
junctions. The voltage-flux characteristics clearly exhibited periodic
modulations with deltaV = 1.4 micro-volt at 14 K, while the intrinsic flux
noise of dc-SQUIDs was 7.8 x 10^-5 fai0/Hz^1/2 above 20 Hz. The rather high
flux noise is mainly attributed to the small voltage modulation depth which
results from the superconductor-normal metal-superconductor junction nature of
the bicrystal grain boundary
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