53 research outputs found
Imaging Local Sources of Intermodulation in Superconducting Microwave Devices
This work presents new experimental results on low-temperature (LT)
characterization of local rf properties of passive superconducting (SC)
microwave devices using a novel Laser Scanning Microscope (LSM). In this
technique, a modulated laser beam is focused onto and scanned over the surface
of a resonant SC device to probe the spatial distribution of rf current. The
highly localized photo-induced change of the kinetic inductance of the SC
device produces both a shift of the resonant frequency f_0 and change of the
quality factor Q. An image of these changes is recorded as the laser spot is
scanned over the device. We present the first measurements of spatially
resolved intermodulation response in a High Temperature Superconducting (HTS)
co-planar waveguide resonator, opening up a new window into the local origins
of nonlinearity in the HTS materials.Comment: 4 Pages, to be published in IEEE Trans. Appl. Supercond., June 200
Observation of breathers in Josephson ladders
We report on the observation of spatially-localized excitations in a ladder
of small Josephson junctions. The excitations are whirling states which persist
under a spatially-homogeneous force due to the bias current. These states of
the ladder are visualized using a low temperature scanning laser microscopy. We
also compute breather solutions with high accuracy in corresponding model
equations. The stability analysis of these solutions is used to interpret the
measured patterns in the I-V characteristics
Roles of intrinsic anisotropy and pi-band pairbreaking effects on critical currents in tilted c-axis MgB2 films probed by magneto-optical and transport measurements
Investigations of MgB2 and Fe-based superconductors in recent years have
revealed many unusual effects of multiband superconductivity but manifestations
of anisotropic multiband effects in the critical current density Jc have not
been addressed experimentally, mostly because of the difficulties to measure Jc
along the c-axis. To investigate the effect of very different intrinsic
anisotropies of sigma and pi electron bands in MgB2 on current transport, we
grew epitaxial films with tilted c-axis (THETA ~ 19.5{\deg}), which enabled us
to measure the components of Jc both along the ab-plane and the c-axis using
magneto-optical and transport techniques. These measurements were combined with
scanning and transmission electron microscopy, which revealed terraced steps on
the surface of the c-axis tilted films. The measured field and temperature
dependencies of the anisotropic Jc(H) show that Jc,L parallel to the terraced
steps is higher than Jc,T perpendicular to the terraced steps, and Jc of
thinner films (50 nm) obtained from transport experiments at 0.1 T reaches ~10%
of the depairing current density Jd in the ab plane, while magneto-optical
imaging revealed much higher Jc at lower fields. To analyze the experimental
data we developed a model of anisotropic vortex pinning which accounts for the
observed behavior of Jc in the c-axis tilted films and suggests that the
apparent anisotropy of Jc is affected by current pairbreaking effects in the
weaker {\pi} band. Our results indicate that the out-of-plane current transport
mediated by the {\pi} band could set the ultimate limit of Jc in MgB2
polycrystals.Comment: 21 pges, 13 figure
Magnetic field induced control of breather dynamics in a single plaquette of Josephson junctions
We present a theoretical study of inhomogeneous dynamic (resistive) states in
a single plaquette consisting of three Josephson junctions. Resonant
interactions of such a breather state with electromagnetic oscillations
manifest themselves by resonant current steps and voltage jumps in the
current-voltage characteristics. An externally applied magnetic field leads to
a variation of the relative shift between the Josephson current oscillations of
two resistive junctions. By making use of the rotation wave approximation
analysis and direct numerical simulations we show that this effect allows to
effectively control the breather instabilities, e. g. to increase (decrease)
the height of the resonant steps and to suppress the voltage jumps in the
current-voltage characteristics.Comment: 4 pages, 3 figure
Penentration of dynamic localized states in DC-driven Josephson junction ladders by discrete jumps
We give a theoretical study of unusual resistive (dynamic) localized states
in anisotropic Josephson junction ladders, driven by a DC current at one edge.
These states comprise nonlinearly coupled rotating Josephson phases in adjacent
cells, and with increasing current they are found to expand into neighboring
cells by a sequence of sudden jumps. We argue that the jumps arise from
instabilities in the ladder's superconducting part, and our analytic
expressions for the peculiar voltage (rotational frequency) ratios and I-V
curves are in very good agreement with direct numerical simulations.Comment: Accepted, Physical Review E. 5 pages, 5 figures. Revtex, with
postscript figure
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Engineering current density over 5 kA mm-2 at 4.2 K, 14 T in thick film REBCO tapes
We report on remarkably high in-field performance at 4.2 K achieved in >4 μm thick rare earth barium copper oxide (REBCO) samples with Zr addition. Two different samples have been measured independently at Lawrence Berkeley National Laboratory and the National High Magnetic Field Laboratory, achieving critical current densities (J ) of 12.21 MA cm and 12.32 MA cm at 4.2 K, 14 T (), respectively, which corresponds to equivalent critical current (I ) values of 2247 and 2119 A/4 mm. These I values are about two times higher than the best reported performance of REBCO tapes to date and more than five times higher than the commercial HTS tapes reported in a recent study. The measured J values, with a pinning force of ∼1.7 T N m are almost identical to the highest value reported for thin (∼1 μm thick) REBCO at the field and temperature, but extended to very thick (>4 μm) films. This results in an engineering current density (J ) above 5 kA mm at 4.2 K, 14 T, which is more than five times higher than Nb Sn and nearly four times higher than the highest reported value of all superconductors other than REBCO at this field and temperature. The reported results have been achieved by utilizing an advanced metal organic chemical vapor deposition system. This study demonstrates the remarkable level of in-field performance achievable with REBCO conductors at 4.2 K and strong potential for high-field magnet applications. c c c c e 3 -2 -2 -3 -
New Fe-based superconductors: properties relevant for applications
Less than two years after the discovery of high temperature superconductivity
in oxypnictide LaFeAs(O,F) several families of superconductors based on Fe
layers (1111, 122, 11, 111) are available. They share several characteristics
with cuprate superconductors that compromise easy applications, such as the
layered structure, the small coherence length, and unconventional pairing, On
the other hand the Fe-based superconductors have metallic parent compounds, and
their electronic anisotropy is generally smaller and does not strongly depend
on the level of doping, the supposed order parameter symmetry is s wave, thus
in principle not so detrimental to current transmission across grain
boundaries. From the application point of view, the main efforts are still
devoted to investigate the superconducting properties, to distinguish intrinsic
from extrinsic behaviours and to compare the different families in order to
identify which one is the fittest for the quest for better and more practical
superconductors. The 1111 family shows the highest Tc, huge but also the most
anisotropic upper critical field and in-field, fan-shaped resistive transitions
reminiscent of those of cuprates, while the 122 family is much less anisotropic
with sharper resistive transitions as in low temperature superconductors, but
with about half the Tc of the 1111 compounds. An overview of the main
superconducting properties relevant to applications will be presented. Upper
critical field, electronic anisotropy parameter, intragranular and
intergranular critical current density will be discussed and compared, where
possible, across the Fe-based superconductor families
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