110 research outputs found
Universal temperature scaling of flux line pinning in high-temperature superconducting thin films
Dissipation-free current transport in high-temperature superconductors is one
of the most crucial properties of this class of materials which is directly
related to the effective inhibition of flux line movement by defect structures.
In this respect epitaxially grown thin films of YBa2Cu3O7-d (YBCO) are proving
to be the strongest candidates for many widescale applications that are close
to realization. We show that the relation between different defect structures
and flux line pinning in these films exhibits universal features which are
clearly displayed in a detailed analysis of the temperature-dependent behaviour
of local critical currents. This allows us to identify different pinning
mechanisms at different temperatures to be responsible for the found critical
currents. Additionally, the presence of grain boundaries with very low
misorientation angles affects the temperature stability of the critical
currents which has important consequences for future applications.Comment: 5 pages, 4 figures To be published in Journal of Physics: Condensed
matte
Influence of vortex-vortex interaction on critical currents across low-angle grain boundaries in YBa2Cu3O7-delta thin films
Low-angle grain boundaries with misorientation angles theta < 5 degrees in
optimally doped thin films of YBCO are investigated by magnetooptical imaging.
By using a numerical inversion scheme of Biot-Savart's law the critical current
density across the grain boundary can be determined with a spatial resolution
of about 5 micrometers. Detailed investigation of the spatially resolved flux
density and current density data shows that the current density across the
boundary varies with varying local flux density. Combining the corresponding
flux and current pattern it is found that there exists a universal dependency
of the grain boundary current on the local flux density. A change in the local
flux density means a variation in the flux line-flux line distance. With this
knowledge a model is developped that explains the flux-current relation by
means of magnetic vortex-vortex interaction.Comment: 7 pages, 14 figure
Doping dependent Irreversible Magnetic Properties of Ba(Fe1-xCox)2As2 Single Crystals
We discuss the irreversible magnetic properties of self-flux grown
Ba(Fe1-xCox)2As2 single crystals for a wide range of concentrations covering
the whole phase diagram from the underdoped to the overdoped regime, x=0.038,
0.047, 0.058, 0.071, 0.074, 0.10, 0.106 and 0.118. Samples were characterized
by a magneto-optical method and show excellent spatial uniformity of the
superconducting state. The overall behavior closely follows classical Bean
model of the critical state. The field-dependent magnetization exhibits second
peak at a temperature and doping - dependent magnetic field, Hp. The evolution
of this fishtail feature with doping is discussed. Magnetic relaxation is
time-logarithmic and unusually fast. Similar to cuprates, there is an apparent
crossover from collective elastic to plastic flux creep above Hp. At high
fields, the field dependence of the relaxation rate becomes doping independent.
We discuss our results in the framework of the weak collective pinning and show
that vortex physics in iron-based pnictide crystals is much closer to high-Tc
cuprates than to conventional s-wave (including MgB2) superconductors.Comment: for the special issue of Physica C on iron-based pnictide
superconductor
Exact analytical solution of the problem of current-carrying states of the Josephson junction in external magnetic fields
The classical problem of the Josephson junction of arbitrary length W in the
presence of externally applied magnetic fields (H) and transport currents (J)
is reconsidered from the point of view of stability theory. In particular, we
derive the complete infinite set of exact analytical solutions for the phase
difference that describe the current-carrying states of the junction with
arbitrary W and an arbitrary mode of the injection of J. These solutions are
parameterized by two natural parameters: the constants of integration. The
boundaries of their stability regions in the parametric plane are determined by
a corresponding infinite set of exact functional equations. Being mapped to the
physical plane (H,J), these boundaries yield the dependence of the critical
transport current Jc on H. Contrary to a wide-spread belief, the exact
analytical dependence Jc=Jc(H) proves to be multivalued even for arbitrarily
small W. What is more, the exact solution reveals the existence of unquantized
Josephson vortices carrying fractional flux and located near one of the
junction edges, provided that J is sufficiently close to Jc for certain finite
values of H. This conclusion (as well as other exact analytical results) is
illustrated by a graphical analysis of typical cases.Comment: 21 pages, 9 figures, to be published in Phys. Rev.
Local threshold field for dendritic instability in superconducting MgB2 films
Using magneto-optical imaging the phenomenon of dendritic flux penetration in
superconducting films was studied. Flux dendrites were abruptly formed in a 300
nm thick film of MgB2 by applying a perpendicular magnetic field. Detailed
measurements of flux density distributions show that there exists a local
threshold field controlling the nucleation and termination of the dendritic
growth. At 4 K the local threshold field is close to 12 mT in this sample,
where the critical current density is 10^7 A/cm^2. The dendritic instability in
thin films is believed to be of thermo-magnetic origin, but the existence of a
local threshold field, and its small value are features that distinctly
contrast the thermo-magnetic instability (flux jumps) in bulk superconductors.Comment: 6 pages, 6 figures, submitted to Phys. Rev.
Strong Pinning in High Temperature Superconductors
Detailed measurements of the critical current density jc of YBa2Cu3O7 films
grown by pulsed laser deposition reveal the increase of jc as function of the
filmthickness. Both this thickness dependence and the field dependence of the
critical current are consistently described using a generalization of the
theory of strong pinning of Ovchinnikov and Ivlev [Phys. Rev. B 43, 8024
(1991)]. From the model, we deduce values of the defect density (10^21 m^-3)
and the elementary pinning force, which are in good agreement with the
generally accepted values for Y2O3-inclusions. In the absence of clear evidence
that the critical current is determined by linear defects or modulations of the
film thickness, our model provides an alternative explanation for the rather
universal field dependence of the critical current density found in YBa2Cu3O7
films deposited by different methods.Comment: 11 pages; 8 Figures; Published Phys. Rev. B 66, 024523 (2002
Interplay of dendritic avalanches and gradual flux penetration in superconducting MgB2 films
Magneto-optical imaging was used to study a zero-field-cooled MgB2 film at
9.6K where in a slowly increasing field the flux penetrates by abrupt formation
of large dendritic structures. Simultaneously, a gradual flux penetration takes
place, eventually covering the dendrites, and a detailed analysis of this
process is reported. We find an anomalously high gradient of the flux density
across a dendrite branch, and a peak value that decreases as the applied field
goes up. This unexpected behaviour is reproduced by flux creep simulations
based on the non-local field-current relation in the perpendicular geometry.
The simulations also provide indirect evidence that flux dendrites are formed
at an elevated local temperature, consistent with a thermo-magnetic mechanism
of the instabilityComment: 5 pages, 5 figures, submitted to Supercond. Sci. Techno
Dynamics of the magnetic flux trapped in fractal clusters of normal phase in a superconductor
The influence of geometry and morphology of superconducting structure on
critical currents and magnetic flux trapping in percolative type-II
superconductor is considered. The superconductor contains the clusters of a
normal phase, which act as pinning centers. It is found that such clusters have
significant fractal properties. The main features of these clusters are studied
in detail: the cluster statistics is analyzed; the fractal dimension of their
boundary is estimated; the distribution of critical currents is obtained, and
its peculiarities are explored. It is examined thoroughly how the finite
resolution capacity of the cluster geometrical size measurement affects the
estimated value of fractal dimension. The effect of fractal properties of the
normal phase clusters on the electric field arising from magnetic flux motion
is investigated in the case of an exponential distribution of cluster areas.
The voltage-current characteristics of superconductors in the resistive state
for an arbitrary fractal dimension are obtained. It is revealed that the
fractality of the boundaries of the normal phase clusters intensifies the
magnetic flux trapping and thereby raises the critical current of a
superconductor.Comment: revtex, 16 pages with 1 table and 5 figures; text and figures are
improved; more detailed version with geometric probability analisys of the
distribution of entry points into weak links over the perimeter of a normal
phase clusters and one additional figure is published in Phys.Rev.B;
alternative e-mail of author is [email protected]
The temperature-dependent magnetization profile across an epitaxial bilayer of ferromagnetic La2/3Ca1/3MnO3 and superconducting YBa2Cu3O7-d
Epitaxial bilayers of ferromagnetic La2/3Ca1/3MnO3 (LCMO) and superconducting
YBa2Cu3O7-d (YBCO) have been grown on single-crystalline SrTiO3 (STO)
substrates by pulsed laser deposition. The Manganese magnetization profile
across the FM layer has been determined with high spatial resolution at low
temperatures by X-ray resonant magnetic reflectivity (XRMR). It is found that
not only the adjacent superconductor but also the substrate underneath
influences the magnetization of the LCMO film at the interfaces at low
temperatures. Both effects can be investigated individually by XRMR
- …