2,219 research outputs found
Flux pinning properties of superconductors with an array of blind holes
We performed ac-susceptibility measurements to explore the vortex dynamics
and the flux pinning properties of superconducting Pb films with an array of
micro-holes (antidots) and non-fully perforated holes (blind holes). A lower
ac-shielding together with a smaller extension of the linear regime for the
lattice of blind holes indicates that these centers provide a weaker pinning
potential than antidots. Moreover, we found that the maximum number of flux
quanta trapped by a pinning site, i.e. the saturation number ns, is lower for
the blind hole array.Comment: 6 figures, 6 page
Novel pinning phenomena in a superconducting film with a square lattice of artificial pinning centers
We study the transport properties of a superconducting Nb film with a square
lattice of artificial pinning centers (APCs) as a function of dc current, at a
temperature close to the superconducting transition temperature of the film. We
find that, at low dc currents, the differential resistance of the film shows
the standard matching field anomaly, that is, the differential resistance has a
local minimum at magnetic fields corresponding to an integer number of flux
lines per APC. However, at higher dc currents, the differential resistance at
each matching field turns to a local maximum, which is exactly opposite to the
low current behavior. This novel effect might indicate that the flux lines in
the APC system change their flow mode as the dc current is increased.Comment: 10 pages, 4 figure
Superconducting films with antidot arrays - novel behavior of the critical current
Novel behavior of the critical current density of a regularly
perforated superconducting film is found, as a function of applied magnetic
field . Previously pronounced peaks of at matching fields were
always found to decrease with increasing . Here we found a {\it reversal of
this behavior} for particular geometrical parameters of the antidot lattice
and/or temperature. This new phenomenon is due to a strong ``caging'' of
interstitial vortices between the pinned ones. We show that this vortex-vortex
interaction can be further tailored by an appropriate choice of the
superconducting material, described by the Ginzburg-Landau parameter .
In effective type-I samples we predict that the peaks in at the
matching fields are transformed into a {\it step-like behavior}.Comment: 5 pages, 4 figure
Remarks on E11 approach
We consider a few topics in approach to superstring/M-theory: even
subgroups ( orbifolds) of , n=11,10,9 and their connection to
Kac-Moody algebras; subgroup of and coincidence of one of
its weights with the weight of , known to contain brane charges;
possible form of supersymmetry relation in ; decomposition of
w.r.t. the and its square root at first few levels; particle orbit
of . Possible relevance of coadjoint orbits method is
noticed, based on a self-duality form of equations of motion in .Comment: Two references adde
Suppression of dissipation in Nb thin films with triangular antidot arrays by random removal of pinning sites
The depinning current Ic versus applied magnetic field B close to the
transition temperature Tc of Nb thin films with randomly diluted triangular
arrays of antidots is investigated. % Our experiments confirm essential
features in Ic(B) as predicted by Reichhardt and Olson Reichhardt [Phys.Rev. B
76, 094512 (2007)]. % We show that, by introducing disorder into periodic
pinning arrays, Ic can be enhanced. % In particular, for arrays with fixed
density n_p of antidots, an increase in dilution Pd induces an increase in Ic
and decrease of the flux-flow voltage for B>Bp=n_p Phi_0.Comment: 5 pages, 4 figure
Direct visualization of magnetic vortex pinning in superconductors
We study the vortex structure in a Pb film deposited on top of a periodic
array of ferromagnetic square microrings by combining two high resolution
imaging techniques: Bitter decoration and scanning Hall probe microscopy
(SHPM). The periodicity and strength of the magnetic pinning potential
generated by the square microrings are controlled by the magnetic history of
the template. When the square rings are in the magnetized dipolar state, known
as the onion state, the strong stray field generated at the domain walls
prevents the decoration of vortices. SHPM images show that the stray field
generated by the dipoles is much stronger than the vortex field in agreement
with the results of simulations. Real space vortex imaging has revealed that,
in the onion state, the corners of the square rings act as effective pinning
centers for vortices.Comment: To be published in Phys. Rev.
Mechanism for flux guidance by micrometric antidot arrays in superconducting films
A study of magnetic flux penetration in a superconducting film patterned with
arrays of micron sized antidots (microholes) is reported. Magneto-optical
imaging (MOI) of a YBCO film shaped as a long strip with perpendicular antidot
arrays revealed both strong guidance of flux, and at the same time large
perturbations of the overall flux penetration and flow of current. These
results are compared with a numerical flux creep simulation of a thin
superconductor with the same antidot pattern. To perform calculations on such a
complex geometry, an efficient numerical scheme for handling the boundary
conditions of the antidots and the nonlocal electrodynamics was developed. The
simulations reproduce essentially all features of the MOI results. In addition,
the numerical results give insight into all other key quantities, e.g., the
electrical field, which becomes extremely large in the narrow channels
connecting the antidots.Comment: 8 pages, 7 figure
Superconducting properties of Nb thin films deposited on porous silicon templates
Porous silicon, obtained by electrochemical etching, has been used as a
substrate for the growth of nanoperforated Nb thin films. The films, deposited
by UHV magnetron sputtering on the porous Si substrates, inherited their
structure made of holes of 5 or 10 nm diameter and of 10 to 40 nm spacing,
which provide an artificial pinning structure. The superconducting properties
were investigated by transport measurements performed in the presence of
magnetic field for different film thickness and substrates with different
interpore spacing. Perpendicular upper critical fields measurements present
peculiar features such as a change in the H_c2(T) curvature and oscillations in
the field dependence of the superconducting resistive transition width at H=1
Tesla. This field value is much higher than typical matching fields in
perforated superconductors, as a consequence of the small interpore distance.Comment: accepted for publication on Journal of Applied Physic
Optical transition radiation in fused quartz under external acoustic field
In the present paper we have investigated the optical transition radiation in a plate excited by a longitudinal acoustic wave. The spectral-angular density of the radiated energy is calculated. The numerical examples are given for
a plate of fused quartz. These results show that the acoustic waves allow to control the parameters of the radiation. In particular, new resonance peaks appear in the
spectral distribution of the radiation intensity. The height of the peaks can be tuned by choosing the parameters of the acoustic wave
Casimir eigenvalues for universal Lie algebra
For two different natural definitions of Casimir operators for simple Lie
algebras we show that their eigenvalues in the adjoint representation can be
expressed polynomially in the universal Vogel's parameters and give explicit formulae for the generating functions of these
eigenvalues.Comment: Slightly revised versio
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