7,651 research outputs found
Irreversible magnetization in thin YBCO films rotated in external magnetic field
The magnetization M of a thin YBaCuO film is measured as a function of the
angle between the applied field H and the c-axis. For fields above
the first critical field, but below the Bean's field for first penetration H*,
M is symmetric with respect to and the magnetization curves for
forward and backward rotation coincide. For H>H* the curves are asymmetric and
they do not coincide. These phenomena have a simple explanation in the
framework of the Bean critical state model.Comment: 14 pages, 7 PostScript figure
Field Induced Nodal Order Parameter in the Tunneling Spectrum of YBaCuO Superconductor
We report planar tunneling measurements on thin films of
YBaCuO at various doping levels under magnetic fields. By
choosing a special setup configuration, we have probed a field induced energy
scale that dominates in the vicinity of a node of the d-wave superconducting
order parameter. We found a high doping sensitivity for this energy scale. At
Optimum doping this energy scale is in agreement with an induced
order parameter. We found that it can be followed down to low fields at optimum
doping, but not away from it.Comment: 9 pages, 8 figures, accepted for publication in Phys. Rev.
Magnetic and thermodynamic properties of Sr_{2}LaFe_{3}O_{9}
Using a Dirac-Heisenberg Hamiltonian with biquadratic exchange interactions,
we study the effect of iron disproportionation on the magnetic ordering, and
describe the first-order magnetic transition occurring in the perovskite
Sr_{2}LaFe_{3}O_{9}. Upon fitting the experimental data, we give an estimate of
the exchange integrals for the antiferromagntic and ferromagnetic interactions,
in agreement with previous works on kindered compounds. Spin-wave theory yields
a magnon spectrum with a gapless antiferromagnetic mode together with two
gapped ferromagnetic ones.Comment: 8 pages of RevTex, 5 figures (available upon request), submitted to
J. Mag. Mag. Ma
Evolution of the fishtail-effect in pure and Ag-doped MG-YBCO
We report on magnetic measurements carried out in a textured
YBaCuO and YBa(CuAg)O (at
0.02) crystals. The so-called fishtail-effect (FE) or second
magnetization peak has been observed in a wide temperature range
0.4~~0.8 for . The origin of the FE arises for
the competition between surface barrier and bulk pinning. This is confirmed in
a non-monotonically behavior of the relaxation rate . The value
for Ag-doped crystals is larger than for the pure one due to the presence of
additional pinning centers, above all on silver atoms.Comment: 6 pages, 6 figure
Universality in D-brane Inflation
We study the six-field dynamics of D3-brane inflation for a general scalar
potential on the conifold, finding simple, universal behavior. We numerically
evolve the equations of motion for an ensemble of more than 7 \times 10^7
realizations, drawing the coefficients in the scalar potential from statistical
distributions whose detailed properties have demonstrably small effects on our
results. When prolonged inflation occurs, it has a characteristic form: the
D3-brane initially moves rapidly in the angular directions, spirals down to an
inflection point in the potential, and settles into single-field inflation. The
probability of N_{e} e-folds of inflation is a power law, P(N_{e}) \propto
N_{e}^{-3}, and we derive the same exponent from a simple analytical model. The
success of inflation is relatively insensitive to the initial conditions: we
find attractor behavior in the angular directions, and the D3-brane can begin
far above the inflection point without overshooting. In favorable regions of
the parameter space, models yielding 60 e-folds of expansion arise
approximately once in 10^3 trials. Realizations that are effectively
single-field and give rise to a primordial spectrum of fluctuations consistent
with WMAP, for which at least 120 e-folds are required, arise approximately
once in 10^5 trials. The emergence of robust predictions from a six-field
potential with hundreds of terms invites an analytic approach to multifield
inflation.Comment: 28 pages, 9 figure
Vector magnetic hysteresis of hard superconductors
Critical state problems which incorporate more than one component for the
magnetization vector of hard superconductors are investigated. The theory is
based on the minimization of a cost functional
which weighs the changes of the magnetic field vector within the sample. We
show that Bean's simplest prescription of choosing the correct sign for the
critical current density in one dimensional problems is just a particular
case of finding the components of the vector . is
determined by minimizing under the constraint , with a bounded set. Upon the selection of
different sets we discuss existing crossed field measurements and
predict new observable features. It is shown that a complex behavior in the
magnetization curves may be controlled by a single external parameter, i.e.:
the maximum value of the applied magnetic field .Comment: 10 pages, 9 figures, accepted in Phys. Rev.
Weak Lensing Effects on the Galaxy Three-Point Correlation Function
We study the corrections to the galaxy three-point correlation function
(3PCF) induced by weak lensing magnification due to the matter distribution
along the line of sight. We consistently derive all the correction terms
arising up to second order in perturbation theory and provide analytic
expressions as well as order of magnitude estimates for their relative
importance. The magnification contributions depend on the geometry of the
projected triangle on the sky plane, and scale with different powers of the
number count slope and redshift of the galaxy sample considered. We evaluate
all terms numerically and show that, depending on the triangle configuration as
well as the galaxy sample considered, weak lensing can in general significantly
contribute to and alter the three-point correlation function observed through
galaxy and quasar catalogs.Comment: 24 pages, 11 figures; version accepted for publication in Phys. Rev.
D; v2: typos corrected, figure caption clarifie
Prominent bulk pinning effect in the MgB_2 superconductor
We report the magnetic-field dependence of the irreversible magnetization of
the recently discovered binary superconductor MgB. For the temperature
region of , the contribution of the bulk pinning to the
magnetization overwhelms that of the surface pinning. This was evident from the
fact that the magnetization curves, , were well described by the
critical-state model without considering the surface pinning effect. It was
also found that the curves at various temperatures scaled when the field
and the magnetization were normalized by the characteristic scaling factors
and , respectively. This feature suggests that the
pinning mechanism determining the hysteresis in is unique below .Comment: 4pages and 4 figures. Phys. Rev. B (accepted
Superconducting gap structure and pinning in disordered MgB2 films
We have performed a comparative study of two thin films of magnesium diboride
(MgB2) grown by different techniques. The critical current density at different
temperatures and magnetic fields was evaluated from magnetisation curves, the
structure of superconducting order parameter was obtained from point-contact
spectroscopy, and the scattering rates were evaluated by fitting the
temperature dependent normal-state resistivity to the two-band model. The films
have similar critical temperatures close to 39 K, but the upper critical fields
were different by a factor of 2 (5.2T and 2.5 T at 20 K). We have found that
the film with higher Hc2 also had stronger scattering in the sigma band and
smaller value of the superconducting gap in this band. As the scattering in
sigma band is primarily due to the defects in boron plane, our results are
consistent with the assumption that disordering the boron planes leads to
enhanced Hc2 and better pinning properties in magnetic field.Comment: Paper presented at EUCAS'0
Enhancement of Critical Current Density in low level Al-doped MgB2
Two sets of MgB2 samples doped with up to 5 at. % of Al were prepared in
different laboratories using different procedures. Decreases in the a and c
lattice parameters were observed with Al doping confirming Al substitution onto
the Mg site. The critical temperature (Tc) remained largely unchanged with Al
doping. For 1 - 2.5 at.% doping, at 20K the in-field critical current densities
(Jc's) were enhanced, particularly at lower fields. At 5K, in-field Jc was
markedly improved, e.g. at 5T Jc was enhanced by a factor of 20 for a doping
level of 1 at.% Al. The improved Jcs correlate with increased sample
resistivity indicative of an increase in the upper critical field, Hc2, through
alloying.Comment: 17 pages, 4 figures, to be published in Superconductor Science and
Technolog
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