46 research outputs found
Magnetoinductance of Josephson junction array with frozen vortex diffusion
The dependence of sheet impedance of a Josephson junction array on the
applied magnetic field is investigated in the regime when vortex diffusion
between array plaquettes is effectively frozen due to low enough temperature.
The field dependent contribution to sheet inductance is found to be
proportional to f*ln(1/f), where f<<1 is the magnitude of the field expressed
in terms of flux quanta per plaquette.Comment: 5 pages, no figure
Temperature and Frequency Dependence of Complex Conductance of Ultrathin YBa2Cu3O7-x Films: A Study of Vortex-Antivortex Pair Unbinding
We have studied the temperature dependencies of the complex sheet conductance
of 1-3 unit cell (UC) thick YBa2Cu3O7-x films sandwiched between semiconducting
Pr0.6Y0.4Ba2Cu3O7-x layers at high frequencies. Experiments have been carried
out in a frequency range between: 2 - 30 MHz with one-spiral coil technique,
100 MHz - 1 GHz frequency range with a new technique using the spiral coil
cavity and at 30 GHz by aid of a resonant cavity technique. The real and
imaginary parts of the mutual-inductance between a coil and a film were
measured and converted to complex conductivity by aid of the inversion
procedure. We have found a quadratic temperature dependence of the kinetic
inductance, L_k^-1(T), at low temperatures independent of frequency, with a
break in slope at T^dc_BKT, the maximum of real part of conductance and a large
shift of the break temperature and the maximum position to higher temperatures
with increasing frequency. We obtain from these data the universal ratio
T^dc_BKT/L_k^-1(T^dc_BKT) = 25, 25, and 17 nHK for 1-, 2- and 3UC films,
respectively in close agreement with theoretical prediction of 12 nHK for
vortex-antivortex unbinding transition. The activated temperature dependence of
the vortex diffusion constant was observed and discussed in the framework of
vortex-antivortex pair pinning.
PACS numbers: 74.80.Dm, 74.25.Nf, 74.72.Bk, 74.76.BzComment: PDF file, 10 pages, 6 figures, to be published in J. Low Temp. Phys.;
Proc. of NATO ARW: VORTEX 200
Dynamic ordering and frustration of confined vortex rows studied by mode-locking experiments
The flow properties of confined vortex matter driven through disordered
mesoscopic channels are investigated by mode locking (ML) experiments. The
observed ML effects allow to trace the evolution of both the structure and the
number of confined rows and their match to the channel width as function of
magnetic field. From a detailed analysis of the ML behavior for the case of
3-rows we obtain ({\it i}) the pinning frequency , ({\it ii}) the onset
frequency for ML ( ordering velocity) and ({\it iii}) the
fraction of coherently moving 3-row regions in the channel. The
field dependence of these quantities shows that, at matching, where is
maximum, the pinning strength is small and the ordering velocity is low, while
at mismatch, where is small, both the pinning force and the ordering
velocity are enhanced. Further, we find that , consistent
with the dynamic ordering theory of Koshelev and Vinokur. The microscopic
nature of the flow and the ordering phenomena will also be discussed.Comment: 10 pages, 7 figure, submitted to PRB. Discussion has been improved
and a figure has been adde
Transverse Phase Locking for Vortex Motion in Square and Triangular Pinning Arrays
We analyze transverse phase locking for vortex motion in a superconductor
with a longitudinal DC drive and a transverse AC drive. For both square and
triangular arrays we observe a variety of fractional phase locking steps in the
velocity versus DC drive which correspond to stable vortex orbits. The locking
steps are more pronounced for the triangular arrays which is due to the fact
that the vortex motion has a periodic transverse velocity component even for
zero transverse AC drive. All the steps increase monotonically in width with AC
amplitude. We confirm that the width of some fractional steps in the square
arrays scales as the square of the AC driving amplitude. In addition we
demonstrate scaling in the velocity versus applied DC driving curves at
depinning and on the main step, similar to that seen for phase locking in
charge-density wave systems. The phase locking steps are most prominent for
commensurate vortex fillings where the interstitial vortices form symmetrical
ground states. For increasing temperature, the fractional steps are washed out
very quickly, while the main step gains a linear component and disappears at
melting. For triangular pinning arrays we again observe transverse phase
locking, with the main and several of the fractional step widths scaling
linearly with AC amplitude.Comment: 10 pages, 14 postscript figure
Directional vortex motion guided by artificially induced mesoscopic potentials
Rectangular pinning arrays of Ni dots define a potential landscape for vortex
motion in Nb films. Magnetotransport experiments in which two in-plane
orthogonal electrical currents are injected simultaneously allow selecting the
direction and magnitude of the Lorentz force on the vortex-lattice, thus
providing the angular dependence of the vortex motion. The background
dissipation depends on angle at low magnetic fields, which is progressively
smeared out with increasing field. The periodic potential locks in the vortex
motion along channeling directions. Because of this, vortex-lattice direction
of motion is up to 85o away from the applied Lorentz force direction.Comment: PDF file includes figure
Vortex states in 2D superconductor at high magnetic field in a periodic pinning potential
The effect of a periodic pinning array on the vortex state in a 2D
superconductor at low temperatures is studied within the framework of the
Ginzburg-Landau approach. It is shown that attractive interaction of vortex
cores to a commensurate pin lattice stabilizes vortex solid phases with long
range positional order against violent shear fluctuations. Exploiting a simple
analytical method, based on the Landau orbitals description, we derive a rather
detailed picture of the low temperatures vortex state phase diagram. It is
predicted that for sufficiently clean samples application of an artificial
periodic pinning array would enable one to directly detect the intrinsic shear
stiffness anisotropy characterizing the ideal vortex lattice.Comment: 8 pages, 5 figure
Temperature dependence and mechanisms for vortex pinning by periodic arrays of Ni dots in Nb films
Pinning interactions between superconducting vortices in Nb and magnetic Ni
dots were studied as a function of current and temperature to clarify the
nature of pinning mechanisms. A strong current dependence is found for a square
array of dots, with a temperature dependent optimum current for the observation
of periodic pinning, that decreases with temperature as (1-T/Tc)3/2. This same
temperature dependence is found for the critical current at the first matching
field with a rectangular array of dots. The analysis of these results allows to
narrow the possible pinning mechanisms to a combination of two: the interaction
between the vortex and the magnetic moment of the dot and the proximity effect.
Moreover, for the rectangular dot array, the temperature dependence of the
crossover between the low field regime with a rectangular vortex lattice to the
high field regime with a square configuration has been studied. It is found
that the crossover field increases with decreasing temperature. This dependence
indicates a change in the balance between elastic and pinning energies,
associated with dynamical effects of the vortex lattice in the high field
range.Comment: 12 text pages (revtex), 6 figures (1st jpeg, 2nd-6th postscript)
accepted in Physical Review
Vortex Pinball Under Crossed AC Drives in Superconductors with Periodic Pinning Arrays
Vortices driven with both a transverse and a longitudinal AC drive which are
out of phase are shown to exhibit a novel commensuration-incommensuration
effect when interacting with periodic substrates. For different AC driving
parameters, the motion of the vortices forms commensurate orbits with the
periodicity of the pinning array. When the commensurate orbits are present,
there is a finite DC critical depinning threshold, while for the incommensurate
phases the vortices are delocalized and the DC depinning threshold is absent.Comment: 4 pages, 4 postscript figure
Quantum corrections to the conductivity of fermion - gauge field models: Application to half filled Landau level and high- superconductors
We calculate the Altshuler-Aronov type quantum correction to the conductivity
of charge carriers in a random potential (or random magnetic field)
coupled to a transverse gauge field. The gauge fields considered simulate the
effect of the Coulomb interaction for the fractional quantum Hall state at half
filling and for the model of high- superconducting compounds. We
find an unusually large quantum correction varying linearly or quadratically
with the logarithm of temperature, in different temperature regimes.Comment: 12 pages REVTEX, 1 figure. The figure is added and minor misprints
are correcte
Flux noise in high-temperature superconductors
Spontaneously created vortex-antivortex pairs are the predominant source of
flux noise in high-temperature superconductors. In principle, flux noise
measurements allow to check theoretical predictions for both the distribution
of vortex-pair sizes and for the vortex diffusivity. In this paper the
flux-noise power spectrum is calculated for the highly anisotropic
high-temperature superconductor Bi-2212, both for bulk crystals and for
ultra-thin films. The spectrum is basically given by the Fourier transform of
the temporal magnetic-field correlation function. We start from a
Berezinskii-Kosterlitz-Thouless type theory and incorporate vortex diffusion,
intra-pair vortex interaction, and annihilation of pairs by means of a
Fokker-Planck equation to determine the noise spectrum below and above the
superconducting transition temperature. We find white noise at low frequencies
omega and a spectrum proportional to 1/omega^(3/2) at high frequencies. The
cross-over frequency between these regimes strongly depends on temperature. The
results are compared with earlier results of computer simulations.Comment: 9 pages, 4 PostScript figures, to be published in Phys. Rev.