216 research outputs found
Growth of Single Unit-Cell Superconducting LaSrCuO Films
We have developed an approach to grow high quality ultrathin films of
LaSrCuO with molecular beam epitaxy, by adding a
homoepitaxial buffer layer in order to minimize the degradation of the film
structure at the interface. The advantage of this method is to enable a further
reduction of the minimal thickness of a superconducting
LaSrCuO film. The main result of our work is that a
single unit cell (only two copper oxide planes) grown on a SrLaAlO
substrate exhibits a superconducting transition at 12.5 K (zero resistance) and
an in-plane magnetic penetration depth = 535 nm.Comment: to be published in "Solid State Electonics" special issue, conference
proceedings of the 9th Workshop on Oxide Electronics, St-Pete Beach, FL,
20-23 november 2002 : 12 pages 4 figures in preprint versio
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Demonstration of Machine Learning-Based Model-Independent Stabilization of Source Properties in Synchrotron Light Sources.
Synchrotron light sources, arguably among the most powerful tools of modern scientific discovery, are presently undergoing a major transformation to provide orders of magnitude higher brightness and transverse coherence enabling the most demanding experiments. In these experiments, overall source stability will soon be limited by achievable levels of electron beam size stability, presently on the order of several microns, which is still 1-2 orders of magnitude larger than already demonstrated stability of source position and current. Until now source size stabilization has been achieved through corrections based on a combination of static predetermined physics models and lengthy calibration measurements, periodically repeated to counteract drift in the accelerator and instrumentation. We now demonstrate for the first time how the application of machine learning allows for a physics- and model-independent stabilization of source size relying only on previously existing instrumentation. Such feed-forward correction based on a neural network that can be continuously online retrained achieves source size stability as low as 0.2 μm (0.4%) rms, which results in overall source stability approaching the subpercent noise floor of the most sensitive experiments
The Three-Dipole Kicker Injection Scheme for the ALS-U Accumulator Ring
The ALS-U light source will implement on-axis single-train swap-out injection
employing an accumulator between the booster and storage rings. The accumulator
ring design is a twelve period triple-bend achromat that will be installed
along the inner circumference of the storage-ring tunnel. A non-conventional
injection scheme will be utilized for top-off off-axis injection from the
booster into the accumulator ring meant to accommodate a large ~nm
emittance beam into a vacuum-chamber with a limiting horizontal aperture radius
as small as mm. The scheme incorporates three dipole kickers distributed
over three sectors, with two kickers perturbing the stored beam and the third
affecting both the stored and the injected beam trajectories. This paper
describes this ``3DK'' injection scheme and how it fits the accumulator ring's
particular requirements. We describe the design and optimization process, and
how we evaluated its fitness as a solution for booster-to-accumulator ring
injection.Comment: 13 pages, 20 figure
Collective pinning of a frozen vortex liquid in ultrathin superconducting YBa_2Cu_3O_7 films
The linear dynamic response of the two-dimensional (2D) vortex medium in
ultrathin YBa_2Cu_3O_7 films was studied by measuring their ac sheet impedance
Z over a broad range of frequencies \omega. With decreasing temperature the
dissipative component of Z exhibits, at a temperature T*(\omega) well above the
melting temperature of a 2D vortex crystal, a crossover from a thermally
activated regime involving single vortices to a regime where the response has
features consistent with a description in terms of a collectively pinned vortex
manifold. This suggests the idea of a vortex liquid which, below T*(\omega),
appears to be frozen at the time scales 1/\omega of the experiments.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Possible first order transition in the two-dimensional Ginzburg-Landau model induced by thermally fluctuating vortex cores
We study the two-dimensional Ginzburg-Landau model of a neutral superfluid in
the vicinity of the vortex unbinding transition. The model is mapped onto an
effective interacting vortex gas by a systematic perturbative elimination of
all fluctuating degrees of freedom (amplitude {\em and} phase of the order
parameter field) except the vortex positions. In the Coulomb gas descriptions
derived previously in the literature, thermal amplitude fluctuations were
neglected altogether. We argue that, if one includes the latter, the vortices
still form a two- dimensional Coulomb gas, but the vortex fugacity can be
substantially raised. Under the assumption that Minnhagen's generic phase
diagram of the two- dimensional Coulomb gas is correct, our results then point
to a first order transition rather than a Kosterlitz-Thouless transition,
provided the Ginzburg-Landau correlation length is large enough in units of a
microscopic cutoff length for fluctuations. The experimental relevance of these
results is briefly discussed. [Submitted to J. Stat. Phys.]Comment: 36 pages, LaTeX, 6 figures upon request, UATP2-DB1-9
The fully frustrated XY model with next nearest neighbor interaction
We introduce a fully frustrated XY model with nearest neighbor (nn) and next
nearest neighbor (nnn) couplings which can be realized in Josephson junction
arrays. We study the phase diagram for ( is the ratio
between nnn and nn couplings). When an Ising and a
Berezinskii-Kosterlitz-Thouless transitions are present. Both critical
temperatures decrease with increasing . For the array
undergoes a sequence of two transitions. On raising the temperature first the
two sublattices decouple from each other and then, at higher temperatures, each
sublattice becomes disorderd.Comment: 11 pages, 5 figure
Fluctuation-dissipation theorem and flux noise in overdamped Josephson junction arrays
The form of the fluctuation-dissipation theorem for a resistively shunted
Josephson juction array is derived with the help of the method which
explicitely takes into acoount screening effects. This result is used to
express the flux noise power spectrum in terms of frequency dependent sheet
impedance of the array. The relation between noise amplitude and parameters of
the detection coil is analysed for the simplest case of a single-loop coil.Comment: ReVTeX, 8 page
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 Scaling of Magnetic Flux Noise Near the KTB Transition in Overdamped Josephson Junction Arrays
We have used a dc Superconducting QUantum Interference Device to measure the
magnetic flux noise generated by the equilibrium vortex density fluctuations
associated with the Kosterlitz-Thouless-Berezinskii (KTB) transition in an
overdamped Josephson junction array. At temperatures slightly above the KTB
transition temperature, the noise is white for and scales as
for . Here , where is the correlation
length and is the dynamic exponent. Moreover, when all frequencies are
scaled by , data for different temperatures and frequencies collapse on
to a single curve. In addition, we have extracted the dynamic exponent and
found .Comment: 5 pages, LaTeX (REVTeX) format, requires epsfig and amstex style
files. 3 figures included. Tentatively scheduled for publication in Physical
Review Letters, 18 March, 199
The G0 Experiment: Apparatus for Parity-Violating Electron Scattering Measurements at Forward and Backward Angles
In the G0 experiment, performed at Jefferson Lab, the parity-violating
elastic scattering of electrons from protons and quasi-elastic scattering from
deuterons is measured in order to determine the neutral weak currents of the
nucleon. Asymmetries as small as 1 part per million in the scattering of a
polarized electron beam are determined using a dedicated apparatus. It consists
of specialized beam-monitoring and control systems, a cryogenic hydrogen (or
deuterium) target, and a superconducting, toroidal magnetic spectrometer
equipped with plastic scintillation and aerogel Cerenkov detectors, as well as
fast readout electronics for the measurement of individual events. The overall
design and performance of this experimental system is discussed.Comment: Submitted to Nuclear Instruments and Method
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