57 research outputs found
A Numerical Treatment of the Rf SQUID: I. General Properties and Noise Energy
We investigate the characteristics and noise performance of rf
Superconducting Quantum Interference Devices (SQUIDs) by solving the
corresponding Langevin equations numerically and optimizing the model
parameters with respect to noise energy. After introducing the basic concepts
of the numerical simulations, we give a detailed discussion of the performance
of the SQUID as a function of all relevant parameters. The best performance is
obtained in the crossover region between the dispersive and dissipative
regimes, characterized by an inductance parameter \beta_L'\equiv 2\pi L
I_0/\Phi_0\approx 1; L is the loop inductance, I_0 the critical current of the
Josephson junction, and \Phi_0 the flux quantum. In this regime, which is not
well explored by previous analytical approaches, the lowest (intrinsic) values
of noise energy are a factor of about 2 above previous estimates based on
analytical approaches. However, several other analytical predictions, such as
the inverse proportionality of the noise energy on the tank circuit quality
factor and the square of the coupling coefficient between the tank circuit and
the SQUID loop, could not be well reproduced. The optimized intrinsic noise
energy of the rf SQUID is superior to that of the dc SQUID at all temperatures.
Although for technologically achievable parameters this advantage shrinks,
particularly at low thermal fluctuation levels, we give an example for
realistic parameters that leads to a noise energy comparable to that of the dc
SQUID even in this regime.Comment: submitted to J. Low Temp. Phy
Semifluxon molecule under control
Josephson junctions with a phase drop pi in the ground state allow to create
vortices of supercurrent carrying only half of the magnetic flux quantum
Phi_0~2.07*10^-15 Wb. Such semifluxons have two-fold degenerate ground states
denoted up (with flux +Phi_0/2 and supercurrent circulating clockwise) and down
(with flux -Phi_0/2 and supercurrent circulating counterclockwise). We
investigate a molecule consisting of two coupled semifluxons in a 0-pi-0 long
Josephson junction. The fluxes (polarities) of semifluxons are measured by two
on-chip SQUIDs. By varying the dc bias current applied to the 0-pi-0 junction,
we demonstrate controllable manipulation and switching between two states,
up-down and down-up, of a semifluxon molecule. These results provide a major
step towards employing semifluxons as bits or qubits for classical and quantum
digital electronics
Improving the performance of superconducting microwave resonators in magnetic fields
The operation of superconducting coplanar waveguide cavities, as used for
circuit quantum electrodynamics and kinetic inductance detectors, in
perpendicular magnetic fields normally leads to a reduction of the device
performance due to energy dissipating Abrikosov vortices. We experimentally
investigate the vortex induced energy losses in such Nb resonators with
different spatial distributions of micropatterned pinning sites (antidots) by
transmission spectroscopy measurements at 4.2 K. In comparison to resonators
without antidots we find a significant reduction of vortex induced losses and
thus increased quality factors over a broad range of frequencies and applied
powers in moderate fields
Magnetic hysteresis effects in superconducting coplanar microwave resonators
We performed transmission spectroscopy experiments on coplanar half
wavelength niobium resonators at a temperature T=4.2 K. We observe not only a
strong dependence of the quality factor Q and the resonance frequency f_res on
an externally applied magnetic field but also on the magnetic history of our
resonators, i.e. on the spatial distribution of trapped Abrikosov vortices in
the device. We find these results to be valid for a broad range of frequencies
and angles between the resonator plane and the magnetic field direction as well
as for resonators with and without antidots near the edges of the center
conductor and the ground planes. In a detailed analysis we show, that
characteristic features of the experimental data can only be reproduced in
calculations, if a highly inhomogeneous rf-current density and a flux density
gradient with maxima at the edges of the superconductor is assumed. We
furthermore demonstrate, that the hysteretic behaviour of the resonator
properties can be used to considerably reduce the vortex induced losses and to
fine-tune the resonance frequency by the proper way of cycling to a desired
magnetic field
Optimizing the spin sensitivity of grain boundary junction nanoSQUIDs -- towards detection of small spin systems with single-spin resolution
We present an optimization study of the spin sensitivity of nanoSQUIDs based
on resistively shunted grain boundary Josephson junctions. In addition the dc
SQUIDs contain a narrow constriction onto which a small magnetic particle can
be placed (with its magnetic moment in the plane of the SQUID loop and
perpendicular to the grain boundary) for efficient coupling of its stray
magnetic field to the SQUID loop. The separation of the location of optimum
coupling from the junctions allows for an independent optimization of the
coupling factor and junction properties. We present different
methods for calculating (for a magnetic nanoparticle placed 10\,nm
above the constriction) as a function of device geometry and show that those
yield consistent results. Furthermore, by numerical simulations we obtain a
general expression for the dependence of the SQUID inductance on geometrical
parameters of our devices, which allows to estimate their impact on the
spectral density of flux noise of the SQUIDs in the thermal white
noise regime. Our analysis of the dependence of and on the
geometric parameters of the SQUID layout yields a spin sensitivity
of a few
( is the Bohr magneton) for optimized parameters, respecting
technological constraints. However, by comparison with experimentally realized
devices we find significantly larger values for the measured white flux noise,
as compared to our theoretical predictions. Still, a spin sensitivity on the
order of for optimized devices seems to be
realistic.Comment: 10 pages, 5 figures, Superconductor Science and Technology
(submitted
Improved tunneling magnetoresistance at low temperature in manganite junctions grown by molecular beam epitaxy
We report resistance versus magnetic field measurements for a
La0.65Sr0.35MnO3/SrTiO3/La0.65Sr0.35MnO3 tunnel junction grown by
molecular-beam epitaxy, that show a large field window of extremely high
tunneling magnetoresistance (TMR) at low temperature. Scanning the in-plane
applied field orientation through 360^/circ, the TMR shows 4-fold symmetry,
i.e. biaxial anisotropy, aligned with the crystalline axes but not the junction
geometrical long axis. The TMR reaches ~ 1900% at 4K, corresponding to an
interfacial spin polarization of > 95% assuming identical interfaces. These
results show that uniaxial anisotropy is not necessary for large TMR, and lay
the groundwork for future improvements in TMR in manganite junctions.Comment: 6 pages, 7 figures; accepted in Applied Physics Letter
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