4,107 research outputs found
Microscopic Analysis of Low-Frequency Flux Noise in YBaCuO Direct Current Superconducting Quantum Interference Devices
We use low-temperature scanning electron microscopy combined with SQUID
detection of magnetic flux to image vortices and to investigate low-frequency
flux noise in YBaCuO thin film SQUIDs. The low-frequency flux noise
shows a nonlinear increase with magnetic cooling field up to 60 T. This
effect is explained by the surface potential barrier at the SQUID hole. By
correlating flux noise data with the spatial distribution of vortices, we
obtain information on spatial fluctuations of vortices on a microscopic scale,
e.g. an average vortex hopping length of approximately 10 nm.Comment: submitted to Applied Physics Letter
Magnetic field dependence of the critical current in stacked Josephson junctions. Evidence for fluxon modes in Bi2Sr2CaCu2O8+x mesas
Modulation of the critical current across layers, Ic(H), of stacked Josephson
junctions (SJJs) as a function of an applied magnetic field parallel to the
junction planes is studied theoretically and experimentally for different
junction lengths and coupling parameters. It is shown that the Ic(H) patterns
of long SJJs are very complicated without periodicity in H. This is due to
interaction between junctions in the stack. This, in turn, gives rise to the
existence of multiple quasi-equilibrium Josephson fluxon modes and submodes
which are different with respect to the symmetry of the phase and the fluxon
sequence in SJJs. The critical current of long SJJs is multiple valued and is
governed by switching between energetically close fluxon modes/submodes. Due to
this, the probability distribution of the critical current may become wide and
may consist of multiple maxima each representing a particular mode/submode.
Experimentally, multiple branched Ic(H) patterns and multiple maxima in the Ic
probability distribution were observed for Bi2Sr2CaCu2O8+x intrinsic SJJs,
which are in a good agreement with numerical simulations and support the idea
of having different quasi-equilibrium fluxon modes/submodes in intrinsic SJJs.Comment: 5 pages, 5 figure
3-junction SQUID rocking ratchet
We investigate 3-junction SQUIDs which show voltage rectification if biased
with an ac current drive with zero mean value. The Josephson phase across the
SQUID experiences an effective ratchet potential, and the device acts as an
efficient rocking ratchet, as demonstrated experimentally for adiabatic and
nonadiabatic drive frequencies. For high-frequency drives the rectified voltage
is quantized due to synchronization of the phase dynamics with the external
drive. The experimental data are in excellent agreement with numerical
simulations including thermal fluctuations.Comment: 5 pages, 4 figures -- Fig.4 revise
Imaging of Thermal Domains in ultrathin NbN films for Hot Electron Bolometers
We present low-temperature scanning electron microscopy (LTSEM)
investigations of superconducting microbridges made from ultrathin NbN films as
used for hot electron bolometers. LTSEM probes the thermal structure within the
microbridges under various dc current bias conditions, either via
electron-beam-induced generation of an unstable hotspot, or via the
beam-induced growth of a stable hotspot. Such measurements reveal
inhomogeneities on a micron scale, which may be due to spatial variations in
the NbN film or film-interface properties. Comparison with model calculations
for the stable hotspot regime confirm the basic features of common hot spot
models.Comment: 3 pages, 3 figure
- …