29 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
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
Observation of Andreev bound states in YBaCuO/Au/Nb ramp-type Josephson junctions
We report on Josephson and quasiparticle tunneling in YBa2Cu3O7-x(YBCO)/Au/Nb
ramp junctions of several geometries. Macroscopically, tunneling occurs in the
ab-plane of YBCO either in the (100) and (010) direction, or in the (110)
direction. These junctions have a stable and macroscopically well defined
geometry. This allows systematic investigations of both quasiparticle and
Josephson tunneling over a wide range of temperature and magnetic field. With
Nb superconducting, its gap appears in the quasiparticle conductance spectra as
Nb coherence peaks and a dip at the center of a broadened zero-bias conductance
peak (ZBCP). As we increase the temperature or an applied magnetic field both
the Nb coherence peaks and the dip get suppressed and the ZBCP fully develops,
while states are conserved. With Nb in the normal state the ZBCP is observed up
to about 77 K and is almost unaffected by an increasing field up to 7 T. The
measurements are consistent with a convolution of density of states with
broadened Andreev bound states formed at the YBCO/Au/Nb junction interfaces.
Since junctions with different geometries are fabricated on the same substrate
under the same conditions one expects to extract reliable tunneling information
that is crystallographic direction sensitive. In high contrast to Josephson
tunneling, however, the quasiparticle conductance spectra are crystallographic
orientation insensitive: independent whether the tunneling occurs in the (100)
or (110) directions, a pronounced ZBCP is always observed, consistent with
microscopic roughness of the junction interfaces. Qualitatively, all these
particularities regarding quasiparticle spectra hold regardless whether the
YBCO thin film is twinned or untwinned.Comment: 13 pages, 10 figure
Sheet-current distribution in a dc SQUID washer probed by vortices
We present a novel method, based on vortex imaging by low-temperature
scanning electron microscopy (LTSEM), to directly image the sheet-current
distribution in YBa2Cu3O7 dc SQUID washers. We show that the LTSEM vortex
signals are simply related to the scalar stream function describing the
vortex-free circulating sheet-current distribution J. Unlike previous inversion
methods that infer the current distribution from the measured magnetic field,
our method uses pinned vortices as local detectors for J. Our experimental
results are in very good agreement with numerical calculations of J.Comment: 4 pages, 3 figures; submitted to Physical Review Letter