111 research outputs found
Influence of Resonances on the Noise Performance of SQUID Susceptometers
Scanning Superconducting Quantum Interference Device (SQUID) Susceptometry simultaneously images the local magnetic fields and susceptibilities above a sample with sub-micron spatial resolution. Further development of this technique requires a thorough understanding of the current, voltage, and flux ( IVĪ¦ ) characteristics of scanning SQUID susceptometers. These sensors often have striking anomalies in their currentāvoltage characteristics, which we believe to be due to electromagnetic resonances. The effect of these resonances on the performance of these SQUIDs is unknown. To explore the origin and impact of the resonances, we develop a model that qualitatively reproduces the experimentally-determined IVĪ¦ characteristics of our scanning SQUID susceptometers. We use this model to calculate the noise characteristics of SQUIDs of different designs. We find that the calculated ultimate flux noise is better in susceptometers with damping resistors that diminish the resonances than in susceptometers without damping resistors. Such calculations will enable the optimization of the signal-to-noise characteristics of scanning SQUID susceptometers
Fluxoid fluctuations in mesoscopic superconducting rings
Rings are a model system for studying phase coherence in one dimension.
Superconducting rings have states with uniform phase windings that are integer
multiples of 2 called fluxoid states. When the energy difference between
these fluxoid states is of order the temperature so that phase slips are
energetically accessible, several states contribute to the ring's magnetic
response to a flux threading the ring in thermal equilibrium and cause a
suppression or downturn in the ring's magnetic susceptibility as a function of
temperature. We review the theoretical framework for superconducting
fluctuations in rings including a model developed by Koshnick which
includes only fluctuations in the ring's phase winding number called fluxoid
fluctuations and a complete model by von Oppen and Riedel that includes all
thermal fluctuations in the Ginzburg-Landau framework. We show that for
sufficiently narrow and dirty rings the two models predict a similar
susceptibility response with a slightly shifted Tc indicating that fluxoid
fluctuations are dominant. Finally we present magnetic susceptibility data for
rings with different physical parameters which demonstrate the applicability of
our models. The susceptibility data spans a region in temperature where the
ring transitions from a hysteretic to a non hysteretic response to a periodic
applied magnetic field. The magnetic susceptibility data, taken where
transitions between fluxoid states are slow compared to the measurement time
scale and the ring response was hysteretic, decreases linearly with increasing
temperature resembling a mean field response with no fluctuations. At higher
temperatures where fluctuations begin to play a larger role a crossover occurs
and the non-hysteretic data shows a fluxoid fluctuation induced suppression of
diamagnetism below the mean field response that agrees well with the models
Meissner response of a bulk superconductor with an embedded sheet of reduced penetration depth
We calculate the change in susceptibility resulting from a thin sheet with
reduced penetration depth embedded perpendicular to the surface of an isotropic
superconductor, in a geometry applicable to scanning Superconducting QUantum
Interference Device (SQUID) microscopy, by numerically solving Maxwell's and
London's equations using the finite element method. The predicted stripes in
susceptibility agree well in shape with the observations of Kalisky et al. of
enhanced susceptibility above twin planes in the underdoped pnictide
superconductor Ba(Fe1-xCox)2As2 (Ba-122). By comparing the predicted stripe
amplitudes with experiment and using the London relation between penetration
depth and superfluid density, we estimate the enhanced Cooper pair density on
the twin planes, and the barrier force for a vortex to cross a twin plane. Fits
to the observed temperature dependence of the stripe amplitude suggest that the
twin planes have a higher critical temperature than the bulk, although stripes
are not observed above the bulk critical temperature.Comment: 16 pages, 9 figure
SuperScreen: An open-source package for simulating the magnetic response of two-dimensional superconducting devices
Quantitative understanding of the spatial distribution of magnetic fields and
Meissner screening currents in two-dimensional (2D) superconductors and
mesoscopic thin film superconducting devices is critical to interpreting the
results of magnetic measurements of such systems. Here, we introduce
SuperScreen, an open-source Python package for simulating the response of 2D
superconductors to trapped flux and applied time-independent or quasi-DC
magnetic fields for any value of the effective magnetic penetration depth,
. Given an applied magnetic field, SuperScreen solves the 2D London
equation using an efficient matrix inversion method to obtain the Meissner
currents and magnetic fields in and around structures composed of one or more
superconducting thin films of arbitrary geometry. SuperScreen can be used to
model screening effects and calculate self- and mutual-inductance in
superconducting devices, and simulate the magnetic response of inhomogeneous 2D
superconductors.Comment: 20 pages, 8 figures. GitHub repository:
https://github.com/loganbvh/superscreen/ . Supplementary Jupyter notebooks:
https://github.com/loganbvh/superscreen-paper . Submitted to Computer Physics
Communication
Persistent currents in normal metal rings
The authors have measured the magnetic response of 33 individual cold
mesoscopic gold rings, one ring at a time. The response of some sufficiently
small rings has a component that is periodic in the flux through the ring and
is attributed to a persistent current. Its period is close to h/e, and its sign
and amplitude vary between rings. The amplitude distribution agrees well with
predictions for the typical h/e current in diffusive rings. The temperature
dependence of the amplitude, measured for four rings, is also consistent with
theory. These results disagree with previous measurements of three individual
metal rings that showed a much larger periodic response than expected. The use
of a scanning SQUID microscope enabled in situ measurements of the sensor
background. A paramagnetic linear susceptibility and a poorly understood
anomaly around zero field are attributed to defect spins.Comment: Journal version. 4+ pages, 3 figures. See
http://stanford.edu/group/moler/publications.html for the auxiliary document
containing additional data and discussion (Ref. 29). Changes w.r.t. v1:
Clarified some details in introduction and regarding experimental procedures,
shortened abstract, added references and fixed some typo
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