12 research outputs found
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
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
A Terraced Scanning Superconducting Quantum Interference Device Susceptometer with Sub-Micron Pickup Loops
Superconducting Quantum Interference Devices (SQUIDs) can have excellent spin
sensitivity depending on their magnetic flux noise, pick-up loop diameter, and
distance from the sample. We report a family of scanning SQUID susceptometers
with terraced tips that position the pick-up loops 300 nm from the sample. The
600 nm - 2 um pickup loops, defined by focused ion beam, are integrated into a
12-layer optical lithography process allowing flux-locked feedback, in situ
background subtraction and optimized flux noise. These features enable a
sensitivity of ~70 electron spins per root Hertz at 4K.Comment: See http://stanford.edu/group/moler/publications.html for an
auxiliary document containing additional fabrication details and discussio
Limits on Superconductivity-Related Magnetization in SrRuO and PrOsSb from Scanning SQUID Microscopy
We present scanning SQUID microscopy data on the superconductors Sr2RuO4 (Tc
= 1.5 K) and PrOsSb (Tc = 1.8 K). In both of these materials,
superconductivity-related time-reversal symmetry-breaking fields have been
observed by muon spin rotation; our aim was to visualize the structure of these
fields. However in neither SrRuO nor PrOsSb do we observe
spontaneous superconductivity-related magnetization. In SrRuO, many
experimental results have been interpreted on the basis of a
superconducting order parameter. This order parameter is expected to give
spontaneous magnetic induction at sample edges and order parameter domain
walls. Supposing large domains, our data restrict domain wall and edge fields
to no more than ~0.1% and ~0.2% of the expected magnitude, respectively.
Alternatively, if the magnetization is of the expected order, the typical
domain size is limited to ~30 nm for random domains, or ~500 nm for periodic
domains.Comment: 8 pages, 7 figures. Submitted to Phys. Rev.
Recommended from our members
Mechanics of Individual, Isolated Vortices in a Cuprate Superconductor
Superconductors often contain quantized microscopic whirlpools of electrons, called vortices, that can be modelled as one-dimensional elastic objects1. Vortices are a diverse area of study for condensed matter because of the interplay between thermal fluctuations, vortex–vortex interactions and the interaction of the vortex core with the three-dimensional disorder landscape. Although vortex matter has been studied extensively, the static and dynamic properties of an individual vortex have not. Here, we use magnetic force microscopy (MFM) to image and manipulate individual vortices in a detwinned YBa2Cu3O6.991 single crystal, directly measuring the interaction of a moving vortex with the local disorder potential. We find an unexpected and marked enhancement of the response of a vortex to pulling when we wiggle it transversely. In addition, we find enhanced vortex pinning anisotropy that suggests clustering of oxygen vacancies in our sample and demonstrates the power of MFM to probe vortex structure and microscopic defects that cause pinning.Physic
Partition asymptotics from one-dimensional quantum entropy and energy currents
We give an alternative method to that of Hardy-Ramanujan-Rademacher to derive
the leading exponential term in the asymptotic approximation to the partition
function p(n,a), defined as the number of decompositions of a positive integer
'n' into integer summands, with each summand appearing at most 'a' times in a
given decomposition. The derivation involves mapping to an equivalent physical
problem concerning the quantum entropy and energy currents of particles flowing
in a one-dimensional channel connecting thermal reservoirs, and which obey
Gentile's intermediate statistics with statistical parameter 'a'. The method is
also applied to partitions associated with Haldane's fractional exclusion
statistics.Comment: Published versio