39 research outputs found
Fluctuation Superconductivity in Mesoscopic Aluminum Rings
Fluctuations are important near phase transitions, where they can be
difficult to describe quantitatively. Superconductivity in mesoscopic rings is
particularly intriguing because the critical temperature is an oscillatory
function of magnetic field. There is an exact theory for thermal fluctuations
in one-dimensional superconducting rings, which are therefore expected to be an
excellent model system. We measure the susceptibility of many rings, one ring
at a time, using a scanning SQUID that can isolate magnetic signals from seven
orders of magnitude larger background applied flux. We find that the
fluctuation theory describes the results and that a single parameter
characterizes the ways in which the fluctuations are especially important at
magnetic fields where the critical temperature is suppressed.Comment: Reprinted with permission from AAA
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.
A possibility of persistent voltage observation in a system of asymmetric superconducting rings
A possibility to observe the persistent voltage in a superconducting ring of
different widths of the arms is experimentally investigated. It was earlier
found that switching of the arms between superconducting and normal states by
ac current induces the dc voltage oscillation in magnetic field with a period
corresponding to the flux quantum inside the ring. We use systems with a large
number of asymmetric rings connected in series in order to investigate the
possibility to observe this quantum phenomenon near the superconducting
transition where thermal fluctuations switch ring segments without external
influence and the persistent current is much smaller than in the
superconducting state.Comment: 7 pages, 4 figure
Universal Signatures of Fractionalized Quantum Critical Points
Groundstates of certain materials can support exotic excitations with a
charge that's a fraction of the fundamental electron charge. The condensation
of these fractionalized particles has been predicted to drive novel quantum
phase transitions, which haven't yet been observed in realistic systems.
Through numerical and theoretical analysis of a physical model of interacting
lattice bosons, we establish the existence of such an exotic critical point,
called XY*. We measure a highly non-classical critical exponent eta = 1.49(2),
and construct a universal scaling function of winding number distributions that
directly demonstrates the distinct topological sectors of an emergent Z_2 gauge
field. The universal quantities used to establish this exotic transition can be
used to detect other fractionalized quantum critical points in future model and
material systems.Comment: 12 pages, 3 figures (+ supplemental
Multiple Current States of Two Phase-Coupled Superconducting Rings
The states of two phase-coupled superconducting rings have been investigated.
Multiple current states have been revealed in the dependence of the critical
current on the magnetic field. The performed calculations of the critical
currents and energy states in a magnetic field have made it possible to
interpret the experiment as the measurement of energy states into which the
system comes with different probabilities because of the equilibrium and
non-equilibrium noises upon the transition from the resistive state to the
superconducting state during the measurement of the critical currentComment: 5 pages, 5 figure
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