18 research outputs found
The role of rare events in the pinning problem
Type II superconductors exhibit a fascinating phenomenology that is
determined by the dynamical properties of the vortex matter hosted by the
material. A crucial element in this phenomenology is vortex pinning by material
defects, e.g., immobilizing vortices at small drives and thereby guaranteeing
dissipation-free current flow. Pinning models for vortices and other
topological defects, such as domain walls in magnets or dislocations in
crystals, come in two standard variants: i) weak collective pinning, where
individual weak defects are unable to pin, while the random accumulation of
many force centers within a collective pinning volume combines into an
effective pin, and ii) strong pinning, where strong defects produce large
vortex displacements and bistabilities that lead to pinning on the level of
individual defects. The transition between strong and weak pinning is
quantified by the Labusch criterion , where
and are the force of one defect and the effective elasticity of
the vortex lattice, respectively ( is the coherence length). Here, we show
that a third generic type of pinning becomes dominant when the pinning force
enters the weak regime, the pinning by rare events. We find that within
an intermediate regime , compact pairs of weak defects define
strong pinning clusters that extend the mechanism of strong pinning into the
weak regime. We present a detailed analysis of this cluster-pinning mechanism
and show that its pinning-force density parametrically dominates over the weak
pinning result. The present work is a first attempt to include correlations
between defects into the discussion of strong pinning
Hessian characterization of a vortex in a maze
Recent advances in vortex imaging allow for tracing the position of
individual vortices with high resolution. Pushing an isolated vortex through
the sample with the help of a controlled transport current and measuring
its local response, the pinning energy landscape could be reconstructed
along the vortex trajectory [, , ]. This setup with linear tilts of the
potential landscape reminds about the dexterity game where a ball is balanced
through a maze. The controlled motion of objects through such tilted energy
landscapes is fundamentally limited to those areas of the landscape developing
local minima under appropriate tilt. We introduce the Hessian stability map and
the Hessian character of a pinning landscape as new quantities to characterize
a pinning landscape. We determine the Hessian character, the area fraction
admitting stable vortex positions, for various types of pinning potentials:
assemblies of cut parabolas, Lorentzian- and Gaussian-shaped traps, as well as
a Gaussian random disordered energy landscape, with the latter providing a
universal result of of stable area. Furthermore,
we discuss various aspects of the vortex-in-a-maze experiment.Comment: 17 pages, 9 figure
Characteristics of First-Order Vortex Lattice Melting: Jumps in Entropy and Magnetization
We derive expressions for the jumps in entropy and magnetization
characterizing the first-order melting transition of a flux line lattice. In
our analysis we account for the temperature dependence of the Landau parameters
and make use of the proper shape of the melting line as determined by the
relative importance of electromagnetic and Josephson interactions. The results
agree well with experiments on anisotropic YBaCuO and
layered BiSrCaCuO materials and reaffirm the validity of
the London model.Comment: 4 pages. We have restructured the paper to emphasize that in the
London scaling regime (appropriate for YBCO) our results are essentially
exact. We have also emphasized that a major controversy over the relevance of
the London model to describe VL melting has been settled by this wor
Quiet SDS Josephson Junctions for Quantum Computing
Unconventional superconductors exhibit an order parameter symmetry lower than
the symmetry of the underlying crystal lattice. Recent phase sensitive
experiments on YBCO single crystals have established the d-wave nature of the
cuprate materials, thus identifying unambiguously the first unconventional
superconductor. The sign change in the order parameter can be exploited to
construct a new type of s-wave - d-wave - s-wave Josephson junction exhibiting
a degenerate ground state and a double-periodic current-phase characteristic.
Here we discuss how to make use of these special junction characteristics in
the construction of a quantum computer. Combining such junctions together with
a usual s-wave link into a SQUID loop we obtain what we call a `quiet' qubit
--- a solid state implementation of a quantum bit which remains optimally
isolated from its environment.Comment: 4 pages, 2 ps-figure