10 research outputs found
Magnetic field enhancement of superconductivity in ultra-narrow wires
We study the effect of an applied magnetic field on sub-10nm wide MoGe and Nb
superconducting wires. We find that magnetic fields can enhance the critical
supercurrent at low temperatures, and does so more strongly for narrower wires.
We conjecture that magnetic moments are present, but their pair-breaking
effect, active at lower magnetic fields, is suppressed by higher fields. The
corresponding microscopic theory, which we have developed, quantitatively
explains all experimental observations, and suggests that magnetic moments have
formed on the wire surfaces.Comment: 4 pages, 3 figures, 1 tabl
Geometrical entanglement of highly symmetric multipartite states and the Schmidt decomposition
In a previous paper we examined a geometric measure of entanglement based on
the minimum distance between the entangled target state of interest and the
space of unnormalized product states. Here we present a detailed study of this
entanglement measure for target states with a large degree of symmetry. We
obtain analytic solutions for the extrema of the distance function and solve
for the Hessian to show that, up to the action of trivial symmetries, the
solutions correspond to local minima of the distance function. In addition, we
show that the conditions that determine the extremal solutions for general
target states can be obtained directly by parametrizing the product states via
their Schmidt decomposition.Comment: 16 pages, references added and discussion expande
High Temperature Macroscopic Entanglement
In this paper I intend to show that macroscopic entanglement is possible at
high temperatures. I analyze multipartite entanglement produced by the
pairing mechanism which features strongly in the fermionic lattice models of
high superconductivity. This problem is shown to be equivalent to
calculating multipartite entanglement in totally symmetric states of qubits. I
demonstrate that we can conclusively calculate the relative entropy of
entanglement within any subset of qubits in an overall symmetric state. Three
main results then follow. First, I show that the condition for
superconductivity, namely the existence of the off diagonal long range order
(ODLRO), is not dependent on two-site entanglement, but on just classical
correlations as the sites become more and more distant. Secondly, the
entanglement that does survive in the thermodynamical limit is the entanglement
of the total lattice and, at half filling, it scales with the log of the number
of sites. It is this entanglement that will exist at temperatures below the
superconducting critical temperature, which can currently be as high as 160
Kelvin. Thirdly, I prove that a complete mixture of symmetric states does not
contain any entanglement in the macroscopic limit. On the other hand, the same
mixture of symmetric states possesses the same two qubit entanglement features
as the pure states involved, in the sense that the mixing does not destroy
entanglement for finite number of qubits, albeit it does decrease it. Maximal
mixing of symmetric states also does not destroy ODLRO and classical
correlations. I discuss various other inequalities between different
entanglements as well as generalizations to the subsystems of any
dimensionality (i.e. higher than spin half).Comment: 14 pages, no figure
Enhancing superconductivity: Magnetic impurities and their quenching by magnetic fields
Magnetic fields and magnetic impurities are each known to suppress
superconductivity. However, as the field quenches (i.e. polarizes) the
impurities, rich consequences, including field-enhanced superconductivity, can
emerge when both effects are present. For the case of superconducting wires and
thin films, this field-spin interplay is investigated via the
Eilenberger-Usadel scheme. Non-monotonic dependence of the critical current on
the field (and therefore field-enhanced superconductivity) is found to be
possible, even in parameter regimes in which the critical temperature decreases
monotonically with increasing field. The present work complements that of
Kharitonov and Feigel'man, which predicts non-monotonic behavior of the
critical temperature.Comment: 8 pages, 2 figures, EPL forma
Evolution of entanglement entropy in one-dimensional systems
We study the unitary time evolution of the entropy of entanglement of a one-dimensional system between the degrees of freedom in an interval of length l and its complement, starting from a pure state which is not an eigenstate of the Hamiltonian. We use path integral methods of quantum field theory as well as explicit computations for the transverse Ising spin chain. In both cases, there is a maximum speed v of propagation of signals. In general the entanglement entropy increases linearly with time t up to t = l/2v, after which it saturates at a value proportional to l, the coefficient depending on the initial state. This behaviour may be understood as a consequence of causality