961 research outputs found
Density functional theory of vortex lattice melting in layered superconductors: a mean-field--substrate approach
We study the melting of the pancake vortex lattice in a layered
superconductor in the limit of vanishing Josephson coupling. Our approach
combines the methodology of a recently proposed mean-field substrate model for
such systems with the classical density functional theory of freezing. We
derive a free-energy functional in terms of a scalar order-parameter profile
and use it to derive a simple formula describing the temperature dependence of
the melting field. Our theoretical predictions are in good agreement with
simulation data. The theoretical framework proposed is thermodynamically
consistent and thus capable of describing the negative magnetization jump
obtained in experiments. Such consistency is demonstrated by showing the
equivalence of our expression for the density discontinuity at the transition
with the corresponding Clausius-Clapeyron relation.Comment: 11 pages, 4 figure
Influence of a random telegraph process on the transport through a point contact
We describe the transport properties of a point contact under the influence of a classical two-level fluctuator. We employ a transfer matrix formalism allowing us to calculate arbitrary correlation functions of the stochastic process by mapping them on matrix products. The result is used to obtain the generating function of the full counting statistics of a classical point contact subject to a classical fluctuator, including extensions to a pair of two-level fluctuators as well as to a quantum point contact. We show that the noise in the quantum point contact is a sum of the (quantum) partitioning noise and the (classical) noise due to the two-level fluctuator. As a side result, we obtain the full counting statistics of a quantum point contact with time-dependent transmission probabilitie
Engineering exotic phases for topologically-protected quantum computation by emulating quantum dimer models
We use a nonperturbative extended contractor renormalization (ENCORE) method
for engineering quantum devices for the implementation of topologically
protected quantum bits described by an effective quantum dimer model on the
triangular lattice. By tuning the couplings of the device, topological
protection might be achieved if the ratio between effective two-dimer
interactions and flip amplitudes lies in the liquid phase of the phase diagram
of the quantum dimer model. For a proposal based on a quantum Josephson
junction array [L. B. Ioffe {\it et al.}, Nature (London) {\bf 415}, 503
(2002)] our results show that optimal operational temperatures below 1 mK can
only be obtained if extra interactions and dimer flips, which are not present
in the standard quantum dimer model and involve three or four dimers, are
included. It is unclear if these extra terms in the quantum dimer Hamiltonian
destroy the liquid phase needed for quantum computation. Minimizing the effects
of multi-dimer terms would require energy scales in the nano-Kelvin regime. An
alternative implementation based on cold atomic or molecular gases loaded into
optical lattices is also discussed, and it is shown that the small energy
scales involved--implying long operational times--make such a device
impractical. Given the many orders of magnitude between bare couplings in
devices, and the topological gap, the realization of topological phases in
quantum devices requires careful engineering and large bare interaction scales.Comment: 12 pages, 10 figure
Instanton classical solutions of SU(3) fixed point actions on open lattices
We construct instanton-like classical solutions of the fixed point action of
a suitable renormalization group transformation for the SU(3) lattice gauge
theory. The problem of the non-existence of one-instantons on a lattice with
periodic boundary conditions is circumvented by working on open lattices. We
consider instanton solutions for values of the size (0.6-1.9 in lattice units)
which are relevant when studying the SU(3) topology on coarse lattices using
fixed point actions. We show how these instanton configurations on open
lattices can be taken into account when determining a few-couplings
parametrization of the fixed point action.Comment: 23 pages, LaTeX, 4 eps figures, epsfig.sty; some comments adde
Interaction of vortices in superconductors with kappa close to 2^(-1/2)
Using a perturbative approach to the infinitely degenerate Bogomolnyi vortex
state for a superconductor with kappa = 2^(-1/2), T -> T_c, we calculate the
interaction of vortices in a superconductor with kappa close to 2^(-1/2). We
find, numerically and analytically, that depending on the material the
interaction potential between the vortices varies with decreasing kappa from
purely repulsive (as in a type-II superconductor) to purely attractive (as in a
type-I superconductor) in two different ways: either vortices form a bound
state and the distance between them changes gradually from infinity to zero, or
this transition occurs in a discontinuous way as a result of a competition
between minima at infinity and zero. We study the discontinuous transition
between the vortex and Meissner states caused by the non-monotonous vortex
interaction and calculate the corresponding magnetization jump.Comment: v1:original submit v2:changed formate of images (gave problems to
some) v3:corrected fig v4v6 (was -v4v6) orthographic corrections (and
U_lat/int) mismatch v4:more small orthographic corrections v5:converted to
revtex4 and bibTex v6:Renamed images to submit to pr
Perfect topological charge for asymptotically free theories
The classical equations of motion of the perfect lattice action in
asymptotically free spin and gauge models possess scale invariant
instanton solutions. This property allows the definition of a topological
charge on the lattice which is perfect in the sense that no topological defects
exist. The basic construction is illustrated in the O(3) non--linear
--model and the topological susceptibility is measured to high
precision in the range of correlation lengths . Our results
strongly suggest that the topological susceptibility is not a physical quantity
in this model.Comment: Contribution to Lattice'94, 3 pages PostScript, uuencoded compresse
Edge Tunneling of Vortices in Superconducting Thin Films
We investigate the phenomenon of the decay of a supercurrent due to the
zero-temperature quantum tunneling of vortices from the edge in a thin
superconducting film in the absence of an external magnetic field. An explicit
formula is derived for the tunneling rate of vortices, which are subject to the
Magnus force induced by the supercurrent, through the Coulomb-like potential
barrier binding them to the film's edge. Our approach ensues from the
non-relativistic version of a Schwinger-type calculation for the decay of the
2D vacuum previously employed for describing vortex-antivortex pair-nucleation
in the bulk of the sample. In the dissipation-dominated limit, our explicit
edge-tunneling formula yields numerical estimates which are compared with those
obtained for bulk-nucleation to show that both mechanisms are possible for the
decay of a supercurrent.Comment: REVTeX file, 15 pages, 1 Postscript figure; to appear in Phys.Rev.
Flux flow resistivity and vortex viscosity of high-Tc films
The flux flow regime of high-T samples of different normal state
resistivities is studied in the temperature range where the sign of the Hall
effect is reversed. The scaling of the vortex viscosity with normal state
resistivity is consistent with the Bardeen-Stephen theory. Estimates of the
influence of possible mechanisms suggested for the sign reversal of the Hall
effect are also given.Comment: 3 pages. 4 figures upon reques
Andreev quantum dot with several conducting channels
We study an Andreev quantum dot, that is a quantum dot inserted in a
superconducting ring, with several levels or conducting channels. We analyze
the degeneracy of the ground state as a function of the phase difference and of
the gate voltage and find its dependence on the Coulomb interaction within and
between channels. We compute a (non integer) charge of the dot region and
Josephson current. The charge-to-phase and current-to-gate voltage
sensitivities are studied. We find that, even in the presence of Coulomb
interaction between the channels, the sensitivity increases with the number of
channels, although it does not scale linearly as in the case with no
interactions. The Andreev quantum dot may therefore be used as a sensitive
detector of magnetic flux or as a Josephson transistor.Comment: 13 pages, 10 figures, minor correction
Using Qubits to Measure Fidelity in Mesoscopic Systems
We point out the similarities in the definition of the `fidelity' of a
quantum system and the generating function determining the full counting
statistics of charge transport through a quantum wire and suggest to use flux-
or charge qubits for their measurement. As an application we use the notion of
fidelity within a first-quantized formalism in order to derive new results and
insights on the generating function of the full counting statistics.Comment: 5 pages, 1 figur
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