409 research outputs found
Quantum Hall effects in layered disordered superconductors
Layered singlet paired superconductors with disorder and broken time reversal
symmetry are studied. The phase diagram demonstrates charge-spin separation in
transport. In terms of the average intergrain transmission and the interlayer
tunnelling we find quantum Hall phases with spin Hall coefficients of 0 and 2
separated by a spin metal phase. We identify a spin metal-insulator
localization exponent as well as a spin conductivity exponent of ~0.9. In
presence of a Zeeman term an additional phase with spin Hall coefficient of 1
appears.Comment: 4 pages, 4 figure
Disorder Induced Transitions in Layered Coulomb Gases and Superconductors
A 3D layered system of charges with logarithmic interaction parallel to the
layers and random dipoles is studied via a novel variational method and an
energy rationale which reproduce the known phase diagram for a single layer.
Increasing interlayer coupling leads to successive transitions in which charge
rods correlated in N>1 neighboring layers are nucleated by weaker disorder. For
layered superconductors in the limit of only magnetic interlayer coupling, the
method predicts and locates a disorder-induced defect-unbinding transition in
the flux lattice. While N=1 charges dominate there, N>1 disorder induced defect
rods are predicted for multi-layer superconductors.Comment: 4 pages, 2 figures, RevTe
Second magnetization peak in flux lattices: the decoupling scenario
The second peak phenomena of flux lattices in layered superconductors is
described in terms of a disorder induced layer decoupling transition. For weak
disorder the tilt mudulus undergoes an apparent discontinuity which leads to an
enhanced critical current and reduced domain size in the decoupled phase. The
Josephson plasma frequency is reduced by decoupling and by Josephson glass
pinning; in the liquid phase it varies as 1/[BT(T+T_0)] where T is temperature,
B is field and T_0 is the disorder dependent temperature of the multicritical
point.Comment: 5 pages, 1 eps figure, Revtex. Minor changes, new reference
Zero temperature geometric spin dephasing on a ring in presence of an Ohmic environment
We study zero temperature spin dynamics of a particle confined to a ring in
presence of spin orbit coupling and Ohmic electromagnetic fluctuations. We show
that the dynamics of the angular position are decoupled from the
spin dynamics and that the latter is mapped to certain correlations of a
spinless particle. We find that the spin correlations in the direction
(perpendicular to the ring) are finite at long times, i.e. do not dephase. The
parallel (in plane) components for spin \half do not dephase at weak
dissipation but they probably decay as a power law with time at strong
dissipation.Comment: 5 pages, submitted to EP
Level statistics for quantum Hall systems
Level statistics for two classes of disordered systems at criticality are analyzed in terms of different
realizations of the Chalker–Coddington network model. These include: 1) Re-examination
of the standard U(1) model describing dynamics of electrons on the lowest Landau level in the
quantum Hall effect, where it is shown that after proper local unfolding the nearest-neighbor spacing
distribution (NNSD) at the critical energy follows the Wigner surmise for Gaussian unitary ensembles
(GUE). 2) Quasi-particles in disordered superconductors with broken time reversal and
spin rotation invariance (in the language of random matrix theory this system is a representative of
symmetry class D in the classification scheme of Altland and Zirnbauer). Here again the NNSD
obeys the Wigner surmise for GUE, reflecting therefore only «basic» discrete symmetries of the
system (time reversal violation) and ignoring particle–hole symmetries and other finer details
(criticality). In the localized regime level repulsion is suppressed
Decoherence of a particle in a ring
We consider a particle coupled to a dissipative environment and derive a
perturbative formula for the dephasing rate based on the purity of the reduced
probability matrix. We apply this formula to the problem of a particle on a
ring, that interacts with a dirty metal environment. At low but finite
temperatures we find a dephasing rate , and identify dephasing
lengths for large and for small rings. These findings shed light on recent
Monte Carlo data regarding the effective mass of the particle. At zero
temperature we find that spatial fluctuations suppress the possibility of
having a power law decay of coherence.Comment: 5 pages, 1 figure, proofed version to be published in EP
Critical Behavior of the Flux-line Tension in Extreme Type-II Superconductors
The entropic corrections to the flux-line energy of extreme type-II
superconductors are computed using a schematic dual Villain model description
of the flux quanta. We find that the temperature profile of the lower-critical
field vanishes polynomially at the transition with an exponent
in the isotropic case, while it exhibits an inflection point for the case of
weakly coupled layers in parallel magnetic field. It is argued that vestiges of
these effects have already been observed in high-temperature superconductors.Comment: 12 pages of plain TeX, 2 postscipt figures, to appear in Phys. Rev.
W(h)ither Fossils? Studying Morphological Character Evolution in the Age of Molecular Sequences
A major challenge in the post-genomics era will be to integrate molecular sequence data from extant organisms with morphological data from fossil and extant taxa into a single, coherent picture of phylogenetic relationships; only then will these phylogenetic hypotheses be effectively applied to the study of morphological character evolution. At least two analytical approaches to solving this problem have been utilized: (1) simultaneous analysis of molecular sequence and morphological data with fossil taxa included as terminals in the analysis, and (2) the molecular scaffold approach, in which morphological data are analyzed over a molecular backbone (with constraints that force extant taxa into positions suggested by sequence data). The perceived obstacles to including fossil taxa directly in simultaneous analyses of morphological and molecular sequence data with extant taxa include: (1) that fossil taxa are missing the molecular sequence portion of the character data; (2) that morphological characters might be misleading due to convergence; and (3) character weighting, specifically how and whether to weight characters in the morphological partition relative to characters in the molecular sequence data partition. The molecular scaffold has been put forward as a potential solution to at least some of these problems. Using examples of simultaneous analyses from the literature, as well as new analyses of previously published morphological and molecular sequence data matrices for extant and fossil Chiroptera (bats), we argue that the simultaneous analysis approach is superior to the molecular scaffold approach, specifically addressing the problems to which the molecular scaffold has been suggested as a solution. Finally, the application of phylogenetic hypotheses including fossil taxa (whatever their derivation) to the study of morphological character evolution is discussed, with special emphasis on scenarios in which fossil taxa are likely to be most enlightening: (1) in determining the sequence of character evolution; (2) in determining the timing of character evolution; and (3) in making inferences about the presence or absence of characteristics in fossil taxa that may not be directly observable in the fossil record.
Published By: Missouri Botanical Garde
Decoupling and decommensuration in layered superconductors with columnar defects
We consider layered superconductors with a flux lattice perpendicular to the
layers and random columnar defects parallel to the magnetic field B. We show
that the decoupling transition temperature Td, at which the Josephson coupling
vanishes, is enhanced by columnar defects by an amount ~B^2 relative to Td.
Decoupling by increasing field can be followed by a reentrant recoupling
transition for strong disorder. We also consider a commensurate component of
the columnar density and show that its pinning potential is renormalized to
zero above a critical long wavelength disorder. This decommnesuration
transition may account for a recently observed kink in the melting line.Comment: 5 pages, Revte
Dephasing of a particle in a dissipative environment
The motion of a particle in a ring of length L is influenced by a dirty metal
environment whose fluctuations are characterized by a short correlation
distance . We analyze the induced decoherence process, and compare
the results with those obtained in the opposing Caldeira-Leggett limit (). A proper definition of the dephasing factor that does not depend on a
vague semiclassical picture is employed. Some recent Monte-Carlo results about
the effect of finite temperatures on "mass renormalization" in this system are
illuminated.Comment: 18 pages, 2 figures, some textual improvements, to be published in
JP
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