873 research outputs found
Dirty quantum Hall ferromagnets and quantum Hall spin glasses
We study quantum Hall ferromagnets in the presence of a random electrostatic
impurity potential, within the framework of a classical non-linear sigma model.
We discuss the behaviour of the system using a heuristic picture for the
competition between exchange and screening, and test our conclusions with
extensive numerical simulations. We obtain a phase diagram for the system as a
function of disorder strength and deviation of the average Landau level filling
factor from unity. Screening of an impurity potential requires distortions of
the spin configuration. In the absence of Zeeman coupling there is a
disorder-driven, zero-temperature phase transition from a ferromagnet at weak
disorder and small deviation from integer filling to a spin glass at stronger
disorder or large charge deviation. We characterise the spin glass phase in
terms of its magnetic and charge response, as well as its ac conductivity.Comment: 12 pages, 6 figures, REVTEX
Critical Conductance of a Mesoscopic System: Interplay of the Spectral and Eigenfunction Correlations at the Metal-Insulator Transition
We study the system-size dependence of the averaged critical conductance
at the Anderson transition. We have: (i) related the correction to the spectral correlations; (ii) expressed
in terms of the quantum return probability; (iii) argued that
-- the critical exponent of eigenfunction correlations. Experimental
implications are discussed.Comment: minor changes, to be published in PR
Density of quasiparticle states for a two-dimensional disordered system: Metallic, insulating, and critical behavior in the class D thermal quantum Hall effect
We investigate numerically the quasiparticle density of states
for a two-dimensional, disordered superconductor in which both time-reversal
and spin-rotation symmetry are broken. As a generic single-particle description
of this class of systems (symmetry class D), we use the Cho-Fisher version of
the network model. This has three phases: a thermal insulator, a thermal metal,
and a quantized thermal Hall conductor. In the thermal metal we find a
logarithmic divergence in as , as predicted from sigma
model calculations. Finite size effects lead to superimposed oscillations, as
expected from random matrix theory. In the thermal insulator and quantized
thermal Hall conductor, we find that is finite at E=0. At the
plateau transition between these phases, decreases towards zero as
is reduced, in line with the result
derived from calculations for Dirac fermions with random mass.Comment: 8 pages, 8 figures, published versio
Transport properties in network models with perfectly conducting channels
We study the transport properties of disordered electron systems that contain
perfectly conducting channels. Two quantum network models that belong to
different universality classes, unitary and symplectic, are simulated
numerically. The perfectly conducting channel in the unitary class can be
realized in zigzag graphene nano-ribbons and that in the symplectic class is
known to appear in metallic carbon nanotubes. The existence of a perfectly
conducting channel leads to novel conductance distribution functions and a
shortening of the conductance decay length.Comment: 4 pages, 6 figures, proceedings of LT2
Recommended from our members
Why do leaves turn red?
All gardeners in temperate areas of the world are aware of seasonal changes in leaf color. Many deciduous leaves turn some shade of red before they fall during the autumn months. The red coloration is due to the production of water-soluble leaf pigments called anthocyanins, which are also found in red, blue, and purple flowers and fruits. But leaves can also turn red for reasons unrelated to autumn color development, such as genetic programming. This publication outlines these reasons and explain when red leaves indicate the need for corrective action by gardeners
Chalker-Coddington model described by an S-matrix with odd dimensions
The Chalker-Coddington network model is often used to describe the transport
properties of quantum Hall systems. By adding an extra channel to this model,
we introduce an asymmetric model with profoundly different transport
properties. We present a numerical analysis of these transport properties and
consider the relevance for realistic systems.Comment: 7 pages, 4 figures. To appear in the EP2DS-17 proceeding
Evaluation of matrix-assisted laser desorption ionisation time-of-flight mass spectrometry (MALDI-TOF MS) for the Identification of Group B Streptococcus.
Objective
Group B Streptococcus (GBS) is a leading cause of neonatal meningitis and sepsis worldwide. Intrapartum antibiotics given to women carrying GBS are an effective means of reducing disease in the first week of life. Rapid and reliable tests are needed to accurately identify GBS from these women for timely intrapartum antibiotic administration to prevent neonatal disease. Many laboratories now use matrix-assisted laser desorption ionisation time-of-flight mass spectrometry (MALDI-TOF MS) by direct plating or cell lysis for the identification of GBS isolates. The cell lysis step increases time to results for clinical samples and is more complex to perform. Therefore, we seek to evaluate the sensitivity and specificity of the quicker and more rapid direct plating method in identifying GBS.
Results
We directly compared swab isolates analysed by both direct plating and cell lysis method and demonstrated that direct plating has a sensitivity and specificity of 0.97 and 1, respectively, compared to an additional cell lysis step. We demonstrated that MALDI-TOF MS can be successfully used for batch processing by the direct plating method which saves time. These results are reassuring for laboratories worldwide who seek to identify GBS from swabs samples as quickly as possible
Tunneling edges at strong disorder
Scattering between edge states that bound one-dimensional domains of opposite
potential or flux is studied, in the presence of strong potential or flux
disorder. A mobility edge is found as a function of disorder and energy, and we
have characterized the extended phase. "paper_FINAL.tex" 439 lines, 20366
characters In the presence of flux and/or potential disorder, the localization
length scales exponentially with the width of the barrier. We discuss
implications for the random-flux problem.Comment: RevTeX, 4 page
Universal eigenvector statistics in a quantum scattering ensemble
We calculate eigenvector statistics in an ensemble of non-Hermitian matrices
describing open quantum systems [F. Haake et al., Z. Phys. B 88, 359 (1992)] in
the limit of large matrix size. We show that ensemble-averaged eigenvector
correlations corresponding to eigenvalues in the center of the support of the
density of states in the complex plane are described by an expression recently
derived for Ginibre's ensemble of random non-Hermitian matrices.Comment: 4 pages, 5 figure
Emergent Moments and Random Singlet Physics in a Majorana Spin Liquid
We exhibit an exactly solvable example of a SU(2) symmetric Majorana spin liquid phase, in which quenched disorder leads to random-singlet phenomenology of emergent magnetic moments. More precisely, we argue that a strong-disorder fixed point controls the low temperature susceptibility chi(T) of an exactly solvable S = 1/2 model on the decorated honeycomb lattice with vacancy and/or bond disorder, leading to chi(T) = C/T + DT alpha(T)-1, where alpha(T) -> 0 slowly as the temperature T -> 0. The first term is a Curie tail that represents the emergent response of vacancy-induced spin textures spread over many unit cells: it is an intrinsic feature of the site-diluted system, rather than an extraneous effect arising from isolated free spins. The second term, common to both vacancy and bond disorder [with different alpha(T) in the two cases] is the response of a random singlet phase, familiar from random antiferromagnetic spin chains and the analogous regime in phosphorus-doped silicon (Si:P)
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