92 research outputs found
Treatment of staphylococcal infections with cyclic lipopeptides
AbstractDaptomycin is the first of a new class of antibiotics, the cyclic lipopeptides, for which a novel mechanism of action is hypothesised. Owing to its mode of action, daptomycin is rapidly bactericidal without being bacteriolytic, is active against static- and growing-phase bacteria, and has a low resistance rate in vitro. Phase III clinical trials have demonstrated that daptomycin is as effective as standard therapy for the treatment of complicated skin and soft-tissue infections associated with Gram-positive infections, and daptomycin-treated patients benefited from a reduced time to clinical resolution. Daptomycin has also been shown to be as effective as standard therapy in the treatment of bacteraemia associated with Staphylococcus aureus, with or without endocarditis. These results indicate that daptomycin is a useful therapeutic option for treating Gram-positive infections, particularly those caused by S. aureus
Behaviour of the energy gap near a commensurate-incommensurate transition in double layer quantum Hall systems at nu=1
The charged excitations in the system of the title are vortex-antivortex
pairs in the spin-texture described in the theory by Yang et al which, in the
commensurate phase, are bound together by a ``string''. It is shown that their
excitation energy drops as the string lengthens as the parallel magnetic field
approaches the critical value, then goes up again in the incommensurate phase.
This produces a sharp downward cusp at the critical point. An alternative
description based on the role of disorder in the tunnelling and which appears
not to produce a minimum in the excitation energy is also discussed. It is
suggested that a similar transition could also occur in compressible
Fermi-liquid-like states.Comment: latex file, 17 page
Hund's Rule for Composite Fermions
We consider the ``fractional quantum Hall atom" in the vanishing Zeeman
energy limit, and investigate the validity of Hund's maximum-spin rule for
interacting electrons in various Landau levels. While it is not valid for {\em
electrons} in the lowest Landau level, there are regions of filling factors
where it predicts the ground state spin correctly {\em provided it is applied
to composite fermions}. The composite fermion theory also reveals a
``self-similar" structure in the filling factor range .Comment: 10 pages, revte
Fractional Quantum Hall States in Low-Zeeman-Energy Limit
We investigate the spectrum of interacting electrons at arbitrary filling
factors in the limit of vanishing Zeeman splitting. The composite fermion
theory successfully explains the low-energy spectrum {\em provided the
composite fermions are treated as hard-core}.Comment: 12 pages, revte
Measurements of the Composite Fermion masses from the spin polarization of 2-D electrons in the region
Measurements of the reflectivity of a 2-D electron gas are used to deduce the
polarization of the Composite Fermion hole system formed for Landau level
occupancies in the regime 1<\nu<2. The measurements are consistent with the
formation of a mixed spin CF system and allow the density of states or
`polarization' effective mass of the CF holes to be determined. The mass values
at \nu=3/2 are found to be ~1.9m_{e} for electron densities of 4.4 x 10^{11}
cm^{-2}, which is significantly larger than those found from measurements of
the energy gaps at finite values of effective magnetic field.Comment: 4 pages, 3 fig
Nonlinear screening and percolative transition in a two-dimensional electron liquid
A novel variational method is proposed for calculating the percolation
threshold, the real-space structure, and the thermodynamical compressibility of
a disordered two-dimensional electron liquid. Its high accuracy is verified
against prior numerical results and newly derived exact asymptotics. The
inverse compressibility is shown to have a strongly asymmetric minimum at a
density that is approximately the triple of the percolation threshold. This
implies that the experimentally observed metal-insulator transition takes place
well before the percolation point is reached.Comment: 4 pages, 2 figures. (v2) minor changes (v3) reference added (v4) few
more references adde
Tunneling from a correlated 2D electron system transverse to a magnetic field
We show that, in a magnetic field parallel to the 2D electron layer, strong
electron correlations change the rate of tunneling from the layer
exponentially. It results in a specific density dependence of the escape rate.
The mechanism is a dynamical Mossbauer-type recoil, in which the Hall momentum
of the tunneling electron is partly transferred to the whole electron system,
depending on the interrelation between the rate of interelectron momentum
exchange and the tunneling duration. We also show that, in a certain
temperature range, magnetic field can enhance rather than suppress the
tunneling rate. The effect is due to the magnetic field induced energy exchange
between the in-plane and out-of-plane motion. Magnetic field can also induce
switching between intra-well states from which the system tunnels, and a
transition from tunneling to thermal activation. Explicit results are obtained
for a Wigner crystal. They are in qualitative and quantitative agreement with
the relevant experimental data, with no adjustable parameters.Comment: 16 pages, 9 figure
Energy, interaction, and photoluminescence of spin-reversed quasielectrons in fractional quantum Hall systems
The energy and photoluminescence spectra of a two-dimensional electron gas in
the fractional quantum Hall regime are studied. The single-particle properties
of reversed-spin quasielectrons (QE's) as well as the
pseudopotentials of their interaction with one another and with Laughlin
quasielectrons (QE's) and quasiholes (QH's) are calculated. Based on the
short-range character of the QE--QE and QE--QE
repulsion, the partially unpolarized incompressible states at the filling
factors and are postulated within Haldane's
hierarchy scheme. To describe photoluminescence, the family of bound
QE states of a valence hole and QE's are
predicted in analogy to the found earlier fractionally charged excitons
QE. The binding energy and optical selection rules for both families are
compared. The QE is found radiative in contrast to the dark QE,
and the QE is found non-radiative in contrast to the bright
QE.Comment: 9 pages, 6 figure
Edge Dynamics in Quantum Hall Bilayers II: Exact Results with Disorder and Parallel Fields
We study edge dynamics in the presence of interlayer tunneling, parallel
magnetic field, and various types of disorder for two infinite sequences of
quantum Hall states in symmetric bilayers. These sequences begin with the 110
and 331 Halperin states and include their fractional descendants at lower
filling factors; the former is easily realized experimentally while the latter
is a candidate for the experimentally observed quantum Hall state at a total
filling factor of 1/2 in bilayers. We discuss the experimentally interesting
observables that involve just one chiral edge of the sample and the correlation
functions needed for computing them. We present several methods for obtaining
exact results in the presence of interactions and disorder which rely on the
chiral character of the system. Of particular interest are our results on the
331 state which suggest that a time-resolved measurement at the edge can be
used to discriminate between the 331 and Pfaffian scenarios for the observed
quantum Hall state at filling factor 1/2 in realistic double-layer systems.Comment: revtex+epsf; two-up postscript at
http://www.sns.ias.edu/~leonid/ntwoup.p
Momentum-Resolved Tunneling into Fractional Quantum Hall Edges
Tunneling from a two-dimensional contact into quantum-Hall edges is
considered theoretically for a case where the barrier is extended, uniform, and
parallel to the edge. In contrast to previously realized tunneling geometries,
details of the microscopic edge structure are exhibited directly in the voltage
and magnetic-field dependence of the differential tunneling conductance. In
particular, it is possible to measure the dispersion of the edge-magnetoplasmon
mode, and the existence of additional, sometimes counterpropagating,
edge-excitation branches could be detected.Comment: 4 pages, 3 figures, RevTex
- …