704 research outputs found
The association of serotype and pulsed-field gel electrophoresis genotype in isolates of Streptococcus pneumoniae isolated in Israel
SummaryThe relationship between Streptococcus pneumoniae isolates causing invasive infections in children admitted to a single center in central Israel was examined by pulsed-field gel electrophoresis (PFGE) and serotyping. Although there was a close correlation between serotype and PFGE clone, the genetic diversity varied by serotype, with some genotypes comprising multiple serotypes. Additionally, clones C and D were associated with higher penicillin minimum inhibitory concentrations. Serotyping alone may be insufficient for epidemiological mapping of pneumococcal isolates in the era of pneumococcal conjugate polysaccharide vaccines
The transverse magnetoresistance of the two-dimensional chiral metal
We consider the two-dimensional chiral metal, which exists at the surface of
a layered, three-dimensional sample exhibiting the integer quantum Hall effect.
We calculate its magnetoresistance in response to a component of magnetic field
perpendicular to the sample surface, in the low temperature, but macroscopic,
regime where inelastic scattering may be neglected. The magnetoresistance is
positive, following a Drude form with a field scale,
, given by the transverse field strength at which
one quantum of flux, , passes through a rectangle with sides set by the
layer-spacing, , and the elastic mean free path, .
Experimental measurement of this magnetoresistance may therefore provide a
direct determination of the elastic mean free path in the chiral metal.Comment: submitted to Phys Rev
Nonadiabatic noncyclic geometric phase and ensemble average spectrum of conductance in disordered mesoscopic rings with spin-orbit coupling
We generalize Yang's theory from the U(1) gauge field to the non-Abelian
gauge field. Based on this generalization and taking
into account the geometric Pancharatnam phase as well as an effective
Aharonov-Bohm (AB) phase in nonadiabatic noncyclic transport, we calculate the
ensemble average Fourier spectrum of the conductance in disordered mesoscopic
rings connected to two leads. Our theory can explain the experimental results
reported by Morpurgo {\sl et al.} [Phys. Rev. Lett. {\bf 80}, 1050 (1998)]
satisfactorily. In particular, we clarify that the experimentally observed
splitting, as well as some structure on the sides of the main peak in the
ensemble average Fourier spectrum, stem from the nonadiabatic noncyclic
Pancharatnam phase and the effective AB phase, both being dependent on
spin-orbit coupling.Comment: 5 pages, 1 figure. A slightly revised version, and re-submitted to
PRL on Mar. 14, 200
Conductance of a quantum point contact in the presence of spin-orbit interaction
A recursive Green's function technique is developed to calculate the
spin-dependent conductance in mesoscopic structures. Using this technique, we
study the spin-dependent electronic transport of quantum point contacts in the
presence of the Rashba spin-orbit interaction. We observed that some
oscillations in the `quantized' conductance are induced by the spin-orbit
interaction, and indicated that the oscillations may stem from the spin-orbit
coupling associated multiple reflections. It is also indicated that the 0.7
structure of the conductance observed in mesoscopic experiments would not stem
from the spin-orbit interaction.Comment: 8 page
Fano resonances and Aharonov-Bohm effects in transport through a square quantum dot molecule
We study the Aharonov-Bohm effect in a coupled 22 quantum dot array
with two-terminals. A striking conductance dip arising from the Fano
interference is found as the energy levels of the intermediate dots are
mismatched, which is lifted in the presence of a magnetic flux. A novel five
peak structure is observed in the conductance for large mismatch. The
Aharonov-Bohm evolution of the linear conductance strongly depends on the
configuration of dot levels and interdot and dot-lead coupling strengths. In
addition, the magnetic flux and asymmetry between dot-lead couplings can induce
the splitting and combination of the conductance peak(s).Comment: 15 pages, 7 figures, Revtex, to be published in Phys. Rev.
Observation of Spin-Orbit Berry's Phase in Magnetoresistance of a Two-Dimensional Hole Anti-dot System
We report observation of spin-orbit Berry's phase in the Aharonov-Bohm (AB)
type oscillation of weak field magnetoresistance in an anti-dot lattice (ADL)
of a two-dimensional hole system. An AB-type oscillation is superposed on the
commensurability peak, and the main peak in the Fourier transform is clearly
split up due to variation in Berry's phase originating from the spin-orbit
interaction. A simulation considering Berry's phase and the phase arising from
the spin-orbit shift in the momentum space shows qualitative agreement with the
experiment.Comment: 13 pages, 5 figure
Current-Carrying Ground States in Mesoscopic and Macroscopic Systems
We extend a theorem of Bloch, which concerns the net orbital current carried
by an interacting electron system in equilibrium, to include mesoscopic
effects. We obtain a rigorous upper bound to the allowed ground-state current
in a ring or disc, for an interacting electron system in the presence of static
but otherwise arbitrary electric and magnetic fields. We also investigate the
effects of spin-orbit and current-current interactions on the upper bound.
Current-current interactions, caused by the magnetic field produced at a point
r by a moving electron at r, are found to reduce the upper bound by an amount
that is determined by the self-inductance of the system. A solvable model of an
electron system that includes current-current interactions is shown to realize
our upper bound, and the upper bound is compared with measurements of the
persistent current in a single ring.Comment: 7 pager, Revtex, 1 figure available from [email protected]
Linear in-plane magnetoconductance and spin susceptibility of a 2D electron gas on a vicinal silicon surface
In this work we have studied the parallel magnetoresistance of a 2DEG near a
vicinal silicon surface. An unusual, linear magnetoconductance is observed in
the fields up to T, which we explain by the effect of spin olarization
on impurity scattering. This linear magnetoresistance shows strong anomalies
near the boundaries of the minigap in the electron spectrum of the vicinal
system.Comment: (accepted to Phys. Rev. B
Shot noise of coupled semiconductor quantum dots
The low-frequency shot noise properties of two electrostatically coupled
semiconductor quantum dot states which are connected to emitter/collector
contacts are studied. A master equation approach is used to analyze the bias
voltage dependence of the Fano factor as a measure of temporal correlations in
tunneling current caused by Pauli's exclusion principle and the Coulomb
interaction. In particular, the influence of the Coulomb interaction on the
shot noise behavior is discussed in detail and predictions for future
experiments will be given. Furthermore, we propose a mechanism for negative
differential conductance and investigate the related super-Poissonian shot
noise.Comment: submitted to PR
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