2,423 research outputs found
Spin-Orbit Assisted Variable-Range Hopping in Strong Magnetic Fields
It is shown that in the presence of strong magnetic fields, spin-orbit
scattering causes a sharp increase in the effective density of states in the
variable-range hopping regime when temperature decreases. This effect leads to
an exponential enhancement of the conductance above its value without
spin-orbit scattering. Thus an experimental study of the hopping conductivity
in a fixed, large magnetic field, is a sensitive tool to explore the spin-orbit
scattering parameters in the strongly localized regime.Comment: 9 pages + 2 figures (enclosed), Revte
The Effects of Resonant Tunneling on Magnetoresistance through a Q uantum Dot
The effect of resonant tunneling on magnetoresistance (MR) is studied
theoretically in a double junction system. We have found that the ratio of the
MR of the resonant peak current is reduced more than that of the single
junction, whereas that of the valley current is enhanced depending on the
change of the discrete energy-level under the change of magnetic field. We also
found that the peak current-valley current (PV) ratio decreases when the
junction conductance increases.Comment: 11 pages, 3 figures(mail if you need), use revtex.st
Random Matrix Theory of Transition Strengths and Universal Magnetoconductance in the Strongly Localized Regime
Random matrix theory of the transition strengths is applied to transport in
the strongly localized regime. The crossover distribution function between the
different ensembles is derived and used to predict quantitatively the {\sl
universal} magnetoconductance curves in the absence and in the presence of
spin-orbit scattering. These predictions are confirmed numerically.Comment: 15 pages and two figures in postscript, revte
A New Spin-Orbit Induced Universality Class in the Quantum Hall Regime ?
Using heuristic arguments and numerical simulations it is argued that the
critical exponent describing the localization length divergence at the
quantum Hall transition is modified in the presence of spin-orbit scattering
with short range correlations. The exponent is very close to , the
percolation correlation length exponent, the prediction of a semi-classical
argument. In addition, a region of weakly localized regime, where the
localization length is exponentially large, is conjectured.Comment: 4 two-column pages including 4 eps figure
Oral White Lesions Associated with Chewing Khat
<p>Abstract</p> <p>Introduction</p> <p>Khat is a cultivated plant whose leaves when chewed elevate mood. Unlike the chewing of betel nut, no association between the white oral mucosal lesions in khat users and oral malignancies has been reported. Chewing of khat has been documented in many countries and has increased with worldwide migration. The impact of chewing khat upon the oral mucosa is essentially unknown.</p> <p>Purpose</p> <p>The purpose of this study was to assess the occurrence of oral white changes in chronic khat chewers. Oral mucosal changes in a group of 47 Yemenite Israeli men over 30 years of age, who had chewed khat more than 3 years, were compared to those of 55 Yemenite men who did not chew.</p> <p>Results</p> <p>White lesions were significantly more prevalent in the khat chewers (83%) compared to the non chewing individuals (16%) (P < 0.001). White oral lesions were identified primarily on the lower buccal attached gingival mucosa, the alveolar mucosa and the lower mucobuccal fold on the chewing side (p < 0.001). There was no significant association between the occurrence of the white lesions and smoking. Even though the majority of the white lesions (85.4%) were homogenous, 71.4% of the non homogenous lesions were identified in khat chewers. Vital staining with toluidine blue and exfoliative cytology was conducted on a subset of patients with homogenous and non-homogenous oral lesions, and there were no findings suspicious for pre-malignant or malignant changes.</p> <p>Discussion</p> <p>This study demonstrated a relationship between khat chewing and oral white lesions, which we attribute to chronic local mechanical and chemical irritation of the mucosa. Our findings also suggest that mucosal changes associated with khat are benign, however, this initial study requires further studies including follow-up of khat users to confirm the current findings, including the likely benign changes associated with chronic use and histologic findings of clinical lesions.</p
The fate of assimilated carbon during drought: impacts on respiration in Amazon rainforests
Interannual variations in CO2 exchange across Amazonia, as deduced from atmospheric inversions, correlate with El Niño occurrence. They are thought to result from changes in net ecosystem exchange and fire incidence that are both related to drought intensity. Alterations to net ecosystem production (NEP) are caused by changes in gross primary production (GPP) and ecosystem respiration (Reco). Here, we analyse observations of the components of Reco (leaves, live and dead woody tissue, and soil) to provide first estimates of changes in Reco during short-term (seasonal to interannual) moisture limitation. Although photosynthesis declines if moisture availability is limiting, leaf dark respiration is generally maintained, potentially acclimating upwards in the longer term. If leaf area is lost, then short-term canopy-scale respiratory effluxes from wood and leaves are likely to decline. Using a moderate short-term drying scenario where soil moisture limitation leads to a loss of 0.5 m2 m−2 yr−1 in leaf area index, we estimate a reduction in respiratory CO2 efflux from leaves and live woody tissue of 1.0 (±0.4) t C ha−1 yr−1. Necromass decomposition declines during drought, but mortality increases; the median mortality increase following a strong El Niño is 1.1% (n=46 tropical rainforest plots) and yields an estimated net short-term increase in necromass CO2 efflux of 0.13–0.18 t C ha−1 yr−1. Soil respiration is strongly sensitive to moisture limitation over the short term, but not to associated temperature increases. This effect is underestimated in many models but can lead to estimated reductions in CO2 efflux of 2.0 (±0.5) t C ha−1 yr−1. Thus, the majority of short-term respiratory responses to drought point to a decline in Reco, an outcome that contradicts recent regional-scale modelling of NEP. NEP varies with both GPP and Reco but robust moisture response functions are clearly needed to improve quantification of the role of Reco in influencing regional-scale CO2 emissions from Amazonia
Revealing Cosmic Rotation
Cosmological Birefringence (CB), a rotation of the polarization plane of
radiation coming to us from distant astrophysical sources, may reveal parity
violation in either the electromagnetic or gravitational sectors of the
fundamental interactions in nature. Until only recently this phenomenon could
be probed with only radio observations or observations at UV wavelengths.
Recently, there is a substantial effort to constrain such non-standard models
using observations of the rotation of the polarization plane of cosmic
microwave background (CMB) radiation. This can be done via measurements of the
-modes of the CMB or by measuring its TB and EB correlations which vanish in
the standard model. In this paper we show that correlations-based
estimator is the best for upcoming polarization experiments. The based
estimator surpasses other estimators because it has the smallest noise and of
all the estimators is least affected by systematics. Current polarimeters are
optimized for the detection of -mode polarization from either primordial
gravitational waves or by large scale structure via gravitational lensing. In
the paper we also study optimization of CMB experiments for the detection of
cosmological birefringence, in the presence of instrumental systematics, which
by themselves are capable of producing correlations; potentially mimicking
CB.Comment: 10 pages, 3 figures, 2 table
Cutting edges at random in large recursive trees
We comment on old and new results related to the destruction of a random
recursive tree (RRT), in which its edges are cut one after the other in a
uniform random order. In particular, we study the number of steps needed to
isolate or disconnect certain distinguished vertices when the size of the tree
tends to infinity. New probabilistic explanations are given in terms of the
so-called cut-tree and the tree of component sizes, which both encode different
aspects of the destruction process. Finally, we establish the connection to
Bernoulli bond percolation on large RRT's and present recent results on the
cluster sizes in the supercritical regime.Comment: 29 pages, 3 figure
Reply to Comment on "Exact analytic solution for the generalized Lyapunov exponent of the 2-dimensional Anderson localization"
We reply to comments by P.Marko, L.Schweitzer and M.Weyrauch
[preceding paper] on our recent paper [J. Phys.: Condens. Matter 63, 13777
(2002)]. We demonstrate that our quite different viewpoints stem for the
different physical assumptions made prior to the choice of the mathematical
formalism. The authors of the Comment expect \emph{a priori} to see a single
thermodynamic phase while our approach is capable of detecting co-existence of
distinct pure phases. The limitations of the transfer matrix techniques for the
multi-dimensional Anderson localization problem are discussed.Comment: 4 pages, accepted for publication in J.Phys.: Condens. Mat
Effect of Quantum Confinement on Electron Tunneling through a Quantum Dot
Employing the Anderson impurity model, we study tunneling properties through
an ideal quantum dot near the conductance minima. Considering the Coulomb
blockade and the quantum confinement on an equal footing, we have obtained
current contributions from various types of tunneling processes; inelastic
cotunneling, elastic cotunneling, and resonant tunneling of thermally activated
electrons. We have found that the inelastic cotunneling is suppressed in the
quantum confinement limit, and thus the conductance near its minima is
determined by the elastic cotunneling at low temperature (,
: dot-reservoir coupling constant), or by the resonant tunneling of
single electrons at high temperature ().Comment: 11 pages Revtex, 2 Postscript figures, To appear in Phys.Rev.
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