430 research outputs found
Single Wall Nanotubes: Atomic Like Behaviour and Microscopic Approach
Recent experiments about the low temperature behaviour of a Single Wall
Carbon Nanotube (SWCNT) showed typical Coulomb Blockade (CB) peaks in the zero
bias conductance and allowed us to investigate the energy levels of interacting
electrons. Other experiments confirmed the theoretical prediction about the
crucial role which the long range nature of the Coulomb interaction plays in
the correlated electronic transport through a SWCNT with two intramolecular
tunneling barriers. In order to investigate the effects on low dimensional
electron systems due to the range of electron electron repulsion, we introduce
a model for the interaction which interpolates well between short and long
range regimes. Our results could be compared with experimental data obtained in
SWCNTs and with those obtained for an ideal vertical Quantum Dot (QD).
For a better understanding of some experimental results we also discuss how
defects and doping can break some symmetries of the bandstructure of a SWCNT.Comment: 8 pages, 4 figure
Antimicrobial Resistance and the Ethics of Drug Development
http://deepblue.lib.umich.edu/bitstream/2027.42/55401/1/Aiello A, et al 2006 Ethical conflicts in public health research and practice antimicrobial resistance and the ethics of drug development.pd
Magnetic Field Dependence of the Level Spacing of a Small Electron Droplet
The temperature dependence of conductance resonances is used to measure the
evolution with the magnetic field of the average level spacing
of a droplet containing electrons created by lateral confinement of a
two-dimensional electron gas in GaAs. becomes very small (eV) near two critical magnetic fields at which the symmetry of the
droplet changes and these decreases of are predicted by
Hartree-Fock (HF) for charge excitations. Between the two critical fields,
however, the largest measured eV is an order of
magnitude smaller than predicted by HF but comparable to the Zeeman splitting
at this field, which suggests that the spin degrees of freedom are important.
PACS: 73.20.Dx, 73.20.MfComment: 11 pages of text in RevTeX, 4 figures in Postscript (files in the
form of uuencoded compressed tar file
The association between indwelling urinary catheter use in the elderly and urinary tract infection in acute care
BACKGROUND: The use of indwelling urinary catheters (IUCs) is thought to be the most significant risk factor for developing nosocomial urinary tract infections (UTIs). However, it is unclear how many elderly patients have preexisting bacteriuria prior to IUC placement. The purpose of this study was to determine 1) the frequency and appropriateness of IUC use in the Emergency Department (ED) in elderly patients admitted to our acute care hospital, 2) the percentage of elderly patients with an IUC who were discharged from the hospital with a diagnosis of UTI, 3) the percentage of patients with IUCs who were diagnosed and treated for UTI in the ED or who had admission bacteriuria ≥10(5 )organisms/ml indicating preexisting UTI, and 4) the percentage of patients with no indication of UTI on admission who had inappropriately placed IUCs and subsequently were diagnosed with a UTI. METHODS: Retrospective chart review. Chi square used to test significance of differences in proportions. RESULTS: Seventy three percent of patients who received an IUC in the ED were elderly (≥65 years old). During the study period, 277 elderly patients received an IUC prior to admission. Of these, 77 (28%) were diagnosed with UTI during their hospitalization. Fifty three (69%) of those diagnosed with a UTI by discharge either had the UTI diagnosed in the ED or had bacteriuria ≥10(5 )organisms/ml prior to IUC placement. Of the 24 elderly patients who developed a catheter-associated UTI (i.e., 9% of the elderly population who received an IUC), 11 of the IUCs were placed inappropriately. Thus, 4% of elderly patients with no indication of UTI on admission who received an inappropriate IUC in the ED had a primary or secondary diagnosis of UTI by discharge. The overall rate of nosocomial UTI due to an inappropriately placed IUC was the same in males and females. CONCLUSION: This study indicates that the strong association between IUC use and UTI may be partly explained by the high prevalence of preexisting UTI prior to IUC placement. Further prospective studies are needed to clarify the true risk vs benefit ratio for IUC use in acutely ill elderly patients
Measuring Temperature Gradients over Nanometer Length Scales
When a quantum dot is subjected to a thermal gradient, the temperature of
electrons entering the dot can be determined from the dot's thermocurrent if
the conductance spectrum and background temperature are known. We demonstrate
this technique by measuring the temperature difference across a 15 nm quantum
dot embedded in a nanowire. This technique can be used when the dot's energy
states are separated by many kT and will enable future quantitative
investigations of electron-phonon interaction, nonlinear thermoelectric
effects, and the effciency of thermoelectric energy conversion in quantum dots.Comment: 6 pages, 5 figure
Signatures of Chaos in the Statistical Distribution of Conductance Peaks in Quantum Dots
Analytical expressions for the width and conductance peak distributions of
irregularly shaped quantum dots in the Coulomb blockade regime are presented in
the limits of conserved and broken time-reversal symmetry. The results are
obtained using random matrix theory and are valid in general for any number of
non-equivalent and correlated channels, assuming that the underlying classical
dynamic of the electrons in the dot is chaotic or that the dot is weakly
disordered. The results are expressed in terms of the channel correlation
matrix which for chaotic systems is given in closed form for both point-like
contacts and extended leads. We study the dependence of the distributions on
the number of channels and their correlations. The theoretical distributions
are in good agreement with those computed in a dynamical model of a chaotic
billiard.Comment: 19 pages, RevTex, 11 Postscript figure
Detection of Coulomb Charging around an Antidot in the Quantum Hall Regime
We have detected oscillations of the charge around a potential hill (antidot)
in a two-dimensional electron gas as a function of a large magnetic field B.
The field confines electrons around the antidot in closed orbits, the areas of
which are quantised through the Aharonov-Bohm effect. Increasing B reduces each
state's area, pushing electrons closer to the centre, until enough charge
builds up for an electron to tunnel out. This is a new form of the Coulomb
blockade seen in electrostatically confined dots. Addition and excitation
spectra in DC bias confirm the Coulomb blockade of tunnelling.Comment: 4 pages, 4 Postscript figure
The Anderson Model out of equilibrium: Time dependent perturbations
The influence of high-frequency fields on quantum transport through a quantum
dot is studied in the low-temperature regime. We generalize the non crossing
approximation for the infinite-U Anderson model to the time-dependent case. The
dc spectral density shows asymmetric Kondo side peaks due to photon-assisted
resonant tunneling. As a consequence we predict an electron-photon pump at zero
bias which is purely based on the Kondo effect. In contrast to the resonant
level model and the time-independent case we observe asymmetric peak amplitudes
in the Coulomb oscillations and the differential conductance versus bias
voltage shows resonant side peaks with a width much smaller than the tunneling
rate. All the effects might be used to clarify the question whether quantum
dots indeed show the Kondo effect.Comment: 13 pages, REVTEX 3.0, 5 figure
Electronic Structure of Dinuclear Gold(I) Complexes
Cyclic voltammetry (CV) experiments on LL(AuSR∗)2
complexes [LL = diphenylphosphinomethane
(dppm), diphenylphosphinopentane (dpppn); R* =
p-SC6H4CH3] show anodic sweeps that broaden by
about 25 mV on going from the longer (dpppn) to the shorter (dppm) bidentate phosphine ligand.
Changing concentrations had no effect on the shape of the waveform. The result suggests a weak
intramolecular metal-metal interaction in dppm(AuSR∗)2
that correlates well with rate acceleration
occurring in the reaction of dppm(AuSR∗)2
with organic disulfides. Quantum yields for cis-dppee(AuX)2
[dppee = 1,2-bis(diphenylphosphino)ethylene; X = Cl, Br, I] complexes, (disappearance) Φ
, are significantly
higher in complexes with a softer X ligand, a trend that correlates well with aurophilicity. This result also
suggests that electronic perturbation caused by Au(I)-Au(I) interactions is important in explaining the
reactivity of some dinuclear gold(I) complexes. The crystal structure for cis-dppee(Aul)2
shows short
intramolecular Au(I)-Au(I) interactions of 2.9526 (6) A°, while the structure
of trans-dppee(AuI)2
, shows intermolecular Au(I)-Au(I) interactions of 3.2292 (9) A°. The substitution of .As for
P results in a ligand, cis-diphenylarsinoethylene
(cis-dpaee), that is photochemically active, in contrast to the cis-dppee ligand.
The complexes, cis-dpaee(AuX)2, are also photochemically active but
with lower quantum yields than the
cis-dppee(AuX)2
complexes
From the Kondo Regime to the Mixed-Valence Regime in a Single-Electron Transistor
We demonstrate that the conductance through a single-electron transistor at
low temperature is in quantitative agreement with predictions of the
equilibrium Anderson model. When an unpaired electron is localized within the
transistor, the Kondo effect is observed. Tuning the unpaired electron's energy
toward the Fermi level in nearby leads produces a cross-over between the Kondo
and mixed-valence regimes of the Anderson model.Comment: 3 pages plus one 2 page postscript file of 5 figures. Submitted to
PR
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