4,536 research outputs found
Comment on "Density Functional Simulation of a Breaking Nanowire"
In a recent Letter, Nakamura et al. [Phys. Rev. Lett. 82, 1538 (1999)]
described first principles calculations for a breaking Na nanocontact. Their
system consists of a periodic one-dimensional array of supercells, each of
which contains 39 Na atoms, originally forming a straight, crystalline wire
with a length of 6 atoms. The system is elongated by increasing the length of
the unit cell. At each step, the atomic configuration is relaxed to a new local
equilibrium, and the tensile force is evaluated from the change of the total
energy with elongation. Aside from a discontinuity of the force occuring at the
transition from a crytalline to an amorphous configuration during the early
stages of elongation, they were unable to identify any simple correlations
between the force and the number of electronic modes transmitted through the
contact. An important question is whether their model is realistic, i.e.,
whether it can be compared to experimental results obtained for a single
nanocontact between two macroscopic pieces of metal. In this Comment, we
demonstrate that with such a small unit cell, the interference effects between
neighboring contacts are of the same size as the force oscillations in a single
nanocontact.Comment: 1 pag
Correlated charge polarization in a chain of coupled quantum dots
Coherent charge transfer in a linear array of tunnel-coupled quantum dots,
electrostatically coupled to external gates, is investigated using the Bethe
ansatz for a symmetrically biased Hubbard chain. Charge polarization in this
correlated system is shown to proceed via two distinct processes: formation of
bound states in the metallic phase, and charge transfer processes corresponding
to a superposition of antibound states at opposite ends of the chain in the
Mott-insulating phase. The polarizability in the insulating phase of the chain
exhibits a universal scaling behavior, while the polarization charge in the
metallic phase of the model is shown to be quantized in units of .Comment: 9 pages, 3 figures, 1 tabl
Many-body theory of electronic transport in single-molecule heterojunctions
A many-body theory of molecular junction transport based on nonequilibrium
Green's functions is developed, which treats coherent quantum effects and
Coulomb interactions on an equal footing. The central quantity of the many-body
theory is the Coulomb self-energy matrix of the junction.
is evaluated exactly in the sequential tunneling limit, and
the correction due to finite tunneling width is evaluated self-consistently
using a conserving approximation based on diagrammatic perturbation theory on
the Keldysh contour. Our approach reproduces the key features of both the
Coulomb blockade and coherent transport regimes simultaneously in a single
unified transport theory. As a first application of our theory, we have
calculated the thermoelectric power and differential conductance spectrum of a
benzenedithiol-gold junction using a semi-empirical -electron Hamiltonian
that accurately describes the full spectrum of electronic excitations of the
molecule up to 8--10eV.Comment: 13 pages, 7 figure
An Outbreak of Salmonella typhimurium at a teaching hospital
An outbreak of Salmonella typhimurium infection in December 1996 affected 52 patients, relatives, and staff of a large teaching hospital in southeast Queensland. Assorted sandwiches were identified as the vehicle of transmission. This article describes the outbreak investigation and demonstrates the importance of food hygiene and timely public health interventions
Universality in metallic nanocohesion: a quantum chaos approach
Convergent semiclassical trace formulae for the density of states and
cohesive force of a narrow constriction in an electron gas, whose classical
motion is either chaotic or integrable, are derived. It is shown that mode
quantization in a metallic point contact or nanowire leads to universal
oscillations in its cohesive force: the amplitude of the oscillations depends
only on a dimensionless quantum parameter describing the crossover from chaotic
to integrable motion, and is of order 1 nano-Newton, in agreement with recent
experiments. Interestingly, quantum tunneling is shown to be described
quantitatively in terms of the instability of the classical periodic orbits.Comment: corrects spelling of one author name on abstract page (paper is
unchanged
Coherent Resonant Tunneling Through an Artificial Molecule
Coherent resonant tunneling through an artificial molecule of quantum dots in
an inhomogeneous magnetic field is investigated using an extended Hubbard
model. Both the multiterminal conductance of an array of quantum dots and the
persistent current of a quantum dot molecule embedded in an Aharanov-Bohm ring
are calculated. The conductance and persistent current are calculated
analytically for the case of a double quantum dot and numerically for larger
arrays using a multi-terminal Breit-Wigner type formula, which allows for the
explicit inclusion of inelastic processes. Cotunneling corrections to the
persistent current are also investigated, and it is shown that the sign of the
persistent current on resonance may be used to determine the spin quantum
numbers of the ground state and low-lying excited states of an artificial
molecule. An inhomogeneous magnetic field is found to strongly suppress
transport due to pinning of the spin-density-wave ground state of the system,
and giant magnetoresistance is predicted to result from the ferromagnetic
transition induced by a uniform external magnetic field.Comment: 23 pages, 12 figure
Stability and Symmetry Breaking in Metal Nanowires
A general linear stability analysis of simple metal nanowires is presented
using a continuum approach which correctly accounts for material-specific
surface properties and electronic quantum-size effects. The competition between
surface tension and electron-shell effects leads to a complex landscape of
stable structures as a function of diameter, cross section, and temperature. By
considering arbitrary symmetry-breaking deformations, it is shown that the
cylinder is the only generically stable structure. Nevertheless, a plethora of
structures with broken axial symmetry is found at low conductance values,
including wires with quadrupolar, hexapolar and octupolar cross sections. These
non-integrable shapes are compared to previous results on elliptical cross
sections, and their material-dependent relative stability is discussed.Comment: 12 pages, 4 figure
Transport Properties of One-Dimensional Hubbard Models
We present results for the zero and finite temperature Drude weight D(T) and
for the Meissner fraction of the attractive and the repulsive Hubbard model, as
well as for the model with next nearest neighbor repulsion. They are based on
Quantum Monte Carlo studies and on the Bethe ansatz. We show that the Drude
weight is well defined as an extrapolation on the imaginary frequency axis,
even for finite temperature. The temperature, filling, and system size
dependence of D is obtained. We find counterexamples to a conjectured
connection of dissipationless transport and integrability of lattice models.Comment: 10 pages, 14 figures. Published versio
Continuous-culture fermentation as a tool for forage evaluation
Ruminal degradation of organic matter and protein in alfalfa and prairie hay were evaluated in vivo, using cannulated cows, and in vitro, using a continuous-culture fermenter to simulate ruminal fermentation. Estimates of organic matter degradability, microbial N flow per unit feed N input, and efficiency of microbial growth were not different (P\u3e.10) between the in vivo and in vitro systems. However, for both forages, estimates of nitrogen degradability were greater with the in vitro system. Despite the differences between in vivo and in vitro techniques for some variables, continuous-culture fermentation will allow us to compare the effects of dietary treatments on forage digestion and will aid in the formulation of supplements to meet specific nutrient requirements for cattle consuming forage-based diets
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