5,906 research outputs found
Formation of Subgap States in Carbon Nanotubes Due to a Local Transverse Electric Field
We introduce two simple models to study the effect of a spatially localized
transverse electric field on the low-energy electronic structure of
semiconducting carbon nanotubes. Starting from the Dirac Hamiltonian for the
low energy states of a carbon nanotube, we use scattering theory to show that
an arbitrarily weak field leads to the formation of localized electronic states
inside the free nanotube band gap. We study the binding energy of these subgap
states as a function of the range and strength of the electrostatic potential.
When the range of the potential is held constant and the strength is varied,
the binding energy shows crossover behavior: the states lie close to the free
nanotube band edge until the potential exceeds a threshold value, after which
the binding energy increases rapidly. When the potential strength is held
constant and the range is varied, we find resonant behavior: the binding energy
passes through a maximum as the range of the potential is increased. Large
electric fields confined to a small region of the nanotube are required to
create localized states far from the band edge.Comment: 15 pages + 5 figures, 1 table in RevTe
Thin-Slice Forecasts of Gubernatorial Elections
We showed 10-second, silent video clips of unfamiliar gubernatorial debates to a group of experimental participants and asked them to predict the election outcomes. The participants' predictions explain more than 20 percent of the variation in the actual two-party vote share across the 58 elections in our study, and their importance survives a range of controls, including state fixed effects. In a horse race of alternative forecasting models, participants' visual forecasts significantly outperform economic variables in predicting vote shares, and are comparable in predictive power to a measure of incumbency status. Adding policy information to the video clips by turning on the sound tends, if anything, to worsen participants' accuracy, suggesting that naïveté may be an asset in some forecasting tasks.
Approximating strongly correlated spin and fermion wavefunctions with correlator product states
We explore correlator product states for the approximation of correlated
wavefunctions in arbitrary dimensions. We show that they encompass many
interesting states including Laughlin's quantum Hall wavefunction, Huse and
Elser's frustrated spin states, and Kitaev's toric code. We further establish
their relation to common families of variational wavefunctions, such as matrix
and tensor product states and resonating valence bond states. Calculations on
the Heisenberg and spinless Hubbard models show that correlator product states
capture both two-dimensional correlations (independent of system width) as well
as non-trivial fermionic correlations (without sign problems). In
one-dimensional simulations, correlator product states appear competitive with
matrix product states with a comparable number of variational parameters,
suggesting they may eventually provide a route to practically generalise the
density matrix renormalisation group to higher dimensions.Comment: Table 1 expanded, Table 2 updated, optimization method discussed,
discussions expanded in some sections, earlier work on similar wavefunctions
included in text and references, see also (arXiv:0905.3898). 5 pages, 1
figure, 2 tables, submitted to Phys. Rev.
The Band Excitation Method in Scanning Probe Microscopy for Rapid Mapping of Energy Dissipation on the Nanoscale
Mapping energy transformation pathways and dissipation on the nanoscale and
understanding the role of local structure on dissipative behavior is a
challenge for imaging in areas ranging from electronics and information
technologies to efficient energy production. Here we develop a novel Scanning
Probe Microscopy (SPM) technique in which the cantilever is excited and the
response is recorded over a band of frequencies simultaneously rather than at a
single frequency as in conventional SPMs. This band excitation (BE) SPM allows
very rapid acquisition of the full frequency response at each point (i.e.
transfer function) in an image and in particular enables the direct measurement
of energy dissipation through the determination of the Q-factor of the
cantilever-sample system. The BE method is demonstrated for force-distance and
voltage spectroscopies and for magnetic dissipation imaging with sensitivity
close to the thermomechanical limit. The applicability of BE for various SPMs
is analyzed, and the method is expected to be universally applicable to all
ambient and liquid SPMs.Comment: 32 pages, 9 figures, accepted for publication in Nanotechnolog
Perfect Reflection of Chiral Fermions in Gated Graphene Nanoribbons
We describe the results of a theoretical study of transport through gated
metallic graphene nanoribbons using a non-equilibrium Green function method.
Although analogies with quantum field theory predict perfect transmission of
chiral fermions through gated regions in one dimension, we find \emph{perfect
reflection} of chiral fermions in armchair ribbons for specific configurations
of the gate. This effect should be measurable in narrow graphene constrictions
gated by a charged carbon nanotube.Comment: 9 pages, 3 figures. Submitted to Nano Letter
Reactive Quenching Of Od A (2)Σ(+) By H-2: Translational Energy Distributions For H- And D-Atom Product Channels
The H- and D-atom products from collisional quenching of OD A (2)Sigma(+) by H-2 are characterized through Doppler spectroscopy using two-photon (2 S-2 \u3c-\u3c- 1 S-2) laser-induced fluorescence. Partial deuteration enables separation of the channel forming H + HOD products, which accounts for 75% of reactive quenching events, from the D + H2O product channel. The Doppler profiles, along with those reported previously for other isotopic variants, are transformed into product translational energy distributions using a robust fitting procedure based on discrete velocity basis functions. The product translational energy distribution for the H- atom channel is strongly peaked at low energy (below 0.5 eV) with a long tail extending to the energetic limit. By contrast, the D-atom channel exhibits a small peak at low translational energy with a distinctive secondary peak at higher translational energy (approximately 1.8 eV) before falling off to higher energy. In both cases, most of the available energy flows into internal excitation of the water products. Similar distributions are obtained upon reanalysis of D- and H- atom Doppler profiles, respectively, from reactive quenching of OH A (2)Sigma(+) by D-2. The sum of the translational energy distributions for H- and D- atom channels is remarkably similar to that obtained for OH A (2)Sigma(+) + H-2, where the two channels cannot be distinguished from one another. The product translational energy distributions from reactive quenching are compared with those obtained from a previous experiment performed at higher collision energy, quasiclassical trajectory calculations of the post-quenching dynamics, and a statistical model. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3644763
- …