10,661 research outputs found
Parity and valley degeneracy in multilayer graphene
We study spatial symmetry in general ABA-stacked multilayer graphene to
illustrate how electronic spectra at the two valleys are related in a magnetic
field. We show that the lattice of multilayers with an even number of layers,
as well as that of monolayer graphene, satisfy spatial inversion symmetry,
which rigorously guarantees valley degeneracy in the absence of time-reversal
symmetry. A multilayer with an odd number of layers (three or more) lacks
inversion symmetry, but there is another transformation imposing an approximate
valley degeneracy, which arises because the low-energy Hamiltonian consists of
separate monolayerlike and bilayerlike parts. We show that an external
electrostatic potential generally breaks valley degeneracy in a magnetic field,
in a markedly different manner in odd and even multilayers.Comment: 6 pages, 3 figure
Degeneracy breaking and intervalley scattering due to short-ranged impurities in finite single-wall carbon nanotubes
We present a theoretical study of degeneracy breaking due to short-ranged
impurities in finite, single-wall, metallic carbon nanotubes. The effective
mass model is used to describe the slowly varying spatial envelope
wavefunctions of spinless electrons near the Fermi level at two inequivalent
valleys (K-points) in terms of the four component Dirac equation for massless
fermions, with the role of spin assumed by pseudospin due to the relative
amplitude of the wave function on the sublattice atoms (``A'' and ``B''). Using
boundary conditions at the ends of the tube that neither break valley
degeneracy nor mix pseudospin eigenvectors, we use degenerate perturbation
theory to show that the presence of impurities has two effects. Firstly, the
position of the impurity with respect to the spatial variation of the envelope
standing waves results in a sinusoidal oscillation of energy level shift as a
function of energy. Secondly, the position of the impurity within the hexagonal
graphite unit cell produces a particular 4 by 4 matrix structure of the
corresponding effective Hamiltonian. The symmetry of this Hamiltonian with
respect to pseudospin flip is related to degeneracy breaking and, for an
armchair tube, the symmetry with respect to mirror reflection in the nanotube
axis is related to pseudospin mixing.Comment: 20 pages, 10 eps figure
Uniform Density Theorem for the Hubbard Model
A general class of hopping models on a finite bipartite lattice is
considered, including the Hubbard model and the Falicov-Kimball model. For the
half-filled band, the single-particle density matrix \uprho (x,y) in the
ground state and in the canonical and grand canonical ensembles is shown to be
constant on the diagonal , and to vanish if and if and
are on the same sublattice. For free electron hopping models, it is shown in
addition that there are no correlations between sites of the same sublattice in
any higher order density matrix. Physical implications are discussed.Comment: 15 pages, plaintex, EHLMLRJM-22/Feb/9
Morphology of Hydrodynamic Winds: A Study of Planetary Winds in Stellar Environments
Bathed in intense ionizing radiation, close-in gaseous planets undergo
hydrodynamic atmospheric escape, which ejects the upper extent of their
atmospheres into the interplanetary medium. Ultraviolet detections of escaping
gas around transiting planets corroborate such a framework. Exposed to the
stellar environment, the outflow is shaped by its interaction with the stellar
wind and by the planet's orbit. We model these effects using Athena to perform
3-D radiative-hydrodynamic simulations of tidally-locked hydrogen atmospheres
receiving large amounts of ionizing extreme-ultraviolet flux in various stellar
environments for the low-magnetic-field case. Through a step-by-step
exploration of orbital and stellar wind effects on the planetary outflow, we
find three structurally distinct stellar wind regimes: weak, intermediate, and
strong. We perform synthetic Lyman- observations and find unique
observational signatures for each regime. A weak stellar
windwhich cannot confine the planetary outflow, leading to a
torus of material around the starhas a pre-transit, red-shifted
dayside arm and a slightly redward-skewed spectrum during transit. The
intermediate regime truncates the dayside outflow at large distances from the
planet and causes periodic disruptions of the outflow, producing observational
signatures that mimic a double transit. The first of these dips is blue-shifted
and precedes the optical transit. Finally, strong stellar winds completely
confine the outflow into a cometary tail and accelerate the outflow outwards,
producing large blue-shifted signals post-transit. Across all three regimes,
large signals occur far outside of transit, offering motivation to continue
ultraviolet observations outside of direct transit.Comment: 33 pages, 21 figures (7 of which have embedded movies viewable with
Adobe Acrobat Pro), Submitted to Ap
Comments: Atlantis Revisited: Recovery under Maryland Law for Purely Economic Loss against Negligent Builders and Manufacturers
Proteomic Signature of the Murine Intervertebral Disc
Low back pain is the most common musculoskeletal problem and the single most common cause of disability, often attributed to degeneration of the intervertebral disc. Lack of effective treatment is directly related to our limited understanding of the pathways responsible for maintaining disc health. While transcriptional analysis has permitted initial insights into the biology of the intervertebral disc, complete proteomic characterization is required. We therefore employed liquid chromatography electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) protein/peptide separation and mass spectrometric analyses to characterize the protein content of intervertebral discs from skeletally mature wild-type mice. A total of 1360 proteins were identified and categorized using PANTHER. Identified proteins were primarily intracellular/plasma membrane (35%), organelle (30%), macromolecular complex (10%), extracellular region (9%). Molecular function categorization resulted in three distinct categories: catalytic activity (33%), binding (molecule interactions) (29%), and structural activity (13%). To validate our list, we confirmed the presence of 14 of 20 previously identified IVD-associated markers, including matrix proteins, transcriptional regulators, and secreted proteins. Immunohistochemical analysis confirmed distinct localization patterns of select protein with the intervertebral disc. Characterization of the protein composition of healthy intervertebral disc tissue is an important first step in identifying cellular processes and pathways disrupted during aging or disease progression
Forward velocity effects on fan noise and the influence of inlet aeroacoustic design as measured in the NASA Ames 40 x 80 foot wind tunnel
The inlet radiated noise of a turbofan engine was studied. The principal research objectives were to characterize or suppress such noise with particular regard to its tonal characteristics. The major portion of this research was conducted by using ground-based static testing without simulation of aircraft forward speed or aircraft installation-related aeroacoustic effects
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