10,502 research outputs found
The role of the disorder range and electronic energy in the graphene nanoribbons perfect transmission
Numerical calculations based on the recursive Green's functions method in the
tight-binding approximation are performed to calculate the dimensionless
conductance in disordered graphene nanoribbons with Gaussian scatterers.
The influence of the transition from short- to long-ranged disorder on is
studied as well as its effects on the formation of a perfectly conducting
channel. We also investigate the dependence of electronic energy on the
perfectly conducting channel. We propose and calculate a backscattering
estimative in order to establish the connection between the perfectly
conducting channel (with ) and the amount of intervalley scattering.Comment: 7 pages, 9 figures. To be published on Phys. Rev.
Measuring Dislocation Density in Aluminum with Resonant Ultrasound Spectroscopy
Dislocations in a material will, when present in enough numbers, change the
speed of propagation of elastic waves. Consequently, two material samples,
differing only in dislocation density, will have different elastic constants, a
quantity that can be measured using Resonant Ultrasound Spectroscopy.
Measurements of this effect on aluminum samples are reported. They compare well
with the predictions of the theory.Comment: 4 pages, 2 figure
Evidence for conventional superconductivity in SrPd2Ge2
Electronic structure of SrPd2Ge2 single crystals is studied by angle-resolved
photoemission spectroscopy (ARPES), scanning tunneling spectroscopy (STS) and
band-structure calculations within the local-density approximation (LDA). The
STS measurements show single s-wave superconducting energy gap \Delta(0) = 0.5
meV. Photon-energy dependence of the observed Fermi surface reveals a strongly
three-dimensional character of the corresponding electronic bands. By comparing
the experimentally measured and calculated Fermi velocities a renormalization
factor of 0.95 is obtained, which is much smaller than typical values reported
in Fe-based superconductors. We ascribe such an unusually low band
renormalization to the different orbital character of the conduction electrons
and using ARPES and STS data argue that SrPd2Ge2 is likely to be a conventional
superconductor, which makes it clearly distinct from isostructural iron
pnictide superconductors of the "122" family.Comment: submitted to PR
Perturbative expansions from Monte Carlo simulations at weak coupling: Wilson loops and the static-quark self-energy
Perturbative coefficients for Wilson loops and the static-quark self-energy
are extracted from Monte Carlo simulations at weak coupling. The lattice
volumes and couplings are chosen to ensure that the lattice momenta are all
perturbative. Twisted boundary conditions are used to eliminate the effects of
lattice zero modes and to suppress nonperturbative finite-volume effects due to
Z(3) phases. Simulations of the Wilson gluon action are done with both periodic
and twisted boundary conditions, and over a wide range of lattice volumes (from
to ) and couplings (from to ).
A high precision comparison is made between the simulation data and results
from finite-volume lattice perturbation theory. The Monte Carlo results are
shown to be in excellent agreement with perturbation theory through second
order. New results for third-order coefficients for a number of Wilson loops
and the static-quark self-energy are reported.Comment: 36 pages, 15 figures, REVTEX documen
Establishment of introduced reptiles increases with the presence and richness of native congeners
Darwin proposed two contradictory hypotheses to explain the influence of congeners on the outcomes of invasion: the naturalization hypothesis, which predicts a negative relationship between the presence of congeners and invasion success, and the pre-adaptation hypothesis, which predicts a positive relationship between the presence of congeners and invasion success. Studies testing these hypotheses have shown mixed support. We tested these hypotheses using the establishment success of non-native reptiles and congener presence/absence and richness across the globe. Our results demonstrated support for the pre-adaptation hypothesis. We found that globally, both on islands and continents, establishment success was higher in the presence than in the absence of congeners and that establishment success increased with increasing congener richness. At the life form level, establishment success was higher for lizards, marginally higher for snakes, and not different for turtles in the presence of congeners; data were insufficient to test the hypotheses for crocodiles. There was no relationship between establishment success and congener richness for any life form. We suggest that we found support for the pre-adaptation hypothesis because, at the scale of our analysis, native congeners represent environmental conditions appropriate for the species rather than competition for niche space. Our results imply that areas to target for early detection of non-native reptiles are those that host closely related species
On the Transport Properties of a Quark-Hadron Coulomb Lattice in the Cores of Neutron Stars
Already more that 40 years ago, it has been suggested that because of the
enormous mass densities in the cores of neutron stars, the hadrons in the
centers of neutron stars may undergo a phase transition to deconfined quark
matter. In this picture, neutron stars could contain cores made of pure (up,
down, strange) quark matter which are surrounded by a mixed phase of quarks and
hadrons. More than that, because of the competition between the Coulomb and the
surface energies associated with the positively charged regions of nuclear
matter and negatively charged regions of quark matter, the mixed phase may
develop geometrical structures, similarly to what is expected of the
sub-nuclear liquid-gas phase transition. In this paper we restrict ourselves to
considering the formation of rare phase blobs in the mixed quark-hadron phase.
The influence of rare phase blobs on the thermal and transport properties of
neutron star matter is investigated. The total specific heat, , thermal
conductivity, , and electron-blob Bremsstrahlung neutrino emissivities,
, of quark-hybrid matter are computed and the results
are compared with the associated thermal and transport properties of standard
neutron star matter. Our results show that the contribution of rare phase blobs
to the specific heat is negligibly small. This is different for the neutrino
emissivity from electron-blob Bremsstrahlung scattering, which turns out to be
of the same order of magnitude as the total contributions from other
Bremsstrahlung processes for temperatures below about K.Comment: minor changes, accepted by Phys. Rev.
Density Variations in the NW Star Stream of M31
The Pan Andromeda Archeological Survey (PAndAS) CFHT Megaprime survey of the
M31-M33 system has found a star stream which extends about 120 kpc NW from the
center of M31. The great length of the stream, and the likelihood that it does
not significantly intersect the disk of M31, means that it is unusually well
suited for a measurement of stream gaps and clumps along its length as a test
for the predicted thousands of dark matter sub-halos. The main result of this
paper is that the density of the stream varies between zero and about three
times the mean along its length on scales of 2 to 20 kpc. The probability that
the variations are random fluctuations in the star density is less than 10^-5.
As a control sample we search for density variations at precisely the same
location in stars with metallicity higher than the stream, [Fe/H]=[0, -0.5] and
find no variations above the expected shot noise. The lumpiness of the stream
is not compatible with a low mass star stream in a smooth galactic potential,
nor is it readily compatible with the disturbance caused by the visible M31
satellite galaxies. The stream's density variations appear to be consistent
with the effects of a large population of steep mass function dark matter
sub-halos, such as found in LCDM simulations, acting on an approximately 10Gyr
old star stream. The effects of a single set of halo substructure realizations
are shown for illustration, reserving a statistical comparison for another
study.Comment: ApJ revised version submitte
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