2,038 research outputs found
Optical Phonons in Carbon Nanotubes: Kohn Anomalies, Peierls Distortions and Dynamic Effects
We present a detailed study of the vibrational properties of Single Wall
Carbon Nanotubes (SWNTs). The phonon dispersions of SWNTs are strongly shaped
by the effects of electron-phonon coupling. We analyze the separate
contributions of curvature and confinement. Confinement plays a major role in
modifying SWNT phonons and is often more relevant than curvature. Due to their
one-dimensional character, metallic tubes are expected to undergo Peierls
distortions (PD) at T=0K. At finite temperature, PD are no longer present, but
phonons with atomic displacements similar to those of the PD are affected by
strong Kohn anomalies (KA). We investigate by Density Functional Theory (DFT)
KA and PD in metallic SWNTs with diameters up to 3 nm, in the electronic
temperature range from 4K to 3000 K. We then derive a set of simple formulas
accounting for all the DFT results. Finally, we prove that the static approach,
commonly used for the evaluation of phonon frequencies in solids, fails because
of the SWNTs reduced dimensionality. The correct description of KA in metallic
SWNTs can be obtained only by using a dynamical approach, beyond the adiabatic
Born-Oppenheimer approximation, by taking into account non-adiabatic
contributions. Dynamic effects induce significant changes in the occurrence and
shape of Kohn anomalies. We show that the SWNT Raman G peak can only be
interpreted considering the combined dynamic, curvature and confinement
effects. We assign the G+ and G- peaks of metallic SWNTs to TO
(circumferential) and LO (axial) modes, respectively, the opposite of
semiconducting SWNTs.Comment: 24 pages, 21 figures, submitted to Phys. Rev.
The Dynamics and Metallicity Distribution of the Distant Dwarf Galaxy VV124
VV124 (UGC 4879) is an isolated, dwarf irregular/dwarf spheroidal (dIrr/dSph) transition-type
galaxy at a distance of 1.36 Mpc. Previous low-resolution spectroscopy yielded inconsistent radial
velocities for different components of the galaxy, and photometry hinted at the presence of a stellar
disk. In order to quantify the stellar dynamics, we observed individual red giants in VV124 with the Keck/DEIMOS spectrograph. We validated members based on their positions in the color-magnitude
diagram, radial velocities, and spectral features. Our sample contains 67 members. The average
radial velocity is = â29.1 ± 1.3 km s^(â1), in agreement with the previous radio measurements
of H I gas. The velocity distribution is Gaussian, indicating that VV124 is supported primarily by
velocity dispersion inside a radius of 1.5 kpc. Outside that radius, our measurements provide only an
upper limit of 8.6 km s^(â1) on any rotation in the photometric disk-like feature. The velocity dispersion
is Ï_v = 9.4± 1.0 km s^(â1), from which we inferred a mass of M_(1/2) = (2.1 ± 0.2)Ă 10^7 M_â and a mass-to-
light ratio of (M/L_V)_(1/2) = 5.2 ± 1.1 M_â/L_â, both measured within the half-light radius. Thus,
VV124 contains dark matter. We also measured the metallicity distribution from neutral iron lines.
The average metallicity, = â1.14 ± 0.06, is consistent with the mass-metallicity relation
defined by dwarf spheroidal galaxies. The dynamics and metallicity distribution of VV124 appear
similar to dSphs of similar stellar mass
Care that Matters: Quality Measurement and Health Care
Barry Saver and colleagues caution against the use of process and performance metrics as health care quality measures in the United States
The Radial Distribution of the Interstellar Medium in Disk Galaxies: Evidence for Secular Evolution
One possible way for spiral galaxies to internally evolve would be for gas to flow to the center and form stars in a central disk (pseudo-bulge). If the inflow rate is faster than the rate of star formation, a central concentration of gas will form. In this paper we present radial profiles of stellar and 8 ÎŒm emission from polycyclic aromatic hydrocarbons (PAHs) for 11 spiral galaxies to investigate whether the interstellar medium in these galaxies contains a central concentration above that expected from the exponential disk. In general, we find that the two-dimensional CO and PAH emission morphologies are similar, and that they exhibit similar radial profiles. We find that in 6 of the 11 galaxies there is a central excess in the 8 ÎŒm and CO emission above the inward extrapolation of an exponential disk. In particular, all four barred galaxies in the sample have strong central excesses in both 8 ÎŒm and CO emission. These correlations suggest that the excess seen in the CO profiles is, in general, not simply due to a radial increase in the CO emissivity. In the inner disk, the ratio of the stellar to the 8 ÎŒm radial surface brightness is similar for 9 of the 11 galaxies, suggesting a physical connection between the average stellar surface brightness and the average gas surface brightness at a given radius. We also find that the ratio of the CO to 8 ÎŒm PAH surface brightness is consistent over the sample, implying that the 8 ÎŒm PAH surface brightness can be used as an approximate tracer of the interstellar medium
A quantum Monte Carlo study of the one-dimensional ionic Hubbard model
Quantum Monte Carlo methods are used to study a quantum phase transition in a
1D Hubbard model with a staggered ionic potential (D). Using recently
formulated methods, the electronic polarization and localization are determined
directly from the correlated ground state wavefunction and compared to results
of previous work using exact diagonalization and Hartree-Fock. We find that the
model undergoes a thermodynamic transition from a band insulator (BI) to a
broken-symmetry bond ordered (BO) phase as the ratio of U/D is increased. Since
it is known that at D = 0 the usual Hubbard model is a Mott insulator (MI) with
no long-range order, we have searched for a second transition to this state by
(i) increasing U at fixed ionic potential (D) and (ii) decreasing D at fixed U.
We find no transition from the BO to MI state, and we propose that the MI state
in 1D is unstable to bond ordering under the addition of any finite ionic
potential. In real 1D systems the symmetric MI phase is never stable and the
transition is from a symmetric BI phase to a dimerized BO phase, with a
metallic point at the transition
Standing Variation and the Capacity for Change: Are Endocrine Phenotypes More Variable That Other Traits?
Circulating steroid hormone levels exhibit high variation both within and between individuals, leading some to hypothesize that these phenotypes are more variable than other morphological, physiological, and behavioral traits. This should have profound implications for the evolution of steroid signaling systems, but few studies have examined how endocrine variation compares to that of other traits or differs among populations. Here we provide such an analysis by first exploring how variation in three measures of corticosterone (CORT)âbaseline, stress-induced, and post-dexamethasone injectionâcompares to variation in key traits characterizing morphology (wing length, mass), physiology (reactive oxygen metabolite concentration [d-ROMs] and antioxidant capacity), and behavior (provisioning rate) in two populations of tree swallow (Tachycineta bicolor). After controlling for measurement precision and within-individual variation, we found that only post-dex CORT was more variable than all other traits. Both baseline and stress-induced CORT exhibit higher variation than antioxidant capacity and provisioning rate, but not oxidative metabolite levels or wing length. Variation in post-dex CORT and d-ROMs was also elevated in the higher-latitude population in that inhabits a less predictable environment. We next studied how these patterns might play out on a macroevolutionary scale, assessing patterns of variation in baseline testosterone (T) and multiple non-endocrine traits (body length, mass, social display rate, and locomotion rate) across 17 species of Anolis lizards. At the macroevolutionary level, we found that circulating T levels and the rate of social display output are higher than other behavioral and morphological traits. Altogether, our results support the idea that within-population variability in steroid levels is substantial, but not exceptionally higher than many other traits that define animal phenotypes. As such, circulating steroid levels in free-living animals should be considered traits that exhibit similar levels of variability from individual to individual in a population
Irrelevant operators in the two-dimensional Ising model
By using conformal-field theory, we classify the possible irrelevant
operators for the Ising model on the square and triangular lattices. We analyze
the existing results for the free energy and its derivatives and for the
correlation length, showing that they are in agreement with the conformal-field
theory predictions. Moreover, these results imply that the nonlinear scaling
field of the energy-momentum tensor vanishes at the critical point. Several
other peculiar cancellations are explained in terms of a number of general
conjectures. We show that all existing results on the square and triangular
lattice are consistent with the assumption that only nonzero spin operators are
present.Comment: 32 pages. Added comments and reference
Where does Cosmological Perturbation Theory Break Down?
We apply the effective field theory approach to the coupled metric-inflaton
system, in order to investigate the impact of higher dimension operators on the
spectrum of scalar and tensor perturbations in the short-wavelength regime. In
both cases, effective corrections at tree-level become important when the
Hubble parameter is of the order of the Planck mass, or when the physical wave
number of a cosmological perturbation mode approaches the square of the Planck
mass divided by the Hubble constant. Thus, the cut-off length below which
conventional cosmological perturbation theory does not apply is likely to be
much smaller than the Planck length. This has implications for the
observability of "trans-Planckian" effects in the spectrum of primordial
perturbations.Comment: 25 pages, uses FeynM
Quantitative Analysis of LISA Pathfinder Test Mass Noise
In this paper we discuss two main problems associated with the analysis of
the data from LISA Pathfinder (LPF): i) Excess noise detection and ii) Noise
parameter identification. The mission is focused on the low frequency region
([0.1; 10] mHz) of the available signal spectrum. In such a region the signal
is dominated by the force noise acting on test masses. Noise analysis is
expected to deal with a limited amount of non-Gaussian data, since the spectrum
statistics will be far from Gaussian and the lowest available frequency is
limited by the data length. In this paper we analyze the details of the
expected statistics for spectral data and develop two suitable excess noise
estimators. One is based on the statistical properties of the integrated
spectrum, the other is based on Kolmogorov-Smirnov test. The sensitivity of the
estimators is discussed theoretically for independent data, then the algorithms
are tested on LPF synthetic data. The test on realistic LPF data allows the
effect of spectral data correlations on the efficiency of the different noise
excess estimators to be highlighted. It also reveals the versatility of the
Kolmogorov-Smirnov approach, which can be adapted to provide reasonable results
on correlated data from a modified version of the standard equations for the
inversion of the test statistic. Closely related to excess noise detection, the
problem of noise parameter identification in non-Gaussian data is approached in
two ways. One procedure is based on maximum likelihood estimator and another is
based on the Kolmogorov-Smirnov goodness of fit estimator. Both approaches
provide unbiased and accurate results for noise parameter estimation and
demonstrate superior performance with respect to standard weighted
least-squares and Huber's norm.Comment: Accepted for publication in Phys Rev
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