810 research outputs found
Spin waves in quasi-equilibrium spin systems
Using the Landau Fermi liquid theory we have discovered a new regime for the
propagation of spin waves in a quasi-equilibrium spin systems. We have
determined the dispersion relation for the transverse spin waves and found that
one of the modes is gapless. The gapless mode corresponds to the precessional
mode of the magnetization in a paramagnetic system in the absence of an
external magnetic field. One of the other modes is gapped which is associated
with the precession of the spin current around the internal field. The gapless
mode has a quadratic dispersion leading to some interesting thermodynamic
properties including a contribution to the specific heat. We also
show that these modes make significant contributions to the dynamic structure
function.Comment: 4 pages, 3 figure
Stellar Chemical Abundances: In Pursuit of the Highest Achievable Precision
The achievable level of precision on photospheric abundances of stars is a
major limiting factor on investigations of exoplanet host star characteristics,
the chemical histories of star clusters, and the evolution of the Milky Way and
other galaxies. While model-induced errors can be minimized through the
differential analysis of spectrally similar stars, the maximum achievable
precision of this technique has been debated. As a test, we derive differential
abundances of 19 elements from high-quality asteroid-reflected solar spectra
taken using a variety of instruments and conditions. We treat the solar spectra
as being from unknown stars and use the resulting differential abundances,
which are expected to be zero, as a diagnostic of the error in our
measurements. Our results indicate that the relative resolution of the target
and reference spectra is a major consideration, with use of different
instruments to obtain the two spectra leading to errors up to 0.04 dex. Use of
the same instrument at different epochs for the two spectra has a much smaller
effect (~0.007 dex). The asteroid used to obtain the solar standard also has a
negligible effect (~0.006 dex). Assuming that systematic errors from the
stellar model atmospheres have been minimized, as in the case of solar twins,
we confirm that differential chemical abundances can be obtained at sub-0.01
dex precision with due care in the observations, data reduction and abundance
analysis.Comment: Accepted for publication in ApJ; 13 pages, 6 figures, 7 table
Absence of Wigner Crystallization in Graphene
Graphene, a single sheet of graphite, has attracted tremendous attention due
to recent experiments which demonstrate that carriers in it are described by
massless fermions with linear dispersion. In this note, we consider the
possibility of Wigner crystallization in graphene in the absence of external
magnetic field. We show that the ratio of potential and kinetic energy is
independent of the carrier density, the tuning parameter that usually drives
Wigner crystallization and find out that for given material parameters
(dielectric constant and Fermi velocity), Wigner crystallization is not
possible. We comment on the how these results change in the presence of a
strong external magnetic field.Comment: 3 pages, 1 figure,Submitted for PR
Large crowding zones in peripheral vision for briefly presented stimuli
YesWhen a target is flanked by distractors, it becomes more
difficult to identify. In the periphery, this crowding effect
extends over a wide range of target-flanker separations,
called the spatial extent of interaction (EoI). A recent
study showed that the EoI dramatically increases in size
for short presentation durations (Chung & Mansfield,
2009). Here we investigate this duration-EoI relation in
greater detail and show that (a) it holds even when
visibility of the unflanked target is equated for different
durations, (b) the function saturates for durations
shorter than 30 to 80 ms, and (c) the largest EoIs
represent a critical spacing greater than 50% of
eccentricity. We also investigated the effect of same or
different polarity for targets and flankers across different
presentation durations. We found that EoIs for target
and flankers having opposite polarity (one white, the
other black) show the same temporal pattern as for
same polarity stimuli, but are smaller at all durations by
29% to 44%. The observed saturation of the EoI for shortduration
stimuli suggests that crowding follows the locus
of temporal integration. Overall, the results constrain
theories that map crowding zones to fixed spatial
extents or to lateral connections of fixed length in the
cortex.This study was supported by the ERC POSITION 324070 (PC) and a visiting professorship to Anglia Ruskin University from the Leverhulme Trust (HEB)
Ordinary and Poor People in 18th-Century Delaware
The authors and their colleagues have recently carried out excavations at three 18th-century farm sites in central Delaware. The Augustine Creek South and Thomas Dawson Sites were both occupied by ordinary property owners in the 1730 to 1770 period. The Augustine Creek North Site was occupied from about 1750 to 1810 by unknown but probably poor tenants, possibly in two chronologically separate occupations. At all three sites, architectural remains and artifact deposits were found. Analysis of these sites has focused on the question of how fully ordinary and poor people participated in the social, economic, and intellectural changes of the 19th century. The answer seems to be that they did embrace some changes, such as tea drinking, but rejected others, such as the reorganization of farms and the separation of public and private space
Pairing symmetry signatures of T1 in superconducting ferromagnets
We study the nuclear relaxation rate 1/T1 as a function of temperature for a
superconducting-ferromagnetic coexistent system using a p-wave triplet model
for the superconducting pairing symmetry. This calculation is contrasted with a
singlet s-wave one done previously, and we see for the s-wave case that there
is a Hebel-Slichter peak, albeit reduced due to the magnetization, and no peak
for the p-wave case. We then compare these results to a nuclear relaxation rate
experiment on UGe2 to determine the possible pairing symmetry signatures in
that material. It is seen that the experimental data is inconclusive to rule
out the possibility of s-wave pairing in .Comment: 4 pages, 4 figure
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