818 research outputs found
Phonon-phonon interactions and phonon damping in carbon nanotubes
We formulate and study the effective low-energy quantum theory of interacting
long-wavelength acoustic phonons in carbon nanotubes within the framework of
continuum elasticity theory. A general and analytical derivation of all three-
and four-phonon processes is provided, and the relevant coupling constants are
determined in terms of few elastic coefficients. Due to the low dimensionality
and the parabolic dispersion, the finite-temperature density of noninteracting
flexural phonons diverges, and a nonperturbative approach to their interactions
is necessary. Within a mean-field description, we find that a dynamical gap
opens. In practice, this gap is thermally smeared, but still has important
consequences. Using our theory, we compute the decay rates of acoustic phonons
due to phonon-phonon and electron-phonon interactions, implying upper bounds
for their quality factor.Comment: 15 pages, 2 figures, published versio
Limitations for change detection in multiple Gabor targets
We investigate the limitations on the ability to detect when a target has changed, using Gabor targets as simple quantifiable stimuli. Using a partial report technique to equalise response variables, we show that the log of the Weber fraction for detecting a spatial frequency change is proportional to the log of the number of targets, with a set-size effect that is greater than that reported for visual search. This is not a simple perceptual limitation, because pre-cueing a single target out of four restores performance to the level found when only one target is present. It is argued that the primary limitation on performance is the division of attention across multiple targets, rather than decay within visual memory. However in a simplified change detection experiment without cueing, where only one target of the set changed, not only was the set size effect still larger, but it was greater at 2000 msec ISI than at 250 msec ISI, indicating a possible memory component. The steepness of the set size effects obtained suggests that even moderate complexity of a stimulus in terms of number of component objects can overload attentional processes, suggesting a possible low-level mechanism for change blindness
Anharmonic Decay of Vibrational States in Amorphous Silicon
Anharmonic decay rates are calculated for a realistic atomic model of
amorphous silicon. The results show that the vibrational states decay on
picosecond timescales and follow the two-mode density of states, similar to
crystalline silicon, but somewhat faster. Surprisingly little change occurs for
localized states. These results disagree with a recent experiment.Comment: 10 pages, 4 Postscript figure
Entangled Rings
Consider a ring of N qubits in a translationally invariant quantum state. We
ask to what extent each pair of nearest neighbors can be entangled. Under
certain assumptions about the form of the state, we find a formula for the
maximum possible nearest-neighbor entanglement. We then compare this maximum
with the entanglement achieved by the ground state of an antiferromagnetic ring
consisting of an even number of spin-1/2 particles. We find that, though the
antiferromagnetic ground state does not maximize the nearest-neighbor
entanglement relative to all other states, it does so relative to other states
having zero z-component of spin.Comment: 19 pages, no figures; v2 includes new results; v3 corrects a
numerical error for the case N=
One-Liners
One liners from: N.M. Martinez-Rossi, C. Andrade-Monteiro and S.R.C. Pombeiro; M. Orbach ; H. Liu and TJ. Schmidhauser; P.A. Hubbard and C.H. Wilso
Between feminism and anorexia: An autoethnography
Critical feminist work on eating disorders has grown substantially since its establishment in the 1980s, and has increasingly incorporated the use of anorexic stories, voices and experiences. Yet rarely do such accounts offer the anorexic a space to respond to the now established feminist conceptions of the problem which structure the books or articles in which they appear. Anorexic, or recovered anorexic, voices are used by the researcher to interpret the role played by gender, even whilst the subjects are invited to respond to and critique, medical and popular discourses on the disorder. This lack of dialogue is all the more striking in the context of the feminist aim to fight ‘back against the tendency to silence anorexic women’s’ own interpretations of their starving, treatment and construction (Saukko, 2008: 34). As someone who suffered from anorexia for 20 years, this article offers an autoethnographic account of my experience of encountering the feminist literature on anorexia in a bid to speak back, or enter into a dialogue between feminist politics and eating disorder experience
Violation of the string hypothesis and Heisenberg XXZ spin chain
In this paper we count the numbers of real and complex solutions to Bethe
constraints in the two particle sector of the XXZ model. We find exact number
of exceptions to the string conjecture and total number of solutions which is
required for completeness.Comment: 15 pages, 7 Postscript figure
High frequency sound waves in vitreous silica
We report a molecular dynamics simulation study of the sound waves in
vitreous silica in the mesoscopic exchanged momentum range. The calculated
dynamical structure factors are in quantitative agreement with recent
experimental inelastic neutron and x-ray scattering data. The analysis of the
longitudinal and transverse current spectra allows to discriminate between
opposite interpretations of the existing experimental data in favour of the
propagating nature of the high frequency sound waves.Comment: 4 pages, Revtex, 4 ps figures; to be published in Phys. Rev. Lett.,
February 198
Deconstructing “real” women: Young women's readings of advertising images of “plus-size” models in the UK
© 2016, © The Author(s) 2016. Critical feminist researchers and others have amply elucidated the perniciousness of contemporary Western beauty ideals and, particularly, the near-ubiquitous idealisations of slenderness. In this context, the advent of media images featuring “plus-size” models has been rightly heralded as a welcome challenge to this hegemony. Yet, little attention has been given to women's interpretations of these images. In this brief report, we outline a preliminary exploration of young women's views about advertising images featuring “plus-size” models in the UK. We used a discourse analytic method to analyse 35 young women's responses to a qualitative questionnaire asking for their views and feelings about three adverts featuring “plus-size” models. Our analysis suggests that, while the models were positively construed, participants also drew on distinctly conservative notions of femininity such that romanticised constructions of a “plus-sized”, traditional and domestic femininity were contrasted with a highly pejorative framing of “stick thin” women as vain, vindictive and self-obsessed. Our analysis thus indicates how representations of women focusing on body weight and shape can, even when reclaiming “fat” or “plus-size” bodies, mobilise derogatory and constricting rather than empowering constructions of femininity
Magnetic Properties of Weakly Doped Antiferromagnets
We study the spin excitations and the transverse susceptibility of a
two-dimensional antiferromagnet doped with a small concentration of holes in
the t-J model. The motion of holes generates a renormalization of the magnetic
properties. The Green's functions are calculated in the self-consistent Born
approximation. It is shown that the long-wavelength spin waves are
significantly softened and the shorter-wavelength spin waves become strongly
damped as the doping increases. The spin wave velocity is reduced by the
coherent motion of holes, and not increased as has been claimed elsewhere. The
transverse susceptibility is found to increase considerably with doping, also
as a result of coherent hole motion. Our results are in agreement with
experimental data for the doped copper oxide superconductors.Comment: 20 page
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