1,127 research outputs found
The Spin of Holographic Electrons at Nonzero Density and Temperature
We study the Green's function of a gauge invariant fermionic operator in a
strongly coupled field theory at nonzero temperature and density using a dual
gravity description. The gravity model contains a charged black hole in four
dimensional anti-de Sitter space and probe charged fermions. In particular, we
consider the effects of the spin of these probe fermions on the properties of
the Green's function. There exists a spin-orbit coupling between the spin of an
electron and the electric field of a Reissner-Nordstrom black hole. On the
field theory side, this coupling leads to a Rashba like dispersion relation. We
also study the effects of spin on the damping term in the dispersion relation
by considering how the spin affects the placement of the fermionic quasinormal
modes in the complex frequency plane in a WKB limit. An appendix contains some
exact solutions of the Dirac equation in terms of Heun polynomials.Comment: 27 pages, 11 figures; v2: minor changes, published versio
Halfvortices in flat nanomagnets
We discuss a new type of topological defect in XY systems where the O(2)
symmetry is broken in the presence of a boundary. Of particular interest is the
appearance of such defects in nanomagnets with a planar geometry. They are
manifested as kinks of magnetization along the edge and can be viewed as
halfvortices with winding numbers \pm 1/2. We argue that halfvortices play a
role equally important to that of ordinary vortices in the statics and dynamics
of flat nanomagnets. Domain walls found in experiments and numerical
simulations are composite objects containing two or more of these elementary
defects. We also discuss a closely related system: the two-dimensional smectic
liquid crystal films with planar boundary condition.Comment: 7 pages, 8 figures, To appear as a chapter in Les Houches summer
school on Quantum Magnetis
'Designer atoms' for quantum metrology
Entanglement is recognized as a key resource for quantum computation and
quantum cryptography. For quantum metrology, the use of entangled states has
been discussed and demonstrated as a means of improving the signal-to-noise
ratio. In addition, entangled states have been used in experiments for
efficient quantum state detection and for the measurement of scattering
lengths. In quantum information processing, manipulation of individual quantum
bits allows for the tailored design of specific states that are insensitive to
the detrimental influences of an environment. Such 'decoherence-free subspaces'
protect quantum information and yield significantly enhanced coherence times.
Here we use a decoherence-free subspace with specifically designed entangled
states to demonstrate precision spectroscopy of a pair of trapped Ca+ ions; we
obtain the electric quadrupole moment, which is of use for frequency standard
applications. We find that entangled states are not only useful for enhancing
the signal-to-noise ratio in frequency measurements - a suitably designed pair
of atoms also allows clock measurements in the presence of strong technical
noise. Our technique makes explicit use of non-locality as an entanglement
property and provides an approach for 'designed' quantum metrology
A thermodynamic unification of jamming
Fragile materials ranging from sand to fire-retardant to toothpaste are able
to exhibit both solid and fluid-like properties across the jamming transition.
Unlike ordinary fusion, systems of grains, foams and colloids jam and cease to
flow under conditions that still remain unknown. Here we quantify jamming via a
thermodynamic approach by accounting for the structural ageing and the
shear-induced compressibility of dry sand. Specifically, the jamming threshold
is defined using a non-thermal temperature that measures the 'fluffiness' of a
granular mixture. The thermodynamic model, casted in terms of pressure,
temperature and free-volume, also successfully predicts the entropic data of
five molecular glasses. Notably, the predicted configurational entropy avoids
the Kauzmann paradox entirely. Without any free parameters, the proposed
equation-of-state also governs the mechanism of shear-banding and the
associated features of shear-softening and thickness-invariance.Comment: 16 pgs double spaced. 4 figure
Observation of the thermal Casimir force
Quantum theory predicts the existence of the Casimir force between
macroscopic bodies, due to the zero-point energy of electromagnetic field modes
around them. This quantum fluctuation-induced force has been experimentally
observed for metallic and semiconducting bodies, although the measurements to
date have been unable to clearly settle the question of the correct
low-frequency form of the dielectric constant dispersion (the Drude model or
the plasma model) to be used for calculating the Casimir forces. At finite
temperature a thermal Casimir force, due to thermal, rather than quantum,
fluctuations of the electromagnetic field, has been theoretically predicted
long ago. Here we report the experimental observation of the thermal Casimir
force between two gold plates. We measured the attractive force between a flat
and a spherical plate for separations between 0.7 m and 7 m. An
electrostatic force caused by potential patches on the plates' surfaces is
included in the analysis. The experimental results are in excellent agreement
(reduced of 1.04) with the Casimir force calculated using the Drude
model, including the T=300 K thermal force, which dominates over the quantum
fluctuation-induced force at separations greater than 3 m. The plasma
model result is excluded in the measured separation range.Comment: 6 page
Mid-infrared plasmons in scaled graphene nanostructures
Plasmonics takes advantage of the collective response of electrons to
electromagnetic waves, enabling dramatic scaling of optical devices beyond the
diffraction limit. Here, we demonstrate the mid-infrared (4 to 15 microns)
plasmons in deeply scaled graphene nanostructures down to 50 nm, more than 100
times smaller than the on-resonance light wavelength in free space. We reveal,
for the first time, the crucial damping channels of graphene plasmons via its
intrinsic optical phonons and scattering from the edges. A plasmon lifetime of
20 femto-seconds and smaller is observed, when damping through the emission of
an optical phonon is allowed. Furthermore, the surface polar phonons in SiO2
substrate underneath the graphene nanostructures lead to a significantly
modified plasmon dispersion and damping, in contrast to a non-polar
diamond-like-carbon (DLC) substrate. Much reduced damping is realized when the
plasmon resonance frequencies are close to the polar phonon frequencies. Our
study paves the way for applications of graphene in plasmonic waveguides,
modulators and detectors in an unprecedentedly broad wavelength range from
sub-terahertz to mid-infrared.Comment: submitte
Understanding Outcomes in Behavior Change Interventions to Prevent Pediatric Obesity: The Role of Dose and Behavior Change Techniques
BACKGROUND: Behavioral interventions to prevent pediatric obesity have shown inconsistent results across the field. Studying what happens within the "black box" of these interventions and how differences in implementation lead to different outcomes will help researchers develop more effective interventions. AIM: To compare the implementation of three features of a phone-based intervention for parents (time spent discussing weight-related behaviors, behavior change techniques used in sessions, and intervention activities implemented by parents between sessions) with study outcomes. METHODS: A random selection of 100 parent-child dyads in the intervention arm of a phone-based obesity prevention trial was included in this analysis. Sessions were coded for overall session length, length of time spent discussing specific weight-related behaviors, number of behavior change techniques used during the sessions, and number of intervention-recommended activities implemented by the parents between sessions (e.g., parent-reported implementation of behavioral practice/rehearsal between sessions). The primary study outcome, prevention of unhealthy increase in child body mass index (BMI) percentile, was measured at baseline and 12 months. RESULTS: Overall session length was associated with decreases in child BMI percentile ( b = -0.02, p = .01). There was no association between the number of behavior change techniques used in the sessions and decreases in child BMI percentile ( b = -0.29, p = .27). The number of activities the parents reported implementing between sessions was associated with decreases in child BMI percentile ( b = -1.25, p = .02). DISCUSSION: To improve future interventions, greater attention should be paid to the intended and delivered session length, and efforts should be made to facilitate parents' implementation of intervention-recommended activities between sessions (ClinicalTrials.gov, No. NCT01084590)
Open Problems on Central Simple Algebras
We provide a survey of past research and a list of open problems regarding
central simple algebras and the Brauer group over a field, intended both for
experts and for beginners.Comment: v2 has some small revisions to the text. Some items are re-numbered,
compared to v
Feasibility of standardized methods to specify behavioral pediatric obesity prevention interventions
Standardized methods are needed to evaluate what occurs within the 'black box' of behavioral interventions to prevent pediatric obesity. The purpose of this research is to evaluate methods to specify the behavior change techniques used and the amount of time spent discussing target weight-related behaviors in an intervention for parents of children at risk for becoming overweight or obese. Independent coders were trained to identify behavior change techniques and time spent discussing weight-related behaviors in audio recordings and transcripts of intervention sessions from 100 randomly selected participants. The behavior change technique taxonomy (BCTTv1) was used to code techniques present in sessions. A newly-developed tool was used to code time spent discussing each target weight-related behavior (e.g., physical activity, screen time). Sessions from a subset of these participants (NÂ =Â 20) were double coded to evaluate inter-rater reliability. After revisions to coding protocols, coders reliably coded behavior change techniques used and time spent discussing target weight-related behaviors in sessions from the subset of 20 participants. The most commonly discussed target weight-related behavior was physical activity followed by energy intake and fruit and vegetable intake. On average, 13.9 (SDÂ =Â 2.8) unique behavior change techniques were present across sessions for a given participant. These results offer reliable methods for systematically identifying behavior change techniques used and time spent discussing weight-related behaviors in a pediatric obesity prevention intervention. This work paves the way for future research to identify which specific target behaviors and techniques are most associated with the prevention of unhealthy weight gain in children
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