9,768 research outputs found
Keldysh-Rutherford model for attoclock
We demonstrate a clear similarity between attoclock offset angles and
Rutherford scattering angles taking the Keldysh tunnelling width as the impact
parameter and the vector potential of the driving pulse as the asymptotic
velocity. This simple model is tested against the solution of the
time-dependent Schr\"odinger equation using hydrogenic and screened (Yukawa)
potentials of equal binding energy. We observe a smooth transition from a
hydrogenic to 'hard-zero' intensity dependence of the offset angle with
variation of the Yukawa screening parameter. Additionally we make comparison
with the attoclock offset angles for various noble gases obtained with the
classical-trajectory Monte Carlo method. In all cases we find a close
correspondence between the model predictions and numerical calculations. This
suggests a largely Coulombic origin of the attoclock offset angle and casts
further doubt on its interpretation in terms of a finite tunnelling time
Survival of a diffusing particle in an expanding cage
We consider a Brownian particle, with diffusion constant D, moving inside an
expanding d-dimensional sphere whose surface is an absorbing boundary for the
particle. The sphere has initial radius L_0 and expands at a constant rate c.
We calculate the joint probability density, p(r,t|r_0), that the particle
survives until time t, and is at a distance r from the centre of the sphere,
given that it started at a distance r_0 from the centre.Comment: 5 page
Defect energy of infinite-component vector spin glasses
We compute numerically the zero temperature defect energy, Delta E, of the
vector spin glass in the limit of an infinite number of spin components m, for
a range of dimensions 2 <= d <= 5. Fitting to Delta E ~ L^theta, where L is the
system size, we obtain: theta = -1.54 (d=2), theta = -1.04 (d=3), theta = -0.67
(d=4) and theta = -0.37 (d=5). These results show that the lower critical
dimension, d_l (the dimension where theta changes sign), is significantly
higher for m=infinity than for finite m (where 2 < d_l < 3).Comment: 4 pages, 5 figure
Electrostatic interactions between discrete helices of charge
We analytically examine the pair interaction for parallel, discrete helices
of charge. Symmetry arguments allow for the energy to be decomposed into a sum
of terms, each of which has an intuitive geometric interpretation. Truncated
Fourier expansions for these terms allow for accurate modeling of both the
axial and azimuthal terms in the interaction energy and these expressions are
shown to be insensitive to the form of the interaction. The energy is evaluated
numerically through application of an Ewald-like summation technique for the
particular case of unscreened Coulomb interactions between the charges of the
two helices. The mode structures and electrostatic energies of flexible helices
are also studied. Consequences of the resulting energy expressions are
considered for both F-actin and A-DNA aggregates
Spin glasses in the limit of an infinite number of spin components
We consider the spin glass model in which the number of spin components, m,
is infinite. In the formulation of the problem appropriate for numerical
calculations proposed by several authors, we show that the order parameter
defined by the long-distance limit of the correlation functions is actually
zero and there is only "quasi long range order" below the transition
temperature. We also show that the spin glass transition temperature is zero in
three dimensions.Comment: 9 pages, 13 figure
Evidence for existence of many pure ground states in 3d Spin Glasses
Ground states of 3d EA Ising spin glasses are calculated for sizes up to
using a combination of genetic algorithms and cluster-exact
approximation . The distribution of overlaps is calculated. For
increasing size the width of converges to a nonzero value, indicating
that many pure ground states exist for short range Ising spin glasses.Comment: 4 pages, 3 figures, 2 tables, 16 reference
Critical properties of the unconventional spin-Peierls system TiOBr
We have performed detailed x-ray scattering measurements on single crystals
of the spin-Peierls compound TiOBr in order to study the critical properties of
the transition between the incommensurate spin-Peierls state and the
paramagnetic state at Tc2 ~ 48 K. We have determined a value of the critical
exponent beta which is consistent with the conventional 3D universality
classes, in contrast with earlier results reported for TiOBr and TiOCl. Using a
simple power law fit function we demonstrate that the asymptotic critical
regime in TiOBr is quite narrow, and obtain a value of beta_{asy} = 0.32 +/-
0.03 in the asymptotic limit. A power law fit function which includes the first
order correction-to-scaling confluent singularity term can be used to account
for data outside the asymptotic regime, yielding a more robust value of
beta_{avg} = 0.39 +/- 0.05. We observe no evidence of commensurate fluctuations
above Tc1 in TiOBr, unlike its isostructural sister compound TiOCl. In
addition, we find that the incommensurate structure between Tc1 and Tc2 is
shifted in Q-space relative to the commensurate structure below Tc1.Comment: 12 pages, 8 figures. Submitted to Physical Review
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Publication rates on the topic of racial and ethnic diversity in dermatology versus other specialties
Background: The population of the U.S. is becoming more diverse every year. The field of dermatology is not following the same trend. Objective: To assess the promotion of diversity in the field of dermatology by analyzing publications focused on diversity, compared to other specialties. Methods: The PubMed database was systematically searched to identify publications focused on diversity from January 2008 to July 2019. The search criteria were as follows: dermatology/radiology/ophthalmology/ anesthesiology/orthopedic surgery/family medicine/ internal medicine/general surgery AND diversity/ diverse/racial/race/ethnic/ethnicity/cultural/culture/competency/competence. Comparisons were made using single-factor ANOVA and two-group t-tests. A qualitative analysis was performed for publications in the field of dermatology. Results: From January 2016 to July 2019, there were 25 publications focused on diversity in dermatology (Mean=6.25, SD=2.06), compared to 6 in radiology (Mean=1.50, SD=1.29, P=0.01), two in ophthalmology (Mean=0.50, SD=0.58, P=0.01), two in anesthesiology (Mean=0.50, SD=1.00, P=0.01), 12 in orthopedic surgery (Mean=3.00, SD=1.41, P=0.04), 23 in family medicine (Mean=5.75, SD=2.22, P=0.75), 9 in internal medicine (Mean=2.25, SD=1.71, P=0.02), and 7 in general surgery (Mean=1.75, SD=0.50, P=0.02). Conclusions: Although the field of dermatology has suffered from a lack of racial/ethnic diversity, efforts to promote diversity via increased publications in the last four years have been stronger in dermatology compared to many other fields
Evidence for the droplet/scaling picture of spin glasses
We have studied the Parisi overlap distribution for the three dimensional
Ising spin glass in the Migdal-Kadanoff approximation. For temperatures T
around 0.7Tc and system sizes upto L=32, we found a P(q) as expected for the
full Parisi replica symmetry breaking, just as was also observed in recent
Monte Carlo simulations on a cubic lattice. However, for lower temperatures our
data agree with predictions from the droplet or scaling picture. The failure to
see droplet model behaviour in Monte Carlo simulations is due to the fact that
all existing simulations have been done at temperatures too close to the
transition temperature so that sytem sizes larger than the correlation length
have not been achieved.Comment: 4 pages, 6 figure
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