6,155 research outputs found
The Bose polaron problem: effect of mass imbalance on binding energy
By means of Quantum Monte Carlo methods we calculate the binding energy of an
impurity immersed in a Bose-Einstein condensate at T = 0. The focus is on the
attractive branch of the Bose polaron and on the role played by the mass
imbalance between the impurity and the surrounding particles. For an impurity
resonantly coupled to the bath, we investigate the dependence of the binding
energy on the mass ratio and on the interaction strength within the medium. In
particular, we determine the equation of state in the case of a static
(infinite mass) impurity, where three-body correlations are irrelevant and the
result is expected to be a universal function of the gas parameter. For the
mass ratio corresponding to K impurities in a gas of Rb atoms we
provide an explicit comparison with the experimental findings of a recent study
carried out at JILA.Comment: 5 pages, 3 figure
Ionic polaron in a Bose-Einstein condensate
The ground state properties of a degenerate bosonic gas doped with an ion are
investigated by means of quantum Monte Carlo simulations in three dimensions.
The system features competing length and energy scales, which result in vastly
different polaronic properties compared to neutral quantum impurities.
Depending on whether a two-body bound state is supported or not by the atom-ion
potential, we identify a transition between a polaron regime amenable to a
perturbative treatment in the limit of weak atom-ion interactions and a
many-body bound state with vanishing quasi-particle residue composed of
hundreds of atoms. In order to analyze the structure of the corresponding
states we examine the atom-ion and atom-atom correlation functions. Our
findings are directly relevant to experiments using hybrid atom-ion setups that
have recently attained the ultracold regime.Comment: 11 pages, 6 figures, 1 tabl
Fifteen years follow-up of photorefractive keratectomy up to 10 D of myopia: outcomes and analysis of the refractive regression
PURPOSE:
To evaluate outcomes of photorefractive keratectomy up to -10.00 D of myopia and -4.50 of astigmatism and to develop a predictive model for the refractive changes in the long term.
SETTING:
Vissum Corporation and Miguel Hernandez University (Alicante, Spain).
DESIGN:
Retrospective-prospective observational series of cases.
METHODS:
This study included 33 eyes of 33 patients aged 46.79±7.04 years (range 40-57) operated with the VISX 20/20 excimer laser with optical zones of 6 mm. No mitomycin C was used in any of these cases. The minimum follow-up was 15 years. The main outcome measures were: uncorrected and corrected distance visual acuity, manifest refraction and corneal topography. Linear regression models were developed from the observed refractive changes over time.
RESULTS:
Safety and efficacy indexes at 15 years were 1.18 and 0.83, respectively. No statistically significant differences were detected for any keratometric variable during the follow-up (p≥0.103). 15 years after the surgery 54.55% of the eyes were within ±1.00 D of spherical equivalent and 84.85% within ±2.00 D. The uncorrected distance visual acuity at 15 years was 20/25 or better in 60.6% of the eyes and 20/40 or better in 72.73% of the eyes. The correlation between the attempted and the achieved refractions was r=0.948 (p<0.001) at 1 year, and r=0.821 (p<0.001) at 15 years. No corneal ectasia was detected in any case during the follow-up.
CONCLUSIONS:
Photorefractive keratectomy is a safe refractive procedure in the long term within the range of myopia currently considered suitable for its use, although its efficacy decreases with time, especially, in high myopia. The model developed predicts a myopic regression of 2.00 D at 15 years for an ablation depth of 130 µm
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