Intensity dependence of Rydberg states

We investigate numerically and analytically the intensity dependence of the fraction of electrons that end up in a Rydberg state after strong-field ionization with linearly polarized light. We find that including the intensity dependent distribution of ionization times and non-adiabatic effects leads to a better understanding of experimental results. Furthermore, we observe using Classical Trajectory Monte Carlo simulations that the intensity dependence of the Rydberg yield changes with wavelength and that the previously observed power-law dependence breaks down at longer wavelengths. Our work suggests that Rydberg yield measurements can be used as an independent test for non-adiabaticity in strong field ionization

Moment-Sum-Of-Squares Approach For Fast Risk Estimation In Uncertain Environments

In this paper, we address the risk estimation problem where one aims at estimating the probability of violation of safety constraints for a robot in the presence of bounded uncertainties with arbitrary probability distributions. In this problem, an unsafe set is described by level sets of polynomials that is, in general, a non-convex set. Uncertainty arises due to the probabilistic parameters of the unsafe set and probabilistic states of the robot. To solve this problem, we use a moment-based representation of probability distributions. We describe upper and lower bounds of the risk in terms of a linear weighted sum of the moments. Weights are coefficients of a univariate Chebyshev polynomial obtained by solving a sum-of-squares optimization problem in the offline step. Hence, given a finite number of moments of probability distributions, risk can be estimated in real-time. We demonstrate the performance of the provided approach by solving probabilistic collision checking problems where we aim to find the probability of collision of a robot with a non-convex obstacle in the presence of probabilistic uncertainties in the location of the robot and size, location, and geometry of the obstacle.Comment: 57th IEEE Conference on Decision and Control 201

Triggering of Imaging Air Cherenkov Telescopes: PMT trigger rates due to night-sky photons

Imaging air Cherenkov telescopes are usually triggered on a coincidence of two or sometimes more pixels, with discriminator thresholds in excess of 20 photoelectrons applied for each pixel. These thresholds required to suppress night-sky background are significantly higher than expected on the basis of a Poisson distribution in the number of night-sky photoelectrons generated during the characteristic signal integration time. We studied noise trigger rates under controlled conditions using an artificial background light source. Large tails in the PMT amplitude response to single photoelectrons are identified as a dominant contribution to noise triggers. The rate of such events is very sensitive to PMT operating parameters.Comment: 19 pages, latex,epsf, 7 figures appended as uuencoded file, submitted to Journal of Physics

Controlling the quantum number distribution and yield of Rydberg states via the duration of the laser pulse

We show that the distribution of quantum numbers of Rydberg states does not only depend on the field strength and wavelength of the laser which the atom is exposed to, but that it also changes significantly with the duration of the laser pulse. We provide an intuitive explanation for the underlying mechanism and derive a scaling law for the position of the peak in the quantum number distribution on the pulse duration. The new analytic description for the electron's movement in the superposed laser and Coulomb field (applied in the study of quantum numbers) is then used to explain the decrease of the Rydberg yield with longer pulse durations. This description stands in contrast to the concepts that explained the decrease so far and also reveals that approximations which neglect Coulomb effects during propagation are not sufficient in cases such as this.Comment: 8 pages, 8 figure

The effect of heart rate variability biofeedback training on stress and anxiety: a meta-analysis

BACKGROUND: Some evidence suggests that heart rate variability (HRV) biofeedback might be an effective way to treat anxiety and stress symptoms. To examine the effect of HRV biofeedback on symptoms of anxiety and stress, we conducted a meta-analysis of studies extracted from PubMed, PsycINFO and the Cochrane Library. METHODS: The search identified 24 studies totaling 484 participants who received HRV biofeedback training for stress and anxiety. We conducted a random-effects meta-analysis. RESULTS: The pre-post within-group effect size (Hedges' g) was 0.81. The between-groups analysis comparing biofeedback to a control condition yielded Hedges' g = 0.83. Moderator analyses revealed that treatment efficacy was not moderated by study year, risk of study bias, percentage of females, number of sessions, or presence of an anxiety disorder. CONCLUSIONS: HRV biofeedback training is associated with a large reduction in self-reported stress and anxiety. Although more well-controlled studies are needed, this intervention offers a promising approach for treating stress and anxiety with wearable devices

Near-threshold fatigue crack growth in bulk metallic glass composites

A major drawback in using bulk metallic glasses (BMGs) as structural materials is their extremely poor fatigue performance. One way to alleviate this problem is through the composite route, in which second phases are introduced into the glass to arrest crack growth. In this paper, the fatigue crack growth behavior of in situ reinforced BMGs with crystalline dendrites, which are tailored to impart significant ductility and toughness to the BMG, was investigated. Three composites, all with equal volume fraction of dendrite phases, were examined to assess the influence of chemical composition on the near-threshold fatigue crack growth characteristics. While the ductility is enhanced at the cost of yield strength vis-à-vis that of the fully amorphous BMG, the threshold stress intensity factor range for fatigue crack initiation in composites was found to be enhanced by more than 100%. Crack blunting and trapping by the dendritic phases and constraining of the shear bands within the interdendritic regions are the micromechanisms responsible for this enhanced fatigue crack growth resistance

Spontaneous particle-hole symmetry breaking of correlated fermions on the Lieb lattice

We study spinless fermions with nearest-neighbor repulsive interactions ($t$-$V$ model) on the two-dimensional three-band Lieb lattice. At half-filling, the free electronic band structure consists of a flat band at zero energy and a single cone with linear dispersion. The flat band is expected to be unstable upon inclusion of electronic correlations, and a natural channel is charge order. However, due to the three-orbital unit cell, commensurate charge order implies an imbalance of electron and hole densities and therefore doping away from half-filling. Our numerical results show that below a finite-temperature Ising transition a charge density wave with one electron and two holes per unit cell and its partner under particle-hole transformation are spontaneously generated. Our calculations are based on recent advances in auxiliary-field and continuous-time quantum Monte Carlo simulations that allow sign-free simulations of spinless fermions at half-filling. It is argued that particle-hole symmetry breaking provides a route to access levels of finite doping, without introducing a sign problem.Comment: 9 pages, 6 figures, added data for strong Coulomb repulsion and classical Ising-limi

Dynamical Mass Generation and Confinement in Maxwell-Chern-Simons Planar Quantum Electrodynamics

We study the non-perturbative phenomena of Dynamical Mass Generation and Confinement by truncating at the non-perturbative level the Schwinger-Dyson equations in Maxwell-Chern-Simons planar quantum electrodynamics. We obtain numerical solutions for the fermion propagator in Landau gauge within the so-called rainbow approximation. A comparison with the ordinary theory without the Chern-Simons term is presented.Comment: 9 pages, 9 figures; prepared for the XIV Mexican School of Particles and Fields, 4-12 November 2010, Morelia, Michoacan, Mexic