Correlation potentials for molecular bond dissociation within the self-consistent random phase approximation

Self-consistent correlation potentials for H$_2$ and LiH for various inter-atomic separations are obtained within the random phase approximation (RPA) of density functional theory. The RPA correlation potential shows a peak at the bond midpoint, which is an exact feature of the true correlation potential, but lacks another exact feature: the step important to preserve integer charge on the atomic fragments in the dissociation limit. An analysis of the RPA energy functional in terms of fractional charge is given which confirms these observations. We find that the RPA misses the derivative discontinuity at odd integer particle numbers but explicitly eliminates the fractional spin error in the exact-exchange functional. The latter finding explains the accurate total energy in the dissociation limit.Comment: 9 pages, 10 figure

Decay dynamics of quantum dots influenced by the local density of optical states of two-dimensional photonic crystal membranes

We have performed time-resolved spectroscopy on InAs quantum dot ensembles in photonic crystal membranes. The influence of the photonic crystal is investigated by varying the lattice constant systematically. We observe a strong slow down of the quantum dots' spontaneous emission rates as the two-dimensional bandgap is tuned through their emission frequencies. The measured band edges are in full agreement with theoretical predictions. We characterize the multi-exponential decay curves by their mean decay time and find enhancement of the spontaneous emission at the bandgap edges and strong inhibition inside the bandgap in good agreement with local density of states calculations.Comment: 9 pages (preprint), 3 figure

Silent Springs: Why Are All the Frogs “Croaking”?

Amphibians are a fabulously successful group of animals; however, it is increasingly clear that they are experiencing extinction rates that far exceed those experienced by other classes of vertebrates. A new book examines the various reasons why amphibians are so threatened, and what can be done about it

Which is the primary factor influencing running stride parameters: age or lower limb strength?

Much still remains unknown about the impact of age, and age-related changes in muscle function, on gait parameters. The aim of this study was to examine the impact of strength on running gait parameters across the adult lifespan. We tested the hypothesis that a greater amount of the variance in peak hip, knee and ankle sagittal plane moments would be explained by peak isometric joint torques as compared to age. Twenty-four healthy adults, ages 20-66 years, completed 5 trials on an overground 20-meter runway at a standardized velocity of 3.5 ms-1 (± 5%). Participants performed maximal isometric plantar flexion and knee extension for three contractions lasting three seconds each. Linear regression analysis between strength, age, and moments were performed. At the ankle, age alone explained 14.4% of the variance in the peak ankle joint moment. There was not a significant increase in the variance explained when strength was added to the model. At the knee, neither age nor strength explained a significant portion of the variance in peak knee moments. However, together age and strength explained 27.9% of the variance in the peak knee moment. No significant associations were found between the hip moments and either knee and ankle strength. These results suggest that other age-related physiological changes may drive changes in gait mechanics more so than maximal torque production. A more dynamic measure of muscle function, such as power or isokinetic torque at varying speeds may have greater predictive value for gait performance

Pressure tuning of structure, superconductivity and novel magnetic order in the Ce-underdoped electron-doped cuprate T'-Pr_1.3-xLa_0.7Ce_xCuO_4 (x = 0.1)

High-pressure neutron powder diffraction, muon-spin rotation and magnetization studies of the structural, magnetic and the superconducting properties of the Ce-underdoped superconducting (SC) electron-doped cuprate system T'-Pr_1.3-xLa_0.7Ce_xCuO_4 with x = 0.1 are reported. A strong reduction of the lattice constants a and c is observed under pressure. However, no indication of any pressure induced phase transition from T' to T structure is observed up to the maximum applied pressure of p = 11 GPa. Large and non-linear increase of the short-range magnetic order temperature T_so in T'-Pr_1.3-xLa_0.7Ce_xCuO_4 (x = 0.1) was observed under pressure. Simultaneously pressure causes a non-linear decrease of the SC transition temperature T_c. All these experiments establish the short-range magnetic order as an intrinsic and a new competing phase in SC T'-Pr_1.2La_0.7Ce_0.1CuO_4. The observed pressure effects may be interpreted in terms of the improved nesting conditions through the reduction of the in-plane and out-of-plane lattice constants upon hydrostatic pressure.Comment: 11 pages, 10 figure

Practical Implementations of Twirl Operations

Twirl operations, which convert impure singlet states into Werner states, play an important role in many schemes for entanglement purification. In this paper we describe strategies for implementing twirl operations, with an emphasis on methods suitable for ensemble quantum information processors such as nuclear magnetic resonance (NMR) quantum computers. We implement our twirl operation on a general two-spin mixed state using liquid state NMR techniques, demonstrating that we can obtain the singlet Werner state with high fidelity.Comment: 6 pages RevTex4 including 2 figures (fig 1 low quality to save space

Ion and polymer dynamics in polymer electrolytes PPO-LiClO4: II. 2H and 7Li NMR stimulated-echo experiment

We use 2H NMR stimulated-echo spectroscopy to measure two-time correlation functions characterizing the polymer segmental motion in polymer electrolytes PPO-LiClO4 near the glass transition temperature Tg. To investigate effects of the salt on the polymer dynamics, we compare results for different ether oxygen to lithium ratios, namely, 6:1, 15:1, 30:1 and infinity. For all compositions, we find nonexponential correlation functions, which can be described by a Kohlrausch function. The mean correlation times show quantitatively that an increase of the salt concentration results in a strong slowing down of the segmental motion. Consistently, for the high 6:1 salt concentration, a high apparent activation energy E_a=4.1eV characterizes the temperature dependence of the mean correlation times at Tg < T< 1.1T_g, while smaller values E_a=2.5eV are observed for moderate salt contents. The correlation functions are most nonexponential for 15:1 PPO-LiClO4, whereas the stretching is reduced for higher and lower salt concentrations. A similar dependence of the correlation functions on the evolution time in the presence and in the absence of ions indicates that addition of salt hardly affects the reorientational mechanism. For all compositions, mean jump angles of about 15 degree characterize the segmental reorientation. In addition, comparison of results from 2H and 7Li NMR stimulated-echo experiments suggests a coupling of ion and polymer dynamics in 15:1 PPO-LiClO4.Comment: 14 pages, 12 figure

Quantification of Ophthalmic Changes After Long-Duration Spaceflight, and Subsequent Recovery

A subset of crewmembers are subjected to ophthalmic structure changes due to long-duration spaceflight (>6 months). Crewmembers who experience these changes are described as having Spaceflight Associated Neuro-Ocular Syndrome (SANS). Characteristics of SANS include optic disk edema, cotton wool spots, choroidal folds, refractive error, and posterior globe flattening. SANS remains a major obstacle to deep-space and planetary missions, requiring a better understanding of its etiology. Quantification of ocular, structural changes will improve our understanding of SANS pathophysiology. Methods were developed to quantify 3D optic nerve (ON) and ON sheath (ONS) geometries, ON tortuosity, and posterior globe deformation using MR imaging

Predicting the influence of a p2-symmetric substrate on molecular self-organization with an interaction-site model

An interaction-site model can a priori predict molecular selforganisation on a new substrate in Monte Carlo simulations. This is experimentally confirmed with scanning tunnelling microscopy on Fre´chet dendrons of a pentacontane template. Local and global ordering motifs, inclusion molecules and a rotated unit cell are correctly predicted