Fast localized wavefront correction using area-mapped phase-shift interferometry

We propose an innovative method for localized wavefront correction based on area-mapped phase-shift (AMPS) interferometry. In this Letter, we present the theory and then experimentally compare it with a previously demonstrated method based on spot-optimized phase-stepping (SOPS) interferometry. We found that AMPS outperforms SOPS interferometry in terms of speed by threefold, although in noisy environments the improvements may be larger. AMPS yielded similar point-spread functions (PSF) as SOPS for moderate system-induced aberrations, but yielded a slightly less ideal PSF for larger aberrations. The method described in this Letter may prove crucial for applications where the phase-stepping solution does not have sufficient speed.     

The interplay between antiferromagnetism and superconductivity in disordered ultrathin high-Tc films

A series of ultrathin films of DyBa₂Cu₃O_7−x, with properties spanning the zero-field superconductor-to-insulator transition, were prepared for the purpose of investigating the magnetic field driven superconductor-insulator transition. The resistance of these films was found to exhibit a peak whose magnitude became larger with the weakening of the superconductivity of the films. For a film just on the superconducting side of the zero-field superconductor-to-insulator transition, three successive changes in the sign of the low temperature dR/dT with increasing field were found. These have been taken as evidence of the superconducting state being successively excited, reentered, and finally exited with increasing field as the sign changes from positive to negative, to positive, and finally to negative. This unusual behavior may result from an interaction of the carriers with Cu²⁺ ions on the copper-oxygen sheets. These spins exhibit an increasing tendency to antiferromagnetic ordering as the superconductivity of the films is weakened. The presence of many grain boundaries in the films may be the reason the effect is strikingly large.     

Prediction of initial motion of a gas bubble in liquids

Numerical predictions are presented for the motion and distortion of a single gas bubble rising through the liquid. The computations were made with an implicit finite-difference procedure which solves the transient equations of motion throughout the bubble and the liquid, such that the free surface between the gas bubble and the liquid is not a boundary of the computational domain.The predictions compare well with the experimental results of others. Computations are presented for bubble sizes from 0.02 to 0.05 m radius and for bubbles of different gas densities rising in liquids of different densities. Surface tension effects are neglected.