Electronic structure calculations of $\beta -{\mathrm{BaB}}_2{\mathrm{O}}_4$ from first principles are performed based on a plane-wave pseudopotential method, and the linear optical properties are then obtained. The static second-harmonic generation (SHG) coefficients are calculated at the independent-particle level with a formalism originally given by Aversa and Sipe [Phys. Rev. B 52, 14 636 (1995)] and later rearranged by Rashkeev et al. [Phys. Rev. B 57, 3905 (1998)] to explicitly show Kleinman’s symmetry. The formalism is improved to be more efficient in reducing the $k$ points necessary for convergence. A real-space atom-cutting method is suggested to analyze the respective contributions of various transitions among ions and ion groups to optical response. The contribution of the cation Ba to SHG effects is found to be not important but non-negligible, while its contribution to birefringence is negligible.

The wide angular-range chopper spectrometer ARCS at the Spallation Neutron Source (SNS) is optimized to provide a high neutron flux at the sample position with a large solid angle of detector coverage. The instrument incorporates modern neutron instrumentation, such as an elliptically focused neutron guide, high speed magnetic bearing choppers, and a massive array of ³He linear position sensitive detectors. Novel features of the spectrometer include the use of a large gate valve between the sample and detector vacuum chambers and the placement of the detectors within the vacuum, both of which provide a window-free final flight path to minimize background scattering while allowing rapid changing of the sample and sample environment equipment. ARCS views the SNS decoupled ambient temperature water moderator, using neutrons with incident energy typically in the range from 15 to 1500 meV. This range, coupled with the large detector coverage, allows a wide variety of studies of excitations in condensed matter, such as lattice dynamics and magnetism, in both powder and single-crystal samples. Comparisons of early results to both analytical and Monte Carlo simulation of the instrument performance demonstrate that the instrument is operating as expected and its neutronic performance is understood. ARCS is currently in the SNS user program and continues to improve its scientific productivity by incorporating new instrumentation to increase the range of science covered and improve its effectiveness in data collection.

The glacial way of wearing away: The rate at which glaciers erode landscapes is an important but poorly constrained relationship. Hermanet al.tackle this issue by considering the Franz Josef alpine glacier in New Zealand. The amount of sediment piling up at the edge of the glacier provided erosion rates, whereas remote sensing allowed for simultaneous tracking of glacial motion. The result was a nonlinear relationship, suggesting that fast glaciers are much more effective at gouging landscapes. This could explain the paradox of why long-term erosion rates are so much lower in polar regions with more permanent glaciers.Science, this issue p.193

Significance: Silicon has a peculiar negative thermal expansion at low temperature. This behavior has been understood with a “quasiharmonic” theory where low-energy phonons decrease in frequency with volume contraction. We report inelastic neutron scattering measurements of phonon dispersions over a wide range of temperatures. These measurements cast doubt upon quasiharmonic theory, which predicts the wrong sign for most phonon shifts with temperature. Fully anharmonic ab initio calculations correctly predict the phonon shifts and thermal expansion. Crystal structure, anharmonicity, and nuclear quantum effects all play important roles in the thermal expansion of silicon, and a simple mechanical explanation is inappropriate. The quantum effect of nuclear vibrations is also expected to be important for thermophysical properties of many materials.

Electronic structure calculations of ${\mathrm{LiB}}_3{\mathrm{O}}_5,$ ${\mathrm{CsB}}_3{\mathrm{O}}_5,$ and ${\mathrm{CsLiB}}_6{\mathrm{O}}_{10}$ crystals from first principles are performed based on a plane-wave pseudopotential method. The static second-harmonic generation (SHG) coefficients are calculated at the independent-particle level with a formalism improved by our group and co-workers [Phys. Rev. B 60, 13 380 (1999)]. A real-space atom-cutting method is adopted to analyze the respective contributions of the cation and anionic groups to optical response. The calculated refractive indices and SHG coefficients are in good agreement with the experimental values. On the basis of these calculations, the influence of the cations on the band gaps and the optical responses is evaluated. The results show that with the increase of their radius their contributions to SHG become slightly more pronounced.