Geodesics around line defects in elastic solids

Topological defects in solids, usually described by complicated boundary conditions in elastic theory, may be described more simply as sources of a gravity- like deformation field in the geometric approach of Katanaev and Volovich. This way, the deformation field is described by non-Euclidean metric that incorporates the boundary imposed by the defects. A possible way of gaining some insight into the motion of particles in a medium with topological defects (e.g., electrons in a dislocated metal) is to look at the geodesics of the medium around the defect. In this work, we find the exact solution for the geodesic equation for elastic medium with a generic line defect, the dispiration, that can either be a screw dislocation or a wedge disclination for particular choices of its parameters.Comment: 10 pages, Latex, 4 figures, accepted for publication in Phys. Lett.

Threading dislocation densities in semiconductor crystals: a geometric approach

In this letter, we introduce a geometric model to explain the origin of the observed shallow levels in semiconductors threaded by a dislocation density. We show that a uniform distribution of screw dislocations acts as an effective uniform magnetic field which yields bound states for a spin-half quantum particle, even in the presence of a repulsive Coulomb-like potential. This introduces energy levels within the band gap, increasing the carrier concentration in the region threaded by the dislocation density and adding additional recombination paths other than the near band-edge recombination.Comment: 9 pages, no figur

Soybean Acreage Response in Brazil

This paper advances Williams and Thompson (1984) by updating their work and by explicitly accounting for price and yield risk in the analysis of acreage response in Brazil for soybeans and by assessing model specification. Empirical equations were estimated using seemingly unrelated regression (SUR). The robustness of the model was evaluated in the battery of misspecification tests suggested by McGuirk et al. (1993) and McGuirk et al. (1995). The results of the testing procedure suggest that the model is fairly robust in terms of normality, heteroscedasticity and functional form. The results point to parameter instability in the soybean model. The approach to the problem of parameter instability involved dividing the data in two periods and estimating regressions for each period. The signs of the significant coefficients were consistent with expectation, particularly for the second period. Soybean acreage is explained mainly by past acreage, expected prices of soybeans and land competing crops (cotton, rice, and corn), and price and yield risk. Results suggest that market signals played a reduced role in the soybean acreage growth in early years. In contrast, in recent years producers in Brazil became more sensitive to changes in prices and risk. Measures of short-run price elasticity of soybean acreage response are similar to the one obtained by William and Thompson (1984) for soybean supply. Long-run elasticities are significantly smaller.Crop Production/Industries,

A liquid crystal analogue of the cosmic string

We consider the propagation of light in a anisotropic medium with a topological line defect in the realm of geometrical optics. It is shown that the effective geometry perceived by light propagating in such medium is that of a spacial section of the cosmic string spacetime.Comment: 3 pages, 2 figures. Modern Physics Letters A, accepted for publicatio