Spatial Variations Of Local Density Research Articles

Optimal Sampling Designs for Monitoring Plant Frequency

Plant frequency is a pragmatic surrogate for plant density in protocols designed for the long-term monitoring of diverse communities. Frequency estimates are based on presence/absence data from plots of fixed size, and plots are usually spatially aggregated into sites (often transects) to reduce field effort. Using a combination of statistical models and computer simulations, we identify sampling designs that maximize statistical power for detecting changes in underlying plant density based on the analysis of plant frequency. The optimal plot size for collecting frequency data decreases both with increasing spatial variation in local density (spatial structure) and with increasing numbers of plots per site. Over realistic ranges for these parameters, plots of optimal size yield mean frequencies that vary from 20% to 80%. However, with the exception of highly overdispersed populations, power is relatively insensitive to plot size; consequently, a plot size that yields a mean frequency of 50% usually provides nearly maximal power. For population monitoring, in which comparisons are made between successive samples from the same population, repeated measures from fixed sites improve statistical power substantially if there is spatial structure among sites, provided that the spatial pattern is at least partially consistent over time. However, there is still a power loss to the extent that the pattern of spatial structure among sites changes over time (a site-by-time interaction). This power loss can be mitigated by increasing the spacing between plots within sites, which has the effect of increasing the within-site structure and reducing the between-site structure. With more than 1 plot per site, there is no statistical advantage to obtaining repeated measures from fixed plots; relocating plots within sites in successive samples may therefore be advisable to minimize disturbance to the community.

Site-specified tunneling spectroscopy on commensurate 1T-TaS2

Abstract Reproducible tunneling spectra were observed using site-specified tunneling spectroscopy in commensurate 1T-TaS2, which indicated the opening of the Mott localization gap. The gap size in both sides of the Fermi level was about 400∼500 mV at 77K. Clear spatial variations of the local density of states near the Fermi level have also been observed in a reproducible manner. The tunneling spectra changed periodically as a function of position on the surface with well defined gap structures. The spatial variations of local density of states were explained by the charge transfer from charge density wave trough to crest. This charge transfer strongly supported the electron localization on the charge density wave crest by Mott localization.

Observation of spatial variations of local density of states in charge density wave state of 1T-TaSe2 at 77K

Spatial variations of the local density of states near the Fermi level were observed in the charge density wave of 1 T -TaSe 2 at 77 K by the site-specified tunneling spectroscopy. The local density of states was significantly different, depending on positions: charge density wave crest and trough. It was observed that Ta 5 d band split into three sub band manifolds as predicted by band calculation. The relative weight of the sub-bands changed with position at the crest and trough of charge density wave.

Electronic density of states and optical spectrum of tetrahedrally bonded amorphous III-V semiconductors

The disorder in an amorphous III-V semiconductor is described in terms of spatial variation in local density. The electronic density of states for the amorphous semiconductors are then simulated by a weighted sum of the crystalline electronic density of states (EDS) with a variation in local density. It is shown that the amorphous electronic density thus obtained is equivalent to its crystalline counterpart with the energy of each electronic state broadened by an individual broadening parameter, which is related to the degree of disorder of the amorphous semiconductor considered and the "sensitivity" of the energy of the particular state to variations in local density. The result of our phenomenological model is similar to that of Kramer's complex-band-structure calculation based on Green's-function formalism. The optical spectra for the corresponding materials are also calculated using the theoretical EDS, along with the nondirect transition model with an energy-dependent matrix element. The results are compared with available experimental data.