54,669 research outputs found
Lorentz Connections and Gravitation
The different roles played by Lorentz connections in general relativity and
in teleparallel gravity are reviewed. Some of the consequences of this
difference are discussed.Comment: Lecture presented at the Sixth International School on Field Theory
and Gravity, Petropolis, Brazil, 201
Spin-charge coupling in quantum wires at zero magnetic field
We discuss an approximation for the dynamic charge response of nonlinear
spin-1/2 Luttinger liquids in the limit of small momentum. Besides accounting
for the broadening of the charge peak due to two-holon excitations, the
nonlinearity of the dispersion gives rise to a two-spinon peak, which at zero
temperature has an asymmetric line shape. At finite temperature the spin peak
is broadened by diffusion. As an application, we discuss the density and
temperature dependence of the Coulomb drag resistivity due to long-wavelength
scattering between quantum wires.Comment: 16 pages, 5 figures. This is an extended version of "Coulomb drag
from spin-charge coupling at zero magnetic field
Accuracy of a DTM derived from full-waveform laser scanning data under unstructured eucalypt forest: a case study
A Digital Terrain Model (DTM) is fundamental for extracting several forest canopy structure metrics from data acquired with small-footprint airborne laser scanning (ALS). This modern remote sensing technology is based on laser measurements from a laser system mounted on an aircraft and integrated with a geodetic GNSS receiver and an inertial measurement unit (IMU) or inertia navigation system (INS). In the context of a research project for deriving forest inventory parameters and fuel variables under eucalypt stands in Mediterranean climates, the vertical precision of the DTM obtained by automatic filtering of full-waveform ALS data had to be evaluated. The DTM accuracy estimation on a study area with peculiar characteristics, which are often avoided in related studies, will also allow verifying the performance of full- waveform ALS systems. The accuracy estimation is carried out in a novel way. By novel way, it is meant an exhaustive, well-planned collection of reliable control data in forest environment. The collection of the control data involves the production of DTM on 43 circular plots (radius = 11.28m) using total stations and geodetic GNSS receivers. These DTM, with a total of 3356 points, allowed one to evaluate consistently and reliably the vertical accuracy of the terrain surface produced with ALS under a eucalypt forest. This global accuracy, expressed by the Root Mean Square Error (RMSE) of the vertical differences between the field surveyed surface and the ALS derived DTM surface is 0.15m (mean=0.08m and std=0.09m). This impressive value indicates that, for an ALS point cloud density of 10pts/m2 and footprint of 20 cm, the methodology used to extract the DTM from full- waveform ALS data under an unstructured eucalypt forest is very accurate. In this article it is addressed both the strategy adopted to collect the control data and the quality assessment of the DTM produced by means of the ALS data
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