7,449 research outputs found
Resistively-Detected NMR in a Two-Dimensional Electron System near : Clues to the Origin of the Dispersive Lineshape
Resistively-detected NMR measurements on 2D electron systems near the quantum Hall state are reported. In contrast to recent results of Gervais
\emph{et al.} [Phys. Rev. Lett. , 196803 (2005)], a dispersive
lineshape is found at all RF powers studied and Korringa-like nuclear
spin-lattice relaxation is observed. The shape of the unexplained dispersive
lineshape is found to invert when the temperature derivative of the
longitudinal resistance changes sign. This suggests that both Zeeman and
thermal effects are important to resistively-detected NMR in this regime.Comment: 5 pages, 4 figures. Version accepted for publication in Phys. Rev.B,
Rapid Communication
Generalization of the Poisson kernel to the superconducting random-matrix ensembles
We calculate the distribution of the scattering matrix at the Fermi level for
chaotic normal-superconducting systems for the case of arbitrary coupling of
the scattering region to the scattering channels. The derivation is based on
the assumption of uniformly distributed scattering matrices at ideal coupling,
which holds in the absence of a gap in the quasiparticle excitation spectrum.
The resulting distribution generalizes the Poisson kernel to the nonstandard
symmetry classes introduced by Altland and Zirnbauer. We show that unlike the
Poisson kernel, our result cannot be obtained by combining the maximum entropy
principle with the analyticity-ergodicity constraint. As a simple application,
we calculate the distribution of the conductance for a single-channel chaotic
Andreev quantum dot in a magnetic field.Comment: 7 pages, 2 figure
Changes in the Spatial Allocation of Cropland in the Ft. Cobb Watershed as a Result of Environmental Restrictions
Pollution runoff estimates from SWAT are used in a mathematical programming model to optimally model site-specific crop and conservation practices for pollution abatement in the Ft. Cobb watershed in Southwestern Oklahoma. Results indicate the tradeoffs between producer income, sediment and nutrient runoff and the spatial allocation of crops in the watershed.Environmental Economics and Policy,
Spin Transition in the Half-Filled Landau Level
The transition from partial to complete spin polarization of two-dimensional
electrons at half filling of the lowest Landau level has been studied using
resistively-detected nuclear magnetic resonance (RDNMR). The nuclear
spin-lattice relaxation time is observed to be density independent in the
partially polarized phase but to increase sharply at the transition to full
polarization. At low temperatures the RDNMR signal exhibits a strong maximum
near the critical density.Comment: 4 pages, 3 postscript figures. As published in Phys. Rev. Lett. 98,
086801 (2007
Surface Acoustic Wave Propagation and Inhomogeneities in Low Density Two-Dimensional Electron Systems Near the Metal-Insulator Transition
We have measured the surface acoustic wave velocity shift in a GaAs/AlGaAs
heterostructure containing a two-dimensional electron system (2DES) in a
low-density regime ( cm) at zero magnetic field. The
interaction of the surface acoustic wave with the 2DES is not well described by
a simple model using low-frequency conductivity measurements. We speculate that
this conflict is a result of inhomogeneities in the 2DES which become very
important at low density. This has implications for the putative
metal-insulator transition in two dimensions.Comment: 5 pages, 5 figure
Finite time corrections in KPZ growth models
We consider some models in the Kardar-Parisi-Zhang universality class, namely
the polynuclear growth model and the totally/partially asymmetric simple
exclusion process. For these models, in the limit of large time t, universality
of fluctuations has been previously obtained. In this paper we consider the
convergence to the limiting distributions and determine the (non-universal)
first order corrections, which turn out to be a non-random shift of order
t^{-1/3} (of order 1 in microscopic units). Subtracting this deterministic
correction, the convergence is then of order t^{-2/3}. We also determine the
strength of asymmetry in the exclusion process for which the shift is zero.
Finally, we discuss to what extend the discreteness of the model has an effect
on the fitting functions.Comment: 34 pages, 5 figures, LaTeX; Improved version including shift of PASEP
height functio
A Noninvasive Method For In situ Determination of Mating Success in Female American Lobsters (Homarus americanus)
Despite being one of the most productive fisheries in the Northwest Atlantic, much remains unknown about the natural reproductive dynamics of American lobsters. Recent work in exploited crustacean populations (crabs and lobsters) suggests that there are circumstances where mature females are unable to achieve their full reproductive potential due to sperm limitation. To examine this possibility in different regions of the American lobster fishery, a reliable and noninvasive method was developed for sampling large numbers of female lobsters at sea. This method involves inserting a blunt-tipped needle into the female\u27s seminal receptacle to determine the presence or absence of a sperm plug and to withdraw a sample that can be examined for the presence of sperm. A series of control studies were conducted at the dock and in the laboratory to test the reliability of this technique. These efforts entailed sampling 294 female lobsters to confirm that the presence of a sperm plug was a reliable indicator of sperm within the receptacle and thus, mating. This paper details the methodology and the results obtained from a subset of the total females sampled. Of the 230 female lobsters sampled from George\u27s Bank and Cape Ann, MA (size range = 71-145 mm in carapace length), 90.3% were positive for sperm. Potential explanations for the absence of sperm in some females include: immaturity (lack of physiological maturity), breakdown of the sperm plug after being used to fertilize a clutch of eggs, and lack of mating activity. The surveys indicate that this technique for examining the mating success of female lobsters is a reliable proxy that can be used in the field to document reproductive activity in natural populations
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