Dielectric function of the semiconductor hole gas

We study the dielectric function of the homogeneous hole gas in p-doped zinc-blende III-V bulk semiconductors within random phase approximation with the valence band being modeled by Luttinger's Hamiltonian in the spherical approximation. In the static limit we find a beating of Friedel oscillations between the two Fermi momenta for heavy and light holes, while at large frequencies dramatic corrections to the plasmon dispersion occur.Comment: 4 pages, 1 figure included. Version to appear in Europhys. Let

The ocean's saltiness and its overturning

Here we explore the relationship between the mean salinity urn:x-wiley:grl:media:grl55555:grl55555-math-0001 of the ocean and the strength of its Atlantic and Pacific Meridional Overturning Circulations (AMOC and PMOC). We compare simulations performed with a realistically configured coarse‐grained ocean model, spanning a range of mean salinities. We find that the AMOC strength increases approximately linearly with urn:x-wiley:grl:media:grl55555:grl55555-math-0002. In contrast, the PMOC strength declines approximately linearly with urn:x-wiley:grl:media:grl55555:grl55555-math-0003 until it reaches a small background value similar to the present‐day ocean. Well‐established scaling laws for the overturning circulation explain both of these dependencies on urn:x-wiley:grl:media:grl55555:grl55555-math-0004

Obliquity pacing of the late Pleistocene glacial terminations

Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature 434 (2005): 491-494, doi:10.1038/nature03401.The timing of glacial/interglacial cycles at intervals of about 100,000 yr (100 kyr) is commonly attributed to control by Earth orbital configuration variations. This “pacemaker” hypothesis has inspired many models, variously depending upon Earth obliquity, orbital eccentricity, and precessional fluctuations, with the latter usually emphasized. A contrasting hypothesis is that glacial cycles arise primarily because of random internal climate variability. Progress requires distinguishing between the more than 30 proposed models of the late Pleistocene glacial variations. Here we present a formal test of the pacemaker hypothesis, focusing on the rapid deglaciation events known as terminations. The null hypothesis that glacial terminations are independent of obliquity can be rejected at the 5% significance level. In contrast, for eccentricity and precession, the corresponding null-hypotheses cannot be rejected. The simplest inference, consistent with the observations, is that ice-sheets terminate every second (80 kyr) or third (120 kyr) obliquity cycle — at times of high obliquity — and similar to the original Milankovitch assumption. Hypotheses not accounting for the obliquity pacing are unlikely to be correct. Both stochastic and deterministic variants of a simple obliquity-paced model describe the observations.PH is supported by the NOAA Postdoctoral Program in Climate and Global Change and CW in part by the National Ocean Partnership Program (ECCO)

The Oceanic Variability Spectrum and Transport Trends

Oceanic meridional transports evaluated over the width of the Pacific Ocean from altimetric observations become incoherent surprisingly rapidly with meridional separation. Even with 15 years of data, surface slopes show no significant coherence beyond 5◦ of latitude separation at any frequency. An analysis of the frequency/zonal-wavenumber spectral density shows a broad continuum of motions at all time and space scales, with a significant excess of energy along a “non-dispersive” line extending between the simple barotropic and first baroclinic mode Rossby waves. It is speculated that much of that excess energy lies with coupled barotropic and first mode Rossby waves. The statistical significance of apparent oceanic transport trends depends upon the existence of a reliable frequency/wavenumber spectrum and for which only a few observational elements now exist.Jet Propulsion Laboratory (U.S.).United States. National Aeronautics and Space Administration (Jason-1 program)National Oceanographic Partnership Program (U.S.

Internal-tide driven tracer transport across the continental slope

The role of the internal tide in driving tracer transport across the continental slope is examined using simplified layered theory, channel model experiments, and observational diagnostics of near shelf-edge moorings. The effect of the internal tide is interpreted in terms of its Stokes' drift, which is separated into two distinct components: a bolus component, driven by the covariance of layer thickness and the velocity, and a shear component, driven by the velocity following the movement of an interface. For a three-layer ocean, in the model experiments and observations, the onshore propagation of an internal tide drives a Stokes' transport directed onshore in the surface and the bottom layers and directed offshore in the pycnocline. This reversing structure is due to the bolus component dominating near the boundaries, while the shear component dominates at the pycnocline. In the observational diagnostics, the Stokes' transport is not canceled by the Eulerian transport, which is mainly directed along bathymetric contours. The Stokes' drift of the internal tide then provides a systematic on shelf tracer transport if there is a tracer sink on the shelf, carried in the surface or bottom layers. Conversely, the tracer transport is directed offshore if there is a tracer source on the shelf with plumes of shelf tracer expected to be carried offshore along the pycnocline. This tracer transport as a result of the internal tide is diagnosed for heat, salt, and nitrate. The depth-integrated nitrate flux is directed onto the shelf supplying nutrients to the productive shelf seas

Resolvent estimates for normally hyperbolic trapped sets

We give pole free strips and estimates for resolvents of semiclassical operators which, on the level of the classical flow, have normally hyperbolic smooth trapped sets of codimension two in phase space. Such trapped sets are structurally stable and our motivation comes partly from considering the wave equation for Kerr black holes and their perturbations, whose trapped sets have precisely this structure. We give applications including local smoothing effects with epsilon derivative loss for the Schr\"odinger propagator as well as local energy decay results for the wave equation.Comment: Further changes to erratum correcting small problems with Section 3.5 and Lemma 4.1; this now also corrects hypotheses, explicitly requiring trapped set to be symplectic. Erratum follows references in this versio

Strained graphene: tight-binding and density functional calculations

We determine the band structure of graphene under strain using density functional calculations. The ab-initio band strucure is then used to extract the best fit to the tight-binding hopping parameters used in a recent microscopic model of strained graphene. It is found that the hopping parameters may increase or decrease upon increasing strain, depending on the orientation of the applied stress. The fitted values are compared with an available parametrization for the dependence of the orbital overlap on the distance separating the two carbon atoms. It is also found that strain does not induce a gap in graphene, at least for deformations up to 10%

The curvature of semidirect product groups associated with two-component Hunter-Saxton systems

In this paper, we study two-component versions of the periodic Hunter-Saxton equation and its $\mu$-variant. Considering both equations as a geodesic flow on the semidirect product of the circle diffeomorphism group \Diff(\S) with a space of scalar functions on $\S$ we show that both equations are locally well-posed. The main result of the paper is that the sectional curvature associated with the 2HS is constant and positive and that 2$\mu$HS allows for a large subspace of positive sectional curvature. The issues of this paper are related to some of the results for 2CH and 2DP presented in [J. Escher, M. Kohlmann, and J. Lenells, J. Geom. Phys. 61 (2011), 436-452].Comment: 19 page

Excitonic condensation in a double-layer graphene system

The possibility of excitonic condensation in a recently proposed electrically biased double-layer graphene system is studied theoretically. The main emphasis is put on obtaining a reliable analytical estimate for the transition temperature into the excitonic state. As in a double-layer graphene system the total number of fermionic "flavors" is equal to N=8 due to two projections of spin, two valleys, and two layers, the large-$N$ approximation appears to be especially suitable for theoretical investigation of the system. On the other hand, the large number of flavors makes screening of the bare Coulomb interactions very efficient, which, together with the suppression of backscattering in graphene, leads to an extremely low energy of the excitonic condensation. It is shown that the effect of screening on the excitonic pairing is just as strong in the excitonic state as it is in the normal state. As a result, the value of the excitonic gap \De is found to be in full agreement with the previously obtained estimate for the mean-field transition temperature $T_c$, the maximum possible value $\Delta^{\rm max},T_c^{\rm max}\sim 10^{-7} \epsilon_F$ ($\epsilon_F$ is the Fermi energy) of both being in $1{\rm mK}$ range for a perfectly clean system. This proves that the energy scale $\sim 10^{-7} \epsilon_F$ really sets the upper bound for the transition temperature and invalidates the recently expressed conjecture about the high-temperature first-order transition into the excitonic state. These findings suggest that, unfortunately, the excitonic condensation in graphene double-layers can hardly be realized experimentally.Comment: 21 pages, 5 figures, invited paper to Graphene special issue in Semiconductor Science and Technolog

Excitonic condensation in a double-layer graphene system

The possibility of excitonic condensation in a recently proposed electrically biased double-layer graphene system is studied theoretically. The main emphasis is put on obtaining a reliable analytical estimate for the transition temperature into the excitonic state. As in a double-layer graphene system the total number of fermionic "flavors" is equal to N=8 due to two projections of spin, two valleys, and two layers, the large-$N$ approximation appears to be especially suitable for theoretical investigation of the system. On the other hand, the large number of flavors makes screening of the bare Coulomb interactions very efficient, which, together with the suppression of backscattering in graphene, leads to an extremely low energy of the excitonic condensation. It is shown that the effect of screening on the excitonic pairing is just as strong in the excitonic state as it is in the normal state. As a result, the value of the excitonic gap \De is found to be in full agreement with the previously obtained estimate for the mean-field transition temperature $T_c$, the maximum possible value $\Delta^{\rm max},T_c^{\rm max}\sim 10^{-7} \epsilon_F$ ($\epsilon_F$ is the Fermi energy) of both being in $1{\rm mK}$ range for a perfectly clean system. This proves that the energy scale $\sim 10^{-7} \epsilon_F$ really sets the upper bound for the transition temperature and invalidates the recently expressed conjecture about the high-temperature first-order transition into the excitonic state. These findings suggest that, unfortunately, the excitonic condensation in graphene double-layers can hardly be realized experimentally.Comment: 21 pages, 5 figures, invited paper to Graphene special issue in Semiconductor Science and Technolog