Frequency correlations in reflection from random media

We present a theoretical study of frequency correlations of light backscattered from a random scattering medium. This statistical quantity provides insight into the dynamics of multiple scattering processes accessible both, in theoretical and experimental investigations. For frequency correlations between field amplitudes, we derive a simple expression in terms of the path length distribution of the underlying backscattering processes. In a second step, we apply this relation to describe frequency correlations between intensities in the regime of weak disorder. Since, with increasing disorder strength, an unexplained breakdown of the angular structure of the frequency correlation function has recently been reported in experimental studies, we explore extensions of our model to the regime of stronger disorder. In particular, we show that closed scattering trajectories tend to suppress the angular dependence of the frequency correlation function.Comment: 10 pages, 8 figure

Semimetallic features in quantum transport through a gate-defined point contact in bilayer graphene

We demonstrate that, at the onset of conduction, an electrostatically defined quantum wire in bilayer graphene (BLG) with an interlayer asymmetry gap may act as a 1D semimetal, due to the multiple minivalley dispersion of its lowest subband. Formation of a non-monotonic subband coincides with a near-degeneracy between the bottom edges of the lowest two subbands in the wire spectrum, suggesting an $8e^2/h$ step at the conduction threshold, and the semimetallic behaviour of the lowest subband in the wire would be manifest as resonance transmission peaks on an $8e^2/h$ conductance plateau.Comment: 9 pages, 8 figures (including appendices

Flux conservation in coherent backscattering and weak localisation of light

The standard theoretical description of coherent backscattering, accord- ing to which maximally crossed diagrams accounting for interference between counter- propagating path amplitudes are added on top of the incoherent background, violates the fundamental condition of flux conservation. In contrast to predictions of previous theories, we show that including maximally crossed diagrams with one additional scat- tering event does not restore flux conservation. Instead, we propose that the latter is recovered when treating the effects of coherent backscattering and weak localisation in a unified framework. On the basis of this framework, we demonstrate explicitly flux conservation in leading order of the weak disorder parameter 1/(kl).Comment: 18 page

A confined-unconfined aquifer model for subglacial hydrology

Modeling the evolution of subglacial channels underneath ice sheets is an urgent need for ice sheet modellers, as channels affect sliding velocities and hence ice discharge. Owing to very limited observations of the subglacial hydraulic system, the development of physical models is quite restricted. Subglacial hydrology models are currently taking two different approaches: either modeling the development of a network of individual channels or modeling an equivalent porous layer where the channels are not resolved individually but modeled as a diffusive process, adjusted to reproduce the characteristic of an efficient system. Here, we use the latter approach, improving it by using a confined-unconfined aquifer model (CUAS), that allows the system to run dry in absence of sufficient water input. This ensures physical values for the water pressure. Channels are represented by adjusting the permeability and storage of the system according to projected locations of channels. The evolution of channel positions is governed by a reduced complexity model that computes channel growths according to simple rules (weighted random walks descending the hydraulic potential). As a proof of concept we present the results of the evolution of the hydrological system over time for a simple artificial glacier geometr

Aluminous websterite and granulite xenoliths from the Chyulu Hills volcanic field, Kenya: gabbro-troctolitic cumulates subjected to lithospheric foundering

Whole rock major and trace element abundances in aluminous garnet-spinel websterite, sapphirine-bearing Mg-Al granulite and hibonite-bearing Ca-Al granulite xenoliths from the Chyulu Hills volcanic field, Kenya, suggest that the samples represent a meta-igneous suite linked by fractionation. The incompatible major element contents increase from the websterites to the Mg-Al granulites and further to the Ca-Al granulites. High bulk rock Mg#s and very low concentrations of most incompatible trace elements indicate that the rocks are cumulates rather than crystallized melts. Elevated Ni abundances, impoverishment in Cr and HFSE and high contents of normative plagioclase and olivine in the granulites indicate that their protoliths were similar to troctolite. The textures and metamorphic reaction paths recorded in the granulites suggest igneous emplacement in the crust and cooling from igneous to ambient crustal temperatures accompanied or followed by compression. For the websterite xenoliths, there is an apparent contradiction between the results of P-T calculations that suggest high P and T of crystallization of early generation pyroxenes and elevated P-T conditions during final equilibration (1.4-2.2GPa/740-980°C) on the one hand and the positive Eu anomaly that suggests shallow-level plagioclase accumulation on the other hand. This contradiction can be reconciled by a model of compression of a plagioclase-bearing (gabbroic) protolith to mantle depths where it recrystallized to an ultramafic assemblage, which requires foundering of dense lower crustal material into the mantl

Grounding line migration as response to cycles of sliding pertubations and initial geometries in the MISMIP3D experiment

The benchmark experiment MISMIP3D (Pattyn et al., 2013) investigated the response of a artificial ice stream-ice shelf system to a sliding perturbation. We continued this experiment by applying cycles of pertubations at different time scales in order to see the long term response of the grounding line positions to changes in basal sliding. For this purpose we applied the finite-difference full-Stokes model TIM-FD3 on 2.5km and 1.25km using three different initial geometries. We found that our steady-state geometry shows a strong dependency of the grounding line position on the horizontal grid size and the chosen initial geometry. Not all experiments show a neutral equilibrium in subsequent basal sliding perturbation simulations