Tunnelling Studies of Two-Dimensional States in Semiconductors with Inverted Band Structure: Spin-orbit Splitting, Resonant Broadening

The results of tunnelling studies of the energy spectrum of two-dimensional (2D) states in a surface quantum well in a semiconductor with inverted band structure are presented. The energy dependence of quasimomentum of the 2D states over a wide energy range is obtained from the analysis of tunnelling conductivity oscillations in a quantizing magnetic field. The spin-orbit splitting of the energy spectrum of 2D states, due to inversion asymmetry of the surface quantum well, and the broadening of 2D states at the energies, when they are in resonance with the heavy hole valence band, are investigated in structures with different strength of the surface quantum well. A quantitative analysis is carried out within the framework of the Kane model of the energy spectrum. The theoretical results are in good agreement with the tunnelling spectroscopy data.Comment: 29 pages, RevTeX, submitted in Phys.Rev.B. Figures available on request from [email protected]

Quantum corrections to conductivity: from weak to strong localization

Results of detailed investigations of the conductivity and Hall effect in gated single quantum well GaAs/InGaAs/GaAs heterostructures with two-dimensional electron gas are presented. A successive analysis of the data has shown that the conductivity is diffusive for $k_F l=25-2$ and behaves like diffusive one for $k_F l=2-0.5$ down to the temperature T=0.4 K. It has been therewith found that the quantum corrections are not small at low temperature when $k_F l\simeq 1$. They are close in magnitude to the Drude conductivity so that the conductivity $\sigma$ becomes significantly less than $e^{2}/h$ (the minimal $\sigma$ value achieved in our experiment is about $3\times 10^{-8}\Omega^{-1}$ at $k_Fl\simeq 0.5$ and $T=0.46$ K). We conclude that the temperature and magnetic field dependences of conductivity in whole $k_Fl$ range are due to changes of quantum corrections.Comment: RevTex 4.0, 10 figures, 7 two-column page

Magnetic-field-dependent zero-bias diffusive anomaly in Pb oxide-n-InAs structures: Coexistence of two- and three-dimensional states

The results of experimental and theoretical studies of zero-bias anomaly (ZBA) in the Pb-oxide-n-InAs tunnel structures in magnetic field up to 6T are presented. A specific feature of the structures is a coexistence of the 2D and 3D states at the Fermi energy near the semiconductor surface. The dependence of the measured ZBA amplitude on the strength and orientation of the applied magnetic field is in agreement with the proposed theoretical model. According to this model, electrons tunnel into 2D states, and move diffusively in the 2D layer, whereas the main contribution to the screening comes from 3D electrons.Comment: 8 double-column pages, REVTeX, 9 eps figures embedded with epsf, published versio

Coherent Matter Wave Transport in Speckle Potentials

This article studies multiple scattering of matter waves by a disordered optical potential in two and in three dimensions. We calculate fundamental transport quantities such as the scattering mean free path $\ell_s$, the Boltzmann transport mean free path \elltrb, and the Boltzmann diffusion constant $D_B$, using a diagrammatic Green functions approach. Coherent multiple scattering induces interference corrections known as weak localization which entail a reduced diffusion constant. We derive the corresponding expressions for matter wave transport in an correlated speckle potential and provide the relevant parameter values for a possible experimental study of this coherent transport regime, including the critical crossover to the regime of strong or Anderson localization.Comment: 33 pages, minor corrections, published versio

Global Ocean Sediment Composition and Burial Flux in the Deep Sea

Quantitative knowledge about the burial of sedimentary components at the seafloor has wide-ranging implications in ocean science, from global climate to continental weathering. The use of 230Th-normalized fluxes reduces uncertainties that many prior studies faced by accounting for the effects of sediment redistribution by bottom currents and minimizing the impact of age model uncertainty. Here we employ a recently compiled global data set of 230Th-normalized fluxes with an updated database of seafloor surface sediment composition to derive atlases of the deep-sea burial flux of calcium carbonate, biogenic opal, total organic carbon (TOC), nonbiogenic material, iron, mercury, and excess barium (Baxs). The spatial patterns of major component burial are mainly consistent with prior work, but the new quantitative estimates allow evaluations of deep-sea budgets. Our integrated deep-sea burial fluxes are 136 Tg C/yr CaCO3, 153 Tg Si/yr opal, 20Tg C/yr TOC, 220 Mg Hg/yr, and 2.6 Tg Baxs/yr. This opal flux is roughly a factor of 2 increase over previous estimates, with important implications for the global Si cycle. Sedimentary Fe fluxes reflect a mixture of sources including lithogenic material, hydrothermal inputs and authigenic phases. The fluxes of some commonly used paleo-productivity proxies (TOC, biogenic opal, and Baxs) are not well-correlated geographically with satellite-based productivity estimates. Our new compilation of sedimentary fluxes provides detailed regional and global information, which will help refine the understanding of sediment preservation

Preservation Factors during Cretaceous Oceanic Anoxic Events in the Espírito Santo Basin, Southeast Brazil

The oceanic anoxic events (OAEs) are characterized by enhanced accumulation of organic matter in marine sediments. However, there is still an ongoing debate regarding the interplay between production and preservation during these events. Moreover, few studies provide quantitative estimations of primary productivity and/or the amount of carbon preserved during the OAEs. Here, we used geochemical data from multiple wells located at the Espírito Santo Basin that cover the intervals of events OAE1d and OAE2 to provide quantitative estimates of preservation factors. Our results show enhanced preservation during OAEs compared to modern conditions and a stronger preservation during OAE1d compared to OAE2 in the Espírito Santo Basin. The amount of preserved carbon could reach up to 8.6% during OAE1d, depending on the productivity of the system. In addition, we show that such improvement in preservation is linked to the bottom water with low-O2 concentrations and not due to fast burial caused by high sedimentation rates. Our findings are extremally relevant for organic carbon and source rock modelling studies since model simulations need quantitative estimations

Preservation Factors during Cretaceous Oceanic Anoxic Events in the Espírito Santo Basin, Southeast Brazil

The oceanic anoxic events (OAEs) are characterized by enhanced accumulation of organic matter in marine sediments. However, there is still an ongoing debate regarding the interplay between production and preservation during these events. Moreover, few studies provide quantitative estimations of primary productivity and/or the amount of carbon preserved during the OAEs. Here, we used geochemical data from multiple wells located at the Espírito Santo Basin that cover the intervals of events OAE1d and OAE2 to provide quantitative estimates of preservation factors. Our results show enhanced preservation during OAEs compared to modern conditions and a stronger preservation during OAE1d compared to OAE2 in the Espírito Santo Basin. The amount of preserved carbon could reach up to 8.6% during OAE1d, depending on the productivity of the system. In addition, we show that such improvement in preservation is linked to the bottom water with low-O2 concentrations and not due to fast burial caused by high sedimentation rates. Our findings are extremally relevant for organic carbon and source rock modelling studies since model simulations need quantitative estimations