459 research outputs found
Lowland river responses to intraplate tectonism and climate forcing quantified with luminescence and cosmogenic 10Be
Intraplate tectonism has produced large-scale folding that steers regional drainage systems, such as the 1600 km-long Cooper Ck, en route to Australia’s continental depocentre at Lake Eyre. We apply cosmogenic 10Be exposure dating in bedrock, and luminescence dating in sediment, to quantify the erosional and depositional response of Cooper Ck where it incises the rising Innamincka Dome. The detachment of bedrock joint-blocks during extreme floods governs the minimum rate of incision (17.4±6.5 mm/ky) estimated using a numerical model of episodic erosion calibrated with our 10Be measurements. The last big-flood phase occurred no earlier than ~112–121ka. Upstream of the Innamincka Dome long-term rates of alluvial deposition, partly reflecting synclinal-basin subsidence, are estimated from 47 luminescence dates in sediments accumulated since ~270 ka. Sequestration of sediment in subsiding basins such as these may account for the lack of Quaternary accumulation in Lake Eyre, and moreover suggests that notions of a single primary depocentre at base-level may poorly represent lowland, arid-zone rivers. Over the period ~75–55 ka Cooper Ck changed from a bedload- dominant, laterally-active meandering river to a muddy anabranching channel network up to 60 km wide. We propose that this shift in river pattern was a product of base-level rise linked with the slowly deforming syncline–anticline structure, coupled with a climate-forced reduction in discharge. The uniform valley slope along this subsiding alluvial and rising bedrock system represents an adjustment between the relative rates of deformation and the ability of greatly enhanced flows at times during the Quaternary to incise the rising anticline. Hence, tectonic and climate controls are balanced in the long term
Two-dimensional negative donors in magnetic fields
A finite-difference solution of the Schroedinger equation for negative donor
centers D^- in two dimensions is presented. Our approach is of exact nature and
allows us to resolve a discrepancy in the literature on the ground state of a
negative donor. Detailed calculations of the energies for a number of states
show that for field strengths less than \gamma=0.117 a.u. the donor possesses
one bound state, for 0.117<\gamma<1.68 a.u. there exist two bound states and
for field strengths \gamma>1.68 a.u. the system possesses three bound states.
Further relevant characteristics of negative donors in magnetic fields are
provided.Comment: 7 pages, 1 figur
Self-Trapping of Polarons in the Rashba-Pekar Model
We performed quantum Monte Carlo study of the exciton-polaron model which
features the self-trapping phenomenon when the coupling strength and/or
particle momentum is varied. For the first time accurate data for energy,
effective mass, the structure of the polaronic cloud, dispersion law, and
spectral function are available throughout the crossover region. We observed
that self-trapping can not be reduced to hybridization of two states with
different lattice deformation, and that at least three states are involved in
the crossover from light- to heavy-mass regimes.Comment: 5 pages, 5 figures, Accepted to Phys. Rev. B Rapid Communication
Donor Centers and Absorption Spectra in Quantum Dots
We have studied the electronic properties and optical absorption spectra of
three different cases of donor centers, D^{0}, D^{-} and D^{2-}, which are
subjected to a perpendicular magnetic field, using the exact diagonalization
method. The energies of the lowest lying states are obtained as function of the
applied magnetic field strength B and the distance zeta between the positive
ion and the confinement xy-plane. Our calculations indicate that the positive
ion induces transitions in the ground-state, which can be observed clearly in
the absorption spectra, but as zeta goes to 0 the strength of the applied
magnetic field needed for a transition to occur tends to infinity.Comment: 5 pages, 4 figures, REVTeX 4, gzipped tar fil
Direct observation by resonant tunneling of the B^+ level in a delta-doped silicon barrier
We observe a resonance in the conductance of silicon tunneling devices with a
delta-doped barrier. The position of the resonance indicates that it arises
from tunneling through the B^+ state of the boron atoms of the delta-layer.
Since the emitter Fermi level in our devices is a field-independent reference
energy, we are able to directly observe the diamagnetic shift of the B^+ level.
This is contrary to the situation in magneto-optical spectroscopy, where the
shift is absorbed in the measured ionization energy.Comment: submitted to PR
Binding Energy of Hydrogen-Like Impurities in Quantum Well Wires of InSb/GaAs in a Magnetic Field
The binding energy of a hydrogen-like impurity in a thin size-quantized wire of the InSb/GaAs semiconductors with Kane’s dispersion law in a magnetic fieldBparallel to the wire axis has been calculated as a function of the radius of the wire and magnitude ofB, using a variational approach. It is shown that when wire radius is less than the Bohr radius of the impurity, the nonparabolicity of dispersion law of charge carriers leads to a considerable increase of the binding energy in the magnetic field, as well as to a more rapid growth of binding energy with growth ofB
- …