366 research outputs found
Modeling Uranium Transport in Koongarra, Australia: The Effect of a Moving Weathering Zone
Natural analogues are an important source of long-term data and may be viewed as naturally occurring experiments that often include processes, phenomena, and scenarios that are important to nuclear waste disposal safety assessment studies. The Koongarra uranium deposit in the Alligator Rivers region of Australia is one of the best-studied natural analogue sites. The deposit has been subjected to chemical weathering over several million years, during which many climatological, hydrological, and geological changes have taken place, resulting in the mobilization and spreading of uranium. Secondary uranium mineralization and dispersed uranium are present from the surface down to the base of the weathering zone, some 25 m deep. In this work, a simple uranium transport model is presented and sensitivity analyses are conducted for key model parameters. Analyses of field and laboratory data show that three layers can be distinguished in the Koongarra area: (1) a top layer that is fully weathered, (2) an intermediate layer that is partially weathered (the weathering zone), and (3) a lower layer that is unweathered. The weathering zone has been moving downward as the weathering process proceeds. Groundwater velocities are found to be largest in the weathering zone. Transport of uranium is believed to take place primarily in this zone. It appears that changes in the direction of groundwater flow have not had a significant effect on the uranium dispersion pattern. The solid-phase uranium data show that the uranium concentration does not significantly change with depth within the fully weathered zone. This implies that uranium transport has stopped in these layers. A two-dimensional vertically integrated model for transport of uranium in the weathering zone has been developed. Simulations with a velocity field constant in time and space have been carried out, taking into account the downward movement of this zone and the dissolution of uranium in the orebody. The latter has been modelled by a nonequilibrium relationship. In these simulations, pseudo-steady state uranium distributions are computed. The main conclusion drawn from this study is that the movement of the weathering zone and the nonequilibrium dissolution of uranium in the orebody play an important role in the transport of uranium. Despite the fact that the model is a gross simplification of what has actually happened in the past two million years, a reasonable fit of calculated and observed uranium distributions was obtained with acceptable values for the model parameters
Four-Majorana qubit with charge readout: dynamics and decoherence
We present a theoretical analysis of a Majorana-based qubit consisting of two
topological superconducting islands connected via a Josephson junction. The
qubit is operated by electrostatic gates which control the coupling of two of
the four Majorana zero modes. At the end of the operation, readout is performed
in the charge basis. Even though the operations are not topologically
protected, the proposed experiment can potentially shed light on the coherence
of the parity degree of freedom in Majorana devices and serve as a first step
towards topological Majorana qubits. We discuss in detail the charge-stability
diagram and its use for characterizing the parameters of the devices, including
the overlap of the Majorana edge states. We describe the multi-level spectral
properties of the system and present a detailed study of its controlled
coherent oscillations, as well as decoherence resulting from coupling to a
non-Markovian environment. In particular, we study a gate-controlled protocol
where conversion between Coulomb-blockade and transmon regimes generates
coherent oscillations of the qubit state due to the overlap of Majorana modes.
We show that, in addition to fluctuations of the Majorana coupling,
considerable measurement errors may be accumulated during the conversion
intervals when electrostatic fluctuations in the superconducting islands are
present. These results are also relevant for several proposed implementations
of topological qubits which rely on readout based on charge detection
Electron-hole interactions in coupled InAs-GaSb quantum dots based on nanowire crystal phase templates
We report growth and characterization of a coupled quantum dot structure that
utilizes nanowire templates for selective epitaxy of radial heterostructures.
The starting point is a zinc blende InAs nanowire with thin segments of
wurtzite structure. These segments have dual roles: they act as tunnel barriers
for electron transport in the InAs core, and they also locally suppress growth
of a GaSb shell, resulting in coaxial InAs-GaSb quantum dots with integrated
electrical probes. The parallel quantum dot structure hosts spatially separated
electrons and holes that interact due to the type-II broken gap of InAs-GaSb
heterojunctions. The Coulomb blockade in the electron and hole transport is
studied, and periodic interactions of electrons and holes are observed and can
be reproduced by modeling. Distorted Coulomb diamonds indicate voltage-induced
ground-state transitions, possibly a result of changes in the spatial
distribution of holes in the thin GaSb shell.Comment: 8 pages, 7 figure
Single-electron transport in InAs nanowire quantum dots formed by crystal phase engineering
We report electrical characterization of quantum dots formed by introducing
pairs of thin wurtzite (WZ) segments in zinc blende (ZB) InAs nanowires.
Regular Coulomb oscillations are observed over a wide gate voltage span,
indicating that WZ segments create significant barriers for electron transport.
We find a direct correlation of transport properties with quantum dot length
and corresponding growth time of the enclosed ZB segment. The correlation is
made possible by using a method to extract lengths of nanowire crystal phase
segments directly from scanning electron microscopy images, and with support
from transmission electron microscope images of typical nanowires. From
experiments on controlled filling of nearly empty dots with electrons, up to
the point where Coulomb oscillations can no longer be resolved, we estimate a
lower bound for the ZB-WZ conduction-band offset of 95 meV.Comment: 9 pages 9 figure
Solute Transport in Soil
Solute transport is of importance in view of the movement of nutrient elements, e.g. towards the plant root system, and because of a broad range of pollutants. Pollution is not necessarily man induced, but may be due to geological or geohydrological causes, e.g. in the cases of pollution with arsenic, and salt. For the polluting species, a distinction can be made between dissolved and immiscible, and between conservative and reactive. Dissolved pollutants (aqueous phase pollutants) will spread with the groundwater due to groundwater flow, diffusion and dispersion
Spectroscopy and level detuning of few-electron spin states in parallel InAs quantum dots
We use tunneling spectroscopy to study the evolution of few-electron spin
states in parallel InAs nanowire double quantum dots (QDs) as a function of
level detuning and applied magnetic field. Compared to the much more studied
serial configuration, parallel coupling of the QDs to source and drain greatly
expands the probing range of excited state transport. Owing to a strong
confinement, we can here isolate transport involving only the very first
interacting single QD orbital pair. For the (2,0)-(1,1) charge transition, with
relevance for spin-based qubits, we investigate the excited (1,1) triplet, and
hybridization of the (2,0) and (1,1) singlets. An applied magnetic field splits
the (1,1) triplet, and due to spin-orbit induced mixing with the (2,0) singlet,
we clearly resolve transport through all triplet states near the avoided
singlet-triplet crossings. Transport calculations, based on a simple model with
one orbital on each QD, fully replicate the experimental data. Finally, we
observe an expected mirrored symmetry between the 1-2 and 2-3 electron
transitions resulting from the two-fold spin degeneracy of the orbitals.Comment: 17 pages, 8 figure
Electrical control of spins and giant g-factors in ring-like coupled quantum dots
Emerging theoretical concepts for quantum technologies have driven a
continuous search for structures where a quantum state, such as spin, can be
manipulated efficiently. Central to many concepts is the ability to control a
system by electric and magnetic fields, relying on strong spin-orbit
interaction and a large g-factor. Here, we present a new mechanism for spin and
orbital manipulation using small electric and magnetic fields. By hybridizing
specific quantum dot states at two points inside InAs nanowires, nearly perfect
quantum rings form. Large and highly anisotropic effective g-factors are
observed, explained by a strong orbital contribution. Importantly, we find that
the orbital and spin-orbital contributions can be efficiently quenched by
simply detuning the individual quantum dot levels with an electric field. In
this way, we demonstrate not only control of the effective g-factor from 80 to
almost 0 for the same charge state, but also electrostatic change of the ground
state spin
A quantum-dot heat engine operating close to the thermodynamic efficiency limits
Cyclical heat engines are a paradigm of classical thermodynamics, but are
impractical for miniaturization because they rely on moving parts. A more
recent concept is particle-exchange (PE) heat engines, which uses energy
filtering to control a thermally driven particle flow between two heat
reservoirs. As they do not require moving parts and can be realized in
solid-state materials, they are suitable for low-power applications and
miniaturization. It was predicted that PE engines could reach the same
thermodynamically ideal efficiency limits as those accessible to cyclical
engines, but this prediction has not been verified experimentally. Here, we
demonstrate a PE heat engine based on a quantum dot (QD) embedded into a
semiconductor nanowire. We directly measure the engine's steady-state electric
power output and combine it with the calculated electronic heat flow to
determine the electronic efficiency . We find that at the maximum power
conditions, is in agreement with the Curzon-Ahlborn efficiency and that
the overall maximum is in excess of 70 of the Carnot efficiency
while maintaining a finite power output. Our results demonstrate that
thermoelectric power conversion can, in principle, be achieved close to the
thermodynamic limits, with direct relevance for future hot-carrier
photovoltaics, on-chip coolers or energy harvesters for quantum technologies
Topological superconductivity in semiconductor-superconductor-magnetic insulator heterostructures
Hybrid superconductor-semiconductor heterostructures are promising platforms
for realizing topological superconductors and exploring Majorana bound states
physics. Motivated by recent experimental progress, we theoretically study how
magnetic insulators offer an alternative to the use of external magnetic fields
for reaching the topological regime. We consider different setups, where: (1)
the magnetic insulator induces an exchange field in the superconductor, which
leads to a splitting in the semiconductor by proximity effect, and (2) the
magnetic insulator acts as a spin-filter tunnel barrier between the
superconductor and the semiconductor. We show that the spin splitting in the
superconductor alone cannot induce a topological transition in the
semiconductor. To overcome this limitation, we propose to use a spin-filter
barrier that enhances the magnetic exchange and provides a mechanism for a
topological phase transition. Moreover, the spin-dependent tunneling introduces
a strong dependence on the band alignment, which can be crucial in
quantum-confined systems. This mechanism opens up a route towards networks of
topological wires with fewer constraints on device geometry compared to
previous devices that require external magnetic fields.Comment: 9+5 pages, 6 figure
Probing Transverse Magnetic Anisotropy by Electronic Transport through a Single-Molecule Magnet
By means of electronic transport, we study the transverse magnetic anisotropy
of an individual Fe single-molecule magnet (SMM) embedded in a
three-terminal junction. In particular, we determine in situ the transverse
anisotropy of the molecule from the pronounced intensity modulations of the
linear conductance, which are observed as a function of applied magnetic field.
The proposed technique works at temperatures exceeding the energy scale of the
tunnel splittings of the SMM. We deduce that the transverse anisotropy for a
single Fe molecule captured in a junction is substantially larger than the
bulk value.Comment: 18 pages with 16 figures; version as publishe
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