2,450 research outputs found
Highly Non-linear Excitonic Zeeman Spin-Splitting in Composition-Engineered Artificial Atoms
Non-linear Zeeman splitting of neutral excitons is observed in composition
engineered In(x)Ga(1-x)As self-assembled quantum dots and its microscopic
origin is explained. Eight-band k.p simulations, performed using realistic dot
parameters extracted from cross-sectional scanning tunneling microscopy, reveal
that a quadratic contribution to the Zeeman energy originates from a spin
dependent mixing of heavy and light hole orbital states in the dot. The dilute
In-composition (x<0.35) and large lateral size (40-50 nm) of the quantum dots
investigated is shown to strongly enhance the non-linear excitonic Zeeman gap,
providing a blueprint to enhance such magnetic non-linearities via growth
engineering
Critical voltage of a mesoscopic superconductor
We study the role of the quasiparticle distribution function f on the
properties of a superconducting nanowire. We employ a numerical calculation
based upon the Usadel equation. Going beyond linear response, we find a
non-thermal distribution for f caused by an applied bias voltage. We
demonstrate that the even part of f (the energy mode f_L) drives a first order
transition from the superconducting state to the normal state irrespective of
the current
Composition profiling InAs quantum dots and wetting layers by atom probe tomography and cross-sectional scanning tunnelling microscopy
This study compares cross-sectional scanning tunnelling microscopy (XSTM) and
atom probe tomography (APT). We use epitaxially grown self-assembled InAs
quantum dots (QDs) in GaAs as an exemplary material with which to compare these
two nanostructural analysis techniques. We studied the composition of the
wetting layer and the QDs, and performed quantitative comparisons of the indium
concentration profiles measured by each method. We show that computational
models of the wetting layer and the QDs, based on experimental data, are
consistent with both analytical approaches. This establishes a link between the
two techniques and shows their complimentary behaviour, an advantage which we
exploit in order to highlight unique features of the examined QD material.Comment: Main article: 8 pages, 6 figures. Appendix: 3 pages, 5 figure
Benchmarking high fidelity single-shot readout of semiconductor qubits
Determination of qubit initialisation and measurement fidelity is important
for the overall performance of a quantum computer. However, the method by which
it is calculated in semiconductor qubits varies between experiments. In this
paper we present a full theoretical analysis of electronic single-shot readout
and describe critical parameters to achieve high fidelity readout. In
particular, we derive a model for energy selective state readout based on a
charge detector response and examine how to optimise the fidelity by choosing
correct experimental parameters. Although we focus on single electron spin
readout, the theory presented can be applied to other electronic readout
techniques in semiconductors that use a reservoir.Comment: 19 pages, 8 figure
Efficient Reactive Brownian Dynamics
We develop a Split Reactive Brownian Dynamics (SRBD) algorithm for particle
simulations of reaction-diffusion systems based on the Doi or volume reactivity
model, in which pairs of particles react with a specified Poisson rate if they
are closer than a chosen reactive distance. In our Doi model, we ensure that
the microscopic reaction rules for various association and disassociation
reactions are consistent with detailed balance (time reversibility) at
thermodynamic equilibrium. The SRBD algorithm uses Strang splitting in time to
separate reaction and diffusion, and solves both the diffusion-only and
reaction-only subproblems exactly, even at high packing densities. To
efficiently process reactions without uncontrolled approximations, SRBD employs
an event-driven algorithm that processes reactions in a time-ordered sequence
over the duration of the time step. A grid of cells with size larger than all
of the reactive distances is used to schedule and process the reactions, but
unlike traditional grid-based methods such as Reaction-Diffusion Master
Equation (RDME) algorithms, the results of SRBD are statistically independent
of the size of the grid used to accelerate the processing of reactions. We use
the SRBD algorithm to compute the effective macroscopic reaction rate for both
reaction- and diffusion-limited irreversible association in three dimensions.
We also study long-time tails in the time correlation functions for reversible
association at thermodynamic equilibrium. Finally, we compare different
particle and continuum methods on a model exhibiting a Turing-like instability
and pattern formation. We find that for models in which particles diffuse off
lattice, such as the Doi model, reactions lead to a spurious enhancement of the
effective diffusion coefficients.Comment: To appear in J. Chem. Phy
Single-charge detection by an atomic precision tunnel junction
We demonstrate sensitive detection of single charges using a planar tunnel junction 8.5 nm wide and 17.2 nm long defined by an atomically precise phosphorus doping profile in silicon. The conductance of the junction responds to a nearby gate potential and also to changes in the charge state of a quantum dot patterned 52 nm away. The response of this detector is monotonic across the entire working voltage range of the device, which will make it particularly useful for studying systems of multiple quantum dots. The charge sensitivity is maximized when the junction is most conductive, suggesting that more sensitive detection can be achieved by shortening the length of the junction to increase its conductance
Applying infrared thermography to soil surface temperature monitoring: Case study of a high-resolution 48 h survey in a vineyard (Anadia, Portugal)
The soil surface albedo decreases with an increasing biochar application rate as a power decay function, but the net impact of biochar application on soil temperature dynamics remains to be clarified. The objective of this study was to assess the potential of infrared thermography (IRT) sensing by monitoring soil surface temperature (SST) with a high spatiotemporal and thermal resolution in a scalable agricultural application. We monitored soil surface temperature (SST) variations over a 48 h period for three treatments in a vineyard: bare soil (plot S), 100% biochar cover (plot B), and biochar-amended topsoil (plot SB). The SST of all plots was monitored at 30 min intervals with a tripod-mounted IR thermal camera. The soil temperature at 10 cm depth in the S and SB plots was monitored continuously with a 5 min resolution probe. Plot B had greater daily SST variations, reached a higher daily temperature peak relative to the other plots, and showed a faster rate of T increase during the day. However, on both days, the SST of plot B dipped below that of the control treatment (plot S) and biochar-amended soil (plot SB) from about 18:00 onward and throughout the night. The diurnal patterns/variations in the IRT-measured SSTs were closely related to those in the soil temperature at a 10 cm depth, confirming that biochar-amended soils showed lower thermal inertia than the unamended soil. The experiment provided interesting insights into SST variations at a local scale. The case study may be further developed using fully automated SST monitoring protocols at a larger scale for a range of environmental and agricultural applications
Shape control of QDs studied by cross-sectional scanning tunneling microscopy
In this cross-sectional scanning tunneling microscopy study we investigated
various techniques to control the shape of self-assembled quantum dots (QDs)
and wetting layers (WLs). The result shows that application of an indium flush
during the growth of strained InGaAs/GaAs QD layers results in flattened QDs
and a reduced WL. The height of the QDs and WLs could be controlled by varying
the thickness of the first capping layer. Concerning the technique of antimony
capping we show that the surfactant properties of Sb result in the preservation
of the shape of strained InAs/InP QDs during overgrowth. This could be achieved
by both a growth interrupt under Sb flux and capping with a thin GaAsSb layer
prior to overgrowth of the uncapped QDs. The technique of droplet epitaxy was
investigated by a structural analysis of strain free GaAs/AlGaAs QDs. We show
that the QDs have a Gaussian shape, that the WL is less than 1 bilayer thick,
and that minor intermixing of Al with the QDs takes place.Comment: 7 pages, 10 figure
Shape control of QDs studied by cross-sectional scanning tunneling microscopy
In this cross-sectional scanning tunneling microscopy study we investigated
various techniques to control the shape of self-assembled quantum dots (QDs)
and wetting layers (WLs). The result shows that application of an indium flush
during the growth of strained InGaAs/GaAs QD layers results in flattened QDs
and a reduced WL. The height of the QDs and WLs could be controlled by varying
the thickness of the first capping layer. Concerning the technique of antimony
capping we show that the surfactant properties of Sb result in the preservation
of the shape of strained InAs/InP QDs during overgrowth. This could be achieved
by both a growth interrupt under Sb flux and capping with a thin GaAsSb layer
prior to overgrowth of the uncapped QDs. The technique of droplet epitaxy was
investigated by a structural analysis of strain free GaAs/AlGaAs QDs. We show
that the QDs have a Gaussian shape, that the WL is less than 1 bilayer thick,
and that minor intermixing of Al with the QDs takes place.Comment: 7 pages, 10 figure
Possible experiment to check the reality of a nonequilibrium temperature
An experiment is proposed to check the physical reality of a nonequilibrium absolute temperature previously proposed from theoretical grounds in the framework of extended irreversible thermodynamics
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