10,462 research outputs found
Optically induced spin to charge transduction in donor spin read-out
The proposed read-out configuration D+D- for the Kane Si:P
architecture[Nature 393, 133 (1998)] depends on spin-dependent electron
tunneling between donors, induced adiabatically by surface gates. However,
previous work has shown that since the doubly occupied donor state is so
shallow the dwell-time of the read-out state is less than the required time for
measurement using a single electron transistor (SET). We propose and analyse
single-spin read-out using optically induced spin to charge transduction, and
show that the top gate biases, required for qubit selection, are significantly
less than those demanded by the Kane scheme, thereby increasing the D+D-
lifetime. Implications for singlet-triplet discrimination for electron spin
qubits are also discussed.Comment: 8 pages, 10 figures; added reference, corrected typ
Donor Electron Wave Functions for Phosphorus in Silicon: Beyond Effective Mass Theory
We calculate the electronic wave-function for a phosphorus donor in silicon
by numerical diagonalisation of the donor Hamiltonian in the basis of the pure
crystal Bloch functions. The Hamiltonian is calculated at discrete points
localised around the conduction band minima in the reciprocal lattice space.
Such a technique goes beyond the approximations inherent in the effective-mass
theory, and can be modified to include the effects of altered donor impurity
potentials, externally applied electro-static potentials, as well as the
effects of lattice strain. Modification of the donor impurity potential allows
the experimentally known low-lying energy spectrum to be reproduced with good
agreement, as well as the calculation of the donor wavefunction, which can then
be used to calculate parameters important to quantum computing applications.Comment: 10 pages, 5 figure
Voltage Control of Exchange Coupling in Phosphorus Doped Silicon
Motivated by applications to quantum computer architectures we study the
change in the exchange interaction between neighbouring phosphorus donor
electrons in silicon due to the application of voltage biases to surface
control electrodes. These voltage biases create electro-static fields within
the crystal substrate, perturbing the states of the donor electrons and thus
altering the strength of the exchange interaction between them. We find that
control gates of this kind can be used to either enhance, or reduce the
strength of the interaction, by an amount that depends both on the magnitude
and orientation of the donor separation.Comment: 5 Pages, 5 Figure
Quantum-Dot Cellular Automata using Buried Dopants
The use of buried dopants to construct quantum-dot cellular automata is
investigated as an alternative to conventional electronic devices for
information transport and elementary computation. This provides a limit in
terms of miniaturisation for this type of system as each potential well is
formed by a single dopant atom. As an example, phosphorous donors in silicon
are found to have good energy level separation with incoherent switching times
of the order of microseconds. However, we also illustrate the possibility of
ultra-fast quantum coherent switching via adiabatic evolution. The switching
speeds are numerically calculated and found to be 10's of picoseconds or less
for a single cell. The effect of decoherence is also simulated in the form of a
dephasing process and limits are estimated for operation with finite dephasing.
The advantages and limitations of this scheme over the more conventional
quantum-dot based scheme are discussed. The use of a buried donor cellular
automata system is also discussed as an architecture for testing several
aspects of buried donor based quantum computing schemes.Comment: Minor changes in response to referees comments. Improved section on
scaling and added plot of incoherent switching time
A theoretical investigation into the microwave spectroscopy of a phosphorus-donor charge-qubit in silicon: Coherent control in the Si:P quantum computer architecture
We present a theoretical analysis of a microwave spectroscopy experiment on a
charge qubit defined by a P donor pair in silicon, for which we calculate
Hamiltonian parameters using the effective-mass theory of shallow donors. We
solve the master equation of the driven system in a dissipative environment to
predict experimental outcomes. We describe how to calculate physical parameters
of the system from such experimental results, including the dephasing time,
, and the ratio of the resonant Rabi frequency to the relaxation rate.
Finally we calculate probability distributions for experimentally relevant
system parameters for a particular fabrication regime
Molecular effects in the ionization of N, O and F by intense laser fields
In this paper we study the response in time of N, O and F to
laser pulses having a wavelength of 390nm. We find single ionization
suppression in O and its absence in F, in accordance with experimental
results at nm. Within our framework of time-dependent density
functional theory we are able to explain deviations from the predictions of
Intense-Field Many-Body -Matrix Theory (IMST). We confirm the connection of
ionization suppression with destructive interference of outgoing electron waves
from the ionized electron orbital. However, the prediction of ionization
suppression, justified within the IMST approach through the symmetry of the
highest occupied molecular orbital (HOMO), is not reliable since it turns out
that, e.g. in the case of F, the electronic response to the laser pulse is
rather complicated and does not lead to dominant depletion of the HOMO.
Therefore, the symmetry of the HOMO is not sufficient to predict ionization
suppression. However, at least for F, the symmetry of the dominantly
ionized orbital is consistent with the non-suppression of ionization.Comment: 19 pages, 5 figure
Why one-size-fits-all vaso-modulatory interventions fail to control glioma invasion: in silico insights
There is an ongoing debate on the therapeutic potential of vaso-modulatory
interventions against glioma invasion. Prominent vasculature-targeting
therapies involve functional tumour-associated blood vessel deterioration and
normalisation. The former aims at tumour infarction and nutrient deprivation
medi- ated by vascular targeting agents that induce occlusion/collapse of
tumour blood vessels. In contrast, the therapeutic intention of normalising the
abnormal structure and function of tumour vascular net- works, e.g. via
alleviating stress-induced vaso-occlusion, is to improve chemo-, immuno- and
radiation therapy efficacy. Although both strategies have shown therapeutic
potential, it remains unclear why they often fail to control glioma invasion
into the surrounding healthy brain tissue. To shed light on this issue, we
propose a mathematical model of glioma invasion focusing on the interplay
between the mi- gration/proliferation dichotomy (Go-or-Grow) of glioma cells
and modulations of the functional tumour vasculature. Vaso-modulatory
interventions are modelled by varying the degree of vaso-occlusion. We
discovered the existence of a critical cell proliferation/diffusion ratio that
separates glioma invasion re- sponses to vaso-modulatory interventions into two
distinct regimes. While for tumours, belonging to one regime, vascular
modulations reduce the tumour front speed and increase the infiltration width,
for those in the other regime the invasion speed increases and infiltration
width decreases. We show how these in silico findings can be used to guide
individualised approaches of vaso-modulatory treatment strategies and thereby
improve success rates
Deformation of the Fermi surface in the extended Hubbard model
The deformation of the Fermi surface induced by Coulomb interactions is
investigated in the t-t'-Hubbard model. The interplay of the local U and
extended V interactions is analyzed. It is found that exchange interactions V
enhance small anisotropies producing deformations of the Fermi surface which
break the point group symmetry of the square lattice at the Van Hove filling.
This Pomeranchuck instability competes with ferromagnetism and is suppressed at
a critical value of U(V). The interaction V renormalizes the t' parameter to
smaller values what favours nesting. It also induces changes on the topology of
the Fermi surface which can go from hole to electron-like what may explain
recent ARPES experiments.Comment: 5 pages, 4 ps figure
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