10,641 research outputs found
Ground-state energy and Wigner crystallization in thick 2D-electron systems
The ground state energy of the 2-D Wigner crystal is determined as a function
of the thickness of the electron layer and the crystal structure. The method of
evaluating the exchange-correlation energy is tested using known results for
the infinitely-thin 2D system. Two methods, one based on the local-density
approximation(LDA), and another based on the constant-density approximation
(CDA) are established by comparing with quantum Monte-Carlo (QMC) results. The
LDA and CDA estimates for the Wigner transition of the perfect 2D fluid are at
and 32 respectively, compared with from QMC. For thick-2D
layers as found in Hetero-junction-insulated-gate field-effect transistors, the
LDA and CDA predictions of the Wigner transition are at and 15.5
respectively. Impurity effects are not considered here.Comment: Last figure and Table are modified in the revised version.
Conclusions regarding the Wigner transition in thick layers are modified in
the revised version. Latex manuscript, four figure
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
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
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
Total energy global optimizations using non orthogonal localized orbitals
An energy functional for orbital based calculations is proposed, which
depends on a number of non orthogonal, localized orbitals larger than the
number of occupied states in the system, and on a parameter, the electronic
chemical potential, determining the number of electrons. We show that the
minimization of the functional with respect to overlapping localized orbitals
can be performed so as to attain directly the ground state energy, without
being trapped at local minima. The present approach overcomes the multiple
minima problem present within the original formulation of orbital based
methods; it therefore makes it possible to perform calculations for an
arbitrary system, without including any information about the system bonding
properties in the construction of the input wavefunctions. Furthermore, while
retaining the same computational cost as the original approach, our formulation
allows one to improve the variational estimate of the ground state energy, and
the energy conservation during a molecular dynamics run. Several numerical
examples for surfaces, bulk systems and clusters are presented and discussed.Comment: 24 pages, RevTex file, 5 figures available upon reques
Probing the band structure of quadri-layer graphene with magneto-phonon resonance
We show how the magneto-phonon resonance, particularly pronounced in sp2
carbon allotropes, can be used as a tool to probe the band structure of
multilayer graphene specimens. Even when electronic excitations cannot be
directly observed, their coupling to the E2g phonon leads to pronounced
oscillations of the phonon feature observed through Raman scattering
experiments with multiple periods and amplitudes detemined by the electronic
excitation spectrum. Such experiment and analysis have been performed up to 28T
on an exfoliated 4-layer graphene specimen deposited on SiO2, and the observed
oscillations correspond to the specific AB stacked 4-layer graphene electronic
excitation spectrum.Comment: 11 pages, 5 Fi
Evidence of breakdown of the spin symmetry in diluted 2D electron gases
Recent claims of an experimental demonstration of spontaneous spin
polarisation in dilute electron gases \cite{young99} revived long standing
theoretical discussions \cite{ceper99,bloch}. In two dimensions, the
stabilisation of a ferromagnetic fluid might be hindered by the occurrence of
the metal-insulator transition at low densities \cite{abra79}. To circumvent
localisation in the two-dimensional electron gas (2DEG) we investigated the low
populated second electron subband, where the disorder potential is mainly
screened by the high density of the first subband. This letter reports on the
breakdown of the spin symmetry in a 2DEG, revealed by the abrupt enhancement of
the exchange and correlation terms of the Coulomb interaction, as determined
from the energies of the collective charge and spin excitations. Inelastic
light scattering experiments and calculations within the time-dependent local
spin-density approximation give strong evidence for the existence of a
ferromagnetic ground state in the diluted regime.Comment: 4 pages, 4 figures, Revte
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