29 research outputs found
Large-scale atomistic density functional theory calculations of phosphorus-doped silicon quantum bits
We present density functional theory calculations of phosphorus dopants in
bulk silicon and of several properties relating to their use as spin qubits for
quantum computation. Rather than a mixed pseudopotential or a Heitler-London
approach, we have used an explicit treatment for the phosphorus donor and
examined the detailed electronic structure of the system as a function of the
isotropic doping fraction, including lattice relaxation due to the presence of
the impurity. Doping electron densities and spin densities are examined in
order to study the properties of the dopant electron as a function of the
isotropic doping fraction. Doping potentials are also calculated for use in
calculations of the scattering cross-sections of the phosphorus dopants, which
are important in the understanding of electrically detected magnetic resonance
experiments. We find that the electron density around the dopant leads to
non-spherical features in the doping potentials, such as trigonal lobes in the
(001) plane at energy scales of +12 eV near the nucleus and of -700 meV
extending away from the dopants. These features are generally neglected in
effective mass theory and will affect the coupling between the donor electron
and the phosphorus nucleus. Our density functional calculations reveal detail
in the densities and potentials of the dopants which are not evident in
calculations that do not include explicit treatment of the phosphorus donor
atom and relaxation of the crystal lattice. These details can also be used to
parameterize tight-binding models for simulation of large-scale devices.Comment: 22 pages, 8 figure
Variational treatment of electron-polyatomic molecule scattering calculations using adaptive overset grids
The Complex Kohn variational method for electron-polyatomic molecule
scattering is formulated using an overset grid representation of the scattering
wave function. The overset grid consists of a central grid and multiple dense,
atom-centered subgrids that allow the simultaneous spherical expansions of the
wave function about multiple centers. Scattering boundary conditions are
enforced by using a basis formed by the repeated application of the free
particle Green's function and potential, on the overset
grid in a "Born-Arnoldi" solution of the working equations. The theory is shown
to be equivalent to a specific Pad\'e approximant to the -matrix, and has
rapid convergence properties, both in the number of numerical basis functions
employed and the number of partial waves employed in the spherical expansions.
The method is demonstrated in calculations on methane and CF in the
static-exchange approximation, and compared in detail with calculations
performed with the numerical Schwinger variational approach based on single
center expansions. An efficient procedure for operating with the free-particle
Green's function and exchange operators (to which no approximation is made) is
also described
Laser pulses for coherent xuv Raman excitation
We combine multi-channel electronic structure theory with quantum optimal
control to derive Raman pulse sequences that coherently populate a valence
excited state. For a neon atom, Raman target populations of up to 13% are
obtained. Superpositions of the ground and valence Raman states with a
controllable relative phase are found to be reachable with up to 4.5%
population and phase control facilitated by the pump pulse carrier envelope
phase. Our results open a route to creating core-hole excitations in molecules
and aggregates that locally address specific atoms and represent the first step
towards realization of multidimensional spectroscopy in the xuv and x-ray
regimes
Continuum-electron interferometry for enhancement of photoelectron circular dichroism and measurement of bound, free, and mixed contributions to chiral response
We develop photoelectron interferometry based on laser-assisted extreme
ultraviolet ionization for flexible and robust control of photoelectron
circular dichroism in randomly oriented chiral molecules. A comb of XUV photons
ionizes a sample of chiral molecules in the presence of a time-delayed infrared
or visible laser pulse promoting interferences between components of the
XUV-ionized photoelectron wave packet. In striking contrast to multicolor phase
control schemes relying on pulse shaping techniques, the magnitude of the
resulting chiral signal is here controlled by the time delay between the XUV
and laser pulses. Furthermore, we show that the relative polarization
configurations of the XUV and IR fields allows for disentangling the
contributions of bound and continuum states to the chiral response. Our
proposal provides a simple, robust and versatile tool for the control of
photoelectron circular dichroism and experimentally feasible protocol for
probing the individual contributions of bound and continuum states to the PECD
in a time-resolved manner
Molecular Mechanics Simulations and Improved Tight-binding Hamiltonians for Artificial Light Harvesting Systems: Predicting Geometric Distributions, Disorder, and Spectroscopy of Chromophores in a Protein Environment
We present molecular mechanics {and spectroscopic} calculations on prototype
artificial light harvesting systems consisting of chromophores attached to a
tobacco mosaic virus (TMV) protein scaffold. These systems have been
synthesized and characterized spectroscopically, but information about the
microscopic configurations and geometry of these TMV-templated chromophore
assemblies is largely unknown. We use a Monte Carlo conformational search
algorithm to determine the preferred positions and orientations of two
chromophores, Coumarin 343 together with its linker, and Oregon Green 488, when
these are attached at two different sites (104 and 123) on the TMV protein. The
resulting geometric information shows that the extent of disorder and
aggregation properties, and therefore the optical properties of the
TMV-templated chromophore assembly, are highly dependent on the choice of
chromophores and protein site to which they are bound. We used the results of
the conformational search as geometric parameters together with an improved
tight-binding Hamiltonian to simulate the linear absorption spectra and compare
with experimental spectral measurements. The ideal dipole approximation to the
Hamiltonian is not valid since the distance between chromophores can be very
small. We found that using the geometries from the conformational search is
necessary to reproduce the features of the experimental spectral peaks