149 research outputs found
Conduction band tight-binding description for silicon applied to phosphorous donors
A tight-binding parametrization for silicon, optimized to correctly reproduce
effective masses as well as the reciprocal space positions of the
conduction-band minima, is presented. The reliability of the proposed
parametrization is assessed by performing systematic comparisons between the
descriptions of donor impurities in Si using this parametrization and
previously reported ones. The spectral decomposition of the donor wavefunction
demonstrates the importance of incorporating full band effects for a reliable
representation, and that an incomplete real space description results from a
truncated reciprocal space expansion as proposed within the effective mass
theory.Comment: 4 pages, 3 figure
Electric field driven donor-based charge qubits in semiconductors
We investigate theoretically donor-based charge qubit operation driven by
external electric fields. The basic physics of the problem is presented by
considering a single electron bound to a shallow-donor pair in GaAs: This
system is closely related to the homopolar molecular ion H_2^+. In the case of
Si, heteropolar configurations such as PSb^+ pairs are also considered. For
both homopolar and heteropolar pairs, the multivalley conduction band structure
of Si leads to short-period oscillations of the tunnel-coupling strength as a
function of the inter-donor relative position. However, for any fixed donor
configuration, the response of the bound electron to a uniform electric field
in Si is qualitatively very similar to the GaAs case, with no valley quantum
interference-related effects, leading to the conclusion that electric field
driven coherent manipulation of donor-based charge qubits is feasible in
semiconductors
Magnetic field-assisted manipulation and entanglement of Si spin qubits
Architectures of donor-electron based qubits in silicon near an oxide
interface are considered theoretically. We find that the precondition for
reliable logic and read-out operations, namely the individual identification of
each donor-bound electron near the interface, may be accomplished by
fine-tuning electric and magnetic fields, both applied perpendicularly to the
interface. We argue that such magnetic fields may also be valuable in
controlling two-qubit entanglement via donor electron pairs near the interface.Comment: 4 pages, 4 figures. 1 ref and 1 footnote adde
Charge qubits in semiconductor quantum computer architectures: Tunnel coupling and decoherence
We consider charge qubits based on shallow donor electron states in silicon
and coupled quantum dots in GaAs. Specifically, we study the feasibility of
P charge qubits in Si, focusing on single qubit properties in terms of
tunnel coupling between the two phosphorus donors and qubit decoherence caused
by electron-phonon interaction. By taking into consideration the multi-valley
structure of the Si conduction band, we show that inter-valley quantum
interference has important consequences for single-qubit operations of P
charge qubits. In particular, the valley interference leads to a
tunnel-coupling strength distribution centered around zero. On the other hand,
we find that the Si bandstructure does not dramatically affect the
electron-phonon coupling and consequently, qubit coherence. We also critically
compare charge qubit properties for Si:P and GaAs double quantum dot
quantum computer architectures.Comment: 10 pages, 3 figure
Polarons in Carbon Nanotubes
We use ab initio total-energy calculations to predict the existence of
polarons in semiconducting carbon nanotubes (CNTs). We find that the CNTs' band
edge energies vary linearly and the elastic energy increases quadratically with
both radial and with axial distortions, leading to the spontaneous formation of
polarons. Using a continuum model parametrized by the ab initio calculations,
we estimate electron and hole polaron lengths, energies and effective masses
and analyze their complex dependence on CNT geometry. Implications of polaron
effects on recently observed electro- and opto-mechanical behavior of CNTs are
discussed.Comment: Revtex preprint format, 12 pages, 2 eps figures, source in LaTeX.
Accepted for publication in Physical Review Letter
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