9 research outputs found
Detection and measurement of the Dzyaloshinskii-Moriya interaction in double quantum dot systems
Spins in quantum dots can act as the qubit for quantum computation. In this
context we point out that spins on neighboring dots will experience an
anisotropic form of the exchange coupling, called the Dzyaloshinskii-Moriya
(DM) interaction, which mixes the spin singlet and triplet states. This will
have an important effect on both qubit interactions and spin-dependent
tunneling. We show that the interaction depends strongly on the direction of
the external field, which gives an unambiguous signature of this effect. We
further propose a new experiment using coupled quantum dots to detect and
characterize the DM interaction.Comment: Updated version. Submitted to Physical Review
Spin-Valley Kondo Effect in Multi-electron Silicon Quantum Dots
We study the spin-valley Kondo effect of a silicon quantum dot occupied by electrons, with up to four. We show that the Kondo
resonance appears in the Coulomb blockade regimes, but not
in the one, in contrast to the spin-1/2 Kondo effect, which
only occurs at odd. Assuming large orbital level spacings, the
energy states of the dot can be simply characterized by fourfold spin-valley
degrees of freedom. The density of states (DOS) is obtained as a function of
temperature and applied magnetic field using a finite-U equation-of-motion
approach. The structure in the DOS can be detected in transport experiments.
The Kondo resonance is split by the Zeeman splitting and valley splitting for
double- and triple-electron Si dots, in a similar fashion to single-electron
ones. The peak structure and splitting patterns are much richer for the
spin-valley Kondo effect than for the pure spin Kondo effect.Comment: 8 pages, 4 figures, in PRB format. This paper is a sequel to the
paper published in Phys. Rev. B 75, 195345 (2007
Valley Splitting Theory of SiGe/Si/SiGe Quantum Wells
We present an effective mass theory for SiGe/Si/SiGe quantum wells, with an
emphasis on calculating the valley splitting. The theory introduces a valley
coupling parameter, , which encapsulates the physics of the quantum well
interface. The new effective mass parameter is computed by means of a tight
binding theory. The resulting formalism provides rather simple analytical
results for several geometries of interest, including a finite square well, a
quantum well in an electric field, and a modulation doped two-dimensional
electron gas. Of particular importance is the problem of a quantum well in a
magnetic field, grown on a miscut substrate. The latter may pose a numerical
challenge for atomistic techniques like tight-binding, because of its
two-dimensional nature. In the effective mass theory, however, the results are
straightforward and analytical. We compare our effective mass results with
those of the tight binding theory, obtaining excellent agreement.Comment: 13 pages, 7 figures. Version submitted to PR
Water Adsorption at Two Unsolvated Peptides with a Protonated Lysine Residue: From Self-Solvation to Solvation
We study the initial steps of the interaction of water
molecules
with two unsolvated peptides: Ac-Ala<sub>5</sub>-LysH<sup>+</sup> and
Ac-Ala<sub>8</sub>-LysH<sup>+</sup>. Each peptide has two primary
candidate sites for water adsorption near the C-terminus: a protonated
carboxyl group and the protonated ammonium group of LysH<sup>+</sup>, which is fully hydrogen-bonded (self-solvated) in the absence of
water. Earlier experimental studies have shown that H<sub>2</sub>O
adsorbs readily at Ac-Ala<sub>5</sub>-LysH<sup>+</sup> (a non-helical
peptide) but with a much lower propensity at Ac-Ala<sub>8</sub>-LysH<sup>+</sup> (a helix) under the same conditions. The helical conformation
of Ac-Ala<sub>8</sub>-LysH<sup>+</sup> has been suggested as the origin
of the different behavior. We here use first-principles conformational
searches (all-electron density functional theory based on a van der
Waals corrected version of the PBE functional, PBE+vdW) to study the
microsolvation of Ac-Ala<sub>5</sub>-LysH<sup>+</sup> with one to
five water molecules and the monohydration of Ac-Ala<sub>8</sub>-LysH<sup>+</sup>. In both cases, the most favorable water adsorption sites
break intramolecular hydrogen bonds associated with the ammonium group,
in contrast to earlier suggestions in the literature. A simple thermodynamic
model yields Gibbs free energies Δ<i>G</i><sup>0</sup>(<i>T</i>) and equilibrium constants in agreement with
experiments. A qualitative change of the first adsorption site does
not occur. For few water molecules, we do not consider carboxyl deprotonation
or finite-temperature dynamics, but in a liquid solvent, both effects
would be important. Exploratory <i>ab initio</i> molecular
dynamics simulations illustrate the short-time effects of a droplet
of 152 water molecules on the initial unsolvated conformation, including
the deprotonation of the carboxyl group. The self-solvation of the
ammonium group by intramolecular hydrogen bonds is lifted in favor
of a solvation by water
Water Adsorption at Two Unsolvated Peptides with a Protonated Lysine Residue: From Self-Solvation to Solvation
We study the initial steps of the interaction of water
molecules
with two unsolvated peptides: Ac-Ala<sub>5</sub>-LysH<sup>+</sup> and
Ac-Ala<sub>8</sub>-LysH<sup>+</sup>. Each peptide has two primary
candidate sites for water adsorption near the C-terminus: a protonated
carboxyl group and the protonated ammonium group of LysH<sup>+</sup>, which is fully hydrogen-bonded (self-solvated) in the absence of
water. Earlier experimental studies have shown that H<sub>2</sub>O
adsorbs readily at Ac-Ala<sub>5</sub>-LysH<sup>+</sup> (a non-helical
peptide) but with a much lower propensity at Ac-Ala<sub>8</sub>-LysH<sup>+</sup> (a helix) under the same conditions. The helical conformation
of Ac-Ala<sub>8</sub>-LysH<sup>+</sup> has been suggested as the origin
of the different behavior. We here use first-principles conformational
searches (all-electron density functional theory based on a van der
Waals corrected version of the PBE functional, PBE+vdW) to study the
microsolvation of Ac-Ala<sub>5</sub>-LysH<sup>+</sup> with one to
five water molecules and the monohydration of Ac-Ala<sub>8</sub>-LysH<sup>+</sup>. In both cases, the most favorable water adsorption sites
break intramolecular hydrogen bonds associated with the ammonium group,
in contrast to earlier suggestions in the literature. A simple thermodynamic
model yields Gibbs free energies Δ<i>G</i><sup>0</sup>(<i>T</i>) and equilibrium constants in agreement with
experiments. A qualitative change of the first adsorption site does
not occur. For few water molecules, we do not consider carboxyl deprotonation
or finite-temperature dynamics, but in a liquid solvent, both effects
would be important. Exploratory <i>ab initio</i> molecular
dynamics simulations illustrate the short-time effects of a droplet
of 152 water molecules on the initial unsolvated conformation, including
the deprotonation of the carboxyl group. The self-solvation of the
ammonium group by intramolecular hydrogen bonds is lifted in favor
of a solvation by water
Validation Challenge of Density-Functional Theory for PeptidesExample of Ac-Phe-Ala<sub>5</sub>‑LysH<sup>+</sup>
We assess the performance of a group
of exchange-correlation functionals
for predicting the secondary structure of peptide chains, up to a
new many-body dispersion corrected hybrid density functional, dubbed
PBE0+MBD* by its original authors. For the purpose of validation,
we first compare to published, high-level benchmark conformational
energy hierarchies (coupled cluster at the singles, doubles, and perturbative
triples level, CCSDÂ(T)) for 73 conformers of small three-residue peptides,
establishing that the van der Waals corrected PBE0 functional yields
an average error of only ∼20 meV (∼0.5 kcal/mol). This
compares to ∼40–50 meV for nondispersion corrected PBE0
and 40–100 meV for different empirical force fields (estimated
for the alanine tetrapeptide). For longer peptide chains that form
a secondary structure, CCSDÂ(T) level benchmark data are currently
unaffordable. We thus turn to the <i>experimentally</i> well
studied Ac-Phe-Ala<sub>5</sub>-LysH<sup>+</sup> peptide, for which
four closely competing conformers were established by infrared spectroscopy.
For comparison, an exhaustive theoretical conformational space exploration
yields at least 11 competing low energy minima. We show that (i) the
many-body dispersion correction, (ii) the hybrid functional nature
of PBE0+MBD*, and (iii) zero-point corrections are needed to reveal
the four experimentally observed structures as the minima that would
be populated at low temperature