6,128 research outputs found
The role of interactions, tunneling and harmonic confinement on the adiabatic loading of bosons in an optical lattice
We calculate entropy-temperature curves for interacting bosons in unit filled
optical lattices for both homogeneous and harmonically trapped situations, and
use them to understand how adiabatic changes in the lattice depth affect the
temperature of the system. In a translationally invariant lattice, the zero
tunneling limit facilitates a rather detailed analytic description. Unlike the
non-interacting bosonic system which is always cooled upon adiabatic loading
for low enough initial temperature, the change in the excitation spectrum
induced by interactions can lead to heating. Finite tunneling helps to reduce
this heating. Finally, we study the spatially inhomogeneous system confined in
a parabolic potential and show that the presence of the trap can significantly
reduce the final available temperature, due to the non-vanishing superfluid
component at the edge of the cloud which is present in trapped systems.Comment: 9 pages and 6 figures. Two typos in Sec.IIIA were corrected and some
references were update
Spin-orbit effects in nanowire-based wurtzite semiconductor quantum dots
We study the effect of the Dresselhaus spin-orbit interaction on the
electronic states and spin relaxation rates of cylindrical quantum dots defined
on quantum wires having wurtzite lattice structure. The linear and cubic
contributions of the bulk Dresselhaus spin-orbit coupling are taken into
account, along with the influence of a weak external magnetic field. The
previously found analytic solution for the electronic states of cylindrical
quantum dots with zincblende lattice structures with Rashba interaction is
extended to the case of quantum dots with wurtzite lattices. For the electronic
states in InAs dots, we determine the spin texture and the effective g-factor,
which shows a scaling collapse when plotted as a function of an effective
renormalized dot-size dependent spin-orbit coupling strength. The
acoustic-phonon-induced spin relaxation rate is calculated and the transverse
piezoelectric potential is shown to be the dominant one.Comment: 12 pages, 5 figure
Signatures of molecular correlations in the few-electron dynamics of coupled quantum dots
We study the effect of Coulomb interaction on the few-electron dynamics in
coupled semiconductor quantum dots by exact diagonalization of the few-body
Hamiltonian. The oscillation of carriers is strongly affected by the number of
confined electrons and by the strength of the interdot correlations.
Single-frequency oscillations are found for either uncorrelated or highly
correlated states, while multi-frequency oscillations take place in the
intermediate regime. Moreover, Coulomb interaction renders few-particle
oscillations sensitive to perturbations in spatial directions other than that
of the tunneling, contrary to the single-particle case. The inclusion of
acoustic phonon scattering does not modify the carrier dynamics substantially
at short times, but can damp oscillation modes selectively at long times.Comment: 4 pages, 5 figures, RevTex4 two-column format, to appear in Phys.
Rev.
Spin relaxation near the metal-insulator transition: dominance of the Dresselhaus spin-orbit coupling
We identify the Dresselhaus spin-orbit coupling as the source of the dominant
spin-relaxation mechanism in the impurity band of doped semiconductors. The
Dresselhaus-type (i.e. allowed by bulk-inversion asymmetry) hopping terms are
derived and incorporated into a tight-binding model of impurity sites, and they
are shown to unexpectedly dominate the spin relaxation, leading to
spin-relaxation times in good agreement with experimental values. This
conclusion is drawn from two complementary approaches employed to extract the
spin-relaxation time from the effective Hamiltonian: an analytical
diffusive-evolution calculation and a numerical finite-size scaling.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
Triplet-Singlet Spin Relaxation in Quantum Dots with Spin-Orbit Coupling
We estimate the triplet-singlet relaxation rate due to spin-orbit coupling
assisted by phonon emission in weakly-confined quantum dots. Our results for
two and four electrons show that the different triplet-singlet relaxation
trends observed in recent experiments under magnetic fields can be understood
within a unified theoretical description, as the result of the competition
between spin-orbit coupling and phonon emission efficiency. Moreover, we show
that both effects are greatly affected by the strength of the confinement and
the external magnetic field, which may give access to very long-lived triplet
states as well as to selective population of the triplet Zeeman sublevels.Comment: 5 pages, 3 figures. Closely related to recent experiments in
cond-mat/060972
An SPQR-tree-like embedding representation for level planarity
An SPQR-tree is a data structure that efficiently represents all planar embeddings of a biconnected planar graph. It is a key tool in a number of constrained planarity testing algorithms, which seek a planar embedding of a graph subject to some given set of constraints. We develop an SPQR-tree-like data structure that represents all level-planar embeddings of a biconnected level graph with a single source, called the LP-tree, and give a simple algorithm to compute it in linear time. Moreover, we show that LP-trees can be used to adapt three constrained planarity algorithms to the level-planar case by using them as a drop-in replacement for SPQR-trees
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