204 research outputs found
Ab initio calculation of the binding energy of impurities in semiconductors: Application to Si nanowires
We discuss the binding energy E_b of impurities in semiconductors within
density functional theory (DFT) and the GW approximation, focusing on donors in
nanowires as an example. We show that DFT succeeds in the calculation of E_b
from the Kohn-Sham (KS) hamiltonian of the ionized impurity, but fails in the
calculation of E_b from the KS hamiltonian of the neutral impurity, as it
misses most of the interaction of the bound electron with the surface
polarization charges of the donor. We trace this deficiency back to the lack of
screened exchange in the present functionals
Describing static correlation in bond dissociation by Kohn-Sham density functional theory
We show that density functional theory within the RPA (random phase
approximation for the exchange-correlation energy) provides a correct
description of bond dissociation in H in a spin-restricted Kohn-Sham
formalism, i.e. without artificial symmetry breaking. We present accurate
adiabatic connection curves both at equilibrium and beyond the Coulson-Fisher
point. The strong curvature at large bond length implies important static
(left-right) correlation, justifying modern hybrid functional constructions but
also demonstrating their limitations. Although exact at infinite and accurate
around the equilibrium bond length, the RPA dissociation curve displays
unphysical repulsion at larger but finite bond lengths. Going beyond the RPA by
including the exact exchange kernel (RPA+X), we find a similar repulsion. We
argue that this deficiency is due to the absence of double excitations in
adiabatic linear response theory. Further analyzing the H dissociation
limit we show that the RPA+X is not size-consistent, in contrast to the RPA.Comment: 15 pages, 5 figure
Topological states in multi-orbital HgTe honeycomb lattices
Research on graphene has revealed remarkable phenomena arising in the
honeycomb lattice. However, the quantum spin Hall effect predicted at the K
point could not be observed in graphene and other honeycomb structures of light
elements due to an insufficiently strong spin-orbit coupling. Here we show
theoretically that 2D honeycomb lattices of HgTe can combine the effects of the
honeycomb geometry and strong spin-orbit coupling. The conduction bands,
experimentally accessible via doping, can be described by a tight-binding
lattice model as in graphene, but including multi-orbital degrees of freedom
and spin-orbit coupling. This results in very large topological gaps (up to 35
meV) and a flattened band detached from the others. Owing to this flat band and
the sizable Coulomb interaction, honeycomb structures of HgTe constitute a
promising platform for the observation of a fractional Chern insulator or a
fractional quantum spin Hall phase.Comment: includes supplementary materia
Boron and nitrogen codoping effect on transport properties of carbon nanotubes
International audienceThis paper reports a theoretical study of the effect of boron and nitrogen codoping on the transport properties of carbon nanotubes (CNTs) at the mesoscopic scale. A new tight-binding parametrization has been set up, based on density functional theory calculations, that enables a reliable description of the electronic structure of realistic BN-doped CNTs. With this model, we have carried out a deep analysis of the electronic mean free path (MFP) exhibited by these nanostructures. The MFP is highly sensitive to the geometry of the scattering centers. We report that the relative distance between B and N atoms in the network influences drastically the electronic conduction. Moreover, we point out that the scattering induced by small hexagonal BN domains in the carbon network is less important than the BN-pair case
The correlation potential in density functional theory at the GW-level: spherical atoms
As part of a project to obtain better optical response functions for nano
materials and other systems with strong excitonic effects we here calculate the
exchange-correlation (XC) potential of density-functional theory (DFT) at a
level of approximation which corresponds to the dynamically- screened-exchange
or GW approximation. In this process we have designed a new numerical method
based on cubic splines which appears to be superior to other techniques
previously applied to the "inverse engineering problem" of DFT, i.e., the
problem of finding an XC potential from a known particle density. The
potentials we obtain do not suffer from unphysical ripple and have, to within a
reasonable accuracy, the correct asymptotic tails outside localized systems.
The XC potential is an important ingredient in finding the particle-conserving
excitation energies in atoms and molecules and our potentials perform better in
this regard as compared to the LDA potential, potentials from GGA:s, and a DFT
potential based on MP2 theory.Comment: 13 pages, 9 figure
Linear density response function within the time-dependent exact-exchange approximation
We have calculated the frequency-dependent exact exchange (EXX) kernel of
time-dependent (TD) density functional theory employing our recently proposed
computational method based on cubic splines. With this kernel we have
calculated the linear density response function and obtained static
polarizabilites, van der Waals coefficients and correlation energies for all
spherical spin compensated atoms up to Argon. Some discrete excitation energies
have also been calculated for Be and Ne. As might be expected, the results of
the TDEXX approximation are close to those of TD Hartree-Fock theory. In
addition, correlation energies obtained by integrating over the strength of the
Coulomb interaction turn out to be highly accurate.Comment: 10 pages, 5 figure
Tunable hole spin-photon interaction based on g-matrix modulation
We consider a spin circuit-QED device where a superconducting microwave
resonator is capacitively coupled to a single hole confined in a semiconductor
quantum dot. Thanks to the strong spin-orbit coupling intrinsic to valence-band
states, the gyromagnetic g-matrix of the hole can be modulated electrically.
This modulation couples the photons in the resonator to the hole spin. We show
that the applied gate voltages and the magnetic-field orientation enable a
versatile control of the spin-photon interaction, whose character can be
switched from fully transverse to fully longitudinal. The longitudinal coupling
is actually maximal when the transverse one vanishes and vice-versa. This
"reciprocal sweetness" results from geometrical properties of the g-matrix and
protects the spin against dephasing or relaxation. We estimate coupling rates
reaching ~ 10 MHz in realistic settings and discuss potential circuit-QED
applications harnessing either the transverse or the longitudinal spin-photon
interaction. Furthermore, we demonstrate that the g-matrix curvature can be
used to achieve parametric longitudinal coupling with enhanced coherence
Hole weak anti-localization in a strained-Ge surface quantum well
We report a magneto-transport study of a two-dimensional hole gas confined to a strained Ge quantum well grown on a relaxed Si0.2Ge0.8 virtual substrate. The conductivity of the hole gas measured as a function of a perpendicular magnetic field exhibits a zero-field peak resulting from weak anti-localization. The peak develops and becomes stronger upon increasing the hole density by means of a top gate electrode. This behavior is consistent with a Rashba-type spin-orbit coupling whose strength is proportional to the perpendicular electric field and hence to the carrier density. In the low-density, the single-subband regime, by fitting the weak anti-localization peak to an analytic model, we extract the characteristic transport time scales and a spin splitting energy ΔSO∼ΔSO∼ 1 meV. Tight-binding calculations show that ΔSO is dominated by a cubic term in the in-plane wave vector. Finally, we observe a weak anti-localization peak also for magnetic fields parallel to the quantum well and associate this finding to an effect of intersubband scattering induced by interface defects
Photoluminescence and photoluminescence excitation studies of lateral size effects in Zn_{1-x}Mn_xSe/ZnSe quantum disc samples of different radii
Quantum disc structures (with diameters of 200 nm and 100 nm) were prepared
from a Zn_{0.72}Mn_{0.28}Se/ZnSe single quantum well structure by electron beam
lithography followed by an etching procedure which combined dry and wet etching
techniques. The quantum disc structures and the parent structure were studied
by photoluminescence and photoluminescence excitation spectroscopy. For the
light-hole excitons in the quantum well region, shifts of the energy positions
are observed following fabrication of the discs, confirming that strain
relaxation occurs in the pillars. The light-hole exciton lines also sharpen
following disc fabrication: this is due to an interplay between strain effects
(related to dislocations) and the lateral size of the discs. A further
consequence of the small lateral sizes of the discs is that the intensity of
the donor-bound exciton emission from the disc is found to decrease with the
disc radius. These size-related effects occur before the disc radius is reduced
to dimensions necessary for lateral quantum confinement to occur but will
remain important when the discs are made small enough to be considered as
quantum dots.Comment: LaTeX2e, 13 pages, 6 figures (epsfig
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