720 research outputs found
Indirect coupling between spins in semiconductor quantum dots
The optically induced indirect exchange interaction between spins in two
quantum dots is investigated theoretically. We present a microscopic
formulation of the interaction between the localized spin and the itinerant
carriers including the effects of correlation, using a set of canonical
transformations. Correlation effects are found to be of comparable magnitude as
the direct exchange. We give quantitative results for realistic quantum dot
geometries and find the largest couplings for one dimensional systems.Comment: 4 pages, 3 figure
Nitrogen-Functionalized Graphene Nanoflakes (GNFs:N): Tunable Photoluminescence and Electronic Structures
This study investigates the strong photoluminescence (PL) and X-ray excited
optical luminescence observed in nitrogen-functionalized 2D graphene nanoflakes
(GNFs:N), which arise from the significantly enhanced density of states in the
region of {\pi} states and the gap between {\pi} and {\pi}* states. The
increase in the number of the sp2 clusters in the form of pyridine-like N-C,
graphite-N-like, and the C=O bonding and the resonant energy transfer from the
N and O atoms to the sp2 clusters were found to be responsible for the blue
shift and the enhancement of the main PL emission feature. The enhanced PL is
strongly related to the induced changes of the electronic structures and
bonding properties, which were revealed by the X-ray absorption near-edge
structure, X-ray emission spectroscopy, and resonance inelastic X-ray
scattering. The study demonstrates that PL emission can be tailored through
appropriate tuning of the nitrogen and oxygen contents in GNFs and pave the way
for new optoelectronic devices.Comment: 8 pages, 6 figures (including toc figure
Control of electron spin decoherence caused by electron-nuclear spin dynamics in a quantum dot
Control of electron spin decoherence in contact with a mesoscopic bath of
many interacting nuclear spins in an InAs quantum dot is studied by solving the
coupled quantum dynamics. The nuclear spin bath, because of its bifurcated
evolution predicated on the electron spin up or down state, measures the
which-state information of the electron spin and hence diminishes its
coherence. The many-body dynamics of nuclear spin bath is solved with a
pair-correlation approximation. In the relevant timescale, nuclear pair-wise
flip-flops, as elementary excitations in the mesoscopic bath, can be mapped
into the precession of non-interacting pseudo-spins. Such mapping provides a
geometrical picture for understanding the decoherence and for devising control
schemes. A close examination of nuclear bath dynamics reveals a wealth of
phenomena and new possibilities of controlling the electron spin decoherence.
For example, when the electron spin is flipped by a -pulse at , its
coherence will partially recover at as a consequence of quantum
disentanglement from the mesoscopic bath. In contrast to the re-focusing of
inhomogeneously broadened phases by conventional spin-echoes, the
disentanglement is realized through shepherding quantum evolution of the bath
state via control of the quantum object. A concatenated construction of pulse
sequences can eliminate the decoherence with arbitrary accuracy, with the
nuclear-nuclear spin interaction strength acting as the controlling small
parameter
Optimized Effective Potential for Extended Hubbard Model
Antiferromagnetic and charge ordered Hartree-Fock solutions of the one-band
Hubbard model with on-site and nearest-neighbor Coulomb repulsions are exactly
mapped onto an auxiliary local Kohn-Sham (KS) problem within a
density-functional theory. The mapping provides a new insight into the
interpretation of the KS equations. (i) With an appropriate choice of the basic
variable, there is a universal form of the KS potential, which is applicable
both for the antiferromagnetic and the charge ordered solutions. (ii) The
Kohn-Sham and Hartree-Fock eigenvalues are interconnected by a scaling
transformation. (iii) The band-gap problem is attributed to the derivative
discontinuity of the basic variable as the function of the electron number,
rather than a finite discontinuity of the KS potential. (iv) It is argued that
the conductivity gap and the energies of spin-wave excitations can be entirely
defined by the parameters of the ground state and the KS eigenvalues.Comment: 21 page, 3 figure
Spin relaxation in low-dimensional systems
We review some of the newest findings on the spin dynamics of carriers and
excitons in GaAs/GaAlAs quantum wells. In intrinsic wells, where the optical
properties are dominated by excitonic effects, we show that exciton-exciton
interaction produces a breaking of the spin degeneracy in two-dimensional
semiconductors. In doped wells, the two spin components of an optically created
two-dimensional electron gas are well described by Fermi-Dirac distributions
with a common temperature but different chemical potentials. The rate of the
spin depolarization of the electron gas is found to be independent of the mean
electron kinetic energy but accelerated by thermal spreading of the carriers.Comment: 1 PDF file, 13 eps figures, Proceedings of the 1998 International
Workshop on Nanophysics and Electronics (NPE-98)- Lecce (Italy
CaB_6: a new semiconducting material for spin electronics
Ferromagnetism was recently observed at unexpectedly high temperatures in
La-doped CaB_6. The starting point of all theoretical proposals to explain this
observation is a semimetallic electronic structure calculated for CaB_6 within
the local density approximation. Here we report the results of parameter-free
quasiparticle calculations of the single-particle excitation spectrum which
show that CaB_6 is not a semimetal but a semiconductor with a band gap of 0.8
eV. Magnetism in La_xCa_{1-x}B_6 occurs just on the metallic side of a Mott
transition in the La-induced impurity band.Comment: 4 pages, 1 postscript figur
DIPPER, a spatiotemporal proteomics atlas of human intervertebral discs for exploring ageing and degeneration dynamics
The spatiotemporal proteome of the intervertebral disc (IVD) underpins its integrity and function. We present DIPPER, a deep and comprehensive IVD proteomic resource comprising 94 genome-wide profiles from 17 individuals. To begin with, protein modules defining key directional trends spanning the lateral and anteroposterior axes were derived from high-resolution spatial proteomes of intact young cadaveric lumbar IVDs. They revealed novel region-specific profiles of regulatory activities and displayed potential paths of deconstruction in the level- and location-matched aged cadaveric discs. Machine learning methods predicted a 'hydration matrisome' that connects extracellular matrix with MRI intensity. Importantly, the static proteome used as point-references can be integrated with dynamic proteome (SILAC/degradome) and transcriptome data from multiple clinical samples, enhancing robustness and clinical relevance. The data, findings, and methodology, available on a web interface (http://www.sbms.hku.hk/dclab/DIPPER/), will be valuable references in the field of IVD biology and proteomic analytics
A theory of ferromagnetism in planar heterostructures of (Mn,III)-V semiconductors
A density functional theory of ferromagnetism in heterostructures of compound
semiconductors doped with magnetic impurities is presented. The variable
functions in the density functional theory are the charge and spin densities of
the itinerant carriers and the charge and localized spins of the impurities.
The theory is applied to study the Curie temperature of planar heterostructures
of III-V semiconductors doped with manganese atoms. The mean-field,
virtual-crystal and effective-mass approximations are adopted to calculate the
electronic structure, including the spin-orbit interaction, and the magnetic
susceptibilities, leading to the Curie temperature. By means of these results,
we attempt to understand the observed dependence of the Curie temperature of
planar -doped ferromagnetic structures on variation of their
properties. We predict a large increase of the Curie Temperature by additional
confinement of the holes in a -doped layer of Mn by a quantum well.Comment: 8 pages, 7 figure
Exact Kohn-Sham exchange kernel for insulators and its long-wavelength behavior
We present an exact expression for the frequency-dependent Kohn-Sham
exact-exchange (EXX) kernel for periodic insulators, which can be employed for
the calculation of electronic response properties within time-dependent (TD)
density-functional theory. It is shown that the EXX kernel has a
long-wavelength divergence behavior of the exact full exchange-correlation
kernel and thus rectifies one serious shortcoming of the adiabatic
local-density approximation and generalized-gradient approximations kernels. A
comparison between the TDEXX and the GW-approximation-Bethe-Salpeter-equation
approach is also made.Comment: two column format 6 pages + 1 figure, to be publisehd in Physical
Review
Detailed Structure of a CDW in a Quenched Random Field
Using high resolution x-ray scattering, we have measured the structure of the
Q_1 CDW in Ta-doped NbSe_3. Detailed line shape analysis of the data
demonstrates that two length scales are required to describe the phase-phase
correlation function. Phase fluctuations with wavelengths less than a new
length scale are suppressed and this is identified with the amplitude
coherence length. We find that xi_a* = 34.4 \pm 10.3 angstroms. Implications
for the physical mechanisms responsible for pinning are discussed.Comment: revtex 3.0, 3 postscript uuencoded figure
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