338 research outputs found
Excitonic absorption in gate controlled graphene quantum dots
We present a theory of excitonic processes in gate controlled graphene
quantum dots. The dependence of the energy gap on shape, size and edge for
graphene quantum dots with up to a million atoms is predicted. Using a
combination of tight-binding, Hartree-Fock and configuration interaction
methods, we show that triangular graphene quantum dots with zigzag edges
exhibit optical transitions simultaneously in the THz, visible and UV spectral
ranges, determined by strong electron-electron and excitonic interactions. The
relationship between optical properties and finite magnetic moment and charge
density controlled by an external gate is predicted.Comment: ~4 pages, 4 figure
An Anderson-Fano Resonance and Shake-Up Processes in the Magneto-Photoluminescence of a Two-Dimensional Electron System
We report an anomalous doublet structure and low-energy satellite in the
magneto-photoluminescence spectra of a two-dimensional electron system. The
doublet structure moves to higher energy with increasing magnetic field and is
most prominent at odd filling factors 5 and 3. The lower-energy satellite peak
tunes to lower energy for increasing magnetic field between filling factor 6
and 2. These features occur at energies below the fundamental band of
recombination originating from the lowest Landau level and display striking
magnetic field and temperature dependence that indicates a many-body origin.
Drawing on a recent theoretical description of Hawrylak and Potemski, we show
that distinct mechanisms are responsible for each feature.Comment: 14 pages including 5 figures. To appear in the April 15th edition of
Phy. Rev. B. rapid com
Electronic shells of Dirac fermions in graphene quantum rings in a magnetic field
We present results of tight binding calculations demonstrating existence of
degenerate electronic shells of Dirac Fermions in narrow, charge neutral
graphene quantum rings. We predict removal of degeneracy with finite magnetic
field. We show, using a combination of tight binding and configuration
interaction methods, that by filling a graphene ring with additional electrons
this carbon based structure with half-filled shell acquires a finite magnetic
moment.Comment: 10 pages, 4 figure
Coulomb effects in semiconductor quantum dots
Coulomb correlations in the optical spectra of semiconductor quantum dots are
investigated using a full-diagonalization approach. The resulting multi-exciton
spectra are discussed in terms of the symmetry of the involved states.
Characteristic features of the spectra like the nearly equidistantly spaced
s-shell emission lines and the approximately constant p-shell transition
energies are explained using simplified Hamiltonians that are derived taking
into account the relative importance of various interaction contributions.
Comparisons with previous results in the literature and their interpretation
are made.Comment: 7 pages, 2 figure
Spin and electronic correlations in gated graphene quantum rings
We present a theory of graphene quantum rings designed to produce degenerate
shells of single particle states close to the Fermi level. We show that
populating these shells with carriers using a gate leads to correlated ground
states with finite total electronic spin. Using a combination of tight-binding
and configuration interaction methods we predict ground state and total spin of
the system as a function of the filling of the shell. We show that for smaller
quantum rings, the spin polarization of the ground state at half filling
depends strongly on the size of the system, but reaches a maximum value after
reaching a critical size.Comment: 7 pages, 8 figure
Electronic properties of gated triangular graphene quantum dots: Magnetism, correlations, and geometrical effects
We present a theory of electronic properties of gated triangular graphene
quantum dots with zigzag edges as a function of size and carrier density. We
focus on electronic correlations, spin and geometrical effects using a
combination of atomistic tight-binding, Hartree-Fock and configuration
interaction methods (TB+HF+CI) including long range Coulomb interactions. The
single particle energy spectrum of triangular dots with zigzag edges exhibits a
degenerate shell at the Fermi level with a degeneracy N_{edge} proportional to
the edge size. We determine the effect of the electron-electron interactions on
the ground state, the total spin and the excitation spectrum as a function of a
shell filling and the degeneracy of the shell using TB+HF+CI for N_{edge} < 12
and approximate CI method for N_{edge}\geq 12. For a half-filled neutral shell
we find spin polarized ground state for structures up to N=500 atoms in
agreement with previous {\it ab initio} and mean-field calculations, and in
agreement with Lieb's theorem for a Hubbard model on a bipartite lattice.
Adding a single electron leads to the complete spin depolarization for
N_{edge}\leq 9. For larger structures, the spin depolarization is shown to
occur at different filling factors. Away from half-fillings excess
electrons(holes) are shown to form Wigner-like spin polarized triangular
molecules corresponding to large gaps in the excitation spectrum. The validity
of conclusions is assessed by a comparison of results obtained from different
levels of approximations. While for the charge neutral system all methods give
qualitatively similar results, away from the charge neutrality an inclusion of
all Coulomb scattering terms is necessary to produce results presented here.Comment: 13 pages, 13 figure
Self Assembled II-VI Magnetic Quantum Dot as a Voltage-Controlled Spin-Filter
A key element in the emergence of a full spintronics technology is the
development of voltage controlled spin filters to selectively inject carriers
of desired spin into semiconductors. We previously demonstrated a prototype of
such a device using a II-VI dilute-magnetic semiconductor quantum well which,
however, still required an external magnetic field to generate the level
splitting. Recent theory suggests that spin selection may be achievable in
II-VI paramagnetic semiconductors without external magnetic field through local
carrier mediated ferromagnetic interactions. We present the first experimental
observation of such an effect using non-magnetic CdSe self-assembled quantum
dots in a paramagnetic (Zn,Be,Mn)Se barrier.Comment: 4 pages, 4 figure
Atomistic theory of electronic and optical properties of InAs/InP self-assembled quantum dots on patterned substrates
We report on a atomistic theory of electronic structure and optical
properties of a single InAs quantum dot grown on InP patterned substrate. The
spatial positioning of individual dots using InP nano-templates results in a
quantum dot embedded in InP pyramid. The strain distribution of a quantum dot
in InP pyramid is calculated using the continuum elasticity theory. The
electron and valence hole single-particle states are calculated using atomistic
effective-bond-orbital model with second nearest-neighbor interactions, coupled
to strain via Bir-Pikus Hamiltonian. The optical properties are determined by
solving many-exciton Hamiltonian for interacting electron and hole complexes
using the configuration-interaction method. The effect of positioning of
quantum dots using nanotemplate on their optical spectra is determined by a
comparison with dots on unpatterned substrates, and with experimental results.
The possibility of tuning the quantum dot properties with varying the
nano-template is explored.Comment: 9 pages, 12 figure
Theory of exciton fine structure in semiconductor quantum dots: quantum dot anisotropy and lateral electric field
Theory of exciton fine structure in semiconductor quantum dots and its
dependence on quantum dot anisotropy and external lateral electric field is
presented. The effective exciton Hamiltonian including long range electron-hole
exchange interaction is derived within the k*p effective mass approximation
(EMA). The exchange matrix elements of the Hamiltonian are expressed explicitly
in terms of electron and hole envelope functions. The matrix element
responsible for the "bright" exciton splitting is identified and analyzed. An
excitonic fine structure for a model quantum dot with quasi- two-dimensional
anisotropic harmonic oscillator (2DLAHO) confining potential is analyzed as a
function of the shape anisotropy, size and applied lateral electric field
Zero-energy states in triangular and trapezoidal graphene structures
We derive analytical solutions for the zero-energy states of degenerate shell
obtained as a singular eigenevalue problem found in tight-binding (TB)
Hamiltonian of triangular graphene quantum dots with zigzag edges. These
analytical solutions are in agreement with previous TB and density functional
theory (DFT) results for small graphene triangles and extend to arbitrary size.
We also generalize these solutions to trapezoidal structure which allow us to
study bowtie graphene devices.Comment: 4 pages, 4 figure
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