370 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
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
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 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
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
Optical properties of charged quantum dots doped with a single magnetic impurity
We present a microscopic theory of the optical properties of self-assembled
quantum dots doped with a single magnetic manganese (Mn) impurity and
containing a controlled number of electrons. The single-particle electron and
heavy-hole electronic shells are described by two-dimensional harmonic
oscillators. The electron-electron, electron-hole Coulomb as well as the
short-range electron spin-Mn spin and hole spin-Mn spin contact exchange
interactions are included. The electronic states of the photo-excited
electron-hole-Mn complex and of the final electron-Mn complex are expanded in a
finite number of configurations and the full interacting Hamiltonian is
diagonalized numerically. The emission spectrum is predicted as a function of
photon energy for a given number of electrons and different number of confined
electronic quantum dot shells. We show how emission spectra allow to identify
the number of electronic shells, the number of electrons populating these
shells and, most importantly, their spin. We show that electrons not
interacting directly with the spin of Mn ion do so via electron-electron
interactions. This indirect interaction is a strong effect even when Mn
impurity is away from the quantum dot center.Comment: 12 pages, 10 figure
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
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
Magnetism and correlations in fractionally filled degenerate shells of graphene quantum dots
When an electron is confined to a triangular atomic thick layer of graphene
[1-5] with zig-zag edges, its energy spectrum collapses to a shell of
degenerate states at the Fermi level (Dirac point) [6-9]. The degeneracy is
proportional to the edge size and can be made macroscopic. This opens up the
possibility to design a strongly correlated electronic system as a function of
fractional filling of the zero-energy shell, in analogy to the fractional
quantum Hall effect in a quasi-two-dimensional electron gas[10], but without
the need for a high magnetic field. In this work we show that electronic
correlations, beyond the Hubbard model[6,7] and mean-field density functional
theory (DFT) [7,8] play a crucial role in determining the nature of the ground
state and the excitation spectrum of triangular graphene quantum dots as a
function of dot size and filling fraction of the shell of zero-energy states.
The interactions are treated by a combination of DFT, tight-binding,
Hartree-Fock and configuration interaction methods (TB-HF-CI) and include all
scattering and exchange terms within second nearest neighbors as well as
interaction with metallic gate. We show that a half filled charge neutral shell
leads to full spin polarization of the island but this magnetic moment is
completely destroyed by the addition of a single electron, in analogy to the
effect of skyrmions on the quantum Hall ferromagnet [11-14] and spin
depolarization in electrostatically defined semiconductor quantum dots[15-18].
The depolarization of the ground state is predicted to result in blocking of
current through a graphene quantum dot due to spin blockade (SB) [18].Comment: v2: minor corrections, new forma
Spin relaxation in a two-electron quantum dot
We discuss the rate of relaxation of the total spin in the two-electron
droplet in the vicinity of the magnetic field driven singlet-triplet
transition. The total spin relaxation is attributed to spin-orbit and
electron-phonon interactions. The relaxation process is found to depend on the
spin of ground and excited states. This asymmetry is used to explain puzzles in
recent high source-drain transport experiments.Comment: 9 pages in the PDF format, 1 figur
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