173 research outputs found
Wigner crystallization at graphene edges
Using many-body configuration interaction techniques we show that Wigner
crystallization occurs at the zigzag edges of graphene at surprisingly high
electronic densities up to \mbox{nm}^{-1}. In contrast with
one-dimensional electron gas, the flat-band structure of the edge states makes
the system interaction dominated, facilitating the electronic localization. The
resulting Wigner crystal manifests itself in pair-correlation functions, and
evolves smoothly as the edge electron density is lowered. We also show that the
crystallization affects the magnetization of the edges. While the edges are
fully polarized when the system is charge neutral (i.e. high density), above
the critical density, the spin-spin correlations between neighboring electrons
go through a smooth transition from antiferromagnetic to magnetic coupling as
the electronic density is lowered.Comment: 4.5 pages, 4 figure
Defect induced Anderson localization and magnetization in graphene quantum dots
We theoretically investigate the effects of atomic defect related short-range
disorders and electron-electron interactions on Anderson type localization and
the magnetic properties of hexagonal armchair graphene quantum dots using an
extended mean-field Hubbard model. We observe that randomly distributed defects
with concentrations between 1-5\% of the total number of atoms leads to
localization alongside magnetic puddle-like structures. We show that
localization lenght is not affected by magnetization if there is an even
distribution of defects between the two sublattices of the honeycomb lattice.
However, for an uneven distributions, localization is found to be significantly
enhanced
Effects of interedge scattering on the Wigner crystallization in graphene nanoribbons
We investigate the effects of coupling between the two zigzag edges of
graphene nanoribbons on the Wigner crystallization of electrons and holes using
a combination of tight-binding, mean field Hubbard and many-body configuration
interaction methods. We show that the thickness of the nanoribbon plays a
crucial role in the formation of Wigner crystal. For ribbon widths smaller than
16 \mbox{\AA}, increased kinetic energy overcomes the long-range Coulomb
repulsion and suppresses the Wigner crystallization. For wider ribbons up to 38
\mbox{\AA} wide, strong Wigner localization is observed for even number of
electrons, revealing an even-odd effect also found in Coulomb blockade addition
spectrum. Interedge correlations are found to be strong enough to allow
simultaneous crystallization on both edges, although an applied electric field
can decouple the two edges. Finally, we show that Wigner crystallization can
also occurs for holes, albeit weaker than for electrons.Comment: Accepted for publication in PR
Effects of long-range disorder and electronic interactions on the optical properties of graphene quantum dots
We theoretically investigate the effects of long-range disorder and
electron-electron interactions on the optical properties of hexagonal armchair
graphene quantum dots consisting of up to 10806 atoms. The numerical
calculations are performed using a combination of tight-binding, mean-field
Hubbard and configuration interaction methods. Imperfections in the graphene
quantum dots are modelled as a long-range random potential landscape, giving
rise to electron-hole puddles. We show that, when the electron-hole puddles are
present, tight-binding method gives a poor description of the low-energy
absorption spectra compared to meanfield and configuration interaction
calculation results. As the size of the graphene quantum dot is increased, the
universal optical conductivity limit can be observed in the absorption
spectrum. When disorder is present, calculated absorption spectrum approaches
the experimental results for isolated monolayer of graphene sheet
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
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
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
The maximum density droplet to lower density droplet transition in quantum dots
We show that, Landau level mixing in two-dimensional quantum dot wave
functions can be taken into account very effectively by multiplying the exact
lowest Landau level wave functions by a Jastrow factor which is optimized by
variance minimization. The comparison between exact diagonalization and fixed
phase diffusion Monte Carlo results suggests that the phase of the many-body
wave functions are not affected much by Landau level mixing. We apply these
wave functions to study the transition from the maximum density droplet state
(incipient integer quantum Hall state with angular momentum L=N(N-1)/2) to
lower density droplet states (L>N(N-1)/2).Comment: 8 pages, 5 figures, accepted for publication in Phys. Rev.
Interaction-Induced Strong Localization in Quantum Dots
We argue that Coulomb blockade phenomena are a useful probe of the cross-over
to strong correlation in quantum dots. Through calculations at low density
using variational and diffusion quantum Monte Carlo (up to r_s ~ 55), we find
that the addition energy shows a clear progression from features associated
with shell structure to those caused by commensurability of a Wigner crystal.
This cross-over (which occurs near r_s ~ 20 for spin-polarized electrons) is,
then, a signature of interaction-driven localization. As the addition energy is
directly measurable in Coulomb blockade conductance experiments, this provides
a direct probe of localization in the low density electron gas.Comment: 4 pages, published version, revised discussio
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