694 research outputs found
Spin and Conductance-Peak-Spacing Distributions in Large Quantum Dots: A Density Functional Theory Study
We use spin-density-functional theory to study the spacing between
conductance peaks and the ground-state spin of 2D model quantum dots with up to
200 electrons. Distributions for different ranges of electron number are
obtained in both symmetric and asymmetric potentials. The even/odd effect is
pronounced for small symmetric dots but vanishes for large asymmetric ones,
suggesting substantially stronger interaction effects than expected. The
fraction of high-spin ground states is remarkably large.Comment: 4 pages, 3 figure
Quantum and frustration effects on fluctuations of the inverse compressibility in two-dimensional Coulomb glasses
We consider interacting electrons in a two-dimensional quantum Coulomb glass
and investigate by means of the Hartree-Fock approximation the combined effects
of the electron-electron interaction and the transverse magnetic field on
fluctuations of the inverse compressibility. Preceding systematic study of the
system in the absence of the magnetic field identifies the source of the
fluctuations, interplay of disorder and interaction, and effects of hopping.
Revealed in sufficiently clean samples with strong interactions is an unusual
right-biased distribution of the inverse compressibility, which is neither of
the Gaussian nor of the Wigner-Dyson type. While in most cases weak magnetic
fields tend to suppress fluctuations, in relatively clean samples with weak
interactions fluctuations are found to grow with the magnetic field. This is
attributed to the localization properties of the electron states, which may be
measured by the participation ratio and the inverse participation number. It is
also observed that at the frustration where the Fermi level is degenerate,
localization or modulation of electrons is enhanced, raising fluctuations.
Strong frustration in general suppresses effects of the interaction on the
inverse compressibility and on the configuration of electrons.Comment: 15 pages, 18 figures, To appear in Phys. Rev.
Evanescent wave approach to diffractive phenomena in convex billiards with corners
What we are going to call in this paper "diffractive phenomena" in billiards
is far from being deeply understood. These are sorts of singularities that, for
example, some kind of corners introduce in the energy eigenfunctions. In this
paper we use the well-known scaling quantization procedure to study them. We
show how the scaling method can be applied to convex billiards with corners,
taking into account the strong diffraction at them and the techniques needed to
solve their Helmholtz equation. As an example we study a classically
pseudointegrable billiard, the truncated triangle. Then we focus our attention
on the spectral behavior. A numerical study of the statistical properties of
high-lying energy levels is carried out. It is found that all computed
statistical quantities are roughly described by the so-called semi-Poisson
statistics, but it is not clear whether the semi-Poisson statistics is the
correct one in the semiclassical limit.Comment: 7 pages, 8 figure
Absence of bimodal peak spacing distribution in the Coulomb blockade regime
Using exact diagonalization numerical methods, as well as analytical
arguments, we show that for the typical electron densities in chaotic and
disordered dots the peak spacing distribution is not bimodal, but rather
Gaussian. This is in agreement with the experimental observations. We attribute
this behavior to the tendency of an even number of electrons to gain on-site
interaction energy by removing the spin degeneracy. Thus, the dot is predicted
to show a non trivial electron number dependent spin polarization. Experimental
test of this hypothesis based on the spin polarization measurements are
proposed.Comment: 13 pages, 3 figures, accepted for publication in PRL - a few small
change
Signatures of spin pairing in a quantum dot in the Coulomb blockade regime
Coulomb blockade resonances are measured in a GaAs quantum dot in which both
shape deformations and interactions are small. The parametric evolution of the
Coulomb blockade peaks shows a pronounced pair correlation in both position and
amplitude, which is interpreted as spin pairing. As a consequence, the
nearest-neighbor distribution of peak spacings can be well approximated by a
smeared bimodal Wigner surmise, provided that interactions which go beyond the
constant interaction model are taken into account.Comment: 5 pages, 3 figure
Interactions in Chaotic Nanoparticles: Fluctuations in Coulomb Blockade Peak Spacings
We use random matrix models to investigate the ground state energy of
electrons confined to a nanoparticle. Our expression for the energy includes
the charging effect, the single-particle energies, and the residual screened
interactions treated in Hartree-Fock. This model is applicable to chaotic
quantum dots or nanoparticles--in these systems the single-particle statistics
follows random matrix theory at energy scales less than the Thouless energy. We
find the distribution of Coulomb blockade peak spacings first for a large dot
in which the residual interactions can be taken constant: the spacing
fluctuations are of order the mean level separation Delta. Corrections to this
limit are studied using the small parameter 1/(kf L): both the residual
interactions and the effect of the changing confinement on the single-particle
levels produce fluctuations of order Delta/sqrt(kf L). The distributions we
find are significantly more like the experimental results than the simple
constant interaction model.Comment: 17 pages, 4 figures, submitted to Phys. Rev.
Detecting the Kondo screening cloud around a quantum dot
A fundamental prediction of scaling theories of the Kondo effect is the
screening of an impurity spin by a cloud of electrons spread out over a
mesoscopic distance. This cloud has never been observed experimentally.
Recently, aspects of the Kondo effect have been observed in experiments on
quantum dots embedded in quantum wires. Since the length of the wire may be of
order the size of the screening cloud, such systems provide an ideal
opportunity to observe it. We point out that persistent current measurements in
a closed ring provide a conceptually simple way of detecting this fundamental
length scale.Comment: 4 pages, RevTex, 1 postscript figur
Disorder Induced Ferromagnetism in Restricted Geometries
We study the influence of on-site disorder on the magnetic properties of the
ground state of the infinite Hubbard model. We find that for one
dimensional systems disorder has no influence, while for two dimensional
systems disorder enhances the spin polarization of the system. The tendency of
disorder to enhance magnetism in the ground state may be relevant to recent
experimental observations of spin polarized ground states in quantum dots and
small metallic grains.Comment: 4 pages, 4 figure
Spin and e-e interactions in quantum dots: Leading order corrections to universality and temperature effects
We study the statistics of the spacing between Coulomb blockade conductance
peaks in quantum dots with large dimensionless conductance g. Our starting
point is the ``universal Hamiltonian''--valid in the g->oo limit--which
includes the charging energy, the single-electron energies (described by random
matrix theory), and the average exchange interaction. We then calculate the
magnitude of the most relevant finite g corrections, namely, the effect of
surface charge, the ``gate'' effect, and the fluctuation of the residual e-e
interaction. The resulting zero-temperature peak spacing distribution has
corrections of order Delta/sqrt(g). For typical values of the e-e interaction
(r_s ~ 1) and simple geometries, theory does indeed predict an asymmetric
distribution with a significant even/odd effect. The width of the distribution
is of order 0.3 Delta, and its dominant feature is a large peak for the odd
case, reminiscent of the delta-function in the g->oo limit. We consider finite
temperature effects next. Only after their inclusion is good agreement with the
experimental results obtained. Even relatively low temperature causes large
modifications in the peak spacing distribution: (a) its peak is dominated by
the even distribution at kT ~ 0.3 Delta (at lower T a double peak appears); (b)
it becomes more symmetric; (c) the even/odd effect is considerably weaker; (d)
the delta-function is completely washed-out; and (e) fluctuation of the
coupling to the leads becomes relevant. Experiments aimed at observing the T=0
peak spacing distribution should therefore be done at kT<0.1 Delta for typical
values of the e-e interaction.Comment: 15 pages, 4 figure
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