4,254 research outputs found
Impurity effects in few-electron quantum dots: Incipient Wigner molecule regime
Numerically exact path-integral Monte Carlo data are presented for
strongly interacting electrons confined in a 2D parabolic quantum dot,
including a defect to break rotational symmetry. Low densities are studied,
where an incipient Wigner molecule forms. A single impurity is found to cause
drastic effects: (1) The standard shell-filling sequence with magic numbers
, corresponding to peaks in the addition energy , is
destroyed, with a new peak at N=8, (2) spin gaps decrease,
(3) for N=8, sub-Hund's rule spin S=0 is induced, and (4) spatial ordering of
the electrons becomes rather sensitive to spin. We also comment on the recently
observed bunching phenomenon.Comment: 7 pages, 1 table, 4 figures, accepted for publication in Europhysics
Letter
Parameter identification in a semilinear hyperbolic system
We consider the identification of a nonlinear friction law in a
one-dimensional damped wave equation from additional boundary measurements.
Well-posedness of the governing semilinear hyperbolic system is established via
semigroup theory and contraction arguments. We then investigte the inverse
problem of recovering the unknown nonlinear damping law from additional
boundary measurements of the pressure drop along the pipe. This coefficient
inverse problem is shown to be ill-posed and a variational regularization
method is considered for its stable solution. We prove existence of minimizers
for the Tikhonov functional and discuss the convergence of the regularized
solutions under an approximate source condition. The meaning of this condition
and some arguments for its validity are discussed in detail and numerical
results are presented for illustration of the theoretical findings
Resonant tunneling in a Luttinger liquid for arbitrary barrier transmission
A numerically exact dynamical quantum Monte Carlo approach has been developed
and applied to transport through a double barrier in a Luttinger liquid with
arbitrary transmission. For strong transmission, we find broad Fabry-Perot
Coulomb blockade peaks, with a lineshape parametrized by a single parameter,
but at sufficiently low temperatures, non-Lorentzian universal lineshapes
characteristic of coherent resonant tunneling emerge, even for strong
interactions. For weak transmission, our data supports the recently proposed
correlated sequential tunneling picture and is consistent with experimental
results on intrinsic nanotube dots.Comment: 4 pages, 4 figure
Conductance quantization and snake states in graphene magnetic waveguides
We consider electron waveguides (quantum wires) in graphene created by
suitable inhomogeneous magnetic fields. The properties of uni-directional snake
states are discussed. For a certain magnetic field profile, two spatially
separated counter-propagating snake states are formed, leading to conductance
quantization insensitive to backscattering by impurities or irregularities of
the magnetic field.Comment: 5 pages, 4 figures, final version accepted as Rapid Comm. in PR
Transport in Double-Crossed Luttinger Liquids
We study transport through two Luttinger liquids (one-dimensional electrons
interacting through a Coulomb repulsion in a metal) coupled together at {\it
two} points. External voltage biases are incorporated through boundary
conditions. We include density-density couplings as well as single-particle
hops at the contacts. For weak repulsive interactions, transport through the
wires remains undisturbed by the inter-wire couplings, which renormalise to
zero. For strong repulsive interactions, the inter-wire couplings become
strong. For symmetric barriers and no external voltage bias, a single gate
voltage is sufficient to tune for resonance transmission in both wires.
However, for asymmetric couplings or for finite external biases, the system is
insulating.Comment: Latex file, 11 pages, one eps figur
A search for soft X-ray emission associated with prominent high-velocity-cloud complexes
We correlate the ROSAT 1/4 keV all-sky survey with the Leiden/Dwingeloo HI
survey, looking for soft X-ray signatures of prominent high-velocity-cloud
(HVC) complexes. We study the transfer of 1/4 keV photons through the
interstellar medium in order to distinguish variations in the soft X-ray
background (SXRB) intensity caused by photoelectric absorption effects from
those due to excess X-ray emission. The X-ray data are modelled as a
combination of emission from the Local Hot Bubble (LHB) and emission from a
distant plasma in the galactic halo and extragalactic sources. The X-ray
radiation intensity of the galactic halo and extragalactic X-ray background is
modulated by the photoelectric absorption of the intervening galactic
interstellar matter. We show that large- and small-scale intensity variations
of the 1/4 keV SXRB are caused by photoelectric absorption which is
predominantly traced by the total N(HI) distribution. The extensive coverage of
the two surveys supports evidence for a hot, X-ray emitting corona. We show
that this leads to a good representation of the SXRB observations. For four
large areas on the sky, we search for regions where the modelled and observed
X-ray emission differ. We find that there is excess X-ray emission towards
regions near HVC complexes C, D, and GCN. We suggest that the excess X-ray
emission is positionally correlated with the high-velocity clouds. Some lines
of sight towards HVCs also pass through significant amounts of
intermediate-velocity gas, so we cannot constrain the possible role played by
IVC gas in these directions of HVC and IVC overlap, in determining the X-ray
excesses.Comment: 16 pages, 8 figures, accepted for publication in Astronomy &
Astrophysics main journa
Paraconductivity in Carbon Nanotubes
We report the calculation of paraconductivity in carbon nanotubes above the
superconducting transition temperature. The complex behavior of
paraconductivity depending upon the tube radius, temperature and magnetic field
strength is analyzed. The results are qualitatively compared with recent
experimental observations in carbon nanotubes of an inherent transition to the
superconducting state and pronounced thermodynamic fluctuations above .
The application of our results to single-wall and multi-wall carbon nanotubes
as well as ropes of nanotubes is discussed.Comment: 7 pages, 1 figur
Electron injection in a nanotube with leads: finite frequency noise-correlations and anomalous charges
The non-equilibrium transport properties of a carbon nanotube which is
connected to Fermi liquid leads, where electrons are injected in the bulk, are
computed. A previous work which considered an infinite nanotube showed that the
zero frequency noise correlations, measured at opposite ends of the nanotube,
could be used to extract the anomalous charges of the chiral excitations which
propagate in the nanotube. Here, the presence of the leads have the effect that
such-noise cross-correlations vanish at zero frequency. Nevertheless,
information concerning the anomalous charges can be recovered when considering
the spectral density of noise correlations at finite frequencies, which is
computed perturbatively in the tunneling amplitude. The spectrum of the noise
cross-correlations is shown to depend crucially on the ratio of the time of
flight of quasiparticles traveling in the nanotube to the ``voltage'' time
which defines the width of the quasiparticle wave-packets injected when an
electron tunnels. Potential applications toward the measurement of such
anomalous charges in non-chiral Luttinger liquids (nanotubes or semiconductor
quantum wires) are discussed.Comment: 11 pages, 5 figure
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