35,654 research outputs found
Coulomb blockade in graphene nanoribbons
We propose that recent transport experiments revealing the existence of an
energy gap in graphene nanoribbons may be understood in terms of Coulomb
blockade. Electron interactions play a decisive role at the quantum dots which
form due to the presence of necks arising from the roughness of the graphene
edge. With the average transmission as the only fitting parameter, our theory
shows good agreement with the experimental data.Comment: 4 pages, 2 figure
Unstable particles versus resonances in impurity systems, conductance in quantum wires
We compute the DC conductance for a homogeneous sine-Gordon model and an
impurity system of Luttinger liquid type by means of the thermodynamic Bethe
ansatz and standard potential scattering theory. We demonstrate that unstable
particles and resonances in impurity systems lead to a sharp increase of the
conductance as a function of the temperature, which is characterized by the
Breit-Wigner formula.Comment: 5 pages Latex, 1 figure replaced, version to appear in J. Phys.
Electronic compressibility of a graphene bilayer
We calculate the electronic compressibility arising from electron-electron
interactions for a graphene bilayer within the Hartree-Fock approximation. We
show that, due to the chiral nature of the particles in this system, the
compressibility is rather different from those of either the two-dimensional
electron gas or ordinary semiconductors. We find that an inherent competition
between the contributions coming from intra-band exchange interactions
(dominant at low densities) and inter-band interactions (dominant at moderate
densities) leads to a non-monotonic behavior of the compressibility as a
function of carrier density.Comment: 4 pages, 4 figures. Final versio
Turning waves and breakdown for incompressible flows
We consider the evolution of an interface generated between two immiscible
incompressible and irrotational fluids. Specifically we study the Muskat and
water wave problems. We show that starting with a family of initial data given
by (\al,f_0(\al)), the interface reaches a regime in finite time in which is
no longer a graph. Therefore there exists a time where the solution of
the free boundary problem parameterized as (\al,f(\al,t)) blows-up: \|\da
f\|_{L^\infty}(t^*)=\infty. In particular, for the Muskat problem, this result
allows us to reach an unstable regime, for which the Rayleigh-Taylor condition
changes sign and the solution breaks down.Comment: 15 page
Cyclic and ruled Lagrangian surfaces in complex Euclidean space
We study those Lagrangian surfaces in complex Euclidean space which are
foliated by circles or by straight lines. The former, which we call cyclic,
come in three types, each one being described by means of, respectively, a
planar curve, a Legendrian curve of the 3-sphere or a Legendrian curve of the
anti de Sitter 3-space. We also describe ruled Lagrangian surfaces. Finally we
characterize those cyclic and ruled Lagrangian surfaces which are solutions to
the self-similar equation of the Mean Curvature Flow. Finally, we give a
partial result in the case of Hamiltonian stationary cyclic surfaces
Gains from the upgrade of the cold neutron triple-axis spectrometer FLEXX at the BER-II reactor
The upgrade of the cold neutron triple-axis spectrometer FLEXX is described.
We discuss the characterisation of the gains from the new primary spectrometer,
including a larger guide and double focussing monochromator, and present
measurements of the energy and momentum resolution and of the neutron flux of
the instrument. We found an order of magnitude gain in intensity (at the cost
of coarser momentum resolution), and that the incoherent elastic energy widths
are measurably narrower than before the upgrade. The much improved count rate
should allow the use of smaller single crystals samples and thus enable the
upgraded FLEXX spectrometer to continue making leading edge measurements.Comment: 8 pages, 7 figures, 5 table
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