2,840 research outputs found
Incorporating Ambipolar and Ohmic Diffusion in the AMR MHD code RAMSES
We have implemented non-ideal Magneto-Hydrodynamics (MHD) effects in the
Adaptive Mesh Refinement (AMR) code RAMSES, namely ambipolar diffusion and
Ohmic dissipation, as additional source terms in the ideal MHD equations. We
describe in details how we have discretized these terms using the adaptive
Cartesian mesh, and how the time step is diminished with respect to the ideal
case, in order to perform a stable time integration. We have performed a large
suite of test runs, featuring the Barenblatt diffusion test, the Ohmic
diffusion test, the C-shock test and the Alfven wave test. For the latter, we
have performed a careful truncation error analysis to estimate the magnitude of
the numerical diffusion induced by our Godunov scheme, allowing us to estimate
the spatial resolution that is required to address non-ideal MHD effects
reliably. We show that our scheme is second-order accurate, and is therefore
ideally suited to study non-ideal MHD effects in the context of star formation
and molecular cloud dynamics
Multibranch Bogoliubov-Bloch spectrum of a cigar shaped Bose condensate in an optical lattice
We study properties of excited states of an array of weakly coupled
quasi-two-dimensional Bose condensates by using the hydrodynamic theory. The
spectrum of the axial excited states strongly depends on the coupling among the
various discrete radial modes in a given symmetry. By including mode-coupling
within a given symmetry, the complete excitation spectrum of axial
quasiparticles with various discrete radial nodes are presented. A single
parameter which determines the strength of the mode coupling is identified. The
excitation spectrum in the zero angular momentum sector can be observed by
using the Bragg scattering experiments.Comment: to apper in Phys. Rev.
Gate-controlled nuclear magnetic resonance in an AlGaAs/GaAs quantum Hall device
We study the resistively detected nuclear magnetic resonance (NMR) in an
AlGaAs/GaAs quantum Hall device with a side gate. The strength of the hyperfine
interaction between electron and nuclear spins is modulated by tuning a
position of the two-dimensional electron systems with respect to the polarized
nuclear spins using the side-gate voltages. The NMR frequency is systematically
controlled by the gate-tuned technique in a semiconductor device.Comment: 3 pages, 4 figures, submitted to Appl. Phys. Let
Alternate two-dimensional quantum walk with a single-qubit coin
We have recently proposed a two-dimensional quantum walk where the
requirement of a higher dimensionality of the coin space is substituted with
the alternance of the directions in which the walker can move [C. Di Franco, M.
Mc Gettrick, and Th. Busch, Phys. Rev. Lett. {\bf 106}, 080502 (2011)]. For a
particular initial state of the coin, this walk is able to perfectly reproduce
the spatial probability distribution of the non-localized case of the Grover
walk. Here, we present a more detailed proof of this equivalence. We also
extend the analysis to other initial states, in order to provide a more
complete picture of our walk. We show that this scheme outperforms the Grover
walk in the generation of - spatial entanglement for any initial
condition, with the maximum entanglement obtained in the case of the particular
aforementioned state. Finally, the equivalence is generalized to wider classes
of quantum walks and a limit theorem for the alternate walk in this context is
presented.Comment: 9 pages, 9 figures, RevTeX
Measurement-induced generation of spatial entanglement in a two-dimensional quantum walk with single-qubit coin
One of the proposals for the exploitation of two-dimensional quantum walks
has been the efficient generation of entanglement. Unfortunately, the
technological effort required for the experimental realization of standard
two-dimensional quantum walks is significantly demanding. In this respect, an
alternative scheme with less challenging conditions has been recently studied,
particularly in terms of spatial-entanglement generation [C. Di Franco, M. Mc
Gettrick, and Th. Busch, Phys. Rev. Lett. 106, 080502 (2011)]. Here, we extend
the investigation to a scenario where a measurement is performed on the coin
degree of freedom after the evolution, allowing a further comparison with the
standard two-dimensional Grover walk.Comment: 9 pages, 4 figures, RevTeX
Saturation of Magnetorotational Instability through Magnetic Field Generation
The saturation mechanism of Magneto-Rotational Instability (MRI) is examined
through analytical quasilinear theory and through nonlinear computation of a
single mode in a rotating disk. We find that large-scale magnetic field is
generated through the alpha effect (the correlated product of velocity and
magnetic field fluctuations) and causes the MRI mode to saturate. If the
large-scale plasma flow is allowed to evolve, the mode can also saturate
through its flow relaxation. In astrophysical plasmas, for which the flow
cannot relax because of gravitational constraints, the mode saturates through
field generation only.Comment: 9 pages, 10 figures to appear in ApJ, Jun 2009, 10 v69
Quantum Hall line junction with impurities as a multi-slit Luttinger liquid interferometer
We report on quantum interference between a pair of counterpropagating
quantum Hall edge states that are separated by a high quality tunnel barrier.
Observed Aharonov-Bohm oscillations are analyzed in terms of resonant tunneling
between coupled Luttinger liquids that creates bound electronic states between
pairs of tunnel centers that act like interference slits. We place a lower
bound in the range of 20-40 m for the phase coherence length and directly
confirm the extended phase coherence of quantum Hall edge states.Comment: 4 pages, 3 figures, 1 tabl
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