4,289 research outputs found
Adiabatic Control of the Electron Phase in a Quantum Dot
A Berry phase can be added to the wavefunction of an isolated quantum dot by
adiabatically modulating a nonuniform electric field along a time-cycle. The
dot is tuned close to a three-level degeneracy, which provides a wide range of
possibilities of control. We propose to detect the accumulated phase by
capacitively coupling the dot to a double-path inteferometer. The effective
Hamiltonian for the phase-sensitive coupling is discussed in detail.Comment: 14 pages, 2 .eps figure
Quantum interference of electrons in a ring: tuning of the geometrical phase
We calculate the oscillations of the DC conductance across a mesoscopic ring,
simultaneously tuned by applied magnetic and electric fields orthogonal to the
ring. The oscillations depend on the Aharonov-Bohm flux and of the spin-orbit
coupling. They result from mixing of the dynamical phase, including the Zeeman
spin splitting, and of geometric phases. By changing the applied fields, the
geometric phase contribution to the conductance oscillations can be tuned from
the adiabatic (Berry) to the nonadiabatic (Ahronov-Anandan) regime. To model a
realistic device, we also include nonzero backscattering at the connection
between ring and contacts, and a random phase for electron wavefunction,
accounting for dephasing due to disorder.Comment: 4 pages, 3 figures, minor change
Thermal transport driven by charge imbalance in graphene in magnetic field, close to the charge neutrality point at low temperature: Non local resistance
Graphene grown epitaxially on SiC, close to the charge neutrality point
(CNP), in an orthogonal magnetic field shows an ambipolar behavior of the
transverse resistance accompanied by a puzzling longitudinal magnetoresistance.
When injecting a transverse current at one end of the Hall bar, a sizeable non
local transverse magnetoresistance is measured at low temperature. While Zeeman
spin effect seems not to be able to justify these phenomena, some dissipation
involving edge states at the boundaries could explain the order of magnitude of
the non local transverse magnetoresistance, but not the asymmetry when the
orientation of the orthogonal magnetic field is reversed. As a possible
contribution to the explanation of the measured non local magnetoresistance
which is odd in the magnetic field, we derive a hydrodynamic approach to
transport in this system, which involves particle and hole Dirac carriers, in
the form of charge and energy currents. We find that thermal diffusion can take
place on a large distance scale, thanks to long recombination times, provided a
non insulating bulk of the Hall bar is assumed, as recent models seem to
suggest in order to explain the appearance of the longitudinal resistance. In
presence of the local source, some leakage of carriers from the edges generates
an imbalance of carriers of opposite sign, which are separated in space by the
magnetic field and diffuse along the Hall bar generating a non local transverse
voltage.Comment: 25 pages, 12 figure
The observed chemical structure of L1544
Prior to star formation, pre-stellar cores accumulate matter towards the
centre. As a consequence, their central density increases while the temperature
decreases. Understanding the evolution of the chemistry and physics in this
early phase is crucial to study the processes governing the formation of a
star. We aim at studying the chemical differentiation of a prototypical
pre-stellar core, L1544, by detailed molecular maps. In contrast with single
pointing observations, we performed a deep study on the dependencies of
chemistry on physical and external conditions. We present the emission maps of
39 different molecular transitions belonging to 22 different molecules in the
central 6.25 arcmin of L1544. We classified our sample in five families,
depending on the location of their emission peaks within the core. Furthermore,
to systematically study the correlations among different molecules, we have
performed the principal component analysis (PCA) on the integrated emission
maps. The PCA allows us to reduce the amount of variables in our dataset.
Finally, we compare the maps of the first three principal components with the
H column density map, and the T map of the core. The results of
our qualitative analysis is the classification of the molecules in our dataset
in the following groups: (i) the -CH family (carbon chain
molecules), (ii) the dust peak family (nitrogen-bearing species), (iii) the
methanol peak family (oxygen-bearing molecules), (iv) the HNCO peak family
(HNCO, propyne and its deuterated isotopologues). Only HCO and
CS do not belong to any of the above mentioned groups. The principal
component maps allow us to confirm the (anti-)correlations among different
families that were described in a first qualitative analysis, but also points
out the correlation that could not be inferred before.Comment: 29 pages, 19 figures, 2 appendices, accepted for publication in A&A,
arXiv abstract has been slightly modifie
Translationally invariant conservation laws of local Lindblad equations
We study the conditions under which one can conserve local translationally
invariant operators by local translationally invariant Lindblad equations in
one-dimensional rings of spin-1/2 particles. We prove that for any 1-local
operator (e.g., particle density) there exist Lindblad dissipators that
conserve that operator, while on the other hand we prove that among 2-local
operators (e.g., energy density) only trivial ones of the Ising type can be
conserved, while all the other cannot be conserved, neither locally nor
globally, by any 2- or 3-local translationally invariant Lindblad equation. Our
statements hold for rings of any finite length larger than some minimal length
determined by the locality of Lindblad equation. These results show in
particular that conservation of energy density in interacting systems is
fundamentally more difficult than conservation of 1-local quantities.Comment: 15 pages, no fig
O2 signature in thin and thick O2-H2O ices
Aims. In this paper we investigate the detectability of the molecular oxygen
in icy dust grain mantles towards astronomical objects. Methods. We present a
systematic set of experiments with O2-H2O ice mixtures designed to disentangle
how the molecular ratio affects the O2 signature in the mid- and near-infrared
spectral regions. All the experiments were conducted in a closed-cycle helium
cryostat coupled to a Fourier transform infrared spectrometer. The ice mixtures
comprise varying thicknesses from 8 10 to 3 m. The
absorption spectra of the O2-H2O mixtures are also compared to the one of pure
water. In addition, the possibility to detect the O2 in icy bodies and in the
interstellar medium is discussed. Results. We are able to see the O2 feature at
1551 cm even for the most diluted mixture of H2O : O2 = 9 : 1,
comparable to a ratio of O2/H2O = 10 % which has already been detected in situ
in the coma of the comet 67P/Churyumov-Gerasimenko. We provide an estimate for
the detection of O2 with the future mission of the James Webb Space Telescope
(JWST).Comment: 11 pages, 10 figures, article in press, to appear in A&A 201
Josephson current through a long quantum wire
The dc Josephson current through a long SNS junction receives contributions
from both Andreev bound states localized in the normal region as well as from
scattering states incoming from the superconducting leads. We show that in the
limit of a long junction, this current, at low temperatures, can be expressed
entirely in terms of properties of the Andreev bound states at the Fermi
energy: the normal and Andreev reflection amplitudes at the left-hand and at
the right-hand S-N interface. This has important implications for treating
interactions in such systems.Comment: 25 pages, 5 figure
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