3,199 research outputs found
From Andreev bound states to Majorana fermions in topological wires on superconducting substrates : a story of mutation
We study the proximity effect in a topological nanowire tunnel coupled to an
s-wave superconducting substrate. We use a general Green's function approach
that allows us to study the evolution of the Andreev bound states in the wire
into Majorana fermions. We show that the strength of the tunnel coupling
induces a topological transition in which the Majorana fermionic states can be
destroyed when the coupling is very strong. Moreover, we provide a
phenomenologial study of the effects of disorder in the superconductor on the
formation of Majorana fermions. We note a non-trivial effect of a quasiparticle
broadening term which can take the wire from a topological into a
non-topological phase in certain ranges of parameters. Our results have also
direct consequences for a nanowire coupled to an inhomogenous superconductor
Majorana Fermions in Honeycomb Lattices
We study the formation of Majorana fermions in honeycomb-lattice structures
in the presence of a Zeeman field, Rashba spin-orbit coupling, and in the
proximity of an s-wave superconductor. We show that an exact mapping exists
between an anisotropic hexagonal-lattice nanoribbon at k = 0 and a
one-dimensional chain, for which the existence of Majorana fermions has been
extensively discussed. Consequently we can predict the conditions for the
emergence of Majorana fermions at the edges of such ribbon, and relate the
existence of Majoranas to a band inversion in the bulk band structure. Moreover
we find that similar situations arise in isotropic lattices and we give some
examples which show the formation of Majorana fermions in these structures.Comment: 7 pages, 9 figure
Effects of finite superconducting coherence lengths and of phase gradients in topological SN and SNS junctions and rings
We study the effect of a finite proximity superconducting (SC) coherence
length in SN and SNS junctions consisting of a semiconducting topological
insulating wire whose ends are connected to either one or two s-wave
superconductors. We find that such systems behave exactly as SN and SNS
junctions made from a single wire for which some regions are sitting on top of
superconductors, the size of the topological SC region being determined by the
SC coherence length. We also analyze the effect of a non-perfect transmission
at the NS interface on the spatial extension of the Majorana fermions.
Moreover, we study the effects of continuous phase gradients in both an open
and closed (ring) SNS junction. We find that such phase gradients play an
important role in the spatial localization of the Majorana fermions
Frequency-Domain Measurement of the Spin Imbalance Lifetime in Superconductors
We have measured the lifetime of spin imbalances in the quasiparticle
population of a superconductor () in the frequency domain. A
time-dependent spin imbalance is created by injecting spin-polarised electrons
at finite excitation frequencies into a thin-film mesoscopic superconductor
(Al) in an in-plane magnetic field (in the Pauli limit). The time-averaged
value of the spin imbalance signal as a function of excitation frequency,
shows a cut-off at . The spin imbalance
lifetime is relatively constant in the accessible ranges of temperatures, with
perhaps a slight increase with increasing magnetic field. Taking into account
sample thickness effects, is consistent with previous measurements and
of the order of the electron-electron scattering time . Our data are
qualitatively well-described by a theoretical model taking into account all
quasiparticle tunnelling processes from a normal metal into a superconductor.Comment: Includes Supplementary Informatio
Response of the warm absorber cloud to a variable nuclear flux in active galactic nuclei
Recent modeling of the warm absorber in active galactic nuclei has proved the
usefulness of constant total (gas plus radiation) pressure models, which are
highly stratified in temperature and density. We explore the consistency of
those models when the typical variation of the flux from the central source is
taken into account. We perform a variability study of the warm absorber
response, based on timescales and our photoionization code TITAN. We show that
the ionization and recombination timescales are much shorter than the dynamical
timescale. Clouds very close to the central black hole will maintain their
equilibrium since the characteristic variability timescales of the nuclear
source are longer than cloud timescales. For more distant clouds, the density
structure has no time to vary, in response to the variations of the temperature
or ionization structure, and such clouds will show the departure from the
constant pressure equilibrium. We explore the impact of this departure on the
observed properties of the transmitted spectrum and soft X-ray variability: (i)
non uniform velocities, of the order of sound speed, appear due to pressure
gradients, up to typical values of 100 km/s. These velocities lead to the
broadening of lines. This broadening is usually observed and very difficult to
explain otherwise. (ii) Energy-dependent fractional variability amplitude in
soft X-ray range has a broader hump around ~ 1-2 keV, and (iv) the plot of the
equivalent hydrogen column density vs. ionization parameter is steeper than for
equilibrium clouds. The results have the character of a preliminary study and
should be supplemented in the future with full time-dependent radiation
transfer and dynamical computations.Comment: 9 pages, 7 figures, accepted for publication by Astronomy &
Astrophysic
Superconductor spintronics: Modeling spin and charge accumulation in out-of-equilibrium NS junctions subjected to Zeeman magnetic fields
We study the spin and charge accumulation in junctions between a
superconductor and a ferromagnet or a normal metal in the presence of a Zeeman
magnetic field, when the junction is taken out of equilibrium by applying a
voltage bias. We write down the most general form for the spin and charge
current in such junctions, taking into account all spin-resolved possible
tunneling processes. We make use of these forms to calculate the spin
accumulation in NS junctions subjected to a DC bias, and to an AC bias,
sinusoidal or rectangular. We observe that in the limit of negligeable changes
on the superconducting gap, the NS dynamical conductance is insensitive to spin
imbalance. Therefore to probe the spin accumulation in the superconductor, one
needs to separate the injection and detection point, i. e. the electrical spin
detection must be non-local. We address also the effect of the spin
accumulation induced in the normal leads by driving a spin current and its
effects on the detection of the spin accumulation in the superconductor.
Finally, we investigate the out-of-equilibrium spin susceptibility of the SC,
and we show that it deviates drastically from it's equilibrium value
Mid-infrared sub-wavelength grating mirror design: tolerance and influence of technological constraints
High polarization selective Si/SiO2 mid-infrared sub-wavelength grating
mirrors with large bandwidth adapted to VCSEL integration are compared. These
mirrors have been automatically designed for operation at \lambda = 2.3 m
by an optimization algorithm which maximizes a specially defined quality
factor. Several technological constraints in relation with the grating
manufacturing process have been imposed within the optimization algorithm and
their impact on the optical properties of the mirror have been evaluated.
Furthermore, through the tolerance computation of the different dimensions of
the structure, the robustness with respect to fabrication errors has been
tested. Finally, it appears that the increase of the optical performances of
the mirror imposes a less tolerant design with severer technological
constraints resulting in a more stringent control of the manufacturing process.Comment: The final publication is available at
http://iopscience.iop.org/2040-8986/13/12/125502
The puzzle of the soft X-ray excess in AGN: absorption or reflection?
The 2-10 keV continuum of AGN is generally well represented by a single power
law. However, at smaller energies the continuum displays an excess with respect
to the extrapolation of this power law, called the ''soft X-ray excess''. Until
now this soft X-ray excess was attributed, either to reflection of the hard
X-ray source by the accretion disk, or to the presence of an additional
comptonizing medium, giving a steep spectrum. An alternative solution proposed
by Gierlinski and Done (2004) is that a single power law well represents both
the soft and the hard X-ray emission and the impression of the soft X-ray
excess is due to absorption of a primary power law by a relativistic wind. We
examine the advantages and drawbacks of reflection versus absorption models,
and we conclude that the observed spectra can be well modeled, either by
absorption (for a strong excess), or by reflection (for a weak excess). However
the physical conditions required by the absorption models do not seem very
realistic: we would prefer an ''hybrid model''.Comment: 4 pages, 3 figures, abstracts SF2A-2005, published by EDP-Sciences
Conference Serie
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