259 research outputs found
Spin and Charge Signatures of Topological Superconductivity in Rashba Nanowires
We consider a Rashba nanowire with proximity gap which can be brought into
the topological phase by tuning external magnetic field or chemical potential.
We study spin and charge of the bulk quasiparticle states when passing through
the topological transition for open and closed systems. We show, analytically
and numerically, that the spin of bulk states around the topological gap
reverses its sign when crossing the transition due to band inversion,
independent of the presence of Majorana fermions in the system. This spin
reversal can be considered as a bulk signature of topological superconductivity
that can be accessed experimentally. We find a similar behaviour for the charge
of the bulk quasiparticle states, also exhibiting a sign reversal at the
transition. We show that these signatures are robust against random static
disorder
Transport signatures of topological phases in double nanowires probed by spin-polarized STM
We study a double-nanowire setup proximity coupled to an -wave
superconductor and search for the bulk signatures of the topological phase
transition that can be observed experimentally, for example, with an STM tip.
Three bulk quantities, namely, the charge, the spin polarization, and the
pairing amplitude of intrawire superconductivity are studied in this work. The
spin polarization and the pairing amplitude flip sign as the system undergoes a
phase transition from the trivial to the topological phase. In order to
identify promising ways to observe bulk signatures of the phase transition in
transport experiments, we compute the spin current flowing between a local
spin-polarized probe, such as an STM tip, and the double-nanowire system in the
Keldysh formalism. We find that the spin current contains information about the
sign flip of the bulk spin polarization and can be used to determine the
topological phase transition point.Comment: 12 pages, 7 figure
Spin-dependent coupling between quantum dots and topological quantum wires
Considering Rashba quantum wires with a proximity-induced superconducting gap
as physical realizations of Majorana fermions and quantum dots, we calculate
the overlap of the Majorana wave functions with the local wave functions on the
dot. We determine the spin-dependent tunneling amplitudes between these two
localized states and show that we can tune into a fully spin polarized
tunneling regime by changing the distance between dot and Majorana fermion.
Upon directly applying this to the tunneling model Hamiltonian, we calculate
the effective magnetic field on the quantum dot flanked by two Majorana
fermions. The direction of the induced magnetic field on the dot depends on the
occupation of the nonlocal fermion formed from the two Majorana end states
which can be used as a readout for such a Majorana qubit.Comment: 18 pages, 11 figure
Zero-energy Andreev bound states from quantum dots in proximitized Rashba nanowires
We study an analytical model of a Rashba nanowire that is partially covered
by and coupled to a thin superconducting layer, where the uncovered region of
the nanowire forms a quantum dot. We find that, even if there is no topological
superconducting phase possible, there is a trivial Andreev bound state that
becomes pinned exponentially close to zero energy as a function of magnetic
field strength when the length of the quantum dot is tuned with respect to its
spin-orbit length such that a resonance condition of Fabry-Perot type is
satisfied. In this case, we find that the Andreev bound state remains pinned
near zero energy for Zeeman energies that exceed the characteristic spacing
between Andreev bound state levels but that are smaller than the spin-orbit
energy of the quantum dot. Importantly, as the pinning of the Andreev bound
state depends only on properties of the quantum dot, we conclude that this
behavior is unrelated to topological superconductivity. To support our
analytical model, we also perform a numerical simulation of a hybrid system
while explicitly incorporating a thin superconducting layer, showing that all
qualitative features of our analytical model are also present in the numerical
results.Comment: Accepted for publication in Phys. Rev.
Topological Phase Detection in Rashba Nanowires with a Quantum Dot
We study theoretically the detection of the topological phase transition
occurring in Rashba nanowires with proximity-induced superconductivity using a
quantum dot. The bulk states lowest in energy of such a nanowire have a spin
polarization parallel or antiparallel to the applied magnetic field in the
topological or trivial phase, respectively. We show that this property can be
probed by the quantum dot created at the end of the nanowire by external gates.
By tuning one of the two spin-split levels of the quantum dot to be in
resonance with nanowire bulk states, one can detect the spin polarization of
the lowest band via transport measurement. This allows one to determine the
topological phase of the Rashba nanowire independently of the presence of
Majorana bound states
Interventional radiology in the management of complications after liver transplantation
The arrival of new surgical transplantation techniques, such as split living donor or auxiliary liver transplantation, have increased the incidence of vascular and biliary complications. The causes, symptoms, and diagnostic modalities of arterial, portal caval, and biliary complications are detailed. Interventional techniques, such as balloon angioplasty and stent placement in the arterial and portal tree, as well as biliary interventional techniques, are discusse
Analysis of the potential of near-ground measurements of CO2 and CH4 in London, UK, for the monitoring of city-scale emissions using an atmospheric transport model
Carbon dioxide (CO2) and methane (CH4) mole fractions were measured at four near-ground sites located in and around London during the summer of 2012 with a view to investigating the potential of assimilating such measurements in an atmospheric inversion system for the monitoring of the CO2 and CH4 emissions in the London area. These data were analysed and compared with simulations using a modelling framework suited to building an inversion system: a 2 km horizontal resolution south of England configuration of the transport model CHIMERE driven by European Centre for Medium-Range Weather Forecasts (ECMWF) meteorological forcing, coupled to a 1 km horizontal resolution emission inventory (the UK National Atmospheric Emission Inventory). First comparisons reveal that local sources, which cannot be represented in the model at a 2 km resolution, have a large impact on measurements. We evaluate methods to filter out the impact of some of the other critical sources of discrepancies between the measurements and the model simulation except that of the errors in the emission inventory, which we attempt to isolate. Such a separation of the impact of errors in the emission inventory should make it easier to identify the corrections that should be applied to the inventory. Analysis is supported by observations from meteorological sites around the city and a 3-week period of atmospheric mixing layer height estimations from lidar measurements. The difficulties of modelling the mixing layer depth and thus CO2 and CH4 concentrations during the night, morning and late afternoon lead to focusing on the afternoon period for all further analyses. The discrepancies between observations and model simulations are high for both CO2 and CH4 (i.e. their root mean square (RMS) is between 8 and 12 parts per million (ppm) for CO2 and between 30 and 55 parts per billion (ppb) for CH4 at a given site). By analysing the gradients between the urban sites and a suburban or rural reference site, we are able to decrease the impact of uncertainties in the fluxes and transport outside the London area and in the model domain boundary conditions. We are thus able to better focus attention on the signature of London urban CO2 and CH4 emissions in the atmospheric CO2 and CH4 concentrations. This considerably improves the statistical agreement between the model and observations for CO2 (with model–data RMS discrepancies that are between 3 and 7 ppm) and to a lesser degree for CH4 (with model–data RMS discrepancies that are between 29 and 38 ppb). Between one of the urban sites and either the rural or suburban reference site, selecting the gradients during periods wherein the reference site is upwind of the urban site further decreases the statistics of the discrepancies in general, though not systematically. In a further attempt to focus on the signature of the city anthropogenic emission in the mole fraction measurements, we use a theoretical ratio of gradients of carbon monoxide (CO) to gradients of CO2 from fossil fuel emissions in the London area to diagnose observation-based fossil fuel CO2 gradients, and compare them with the fossil fuel CO2 gradients simulated with CHIMERE. This estimate increases the consistency between the model and the measurements when considering only one of the two urban sites, even though the two sites are relatively close to each other within the city. While this study evaluates and highlights the merit of different approaches for increasing the consistency between the mesoscale model and the near-ground data, and while it manages to decrease the random component of the analysed model–data discrepancies to an extent that should not be prohibitive to extracting the signal from the London urban emissions, large biases, the sign of which depends on the measurement sites, remain in the final model–data discrepancies. Such biases are likely related to local emissions to which the urban near-ground sites are highly sensitive. This questions our current ability to exploit urban near-ground data for the atmospheric inversion of city emissions based on models at spatial resolution coarser than 2 km. Several measurement and modelling concepts are discussed to overcome this challenge
Gender, style diversity and their effect on fund performance
© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license(http://creativecommons.org/licenses/by/4.0/).This paper examines the performance of 358 European diversified equity mutual funds controlling for gender diversity. Fund performance is evaluated against funds’ designated market indices and representative style portfolios. Consistently with previous studies, proper statistical tests point to the absence of significant differences in performance and risk between female and male managed funds. However, perverse market timing manifests itself mainly in female managed funds and in the left tail of the returns distribution. Interestingly, at fund level there is evidence of significant overperformance that survives even after accounting for funds’ exposure to known risk factors. Employing a quantile regression approach reveals that fund performance is highly dependent on the selection of the specific quantile of the returns distribution; also, style consistency for male and female managers manifests itself across different quantiles. These results have important implications for fund management companies and for retail investors’ asset allocation strategies
Disentangling dark energy and cosmic tests of gravity from weak lensing systematics
We consider the impact of key astrophysical and measurement systematics on
constraints on dark energy and modifications to gravity on cosmic scales. We
focus on upcoming photometric "Stage III" and "Stage IV" large scale structure
surveys such as DES, SuMIRe, Euclid, LSST and WFIRST. We illustrate the
different redshift dependencies of gravity modifications compared to intrinsic
alignments, the main astrophysical systematic. The way in which systematic
uncertainties, such as galaxy bias and intrinsic alignments, are modelled can
change dark energy equation of state and modified gravity figures of merit by a
factor of four. The inclusion of cross-correlations of cosmic shear and galaxy
position measurements helps reduce the loss of constraining power from the
lensing shear surveys. When forecasts for Planck CMB and Stage IV surveys are
combined, constraints on the dark energy equation of state and modified gravity
model are recovered, relative to those from shear data with no systematic
uncertainties, if fewer than 36 free parameters in total are used to describe
the galaxy bias and intrinsic alignment models as a function of scale and
redshift. To facilitate future investigations, we also provide a fitting
function for the matter power spectrum arising from the phenomenological
modified gravity model we consider.Comment: 18 pages, 8 figure
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