6,622 research outputs found
Quasiclassical theory for the superconducting proximity effect in Dirac materials
We derive the quasiclassical non-equilibrium Eilenberger and Usadel equations
to first order in quantities small compared to the Fermi energy, valid for
Dirac edge and surface electrons with spin-momentum locking, as relevant for
topological insulators. We discuss in detail several of the key technical
points and assumptions of the derivation, and provide a Riccati-parametrization
of the equations. Solving first the equilibrium equations for S/N and S/F
bilayers and Josephson junctions, we study the superconducting proximity effect
in Dirac materials. Similarly to related works, we find that the effect of an
exchange field depends strongly on the direction of the field. Only components
normal to the transport direction lead to attenuation of the Cooper pair
wavefunction inside the F. Fields parallel to the transport direction lead to
phase-shifts in the dependence on the superconducting phase difference for both
the charge current and density of states in an S/F/S-junction. Moreover, we
compute the differential conductance in S/N and S/F bilayers with an applied
voltage bias, and determine the dependence on the length of the N and F regions
and the exchange field.Comment: 13 pages, 5 figures. Accepted for publication in Phys. Rev.
Critical Temperature and Tunneling Spectroscopy of Superconductor-Ferromagnet Hybrids with Intrinsic Rashba-Dresselhaus Spin-Orbit Coupling
We investigate theoretically how the proximity effect in
superconductor/ferromagnet hybrid structures with intrinsic spin-orbit coupling
manifests in the density of states and critical temperature. To describe a
general scenario, we allow for both Rashba and Dresselhaus type spin-orbit
coupling. Our results are obtained via the quasiclassical theory of
superconductivity, extended to include spin-orbit coupling in the Usadel
equation and Kupriyanov--Lukichev boundary conditions. Unlike previous works,
we have derived a Riccati parametrization of the Usadel equation with
spin-orbit coupling which allows us to address the full proximity regime.
First, we consider the density of states in both SF bilayers and SFS trilayers,
where the spectroscopic features in the latter case are sensitive to the phase
difference between the two superconductors. We find that the presence of
spin-orbit coupling leaves clear spectroscopic fingerprints in the density of
states due to its role in creating spin-triplet Cooper pairs. Unlike SF and SFS
structures without spin-orbit coupling, the density of states in the present
case depends strongly on the direction of magnetization. We show that the
spin-orbit coupling can stabilize singlet superconductivity even in the
presence of a strong exchange field . This leads to the
possibility of a magnetically tunable minigap: changing the direction of the
exchange field opens and closes the minigap. We also determine how the critical
temperature of an SF bilayer is affected by spin-orbit coupling and
demonstrate that one can achieve a spin-valve effect with a single ferromagnet.
We find that displays highly non-monotonic behavior both as a function of
the magnetization direction and the type and direction of the spin-orbit
coupling, offering a new way to exert control over the superconductivity of
proximity structures.Comment: 25 pages, 21 figures. Accepted for publication in Phys. Rev.
Physics of SNeIa and Cosmology
We give an overview of the current understanding of Type Ia supernovae
relevant for their use as cosmological distance indicators. We present the
physical basis to understand their homogeneity of the observed light curves and
spectra and the observed correlations. This provides a robust method to
determine the Hubble constant, 67 +- 8 (2 sigma) km/Mpc/sec, independently from
primary distance indicators.
We discuss the uncertainties and tests which include SNe Ia based distance
determinations prior to delta-Ceph. measurements for the host galaxies. Based
on detailed models, we study the small variations from homogeneities and their
observable consequences. In combination with future data, this underlines the
suitability and promises the refinements needed to determine accurate relative
distances within 2 to 3 % and to use SNe Ia for high precision cosmology.Comment: to be published in "Stellar Candles", eds. Gieren et al. Lecture
Notes in Physics (http://link.springer.de/series/lnpp
Strong Gravitational Lensing and Dark Energy Complementarity
In the search for the nature of dark energy most cosmological probes measure
simple functions of the expansion rate. While powerful, these all involve
roughly the same dependence on the dark energy equation of state parameters,
with anticorrelation between its present value w_0 and time variation w_a.
Quantities that have instead positive correlation and so a sensitivity
direction largely orthogonal to, e.g., distance probes offer the hope of
achieving tight constraints through complementarity. Such quantities are found
in strong gravitational lensing observations of image separations and time
delays. While degeneracy between cosmological parameters prevents full
complementarity, strong lensing measurements to 1% accuracy can improve
equation of state characterization by 15-50%. Next generation surveys should
provide data on roughly 10^5 lens systems, though systematic errors will remain
challenging.Comment: 7 pages, 5 figure
The massive star binary fraction in young open clusters I. NGC 6231 revisited
We present the results of a long-term high-resolution spectroscopy campaign
on the O-type stars in NGC 6231. We revise the spectral classification and
multiplicity of these objects and we constrain the fundamental properties of
the O-star population. Almost three quarters of the O-type stars in the cluster
are members of a binary system. The minimum binary fraction is 0.63, with half
the O-type binaries having an orbital period of the order of a few days. The
eccentricities of all the short-period binaries are revised downward, and
henceforth match a normal period-eccentricity distribution. The mass-ratio
distribution shows a large preference for O+OB binaries, ruling out the
possibility that, in NGC 6231, the companion of an O-type star is randomly
drawn from a standard IMF. Obtained from a complete and homogeneous population
of O-type stars, our conclusions provide interesting observational constraints
to be confronted with the formation and early-evolution theories of O stars.Comment: 16 pages, 14 figures. Accepted by MNRA
Vegetative morphology and interfire survival strategies in the Cape Fynbos grasses
It is shown that there is a wide range of structural variation in the habit of the Arundineae and Ehrharteae of the fynbos of the Cape Floristic Region (Cape Province, South Africa). Structural differences in the bases of the fynbos grasses have been classified into four groups: swollen, knotty tillering, weak and annual. Variation in the position of the innovation buds occurs with one group having basal perennating buds, implying that all the culm material is annual, while the second group has cauline innovation buds, leading to the development of a divaricate perennial herb. The recognition of caducous, mesic (orthophyllous) and sclerophyllous leaf blades is also possible, based on leaf morphology and anatomy. These variations in growth forms allow the classification of the Cape grasses into five guilds adapted for survival in the dense fynbos vegetation that develops between the well-spaced fires in these heathlands. The following guilds have been recognized: competition avoiders that grow on rock ledges and outcrops where competition from shrubby vegetation is reduced; reseeders, that survive the protracted interfire period as seed; geophytes, that survive this period as underground organs; coppicers, that survive as small plants; and competitors, that grow tall by means of cauline innovation buds, and so are able to compete with the shrubby heath vegetation
Paper Session I-A - An Examination of the Human Factors Support of NASA\u27s Safety Directorate on the Space Station Processing Facility
The goal of the Human Factors Engineering (HFE) pilot project undertaken by NASA on the Space Station Processing Facility (SSPF) at Kennedy Space Center, Florida, is to demonstrate the advantages of using Human Factors to support NASA Safety. The primary objective of the project is to demonstrate how Human Factors can assist in decreasing the causes of accidents by reducing error producing situations. The project began with a review of design drawings for the SSPF, in which all Human Factors (HF) concerns were identified especially those that affected personnel safety, pay load protection, and operational efficiency. Visits to other KSC facilities produced insights that could be applied to the drawing critiques when the drawings were not sufficient to disclose how the facility\u27s characteristics would fulfill operational needs. Overall, the drawing review revealed a broad range of HF and Safety concerns. When possible, these concerns were discussed with the appropriate engineering personnel to effect workable solutions. To date, some of these HF & Safety concerns have been resolved by incorporating HF principles. Thus, this project has reduced potential problems that can contribute to accidents and costly delays, such as the Magellan Spacecraft incident in October of 1988. This incident typifies payload processing problems that can develop unexpectedly within any processing facility when Human Factors issues are either ignored or overlooked in the initial design of the spacecraft or in developing appropriate service and checkout procedures. Although the problem occurred on a spacecraft, this type of problem also could easily occur within a processing facility, on payloads that are being processed, or on the ground support equipment being used to process the payloads. In addition, this project has led to the evaluation of candidate methods for the implementation of HF. Among these, a means of conducting HF evaluations during Engineering Prototyping in a Computer Aided Design environment. This innovative technique is expected to demonstrate the Safety advantage and substantial cost savings of incorporating HF principles
The Struve-Sahade effect in the optical spectra of O-type binaries I. Main-sequence systems
We present a spectroscopic analysis of four massive binary systems that are
known or are good candidates to display the Struve-Sahade effect (defined as
the apparent strengthening of the secondary spectrum of the binary when the
star is approaching, and the corresponding weakening of the lines when it is
receding).
We use high resolution optical spectra to determine new orbital solutions and
spectral types of HD 165052, HD 100213, HD 159176 and DH Cep. As good knowledge
of the fundamental parameters of the considered systems is necessary to examine
the Struve-Sahade effect. We then study equivalent width variations in the
lines of both components of these binaries during their orbital cycle.
In the case of these four systems, variations appear in the equivalent widths
of some lines during the orbital cycle, but the definition given above can any
longer be valid, since it is now clear that the effect modifies the primary
spectrum as much as the secondary spectrum. Furthermore, the lines affected,
and the way in which they are affected, depend on the considered system. For at
least two of them (HD 100213 and HD 159176) these variations probably reflect
the ellipsoidal variable nature of the system.Comment: 12 pages, 20 figures, in press A&
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