764 research outputs found
Transport and thermoelectric properties of the LaAlO/SrTiO interface
The transport and thermoelectric properties of the interface between
SrTiO and a 26-monolayer thick LaAlO-layer grown at high
oxygen-pressure have been investigated at temperatures from 4.2 K to 100 K and
in magnetic fields up to 18 T. For 4.2 K, two different electron-like
charge carriers originating from two electron channels which contribute to
transport are observed. We probe the contributions of a degenerate and a
non-degenerate band to the thermoelectric power and develop a consistent model
to describe the temperature dependence of the thermoelectric tensor. Anomalies
in the data point to an additional magnetic field dependent scattering.Comment: 7 pages, 4 figure
Coronal density diagnostics with Helium-like triplets: CHANDRA--LETGS observations of Algol, Capella, Procyon, Eps Eri, Alpha Cen A&B, UX Ari, AD Leo, YY Gem, and HR1099
We present an analysis of ten cool stars (Algol, Capella, Procyon, Eps Eri,
Alpha Cen A&B, UX Ari, AD Leo, YY Gem, and HR1099) observed with the Low Energy
Transmission Grating Spectrometer (LETGS) on board the Chandra X-ray
Observatory. This sample contains all cools stars observed with the LETGS
presently available to us with integration times sufficiently long to warrant a
meaningful spectral analysis. Our sample comprises inactive, moderately active,
and hyperactive stars and samples the bulk part of activity levels encountered
in coronal X-ray sources. We use the LETGS spectra to carry out density and
temperature diagnostics with an emphasis on the H-like and the He-like ions. We
find a correlation between line flux ratios of the Lyman-Alpha and He-like
resonance lines with the mean X-ray surface flux. We determine densities using
the He-like triplets. For active stars we find no significant deviations from
the low-density limit for the ions of Ne, Mg, and Si, while the measured line
ratios for the ions of C, N, and O do show evidence for departures from the
low-density limit in the active stars, but not in the inactive stars. Best
measurements can be made for the OVII triplet where we find significant
deviations from the low-density limit for the stars Algol, Procyon, YY Gem, Eps
Eri, and HR1099. We discuss the influence of radiation fields on the
interpretation of the He-like triplet line ratios in the low-Z ions, which is
relevant for Algol, and the influence of dielectronic satellite lines, which is
relevant for Procyon. For the active stars YY Gem, Eps Eri, and HR1099 the low
f/i ratios can unambiguously be attributed to high densties in the range 1--3
10^10 cm^-3 at OVII temperatures. We find our LETGS spectra to be an extremely
useful tool for plasma diagnostics of stellar coronae.Comment: 17 pages, Latex2e, 12 figures. accepted for A&A under MS262
Multifrequency nature of the 0.75 mHz feature in the X-ray light curves of the nova V4743 Sgr
We present timing analyses of eight X-ray light curves and one optical/UV
light curve of the nova V4743 Sgr (2002) taken by CHANDRA and XMM on days after
outburst: 50 (early hard emission phase), 180, 196, 302, 371, 526 (super soft
source, SSS, phase), and 742 and 1286 (quiescent emission phase). We have
studied the multifrequency nature and time evolution of the dominant peak at
~0.75 mHz using the standard Lomb-Scargle method and a 2-D sine fitting method.
We found a double structure of the peak and its overtone for days 180 and 196.
The two frequencies were closer together on day 196, suggesting that the
difference between the two peaks is gradually decreasing. For the later
observations, only a single frequency can be detected, which is likely due to
the exposure times being shorter than the beat period between the two peaks,
especially if they are moving closer together. The observations on days 742 and
1286 are long enough to detect two frequencies with the difference found for
day 196, but we confidently find only a single frequency. We found significant
changes in the oscillation frequency and amplitude. We have derived blackbody
temperatures from the SSS spectra, and the evolution of changes in frequency
and blackbody temperature suggests that the 0.75-mHz peak was modulated by
pulsations. Later, after nuclear burning had ceased, the signal stabilised at a
single frequency, although the X-ray frequency differs from the optical/UV
frequency obtained consistently from the OM onboard XMM and from ground-based
observations. We believe that the late frequency is the white dwarf rotation
and that the ratio of spin/orbit period strongly supports that the system is an
intermediate polar.Comment: 17 pages, 22 figures, 7 tables, accepted for publication in MNRA
First Light Measurements of Capella with the Low Energy Transmission Grating Spectrometer aboard the Chandra X-ray Observatory
We present the first X-ray spectrum obtained by the Low Energy Transmission
Grating Spectrometer (LETGS) aboard the Chandra X-ray Observatory. The spectrum
is of Capella and covers a wavelength range of 5-175 A (2.5-0.07 keV). The
measured wavelength resolution, which is in good agreement with ground
calibration, is 0.06 A (FWHM). Although in-flight
calibration of the LETGS is in progress, the high spectral resolution and
unique wavelength coverage of the LETGS are well demonstrated by the results
from Capella, a coronal source rich in spectral emission lines. While the
primary purpose of this letter is to demonstrate the spectroscopic potential of
the LETGS, we also briefly present some preliminary astrophysical results. We
discuss plasma parameters derived from line ratios in narrow spectral bands,
such as the electron density diagnostics of the He-like triplets of carbon,
nitrogen, and oxygen, as well as resonance scattering of the strong Fe XVII
line at 15.014 A.Comment: 4 pages (ApJ letter LaTeX), 2 PostScript figures, accepted for
publication in ApJ Letters, 200
Testing and healthcare seeking behavior preceding HIV diagnosis among migrant and non-migrant individuals living in the Netherlands: Directions for early-case finding
Propagation of a hole on a Neel background
We analyze the motion of a single hole on a N\'eel background, neglecting
spin fluctuations. Brinkman and Rice studied this problem on a cubic lattice,
introducing the retraceable-path approximation for the hole Green's function,
exact in a one-dimensional lattice. Metzner et al. showed that the
approximationalso becomes exact in the infinite-dimensional limit. We introduce
a new approach to this problem by resumming the Nagaoka expansion of the
propagator in terms of non-retraceable skeleton-paths dressed by
retraceable-path insertions. This resummation opens the way to an almost
quantitative solution of the problemin all dimensions and, in particular sheds
new light on the question of the position of the band-edges. We studied the
motion of the hole on a double chain and a square lattice, for which deviations
from the retraceable-path approximation are expected to be most pronounced. The
density of states is mostly adequately accounted for by the
retra\-ce\-able-path approximation. Our band-edge determination points towards
an absence of band tails extending to the Nagaoka energy in the spectrums of
the double chain and the square lattice. We also evaluated the spectral density
and the self-energy, exhibiting k-dependence due to finite dimensionality. We
find good agreement with recent numerical results obtained by Sorella et al.
with the Lanczos spectra decoding method. The method we employ enables us to
identify the hole paths which are responsible for the various features present
in the density of states and the spectral density.Comment: 26 pages,Revte
Josephson supercurrent through a topological insulator surface state
Topological insulators are characterized by an insulating bulk with a finite
band gap and conducting edge or surface states, where charge carriers are
protected against backscattering. These states give rise to the quantum spin
Hall effect without an external magnetic field, where electrons with opposite
spins have opposite momentum at a given edge. The surface energy spectrum of a
threedimensional topological insulator is made up by an odd number of Dirac
cones with the spin locked to the momentum. The long-sought yet elusive
Majorana fermion is predicted to arise from a combination of a superconductor
and a topological insulator. An essential step in the hunt for this emergent
particle is the unequivocal observation of supercurrent in a topological phase.
Here, we present the first measurement of a Josephson supercurrent through a
topological insulator. Direct evidence for Josephson supercurrents in
superconductor (Nb) - topological insulator (Bi2Te3) - superconductor e-beam
fabricated junctions is provided by the observation of clear Shapiro steps
under microwave irradiation, and a Fraunhofer-type dependence of the critical
current on magnetic field. The dependence of the critical current on
temperature and length shows that the junctions are in the ballistic limit.
Shubnikov-de Haas oscillations in magnetic fields up to 30 T reveal a
topologically non-trivial two-dimensional surface state. We argue that the
ballistic Josephson current is hosted by this surface state despite the fact
that the normal state transport is dominated by diffusive bulk conductivity.
The lateral Nb-Bi2Te3-Nb junctions hence provide prospects for the realization
of devices supporting Majorana fermions
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