2,547 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
Electronically coupled complementary interfaces between perovskite band insulators
Perovskite oxides exhibit a plethora of exceptional electronic properties,
providing the basis for novel concepts of oxide-electronic devices. The
interest in these materials is even extended by the remarkable characteristics
of their interfaces. Studies on single epitaxial connections between the two
wide-bandgap insulators LaAlO3 and SrTiO3 have revealed them to be either
high-mobility electron conductors or insulating, depending on the atomic
stacking sequences. In the latter case they are conceivably positively charged.
For device applications, as well as for basic understanding of the interface
conduction mechanism, it is important to investigate the electronic coupling of
closely-spaced complementary interfaces. Here we report the successful
realization of such electronically coupled complementary interfaces in SrTiO3 -
LaAlO3 thin film multilayer structures, in which the atomic stacking sequence
at the interfaces was confirmed by quantitative transmission electron
microscopy. We found a critical separation distance of 6 perovskite unit cell
layers, corresponding to approximately 2.3 nm, below which a decrease of the
interface conductivity and carrier density occurs. Interestingly, the high
carrier mobilities characterizing the separate electron doped interfaces are
found to be maintained in coupled structures down to sub-nanometer interface
spacing
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
Gate-tunable band structure of the LaAlO-SrTiO interface
The 2-dimensional electron system at the interface between LaAlO and
SrTiO has several unique properties that can be tuned by an externally
applied gate voltage. In this work, we show that this gate-tunability extends
to the effective band structure of the system. We combine a magnetotransport
study on top-gated Hall bars with self-consistent Schr\"odinger-Poisson
calculations and observe a Lifshitz transition at a density of
cm. Above the transition, the carrier density of one
of the conducting bands decreases with increasing gate voltage. This surprising
decrease is accurately reproduced in the calculations if electronic
correlations are included. These results provide a clear, intuitive picture of
the physics governing the electronic structure at complex oxide interfaces.Comment: 14 pages, 4 figure
Parallel electron-hole bilayer conductivity from electronic interface reconstruction
The perovskite SrTiO-LaAlO structure has advanced to a model system
to investigate the rich electronic phenomena arising at polar interfaces. Using
first principles calculations and transport measurements we demonstrate that an
additional SrTiO capping layer prevents structural and chemical
reconstruction at the LaAlO surface and triggers the electronic
reconstruction at a significantly lower LaAlO film thickness than for the
uncapped systems. Combined theoretical and experimental evidence (from
magnetotransport and ultraviolet photoelectron spectroscopy) suggests two
spatially separated sheets with electron and hole carriers, that are as close
as 1 nm.Comment: Phys. Rev. Lett., in pres
Evidence of two-dimensional macroscopic quantum tunneling of a current-biased DC-SQUID
The escape probability out of the superconducting state of a hysteretic
DC-SQUID has been measured at different values of the applied magnetic flux. At
low temperature, the escape current and the width of the probability
distribution are temperature independent but they depend on flux. Experimental
results do not fit the usual one-dimensional (1D) Macroscopic Quantum Tunneling
(MQT) law but are perfectly accounted for by the two-dimensional (2D) MQT
behaviour as we propose here. Near zero flux, our data confirms the recent MQT
observation in a DC-SQUID \cite{Li02}.Comment: 4 pages, 4 figures Accepted to PR
- …