2,619 research outputs found
Experimental realization of SQUIDs with topological insulator junctions
We demonstrate topological insulator (BiTe) dc SQUIDs, based on
superconducting Nb leads coupled to nano-fabricated Nb-BiTe-Nb
Josephson junctions. The high reproducibility and controllability of the
fabrication process allows the creation of dc SQUIDs with parameters that are
in agreement with design values. Clear critical current modulation of both the
junctions and the SQUID with applied magnetic fields have been observed. We
show that the SQUIDs have a periodicity in the voltage-flux characteristic of
, of relevance to the ongoing pursuit of realizing interferometers for
the detection of Majorana fermions in superconductor- topological insulator
structures
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
Hubbard physics in the symmetric half-filled periodic Anderson-Hubbard model
Two very different methods -- exact diagonalization on finite chains and a
variational method -- are used to study the possibility of a metal-insulator
transition in the symmetric half-filled periodic Anderson-Hubbard model. With
this aim we calculate the density of doubly occupied sites as a function of
various parameters. In the absence of on-site Coulomb interaction ()
between electrons, the two methods yield similar results. The double
occupancy of levels remains always finite just as in the one-dimensional
Hubbard model. Exact diagonalization on finite chains gives the same result for
finite , while the Gutzwiller method leads to a Brinkman-Rice transition
at a critical value (), which depends on and .Comment: 10 pages, 5 figure
The Impact of Phorate on the Genetic Diversity of Wetland Aquatic Invertebraes
Impacts of the insecticide phorate on the genetic diversity of wetland invertebrates were investigated using field and laboratory studies in 1991. Electrophoretic methods were evaluated for revealing the impact of insecticides. Objectives were to determine the ability of electrophoresis to reveal the impact of phorate on invertebrates and to determine the influence of phorate on the genetic diversity in two common invertebrates. Amphipods, Hyallela azteca and mayflies, Callibaetis ferrugineus (Walsh) were placed in constructed mesocosms in wetlands and were exposed to varying amounts of phorate. Survivors and individuals from the parent population were genetically tested using cellulose acetate electrophoresis techniques. Allele frequencies were calculated for invertebrates in treatments and invertebrates from populations not exposed to phorate. Mortality of test invertebrates was significantly greater in phorate treatments than in controls (F = 5.97, P = 0.019). Chi-square analysis revealed differences in allele frequencies between the untreated populations and individuals of both species treated with phorate cx2 \u3e 8.5; df = 1,2; p \u3c 0.05). In addition, phorate appeared to eliminate, or reduce the frequency of certain genotypes in both species. Results indicate phorate selected against sensitive individuals and electrophoresis was effective at detecting differences between untreated populations and invertebrates that survived treatments. Genetic techniques should enable wetland scientists to detect the effects of pollution on invertebrate populations by monitoring genetic composition
High-Resolution spectroscopy of the low-mass X-ray binary EXO 0748-67
We present initial results from observations of the low-mass X-ray binary EXO
0748-67 with the Reflection Grating Spectrometer on board the XMM-Newton
Observatory. The spectra exhibit discrete structure due to absorption and
emission from ionized neon, oxygen, and nitrogen. We use the quantitative
constraints imposed by the spectral features to develop an empirical model of
the circumsource material. This consists of a thickened accretion disk with
emission and absorption in the plasma orbiting high above the binary plane.
This model presents challenges to current theories of accretion in X-ray binary
systems.Comment: 5 pages, 4 figures, accepted by A&A letters, XMM special issu
Optimizing the Majorana character of SQUIDs with topologically non-trivial barriers
We have modeled SQUIDs with topologically non-trivial superconducting
junctions and performed an optimization study on the Majorana fermion
detection. We find that the SQUID parameters beta_L, and beta_C can be used to
increase the ratio of Majorana tunneling to standard Cooper pair tunneling by
more than two orders of magnitude. Most importantly, we show that dc SQUIDs
including topologically trivial components can still host strong signatures of
the Majorana fermion. This paves the way towards the experimental verification
of the theoretically predicted Majorana fermion.Comment: accepted by Physical Review
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
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