7,135 research outputs found
Cooper pair insulator in amorphous films induced by nanometer-scale thickness variations
Unusual transport properties of superconducting (SC) materials, such as the
under doped cuprates, low dimensional superconductors in strong magnetic
fields, and insulating films near the Insulator Superconductor Transition
(IST), have been attributed to the formation of inhomogeneous phases.
Difficulty correlating the behaviors with observations of the inhomogeneities
make these connections uncertain. Of primary interest here are proposals that
insulating films near the IST, which show an activated resistance and giant
positive magnetoresistance, contain islands of Cooper Pairs (CPs). Here we
present evidence that these types of inhomogeneities are essential to such an
insulating phase in amorphous Bi (a-Bi) films deposited on substrates patterned
with nanometer-sized holes. The patterning induces film thickness variations,
and corresponding coupling constant variations, that transform the composition
of the insulator from localized electrons to CPs. Analyses near the
thickness-tuned ISTs of films on nine different substrates show that weak links
between SC islands dominate the transport. In particular, the ISTs all occur
when the link resistance approaches the resistance quantum for pairs. These
observations lead to a detailed picture of CPs localized by spatial variations
of the superconducting coupling constant.Comment: 4 pages, 3 figures, 1 supplemental page with 1 supplemental figur
Cooper-pair insulator phase in superconducting amorphous Bi films induced by nanometer-scale thickness variations
Ultrathin films near the quantum insulator-superconductor transition (IST) can exhibit Cooper-pair transport in their insulating state. This Cooper-pair insulator (CPI) state is achieved in amorphous Bi films evaporated onto substrates with a topography varying on lengths slightly greater than the superconducting coherence length. We present evidence that this topography induces film thickness and corresponding superconducting coupling constant variations that promote Cooper-pair island formation. Analyses of many thickness-tuned ISTs show that weak links between superconducting islands dominate the transport. In particular, the IST occurs when the link resistance approaches the resistance quantum for pairs. These results support conjectures that the CPI is an inhomogeneous state of matter
Observation of giant positive magnetoresistance in a Cooper pair insulator.
Ultrathin amorphous Bi films, patterned with a nanohoneycomb array of holes, can exhibit an insulating phase with transport dominated by the incoherent motion of Cooper pairs (CP) of electrons between localized states. Here, we show that the magnetoresistance (MR) of this Cooper pair insulator (CPI) phase is positive and grows exponentially with decreasing temperature T, for T well below the pair formation temperature. It peaks at a field estimated to be sufficient to break the pairs and then decreases monotonically into a regime in which the film resistance assumes the T dependence appropriate for weakly localized single electron transport. We discuss how these results support proposals that the large MR peaks in other unpatterned, ultrathin film systems disclose a CPI phase and provide new insight into the CP localization
Superconductor-insulator transitions in films patterned with a disordered nanohoneycomb hole array
High field magnetoresistance peak near the superconductor insulator transition in amorphous Bi films patterned with a nanohoneycomb array of holes
Movement of transgenic plant-expressed Bt Cry1Ac proteins through high trophic levels
The movement of Bacillus thuringiensis (Berliner) (Bt) Cry1Ac endotoxin through high trophic levels was assessed to help elucidate the effects of Bt toxin on non-target insects. The diamondback moth (Plutella xylostella L., Lepidoptera: Plutellidae), the parasitic wasp (Cotesia vestalis Haliday, Hymenoptera: Braconidae) and the predatory green lacewing Chrysoperla carnea (Stephen) (Neuroptera: Chrysopidae) were used as a model system in this laboratory study. Bt-resistant P. xylostella larvae fed Cry1Ac-expressing transgenic oilseed rape (OSR, Brassica napus L., Cruciferae), before and after parasitization by C. vestalis, consumed Cry1Ac with the ingested plant material but only a proportion of Cry1Ac consumed was recovered from the bodies and faeces of P. xylostella larvae. Cry1Ac was not detected in newly emerged parasitoid larvae. In contrast, Cry1Ac was detected in C. carnea larvae fed on resistant P. xylostella larvae reared on Bt OSR. However, no Cry1Ac could be detected in C. carnea larvae when the lacewings were transferred to P. xylostella larvae reared on conventional OSR and tested 24-48 h. The metabolizing ability of Cry1Ac is discussed for the larvae of P. xylostella and C. carnea
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Design of standards for nondestructive assay of special nuclear material
Nondestructive assay (NDA) of special nuclear material (SNM) involves a variety of measurement techniques, instruments, and nuclear materials. High-quality measurements require well-characterized SNM standards that represent the expected range of mass, chemical composition, and physical properties of the SNM to be measured. Due to the very limited commercial availability of NDA standards, facilities must usually produce their own standards, both to meet their specific measurement needs and to comply with existing regulations. This paper will describe the current extent to which NDA standards are commercially available. The authors will further describe the types of NDA standards used to calibrate and verify the measurement techniques commonly used in the safeguards of SNM. Several types of NDA standards will be discussed in detail to illustrate the considerations that go into specifying and designing traceable, representative standards for materials accounting measurements
Adjusted ADM systems and their expected stability properties: constraint propagation analysis in Schwarzschild spacetime
In order to find a way to have a better formulation for numerical evolution
of the Einstein equations, we study the propagation equations of the
constraints based on the Arnowitt-Deser-Misner formulation. By adjusting
constraint terms in the evolution equations, we try to construct an
"asymptotically constrained system" which is expected to be robust against
violation of the constraints, and to enable a long-term stable and accurate
numerical simulation. We first provide useful expressions for analyzing
constraint propagation in a general spacetime, then apply it to Schwarzschild
spacetime. We search when and where the negative real or non-zero imaginary
eigenvalues of the homogenized constraint propagation matrix appear, and how
they depend on the choice of coordinate system and adjustments. Our analysis
includes the proposal of Detweiler (1987), which is still the best one
according to our conjecture but has a growing mode of error near the horizon.
Some examples are snapshots of a maximally sliced Schwarzschild black hole. The
predictions here may help the community to make further improvements.Comment: 23 pages, RevTeX4, many figures. Revised version. Added subtitle,
reduced figures, rephrased introduction, and a native checked. :-
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