19,605 research outputs found
Superconducting nanobridges under magnetic fields
We report on the study of superconducting nanotips and nanobridges of lead
with a Scanning Tunnelling Microscope in tunnel and point contact regimes. We
deal with three different structures. A nanotip that remains superconducting
under a field of 2 T. For this case we present model calculations of the order
parameter, which are in good agreement with the experiments. An asymmetric
nanobridge of lead showing a two steps loss of the Andreev excess current due
to different heating and dissipation phenomena in each side of the structure. A
study of the effect of the thermal fluctuations on the Josephson coupling
between the two sides of a superconducting nanobridge submitted to magnetic
fields. The different experiments were made under magnetic fields up to twenty
five times the volume critical field of lead, and in a temperature range
between 0.6 K and 7.2 K.Comment: 17 pages, 7 figure
Visual observation to identify sexes in subspecies of adult Black Skimmers (Rynchops niger)
Identifying sexes in birds from visual observations could be a useful and inexpensive method. While sexual dichromatism and ornaments are readily used by observers, sexual size dimorphism can also be used to identify sexes in some bird species. This study assessed the applicability of visual observation of size differences to identify sexes in adult Black Skimmers (Rynchops niger). Black Skimmers do not have sexual dichromatism; however, Black Skimmer males are skeletally larger (6.7–31.7% depending on trait) and 33.3–37.5% heavier than females. The study focused on two subspecies: Amazonian (R. n. cinerascens) and South American (R. n. intercedens) Black Skimmers. Sex identified by visually observing size differences was consistent with the sex identified at specimen preparation from examining gonads (RGLMM = 0.996 ± 0.004). The identification of sexes from photographs using visual observation of size had a very high within- (RGLMM = 0.995 ± 0.001) and between- (RGLMM = 0.984 ± 0.002) observer repeatability. Non-invasive methods for identifying sex by visual observation may allow enhanced use of data from photographic datasets, citizen science projects, and surveys using direct observation or images
Spin-independent v-representability of Wigner crystal oscillations in one-dimensional Hubbard chains: The role of spin-charge separation
Electrons in one-dimension display the unusual property of separating their
spin and charge into two independent entities: The first, which derive from
uncharged spin-1/2 electrons, can travel at different velocities when compared
with the second, built from charged spinless electrons. Predicted theoretically
in the early sixties, the spin-charge separation has attracted renewed
attention since the first evidences of experimental observation, with usual
mentions as a possible explanation for high-temperature superconductivity. In
one-dimensional (1D) model systems, the spin-charge separation leads the
frequencies of Friedel oscillations to suffer a 2k_F -- 4k_F crossover, mainly
when dealing with strong correlations, where they are referred to as Wigner
crystal oscillations. In non-magnetized systems, the current density
functionals which are applied to the 1D Hubbard model are not seen to reproduce
this crossover, referring to a more fundamental question: Are the Wigner
crystal oscillations in 1D systems non-interacting v-representable? Or, is
there a spin-independent Kohn-Sham potential which is able to yield spin-charge
separation? Finding an appropriate answer to both questions is our main task
here. By means of exact and DMRG solutions, as well as, a new approach of
exchange-correlation potential, we show the answer to be positive.
Specifically, the v-representable 4k_F oscillations emerge from attractive
interactions mediated by positively charged spinless holes -- the holons -- as
an additional contribution to the repulsive on-site Hubbard interaction
Using field photography to study avian moult
Methods to obtain moult data from wild birds have not changed much over the last century and most studies still depend on checking museum specimens or capturing birds. Here we assess the applicability of systematic field photography for detecting and scoring moult in adult Black Skimmers Rynchops niger from southern Brazil. Moult data extracted from photographs have a high within- (RGLMM = 0.98) and between-observer repeatability (RGLMM = 0.97) and show very good fit to current Underhill–Zucchini moult models (R2 = 0.75). Photography offers the advantages of being less invasive, requiring less equipment and human effort, being feasible in areas where captures may not be possible, and causing less disturbance, so enhancing the number of sampled individuals
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