306 research outputs found
X-ray photoelectron spectroscopy studies of non-stoichiometric superconducting NbB2+x
Polycrystalline samples of NbB2+x with nominal composition (B/Nb) = 2.0, 2.1,
2.2, 2.3, 2.4 and 2.5 were studied by X-ray photoelectron spectroscopy (XPS).
The spectra revealed Nb and B oxides on the surface of the samples, mainly B2O3
and Nb2O5. After Ar ion etching the intensity of Nb and B oxides decreased. The
Nb 3d5/2 and B 1s core levels associated with the chemical states (B/Nb) were
identified and they do not change with etching time. The Binding Energy of the
Nb 3d5/2 and B 1s core levels increase as boron content increases, suggesting a
positive chemical shift in the core levels. On the other hand, analysis of
Valence Band spectra showed that the contribution of the Nb 4d states slightly
decreased while the contribution of the B 2p(pi) states increased as the boron
content increased. As a consequence, the electronic and superconducting
properties were substantially modified, in good agreement with band-structure
calculations.Comment: 10 pages, 7 figures, 1 tabl
Unusual features in the nonlinear microwave surface impedance of Y-Ba-Cu-O thin films
Striking features have been found in the nonlinear microwave (8 GHz) surface
impedance of high-quality YBaCuO thin films with comparable
low power characteristics [ and ]. The surface resistance is found to increase,
decrease, or remain independent of the microwave field (up to 60 mT)
at different temperatures and for different samples. However, the surface
reactance always follows the same functional form. Mechanisms which may
be responsible for the observed variations in and are briefly
discussed.Comment: 4 pages, 4 figure
Evaluating an Alleged Mimic of the Monarch Butterfly: \u3ci\u3eNeophasia\u3c/i\u3e (Lepidoptera: Pieridae) Butterflies are Palatable to Avian Predators
Some taxa have adopted the strategy of mimicry to protect themselves from predation. Butterflies are some of the best representatives used to study mimicry, with the monarch butterfly, Danaus plexippus (Lepidoptera: Nymphalidae) a well-known model. We are the first to empirically investigate a proposed mimic of the monarch butterfly: Neophasia terlooii, the Mexican pine white butterfly (Lepidoptera: Pieridae). We used captive birds to assess the palatability of N. terlooii and its sister species, N. menapia, to determine the mimicry category that would best fit this system. The birds readily consumed both species of Neophasia and a palatable control species but refused to eat unpalatable butterflies such as D. plexippus and Heliconius charithonia (Lepidoptera: Nymphalidae). Given some evidence for mild unpalatability of Neophasia, we discuss the results considering modifications to classic mimicry theory, i.e., a palatability-based continuum between Batesian and Müllerian mimicry, with a quasi-Batesian intermediate. Understanding the ecology of Neophasia in light of contemporary and historical sympatry with D. plexippus could shed light on the biogeography of, evolution of, and predation pressure on the monarch butterfly, whose migration event has become a conservation priority
Search for Magnetic Field Induced Gap in a High-Tc Superconductor
Break junctions made of the optimally doped high temperature superconductor
Bi2Sr2Ca2CuO8 with Tc of 90 K has been investigated in magnetic fields up to 12
T, at temperatures from 4.2 K to Tc. The junction resistance varied between
1kOhm and 300kOhm. The differential conductance at low biases did not exhibit a
significant magnetic field dependence, indicating that a magnetic-field-induced
gap (Krishana et al., Science 277 83 (1997)), if exists, must be smaller than
0.25 meV.Comment: 3 pages, 2 figure
Identification of the bulk pairing symmetry in high-temperature superconductors: Evidence for an extended s-wave with eight line nodes
we identify the intrinsic bulk pairing symmetry for both electron and
hole-doped cuprates from the existing bulk- and nearly bulk-sensitive
experimental results such as magnetic penetration depth, Raman scattering,
single-particle tunneling, Andreev reflection, nonlinear Meissner effect,
neutron scattering, thermal conductivity, specific heat, and angle-resolved
photoemission spectroscopy. These experiments consistently show that the
dominant bulk pairing symmetry in hole-doped cuprates is of extended s-wave
with eight line nodes, and of anisotropic s-wave in electron-doped cuprates.
The proposed pairing symmetries do not contradict some surface- and
phase-sensitive experiments which show a predominant d-wave pairing symmetry at
the degraded surfaces. We also quantitatively explain the phase-sensitive
experiments along the c-axis for both Bi_{2}Sr_{2}CaCu_{2}O_{8+y} and
YBa_{2}Cu_{3}O_{7-y}.Comment: 11 pages, 9 figure
Infrared Conductivity in Layered -wave Superconductors
We calculate the infrared conductivity of a stack of coupled, two-dimensional
superconducting planes within the Fermi liquid theory of superconductivity. We
include the effects of random scattering processes and show that the presence
of even a small concentration of resonant impurities, in a -wave
superconductor, has an important effect on both the in-plane and -axis
transport properties, which could serve as signatures for -wave pairing.Comment: 18 pages in a RevTex (3.0) file plus 5 postscript figures
(uuencoded). Replaced with minor changes as it will appear in the Physical
Review B {\bf 52} issue 1 Oct. 199
Influence of vortex-vortex interaction on critical currents across low-angle grain boundaries in YBa2Cu3O7-delta thin films
Low-angle grain boundaries with misorientation angles theta < 5 degrees in
optimally doped thin films of YBCO are investigated by magnetooptical imaging.
By using a numerical inversion scheme of Biot-Savart's law the critical current
density across the grain boundary can be determined with a spatial resolution
of about 5 micrometers. Detailed investigation of the spatially resolved flux
density and current density data shows that the current density across the
boundary varies with varying local flux density. Combining the corresponding
flux and current pattern it is found that there exists a universal dependency
of the grain boundary current on the local flux density. A change in the local
flux density means a variation in the flux line-flux line distance. With this
knowledge a model is developped that explains the flux-current relation by
means of magnetic vortex-vortex interaction.Comment: 7 pages, 14 figure
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