39 research outputs found
Superconducting Properties of MgCNi3 Films
We report the magnetotransport properties of thin polycrystalline films of
the recently discovered non-oxide perovskite superconductor MgCNi3. CNi3
precursor films were deposited onto sapphire substrates and subsequently
exposed to Mg vapor at 700 C. We report transition temperatures (Tc) and
critical field values (Hc2) of MgCNi3 films ranging in thickness from 7.5 nm to
100 nm. Films thicker than ~40 nm have a Tc ~ 8 K, and an upper critical field
Hc2 ~ 14 T, which are both comparable to that of polycrystalline powders. Hall
measurements in the normal state give a carrier density, n =-4.2 x 10^22 cm^-3,
that is approximately 4 times that reported for bulk samples.Comment: submitted to PR
Photoemission and x-ray absorption study of MgC_(1-x)Ni_3
We investigated electronic structure of MgC_(1-x)Ni_3 with photoemission and
x-ray absorption spectroscopy. Both results show that overall band structure is
in reasonable agreement with band structure calculations including the
existence of von Hove singularity (vHs)near E_F. However, we find that the
sharp vHs peak theoretically predicted near the E_F is substantially
suppressed. As for the Ni core level and absorption spectrum, there exist the
satellites of Ni 2p which have a little larger energy separation and reduced
intensity compared to the case of Ni-metal. These facts indicate that
correlation effects among Ni 3d electrons may be important to understand
various physical properties.Comment: 12 pages, 4 figure
Fluctuation induced hopping and spin polaron transport
We study the motion of free magnetic polarons in a paramagnetic background of
fluctuating local moments. The polaron can tunnel only to nearby regions of
local moments when these fluctuate into alignment. We propose this fluctuation
induced hopping as a new transport mechanism for the spin polaron. We calculate
the diffusion constant for fluctuation induced hopping from the rate at which
local moments fluctuate into alignment. The electrical resistivity is then
obtained via the Einstein relation. We suggest that the proposed transport
mechanism is relevant in the high temperature phase of the Mn pyrochlore
colossal magneto resistance compounds and Europium hexaboride.Comment: 8 pages, 3 figure
Two-bands superconductivity with intra- and interband pairing for synthetic superlattices
We consider a model for superconductivity in a two-band superconductor,
having an anisotropic electronic structure made of two partially overlapping
bands with a first hole-like and a second electron-like fermi surface. In this
pairing scenario, driven by the interplay between interband and
intraband pairing terms, we have solved the two gap equations at the
critical temperature and calculate and the chemical potential
as a function of the number of carriers for various values of pairing
interactions, , , and . The results show the
complexity of the physics of condensates with multiple order parameters with
the chemical potential near band edges.Comment: 6 pages, 2 figure
Critical Currents and Vortex States at Fractional Matching Fields in Superconductors with Periodic Pinning
We study vortex states and dynamics in 2D superconductors with periodic
pinning at fractional sub-matching fields using numerical simulations. For
square pinning arrays we show that ordered states form at 1/1, 1/2, and 1/4
filling fractions while only partially ordered states form at other filling
fractions, such as 1/3 and 1/5, in agreement with recent imaging experiments.
For triangular pinning arrays we observe matching effects at filling fractions
of 1/1, 6/7, 2/3, 1/3, 1/4, 1/6, and 1/7. For both square and triangular
pinning arrays we also find that, for certian sub-matching fillings, vortex
configurations depend on pinning strength. For weak pinning, ordering in which
a portion of the vortices are positioned between pinning sites can occur.
Depinning of the vortices at the matching fields, where the vortices are
ordered, is elastic while at the incommensurate fields the motion is plastic.
At the incommensurate fields, as the applied driving force is increased, there
can be a transition to elastic flow where the vortices move along the pinning
sites in 1D channels and a reordering transition to a triangular or distorted
triangular lattice. We also discuss the current-voltage curves and how they
relate to the vortex ordering at commensurate and incommensurate fields.Comment: 14 figure
Matching and surface barrier effects of the flux-line lattice in superconducting films and multilayers.
The flux-line lattice dissipation and the pinning force of Bi2Sr2CaCu2O8 and YBa2Cu3O7 films and a Nb/Cu multilayer are investigated with the vibrating reed technique. In magnetic fields oriented under a small angle with respect to the film surfaces the Bi-2:2:1:2 film shows a series of pronounced dissipation maxima at matching fields BN in the irreversible region of the magnetic phase diagram. The Y-1:2:3 film shows tiny damping maxima, whereas no structure in the dissipation of the Nb/Cu multilayer is detected below the upper critical field. The comparison of the matching fields to an anisotropic London model shows that the dissipation maxima are caused by rearrangements of the flux-line lattice configuration due to interactions with the sample surface. The different behavior of the high-temperature superconductors and the Nb/Cu multilayer is understood by explicitly taking the surface barrier into account. Deviations from the surface induced commensurability of the flux-line lattice due to the intrinsic pinning are discussed. Our results indicate that pancake vortices in the Bi-2:2:1:2 film should be coupled below the irreversibility line and below magnetic fields B??0.5 T perpendicular to the film surface
Trophic relationships between sperm whales and the jumbo squid using stable isotopes of C and N
The trophic position and the predator–prey relationship between the sperm whale Physeter macrocephalus and the jumbo squid Dosidicus gigas were examined by measuring stable isotope ratios of carbon and nitrogen. Skin samples of sperm whales and muscle samples of small and large jumbo squid were collected between 1996 and 1999 in the Gulf of California. Gender determination through molecular analysis and field identification of size were used to identify adult male, female and immature male sperm whales. The stable isotope ratios of C and N of females and immature males were significantly different from those of adult male sperm whales; however, between females and immature males they did not differ significantly. The δ13C and δ15N values of females and immature males were higher than large jumbo squid by 1.1‰ and 2.7‰ respectively, suggesting a predator–prey relationship between them. A low isotopic interannual variation among the years 1997 to 1999 was observed in the isotopic signature of females and males. Adult males exhibited a lower isotopic signature than females and immature males, and did not show a trophic relationship with D. gigas. We hypothesized that the stable isotopic signature of mature males reflected their diet from an earlier high-latitude feeding ground. This study shows that stable isotope analysis of sloughed skin samples from free-ranging sperm whales can be an alternative method to stomach content and fecal analyses for evaluating trophic relationships