621 research outputs found
Krill faecal pellets drive hidden pulses of particulate organic carbon in the marginal ice zone
The biological carbon pump drives a flux of particulate organic carbon (POC) through the ocean and affects atmospheric levels of carbon dioxide. Short term, episodic flux events are hard to capture with current observational techniques and may thus be underrepresented in POC flux estimates. We model the potential hidden flux of POC originating from Antarctic krill, whose swarming behaviour could result in a major conduit of carbon to depth through their rapid exploitation of phytoplankton blooms and bulk egestion of rapidly sinking faecal pellets (FPs). Our model results suggest a seasonal krill FP export flux of 0.039 GT C across the Southern Ocean marginal ice zone, corresponding to 17–61% (mean 35%) of current satellite-derived export estimates for this zone. The magnitude of our conservatively estimated flux highlights the important role of large, swarming macrozooplankton in POC export and, the need to incorporate such processes more mechanistically to improve model projections
Electronic Spectra and Their Relation to the (pi,pi) Collective Mode in High-Tc Superconductors
Photoemission spectra of Bi2Sr2CaCu2O8 reveal that the high energy feature
near (pi,0), the "hump", scales with the superconducting gap and persists above
Tc in the pseudogap phase. As the doping decreases, the dispersion of the hump
increasingly reflects the wavevector (pi,pi) characteristic of the undoped
insulator, despite the presence of a large Fermi surface. This can be
understood from the interaction of the electrons with a collective mode,
supported by our observation that the doping dependence of the resonance
observed by neutron scattering is the same as that inferred from our data.Comment: 4 pages (revtex), 5 figures (eps
Relationship between soil fungal diversity and temperature in the maritime Antarctic
Soil fungi have pivotal ecological roles as decomposers, pathogens and symbionts1, 2. Alterations to their diversity arising from climate change could have substantial effects on ecosystems, particularly those undergoing rapid warming that contain few species3, 4. Here, we report a study using pyrosequencing to assess fungal diversity in 29 soils sampled from a 1,650 km climatic gradient through the maritime Antarctic, the most rapidly warming region in the Southern Hemisphere5, 6. Using a ‘space-for-time’ substitution approach, we show that soil fungal diversity is higher in warmer habitats, with increases of 4.7 (observed) and 11.3 (predicted) fungal taxa per degree Celsius rise in surface temperature along the transect. Among 22 predictor variables, air temperature was the strongest and most consistent predictor of diversity. We propose that the current rapid warming in the maritime Antarctic (0.34 °C per decade6) will facilitate the colonization of soil by a wider diversity of fungi than at present, with data from regression models suggesting 20–27% increases in fungal species richness in the southernmost soils by 2100. Such increases in diversity, which provide a sentinel for changes at lower latitudes, are likely to have substantial effects on nutrient cycling and, ultimately, productivity in the species-poor soils of maritime Antarctica
Quasiparticles in the superconducting state of Bi2Sr2CaCu2O8
Recent improvements in momentum resolution by a factor of 32 lead to
qualitatively new ARPES results on the spectra of Bi2Sr2CaCu2O8 (Bi2212) along
the (pi,pi) direction, where there is a node in the superconducting gap. With
improved resolution, we now see the intrinsic lineshape, which indicates the
presence of true quasiparticles at the Fermi momentum in the superconducting
state, and lack thereof in the normal state. The region of momentum space
probed here is relevant for charge transport, motivating a comparison of our
results to conductivity measurements by infrared reflectivity.Comment: revised paper with new figure
Origins of large critical temperature variations in single layer cuprates
We study the electronic structures of two single layer superconducting
cuprates, TlBaCuO (Tl2201) and
(BiPb)(SrLa)CuO (Bi2201) which
have very different maximum critical temperatures (90K and 35K respectively)
using Angular Resolved Photoemission Spectroscopy (ARPES). We are able to
identify two main differences in their electronic properties. First, the shadow
band that is present in double layer and low T single layer cuprates
is absent in Tl2201. Recent studies have linked the shadow band to structural
distortions in the lattice and the absence of these in Tl2201 may be a
contributing factor in its T.Second, Tl2201's Fermi surface (FS)
contains long straight parallel regions near the anti-node, while in Bi2201 the
anti-nodal region is much more rounded. Since the size of the superconducting
gap is largest in the anti-nodal region, differences in the band dispersion at
the anti-node may play a significant role in the pairing and therefore affect
the maximum transition temperature.Comment: 6 pages, 5 figures,1 tabl
On the determination of the Fermi surface in high-Tc superconductors by angle-resolved photoemission spectroscopy
We study the normal state electronic excitations probed by angle resolved
photoemission spectroscopy (ARPES) in Bi2201 and Bi2212. Our main goal is to
establish explicit criteria for determining the Fermi surface from ARPES data
on strongly interacting systems where sharply defined quasiparticles do not
exist and the dispersion is very weak in parts of the Brillouin zone.
Additional complications arise from strong matrix element variations within the
zone. We present detailed results as a function of incident photon energy, and
show simple experimental tests to distinguish between an intensity drop due to
matrix element effects and spectral weight loss due to a Fermi crossing. We
reiterate the use of polarization selection rules in disentangling the effect
of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite
all the complications, the Fermi surface can be determined unambiguously: it is
a single large hole barrel centered about (pi,pi) in both materials.Comment: Expanded discussion of symmetrization method in Section 5, figures
remain the sam
The Fermi surface of Bi2Sr2CaCu2O8
We study the Fermi surface of Bi2Sr2CaCu2O8 (Bi2212) using angle resolved
photoemission (ARPES) with a momentum resolution of ~ 0.01 of the Brillouin
zone. We show that, contrary to recent suggestions, the Fermi surface is a
large hole barrel centered at (pi,pi), independent of the incident photon
energy.Comment: 4 pages (revtex), 4 figures (eps, 2 color
ARPES study of Pb doped Bi_2Sr_2CaCu_2O_8 - a new Fermi surface picture
High resolution angle resolved photoemission data from Pb doped
Bi_2Sr_2CaCu_2O_8 (Bi2212) with suppressed superstructure is presented.
Improved resolution and very high momentum space sampling at various photon
energies reveal the presence of two Fermi surface pieces. One has the hole-like
topology, while the other one has its van Hove singularity very close to
(pi,0), its topology at some photon energies resembles the electron-like piece.
This result provides a unifying picture of the Fermi surface in the Bi2212
compound and reconciles the conflicting reports.Comment: 4 pages, 4 figure
Doubling of the bands in overdoped Bi2Sr2CaCu2O8-probable evidence for c-axis bilayer coupling
We present high resolution ARPES data of the bilayer superconductor
Bi2Sr2CaCu2O8 (Bi2212) showing a clear doubling of the near EF bands. This
splitting approaches zero along the (0,0)-(pi,pi) nodal line and is not
observed in single layer Bi2Sr2CuO6 (Bi2201), suggesting that the splitting is
due to the long sought after bilayer splitting effect. The splitting has a
magnitude of approximately 75 meV near the middle of the zone, extrapolating to
about 100 meV near the (pi,0) poin
Renormalization of spectral lineshape and dispersion below Tc in Bi2Sr2CaCu2O8+d
Angle-resolved photoemission (ARPES) data in the superconducting state of
Bi2Sr2CaCu2O8+d show a kink in the dispersion along the zone diagonal, which is
related via a Kramers-Kronig analysis to a drop in the low-energy scattering
rate. As one moves towards (pi,0), this kink evolves into a spectral dip. The
occurrence of these anomalies in the dispersion and lineshape throughout the
zone indicate the presence of a new energy scale in the superconducting state.Comment: New Figure 3 with expanded discussio
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