2,923 research outputs found
The chemistry of Antarctic ozone 1960-1987
The factors that influence Antarctic ozone are examined with a view to understanding the observed historical trend. Researchers show that reduced ambient temperatures can dramatically enhance the efficiency of chemical removal processes. Attention is focused on positive feedback between levels of ozone, temperature, and rates of heterogeneous chemical reactions. ClO and its dimer, and high levels of these gases are maintained until the clouds evaporate, on 15 September for the simulation shown here
Vortex avalanches in the non-centrosymmetric superconductor Li2Pt3B
We investigated the vortex dynamics in the non-centrosymmetric superconductor
Li_2Pt_3B in the temperature range 0.1 K - 2.8 K. Two different logarithmic
creep regimes in the decay of the remanent magnetization from the Bean critical
state have been observed. In the first regime, the creep rate is
extraordinarily small, indicating the existence of a new, very effective
pinning mechanism. At a certain time a vortex avalanche occurs that increases
the logarithmic creep rate by a factor of about 5 to 10 depending on the
temperature. This may indicate that certain barriers against flux motion are
present and they can be opened under increased pressure exerted by the
vortices. A possible mechanism based on the barrier effect of twin boundaries
is briefly discussed
Evolution of the electronic excitation spectrum with strongly diminishing hole-density in superconducting Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta}
A complete knowledge of its excitation spectrum could greatly benefit efforts
to understand the unusual form of superconductivity occurring in the lightly
hole-doped copper-oxides. Here we use tunnelling spectroscopy to measure the
T\to 0 spectrum of electronic excitations N(E) over a wide range of
hole-density p in superconducting Bi_{2}Sr_{2}CaCu_{2}O_{8+/delta}. We
introduce a parameterization for N(E) based upon an anisotropic energy-gap
/Delta (\vec k)=/Delta_{1}(Cos(k_{x})-Cos(k_{y}))/2 plus an effective
scattering rate which varies linearly with energy /Gamma_{2}(E) . We
demonstrate that this form of N(E) allows successful fitting of differential
tunnelling conductance spectra throughout much of the
Bi_{2}Sr_{2}CaCu_{2}O_{8+/delta} phase diagram. The resulting average
/Delta_{1} values rise with falling p along the familiar trajectory of
excitations to the 'pseudogap' energy, while the key scattering rate
/Gamma_{2}^{*}=/Gamma_{2}(E=/Delta_{1}) increases from below ~1meV to a value
approaching 25meV as the system is underdoped from p~16% to p<10%. Thus, a
single, particle-hole symmetric, anisotropic energy-gap, in combination with a
strongly energy and doping dependent effective scattering rate, can describe
the spectra without recourse to another ordered state. Nevertheless we also
observe two distinct and diverging energy scales in the system: the energy-gap
maximum /Delta_{1} and a lower energy scale /Delta_{0} separating the spatially
homogeneous and heterogeneous electronic structures.Comment: High resolution version available at:
http://people.ccmr.cornell.edu/~jcdavis/files/Alldredge-condmat08010087-highres.pd
Nitrous oxide in fresh water systems: An estimate for the yield of atmospheric N2O associated with disposal of human waste
The N2O content of waters in the Potomac and Merrimack Rivers was measured on a number of occasions over the period April to July 1977. The concentrations of dissolved N2O exceeded those which would apply in equilibrium with air by factors ranging from about 46 in the Potomac to 1.2 in the Merrimack. Highest concentrations of dissolved N2O were associated with sewage discharges from the vicinity of Washington, D. C., and analysis indicates a relatively high yield, 1.3 to 11%, for prompt conversion of waste nitrogen to N2O. Measurements of dissolved N2O in fresh water ponds near Boston demonstrated that aquatic systems provide both strong sources and sinks for atmospheric N2O
Spectroscopic Fingerprint of Phase-Incoherent Superconductivity in the Cuprate Pseudogap State
A possible explanation for the existence of the cuprate "pseudogap" state is
that it is a d-wave superconductor without quantum phase rigidity. Transport
and thermodynamic studies provide compelling evidence that supports this
proposal, but few spectroscopic explorations of it have been made. One
spectroscopic signature of d-wave superconductivity is the particle-hole
symmetric "octet" of dispersive Bogoliubov quasiparticle interference
modulations. Here we report on this octet's evolution from low temperatures to
well into the underdoped pseudogap regime. No pronounced changes occur in the
octet phenomenology at the superconductor's critical temperature Tc, and it
survives up to at least temperature T ~ 1.5Tc. In the pseudogap regime, we
observe the detailed phenomenology that was theoretically predicted for
quasiparticle interference in a phase-incoherent d-wave superconductor. Thus,
our results not only provide spectroscopic evidence to confirm and extend the
transport and thermodynamics studies, but they also open the way for
spectroscopic explorations of phase fluctuation rates, their effects on the
Fermi arc, and the fundamental source of the phase fluctuations that suppress
superconductivity in underdoped cuprates.Comment: 27 pages, 12 figure
Ozone trends estimated from Umkehr observations made at Edmonton, Alberta, Canada
A Brewer Ozone Spectrophotometer has been in service at the Canadian ozone monitoring station at Stony Plain (53.55 deg N, 114.10 deg W), near Edmonton, Alberta, since 1984. During that time, the instrument has been operated in a fully automated mode that includes the collection of morning and evening Umkehr observations. Some 197 Umkehr observations have been analyzed to make an estimate of the temporal trend in ozone amount at high altitude over the station during the last 8 years. This work has shown that at 40 km the trend in the ozone concentration has been observed to be 0.14 plus or minus 0.10 percent per year
Fourier-Transformed Local Density of States and Tunneling into a -Wave Superconductor with Bosonic Modes
We analyze the effects of the electronic coupling to bosonic modes in a
d-wave superconductor. The role of the scattering due to boson on the momentum
transfer between electronic states in the Brilloine zone is addressed. We
consider specific examples of phonon, breathing mode phonon and spin
resonance at . The Fourier spectrum of the energy derivative local
density of states (LDOS) is calculated. To properly calibrate the effects of
different modes we fix the quasipartilce renormalization at specific momentum
points. It is found that the mode with highly anisotropic
momentum-dependent coupling matrix element gives rise to well definded features
in the Fourier spectrum, at the energy of mode plus gap, with a momentum
transfer along the Cu-O bond direction of cuprates. This result is in a
striking contrast to the cases of the coupling to other modes and also to the
case of no mode coupling. The origin of this difference is explored in detail.
A comparison with the recent STM experiments is briefly discussed.Comment: 9 pages, 4 eps figures include
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