1,061 research outputs found
Photoemission and the Origin of High Temperature Superconductivity
The condensation energy can be shown to be a moment of the change in the
occupied part of the spectral function when going from the normal to the
superconducting state. As a consequence, there is a one to one correspondence
between the energy gain associated with forming the superconducting ground
state, and the dramatic changes seen in angle resolved photoemission spectra.
Some implications this observation has are offered.Comment: 4 pages, M2S conference proceeding
Electron Self-Energy of High Temperature Superconductors as Revealed by Angle Resolved Photoemission
In this paper, we review some of the work our group has done in the past few
years to obtain the electron self-energy of high temperature superconductors by
analysis of angle-resolved photoemission data. We focus on three examples which
have revealed: (1) a d-wave superconducting gap, (2) a collective mode in the
superconducting state, and (3) pairing correlations in the pseudogap phase. In
each case, although a novel result is obtained which captures the essense of
the data, the conventional physics used leads to an incomplete picture. This
indicates that new physics needs to be developed to obtain a proper
understanding of these materials.Comment: 5 pages, revtex, 3 encapsulated postscript figures, SNS97 proceeding
Calculation of Effective Coulomb Interaction for , , and
In this paper, the Slater integrals for a screened Coulomb interaction of the
the Yukawa form are calculated and by fitting the Thomas-Fermi wavevector, good
agreement is obtained with experiment for the multiplet spectra of
and ions. Moreover, a predicted multiplet spectrum for the heavy
fermion superconductor is shown with a calculated Coulomb U of 1.6 eV.
These effective Coulomb interactions, which are quite simple to calculate,
should be useful inputs to further many-body calculations in correlated
electron metals.Comment: 8 pages, revtex, 3 uuencoded postscript figure
The State of Self-Organized Criticality of the Sun During the Last Three Solar Cycles. II. Theoretical Model
The observed powerlaw distributions of solar flare parameters can be
interpreted in terms of a nonlinear dissipative system in the state of
self-organized criticality (SOC). We present a universal analytical model of a
SOC process that is governed by three conditions: (i) a multiplicative or
exponential growth phase, (ii) a randomly interrupted termination of the growth
phase, and (iii) a linear decay phase. This basic concept approximately
reproduces the observed frequency distributions. We generalize it to a
randomized exponential-growth model, which includes also a (log-normal)
distribution of threshold energies before the instability starts, as well as
randomized decay times, which can reproduce both the observed occurrence
frequency distributions and the scatter of correlated parametyers more
realistically. With this analytical model we can efficiently perform
Monte-Carlo simulations of frequency distributions and parameter correlations
of SOC processes, which are simpler and faster than the iterative simulations
of cellular automaton models. Solar cycle modulations of the powerlaw slopes of
flare frequency distributions can be used to diagnose the thresholds and growth
rates of magnetic instabilities responsible for solar flares.Comment: Part II of Paper I: The State of Self-Organized Criticality of the
Sun During the Last Three Solar Cycles. I. Observation
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
Evolution of the pairing pseudogap in the spectral function with interplane anisotropy
We study the pairing pseudogap in the spectral function as a function of
interplane coupling. The analytical expressions for the self-energy in the
critical regime are obtained for any degree of anisotropy. The frequency
dependence of the self-energy is found to be qualitatively different in two and
three dimensions, and the crossover from two to three dimensional behavior is
discussed. In particular, by considering the anisotropy of the Fermi velocity
and gap along the Fermi surface, we can qualitatively explain recent
photoemission experiments on high temperature superconductors concerning the
temperature dependent Fermi arcs seen in the pseudogap phase.Comment: 20 pages, revtex, 5 encapsulated postscript figures include
The physiological and behavioral responses of steers to gaseous ammonia in simulated long distance transport by ship.
Ammonia (NH3) can accumulate in high density cattle accommodation during live export shipments and could potentially threaten the animals' health and welfare. The effects of 4 NH3 concentrations, control ( 0.05) on hematological parameters or body weight. Twenty-eight days after exposure to NH3, the steers' pulmonary macrophage activity and neutrophil levels had returned to normal. It was concluded that ammonia concentrations of 30 and 45 ppm induced temporary inflammatory responses which indicate an adverse effect on the welfare of steers
Absence of Persistent Magnetic Oscillations in Type-II Superconductors
We report on a numerical study intended to examine the possibility that
magnetic oscillations persist in type II superconductors beyond the point where
the pairing self-energy exceeds the normal state Landau level separation. Our
work is based on the self-consistent numerical solution for model
superconductors of the Bogoliubov-deGennes equations for the vortex lattice
state. In the regime where the pairing self-energy is smaller than the
cyclotron energy, magnetic oscillations resulting from Landau level
quantization are suppressed by the broadening of quasiparticle Landau levels
due to the non-uniform order parameter of the vortex lattice state, and by
splittings of the quasiparticle bands. Plausible arguments that the latter
effect can lead to a sign change of the fundamental harmonic of the magnetic
oscillations when the pairing self-energy is comparable to the cyclotron energy
are shown to be flawed. Our calculations indicate that magnetic oscillations
are strongly suppressed once the pairing self-energy exceeds the Landau level
separation.Comment: 7 pages, revtex, 7 postscript figure
Disbudding effects on growth analysis of Celosia (Celosia cristata)
Experiments to investigate the effects of disbudding on growth analysis of two celosia cultivars, ‘Carmine’ and ‘Chief Gold’, were carried out on the field in 2009 and 2010 at the Sinna Garden of Department of Crop Science, University of Ghana, Legon, Accra, Ghana. The treatments consisted of disbudding once, disbudding twice, and no disbudding, as control, and were arranged in a 3x2 split plot in a randomized complete block design with four replications in 2009 (experiment 1) and three in 2010 (experiment 2). The two cultivars were harvested weekly during the growing period and separated into the various plant parts and oven-dried for dry weights, using appropriate formulae to calculate the various growth parameters. Analysis of variance (ANOVA) was used to analyse the data and a correlation coefficient matrix showed relationships among growth parameters. Disbudding resulted in increased leaf area index, leaf area ratio, leaf area duration, relative growth rate, and harvest index, but reduced crop growth rate and net assimilation rate. ‘Chief Gold’ had a higher harvest index than ‘Carmine’. Disbudding plants once gave the best flower head size and weight result. ‘Carmine’ gave the best flower yield and quality results in experiment 1 and ‘Chief Gold’ in experiment 2
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