5,326 research outputs found
Supercritical multicomponent solvent coal extraction
The yield of organic extract from the supercritical extraction of coal with larger diameter organic solvents such as toluene is increased by use of a minor amount of from 0.1 to 10% by weight of a second solvent such as methanol having a molecular diameter significantly smaller than the average pore diameter of the coal
Magnetic resonance peak and nonmagnetic impurities
Nonmagnetic Zn impurities are known to strongly suppress superconductivity.
We review their effects on the spin excitation spectrum in , as investigated by inelastic neutron scattering measurements.Comment: Proceedings of Mato Advanced Research Workshop BLED 2000. To appear
in Nato Science Series: B Physic
Pi excitation of the t-J model
In this paper, we present analytical and numerical calculations of the pi
resonance in the t-J model. We show in detail how the pi resonance in the
particle-particle channel couples to and appears in the dynamical spin
correlation function in a superconducting state. The contribution of the pi
resonance to the spin excitation spectrum can be estimated from general
model-independent sum rules, and it agrees with our detailed calculations. The
results are in overall agreement with the exact diagonalization studies of the
t-J model. Earlier calculations predicted the correct doping dependence of the
neutron resonance peak in the YBCO superconductor, and in this paper detailed
energy and momentum dependence of the spin correlation function is presented.
The microscopic equations of motion obtained within current formalism agree
with that of the SO(5) nonlinear sigma model, where the pi resonance is
interpreted as a pseudo Goldstone mode of the spontaneous SO(5) symmetry
breaking.Comment: 33 pages, LATEX, 14 eps fig
Excitations in antiferromagnetic cores of superconducting vortices
We study excitations of the predicted antiferromagnetically ordered vortex
cores in the superconducting phase of the newly proposed SO(5) model of
strongly correlated electrons. Using experimental data from the literature we
show that the susceptibilities in the spin sector and the charge sector are
nearly equal, and likewise for the stiffnesses. In the case of strict equality
SO(5) symmetry is possible, and we find that if present the vortices give rise
to an enhanced neutron scattering cross section near the so called pi resonance
at 41 meV. In the case of broken SO(5) symmetry two effects are predicted.
Bound excitations can exist in the vortex cores with ``high'' excitation
energies slightly below 41 meV, and the massless Goldstone modes corresponding
to the antiferromagnetic ordering of the core can acquire a mass and show up as
core excitation with ``low'' excitation energies around 2 meV.Comment: 9 pages, RevTeX, including 3 postscript figures, submitted to Phys.
Rev. B, July 10, 199
What the resonance peak cannot do
In certain cuprates, a spin 1 resonance mode is prominent in the magnetic
structure measured by neutron scattering. It has been proposed that this mode
is responsible for significant features seen in other spectroscopies, such as
photoemission and optical absorption, which are sensitive to the charge
dynamics, and even that this mode is the boson responsibile for ``mediating''
the superconducting pairing. We show that its small (measured) intensity and
weak coupling to electron-hole pairs (as deduced from the measured lifetime)
disqualifies the resonant mode from either proposed role.Comment: 4 pages, no figur
Flavoured soft leptogenesis and natural values of the B term
We revisit flavour effects in soft leptogenesis relaxing the assumption of
universality for the soft supersymmetry breaking terms. We find that with
respect to the case in which the heavy sneutrinos decay with equal rates and
equal CP asymmetries for all lepton flavours, hierarchical flavour
configurations can enhance the efficiency by more than two orders of magnitude.
This translates in more than three order of magnitude with respect to the
one-flavour approximation. We verify that lepton flavour equilibration effects
related to off-diagonal soft slepton masses are ineffective for damping these
large enhancements. We show that soft leptogenesis can be successful for
unusual values of the relevant parameters, allowing for and for values of the washout parameter up to .Comment: 23 pages, 5 figures postscript, Minor changes to match the published
version in JHE
Neutron scattering search for static magnetism in oxygen ordered YBa2Cu3O6.5
We present elastic and inelastic neutron scattering results on highly oxygen
ordered YBa2Cu3O6.5 ortho-II. We find no evidence for the presence of ordered
magnetic moments to a sensitivity of 0.003 Bohr magnetons, an order of
magnitude smaller than has been suggested in theories of orbital or
d-density-wave (DDW) currents. The absence of sharp elastic peaks, shows that
the d-density-wave phase is not present, at least for the superconductor with
the doping of 6.5 and the ordered ortho-II structure. We cannot exclude the
possibility that a broad peak may exist with extremely short-range DDW
correlations. For less ordered or more doped crystals it is possible that
disorder may lead to static magnetism. We have also searched for the large
normal state spin gap that is predicted to exist in an ordered DDW phase.
Instead of a gap we find that the Q-correlated spin susceptibility persists to
the lowest energies studied, 6 meV. Our results are compatible with the
coexistence of superconductivity with orbital currents, but only if they are
dynamic, and exclude a sharp phase transition to an ordered d-density-wave
phase.Comment: 6 pages 4 figures RevTex Submitted to Phys Rev B January 23, 200
Quantum Gravity, the Origin of Time and Time's Arrow
The local Lorentz and diffeomorphism symmetries of Einstein's gravitational
theory are spontaneously broken by a Higgs mechanism by invoking a phase
transition in the early Universe, at a critical temperature below which
the symmetry is restored. The spontaneous breakdown of the vacuum state
generates an external time and the wave function of the Universe satisfies a
time dependent Schrodinger equation, which reduces to the Wheeler-deWitt
equation in the classical regime for , allowing a semi-classical WKB
approximation to the wave function. The conservation of energy is spontaneously
violated for and matter is created fractions of seconds after the big
bang, generating the matter in the Universe. The time direction of the vacuum
expectation value of the scalar Higgs field generates a time asymmetry, which
defines the cosmological arrow of time and the direction of increasing entropy
as the Lorentz symmetry is restored at low temperatures.Comment: 37 page
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