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Self-sustained asymmetry of lepton-number emission: A new phenomenon during the supernova shock-accretion phase in three dimensions
During the stalled-shock phase of our 3D hydrodynamical core-collapse
simulations with energy-dependent, 3-flavor neutrino transport, the
lepton-number flux (nue minus antinue) emerges predominantly in one hemisphere.
This novel, spherical-symmetry breaking neutrino-hydrodynamical instability is
termed LESA for "Lepton-number Emission Self-sustained Asymmetry." While the
individual nue and antinue fluxes show a pronounced dipole pattern, the
heavy-flavor neutrino fluxes and the overall luminosity are almost spherically
symmetric. Initially, LESA seems to develop stochastically from convective
fluctuations, it exists for hundreds of milliseconds or more, and it persists
during violent shock sloshing associated with the standing accretion shock
instability. The nue minus antinue flux asymmetry originates mainly below the
neutrinosphere in a region of pronounced proto-neutron star (PNS) convection,
which is stronger in the hemisphere of enhanced lepton-number flux. On this
side of the PNS, the mass-accretion rate of lepton-rich matter is larger,
amplifying the lepton-emission asymmetry, because the spherical stellar infall
deflects on a dipolar deformation of the stalled shock. The increased shock
radius in the hemisphere of less mass accretion and minimal lepton-number flux
(antinue flux maximum) is sustained by stronger convection on this side, which
is boosted by stronger neutrino heating because the average antinue energy is
higher than the average nue energy. Asymmetric heating thus supports the global
deformation despite extremely nonstationary convective overturn behind the
shock. While these different elements of LESA form a consistent picture, a full
understanding remains elusive at present. There may be important implications
for neutrino-flavor oscillations, the neutron-to-proton ratio in the
neutrino-heated supernova ejecta, and neutron-star kicks, which remain to be
explored.Comment: 21 pages, 15 figures; new results and new figure added; accepted by
Ap
Comment on "Quantum Monte Carlo Evidence for Superconductivity in the Three-Band Hubbard Model in Two Dimensions"
In a recent Letter, Kuroki and Aoki [Phys. Rev. Lett. 76, 440 (1996)]
presented quantum Monte-Carlo (QMC) results for pairing correlations in the
three-band Hubbard model, which describes the Cu-d_{x^2-y^2} and O-p_{x,y}
orbitals present in the CuO_2 planes of high-T_c materials. In this comment we
argue that (i) the used parameter set is not appropriate for the description of
high-T_c materials since it does not satisfy the minimal requirement of a
charge-transfer gap at half-filling, and (ii) the observed increase in the
d_{x^2-y^2} channel is dominantly produced by the pair-field correlations
without the vertex part. Hence, the claim of evidence of ODLRO is not
justified.Comment: 1 page latex and 2 eps-figures, uses epsfig, submitted to PR
Phase diagram and single-particle spectrum of CuO layers within a variational cluster approach to the 3-band Hubbard model
We carry out a detailed numerical study of the three-band Hubbard model in
the underdoped region both in the hole- as well as in the electron-doped case
by means of the variational cluster approach. Both the phase diagram and the
low-energy single-particle spectrum are very similar to recent results for the
single-band Hubbard model with next-nearest-neighbor hoppings. In particular,
we obtain a mixed antiferromagnetic+superconducting phase at low doping with a
first-order transition to a pure superconducting phase accompanied by phase
separation. In the single-particle spectrum a clear Zhang-Rice singlet band
with an incoherent and a coherent part can be seen, in which holes enter upon
doping around . The latter is very similar to the coherent
quasi-particle band crossing the Fermi surface in the single-band model. Doped
electrons go instead into the upper Hubbard band, first filling the regions of
the Brillouin zone around . This fact can be related to the enhanced
robustness of the antiferromagnetic phase as a function of electron doping
compared to hole doping.Comment: 14 pages, 15 eps figure
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