13,572 research outputs found
The Reionization History and Early Metal Enrichment inferred from the Gamma-Ray Burst Rate
Based on the gamma-ray burst (GRB) event rate at redshifts of , which is assessed by the spectral peak energy-to-luminosity relation
recently found by Yonetoku et al., we observationally derive the star formation
rate (SFR) for Pop III stars in a high redshift universe. As a result, we find
that Pop III stars could form continuously at . Using the
derived Pop III SFR, we attempt to estimate the ultraviolet (UV) photon
emission rate at in which redshift range no observational
information has been hitherto obtained on ionizing radiation intensity. We find
that the UV emissivity at can make a noticeable contribution
to the early reionization. The maximal emissivity is higher than the level
required to keep ionizing the intergalactic matter at .
However, if the escape fraction of ionizing photons from Pop III objects is
smaller than 10%, then the IGM can be neutralized at some redshift, which may
lead to the double reionization. As for the enrichment, the ejection of all
metals synthesized in Pop III objects is marginally consistent with the IGM
metallicity, although the confinement of metals in Pop III objects can reduce
the enrichment significantly.Comment: 12 pages, 2 figures, ApJL accepte
Superconducting phase diagram of itinerant antiferromagnets
We study the phase diagram of the Hubbard model in the weak-coupling limit
for coexisting spin-density-wave order and spin-fluctuation-mediated
superconductivity. Both longitudinal and transverse spin fluctuations
contribute significantly to the effective interaction potential, which creates
Cooper pairs of the quasi-particles of the antiferromagnetic metallic state. We
find a dominant -wave solution in both electron- and hole-doped
cases. In the quasi-spin triplet channel, the longitudinal fluctuations give
rise to an effective attraction supporting a -wave gap, but are overcome by
repulsive contributions from the transverse fluctuations which disfavor
-wave pairing compared to . The sub-leading pair instability is
found to be in the -wave channel, but complex admixtures of and are
not energetically favored since their nodal structures coincide. Inclusion of
interband pairing, in which each fermion in the Cooper pair belongs to a
different spin-density-wave band, is considered for a range of electron dopings
in the regime of well-developed magnetic order. We demonstrate that these
interband pairing gaps, which are non-zero in the magnetic state, must have the
same parity under inversion as the normal intraband gaps. The self-consistent
solution to the full system of five coupled gap equations give intraband and
interband pairing gaps of structure and similar gap magnitude. In
conclusion, the gap dominates for both hole and electron doping
inside the spin-density-wave phase.Comment: 14 pages, 9 figure
Superconducting Junctions with Ferromagnetic, Antiferromagnetic or Charge-Density-Wave Interlayers
Spectra and spin structures of Andreev interface states and the Josephson
current are investigated theoretically in junctions between clean
superconductors (SC) with ordered interlayers. The Josephson current through
the ferromagnet-insulator-ferromagnet interlayer can exhibit a nonmonotonic
dependence on the misorientation angle. The characteristic behavior takes place
if the pi state is the equilibrium state of the junction in the particular case
of parallel magnetizations. We find a novel channel of quasiparticle reflection
(Q reflection) from the simplest two-sublattice antiferromagnet (AF) on a
bipartite lattice. As a combined effect of Andreev and Q reflections, Andreev
states arise at the AF/SC interface. When the Q reflection dominates the
specular one, Andreev bound states have almost zero energy on AF/ s-wave SC
interfaces, whereas they lie near the edge of the continuous spectrum for
AF/d-wave SC boundaries. For an s-wave SC/AF/s-wave SC junction, the bound
states are found to split and carry the supercurrent. Our analytical results
are based on a novel quasiclassical approach, which applies to interfaces
involving itinerant antiferromagnets. Similar effects can take place on
interfaces of superconductors with charge density wave materials (CDW),
including the possible d-density wave state (DDW) of the cuprates.Comment: LT24 conference proceeding, 2 pages, 1 figur
Electronic Structure of the Chevrel-Phase Compounds SnMoSe: Photoemission Spectroscopy and Band-structure Calculations
We have studied the electronic structure of two Chevrel-phase compounds,
MoSe and SnMoSe, by combining photoemission
spectroscopy and band-structure calculations. Core-level spectra taken with
x-ray photoemission spectroscopy show systematic core-level shifts, which do
not obey a simple rigid-band model. The inverse photoemission spectra imply the
existence of an energy gap located eV above the Fermi level, which is
a characteristic feature of the electronic structure of the Chevrel compounds.
Quantitative comparison between the photoemission spectra and the
band-structure calculations have been made. While good agreement between theory
and experiment in the wide energy range was obtained as already reported in
previous studies, we found that the high density of states near the Fermi level
predicted theoretically due to the Van Hove singularity is considerably reduced
in the experimental spectra taken with higher energy resolution than in the
previous reports. Possible origins are proposed to explain this observation.Comment: 8 pages, 5 figure
Experimental investigation of the Landau-Pomeranchuk-Migdal effect in low-Z targets
In the CERN NA63 collaboration we have addressed the question of the
potential inadequacy of the commonly used Migdal formulation of the
Landau-Pomeranchuk-Migdal (LPM) effect by measuring the photon emission by 20
and 178 GeV electrons in the range 100 MeV - 4 GeV, in targets of
LowDensityPolyEthylene (LDPE), C, Al, Ti, Fe, Cu, Mo and, as a reference
target, Ta. For each target and energy, a comparison between simulated values
based on the LPM suppression of incoherent bremsstrahlung is shown, taking
multi-photon effects into account. For these targets and energies, we find that
Migdal's theoretical formulation is adequate to a precision of better than
about 5%, irrespective of the target substance.Comment: 8 pages, 13 figure
W Plus Multiple Jets at the LHC with High Energy Jets
We study the production of a W boson in association with n hard QCD jets (for
n>=2), with a particular emphasis on results relevant for the Large Hadron
Collider (7 TeV and 8 TeV). We present predictions for this process from High
Energy Jets, a framework for all-order resummation of the dominant
contributions from wide-angle QCD emissions. We first compare predictions
against recent ATLAS data and then shift focus to observables and regions of
phase space where effects beyond NLO are expected to be large.Comment: 19 pages, 9 figure
Knight Shift and Leading Superconducting Instability From Spin Fluctuations in Sr2RuO4
Recent nuclear magnetic resonance studies [A. Pustogow {\it et al.},
arXiv:1904.00047] have challenged the prevalent chiral triplet pairing scenario
proposed for SrRuO. To provide guidance from microscopic theory as to
which other pair states might be compatible with the new data, we perform a
detailed theoretical study of spin-fluctuation mediated pairing for this
compound. We map out the phase diagram as a function of spin-orbit coupling,
interaction parameters, and band-structure properties over physically
reasonable ranges, comparing when possible with photoemission and inelastic
neutron scattering data information. We find that even-parity pseudospin
singlet solutions dominate large regions of the phase diagram, but in certain
regimes spin-orbit coupling favors a near-nodal odd-parity triplet
superconducting state, which is either helical or chiral depending on the
proximity of the band to the van Hove points. A surprising
near-degeneracy of the nodal - and -wave solutions leads
to the possibility of a near-nodal time-reversal symmetry broken
pair state. Predictions for the temperature dependence
of the Knight shift for fields in and out of plane are presented for all
states.Comment: 5 pages (3 figures) + supplementary informatio
Pairing symmetry of the one-band Hubbard model in the paramagnetic weak-coupling limit: a numerical RPA study
We study the spin-fluctuation-mediated superconducting pairing gap in a
weak-coupling approach to the Hubbard model for a two dimensional square
lattice in the paramagnetic state. Performing a comprehensive theoretical study
of the phase diagram as a function of filling, we find that the superconducting
gap exhibits transitions from p-wave at very low electron fillings to
d_{x^2-y^2}-wave symmetry close to half filling in agreement with previous
reports. At intermediate filling levels, different gap symmetries appear as a
consequence of the changes in the Fermi surface topology and the associated
structure of the spin susceptibility. In particular, the vicinity of a van Hove
singularity in the electronic structure close to the Fermi level has important
consequences for the gap structure in favoring the otherwise sub-dominant
triplet solution over the singlet d-wave solution. By solving the full gap
equation, we find that the energetically favorable triplet solutions are chiral
and break time reversal symmetry. Finally, we also calculate the detailed
angular gap structure of the quasi-particle spectrum, and show how
spin-fluctuation-mediated pairing leads to significant deviations from the
first harmonics both in the singlet d_{x^2-y^2} gap as well as the chiral
triplet gap solution.Comment: 11 pages 11 figure
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