2,404 research outputs found
Magneto-elastic quantum fluctuations and phase transitions in the iron superconductors
We examine the relevance of magneto-elastic coupling to describe the complex
magnetic and structural behaviour of the different classes of the iron
superconductors. We model the system as a two-dimensional metal whose magnetic
excitations interact with the distortions of the underlying square lattice.
Going beyond mean field we find that quantum fluctuation effects can explain
two unusual features of these materials that have attracted considerable
attention. First, why iron telluride orders magnetically at a non-nesting
wave-vector and not at the nesting wave-vector as
in the iron arsenides, even though the nominal band structures of both these
systems are similar. And second, why the magnetic transition in the
iron arsenides is often preceded by an orthorhombic structural transition.
These are robust properties of the model, independent of microscopic details,
and they emphasize the importance of the magneto-elastic interaction.Comment: 4 pages, 3 figures; minor change
Accounting for spin fluctuations beyond LSDA in the density functional theory
We present a method to correct the magnetic properties of itinerant systems
in local spin density approximation (LSDA) and we apply it to the
ferromagnetic-paramagnetic transition under pressure in a typical itinerant
system, NiAl. We obtain a scaling of the critical fluctuations as a
function of pressure equivalent to the one obtained within Moryia's theory.
Moreover we show that in this material the role of the bandstructure is crucial
in driving the transition. Finally we calculate the magnetic moment as a
function of pressure, and find that it gives a scaling of the Curie temperature
that is in good agreement with the experiment. The method can be easily
extended to the antiferromagnetic case and applied, for instance, to the
Fe-pnictides in order to correct the LSDA magnetic moment.Comment: 7 pages, 4 figure
The Impact of the Macroeconomy on Health Insurance Coverage: Evidence from the Great Recession
This paper investigates the impact of the macroeconomy on the health insurance coverage of Americans using panel data from the Survey of Income and Program Participation (SIPP) for 2004-2010, a period that includes the Great Recession of 2007-09. We find that a one percentage point increase in the state unemployment rate is associated with a 1.67 percentage point (2.12%) reduction in the probability that men have health insurance; this effect is strongest among college-educated, white, and older (50-64 year old) men. For women and children, health insurance coverage is not significantly correlated with the unemployment rate, which may be the result of public health insurance acting as a social safety net. Compared to the previous recession, the health insurance coverage of men is more sensitive to the unemployment rate, which may be due to the nature of the Great Recession.
Electron-capture supernovae exploding within their progenitor wind
The most massive stars on the asymptotic giant branch (AGB), so called
super-AGB stars, are thought to produce supernovae (SNe) triggered by electron
captures in their degenerate O+Ne+Mg cores. Super-AGB stars are expected to
have slow winds with high mass-loss rates, so their wind density is high. The
explosions of super-AGB stars are therefore presumed to occur in this dense
wind. We provide the first synthetic light curves (LCs) for such events by
exploding realistic electron-capture supernova (ecSN) progenitors within their
super-AGB winds. We find that the early LC, i.e. before the recombination wave
reaches the bottom of the H-rich envelope of SN ejecta (the plateau phase), is
not affected by the dense wind. However, after the plateau phase, the
luminosity remains higher when the super-AGB wind is taken into account. We
compare our results to the historical LC of SN 1054, the progenitor of the Crab
Nebula, and show that the explosion of an ecSN within an ordinary super-AGB
wind can explain the LC features. We conclude that SN 1054 could have been a
Type IIn SN without any extra extreme mass loss which was previously suggested
to be necessary to account for its early high luminosity. We also show that our
LCs match Type IIn SNe with an early plateau phase (`Type IIn-P') and suggest
that they are ecSNe within super-AGB winds. Although some ecSNe can be bright
in the optical spectral range due to the large progenitor radius, their X-ray
luminosity from the interaction does not necessarily get as bright as other
Type IIn SNe whose optical luminosities are also powered by the interaction.
Thus, we suggest that optically-bright X-ray-faint Type IIn SNe can emerge from
ecSNe. Optically-faint Type IIn SNe, such as SN 2008S, can also originate from
ecSNe if their H-rich envelope masses are small. Some of them can be observed
as `Type IIn-b' SNe due to the small H-rich envelope mass.Comment: 8 pages, 6 figures, accepted by Astronomy & Astrophysics, abstract
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Synthetic Light Curves of Shocked Dense Circumstellar Shells
We numerically investigate light curves (LCs) of shocked circumstellar shells
which are suggested to reproduce the observed LC of superluminous SN 2006gy
analytically. In the previous analytical model, the effects of the
recombination and the bolometric correction on LCs are not taken into account.
To see the effects, we perform numerical radiation hydrodynamic calculations of
shocked shells by using STELLA, which can numerically treat multigroup
radiation transfer with realistic opacities. We show that the effects of the
recombination and the bolometric correction are significant and the analytical
model should be compare to the bolometric LC instead of a single band LC. We
find that shocked circumstellar shells have a rapid LC decline initially
because of the adiabatic expansion rather than the luminosity increase and the
shocked shells fail to explain the LC properties of SN 2006gy. However, our
synthetic LCs are qualitatively similar to those of superluminous SN 2003ma and
SN 1988Z and they may be related to shocked circumstellar shells.Comment: 7 pages, 7 figures, 1 table, accepted by Monthly Notices of the Royal
Astronomical Societ
A factorization of a super-conformal map
A super-conformal map and a minimal surface are factored into a product of
two maps by modeling the Euclidean four-space and the complex Euclidean plane
on the set of all quaternions. One of these two maps is a holomorphic map or a
meromorphic map. These conformal maps adopt properties of a holomorphic
function or a meromorphic function. Analogs of the Liouville theorem, the
Schwarz lemma, the Schwarz-Pick theorem, the Weierstrass factorization theorem,
the Abel-Jacobi theorem, and a relation between zeros of a minimal surface and
branch points of a super-conformal map are obtained.Comment: 21 page
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