14,715 research outputs found
Sensitivity analysis of the solar rotation to helioseismic data from GONG, GOLF and MDI observations
Accurate determination of the rotation rate in the radiative zone of the sun
from helioseismic observations requires rotational frequency splittings of
exceptional quality as well as reliable inversion techniques. We present here
inferences based on mode parameters calculated from 2088-days long MDI, GONG
and GOLF time series that were fitted to estimate very low frequency rotational
splittings (nu < 1.7 mHz). These low frequency modes provide data of
exceptional quality, since the width of the mode peaks is much smaller than the
rotational splitting and hence it is much easier to separate the rotational
splittings from the effects caused by the finite lifetime and the stochastic
excitation of the modes. We also have implemented a new inversion methodology
that allows us to infer the rotation rate of the radiative interior from mode
sets that span l=1 to 25. Our results are compatible with the sun rotating like
a rigid solid in most of the radiative zone and slowing down in the core (R_sun
< 0.2). A resolution analysis of the inversion was carried out for the solar
rotation inverse problem. This analysis effectively establishes a direct
relationship between the mode set included in the inversion and the sensitivity
and information content of the resulting inferences. We show that such an
approach allows us to determine the effect of adding low frequency and low
degree p-modes, high frequency and low degree p-modes, as well as some g-modes
on the derived rotation rate in the solar radiative zone, and in particular the
solar core. We conclude that the level of uncertainties that is needed to infer
the dynamical conditions in the core when only p-modes are included is unlikely
to be reached in the near future, and hence sustained efforts are needed
towards the detection and characterization of g-modes.Comment: Accepted for publication in Astrophysical journal. 15 pages, 19
figure
Bose-Einstein condensation in antiferromagnets close to the saturation field
At zero temperature and strong applied magnetic fields the ground sate of an
anisotropic antiferromagnet is a saturated paramagnet with fully aligned spins.
We study the quantum phase transition as the field is reduced below an upper
critical and the system enters a XY-antiferromagnetic phase. Using a
bond operator representation we consider a model spin-1 Heisenberg
antiferromagnetic with single-ion anisotropy in hyper-cubic lattices under
strong magnetic fields. We show that the transition at can be
interpreted as a Bose-Einstein condensation (BEC) of magnons. The theoretical
results are used to analyze our magnetization versus field data in the organic
compound - (DTN) at very low temperatures. This is the
ideal BEC system to study this transition since is sufficiently low to
be reached with static magnetic fields (as opposed to pulsed fields). The
scaling of the magnetization as a function of field and temperature close to
shows excellent agreement with the theoretical predictions. It allows
to obtain the quantum critical exponents and confirm the BEC nature of the
transition at .Comment: 4 pages, 1 figure. Accepted for publication in PRB
Endogenous Sanctioning Institutions and Migration Patterns: Experimental Evidence
We experimentally analyze the effect of the endogenous choice of sanctioning institutions on cooperation and migration patterns across societies. In our experiment, subjects are allocated to one of two groups, are endowed with group-specific preferences,and play a public goods game for 30 periods. Each period, subjects can move between groups and, at fixed intervals, can vote on whether to implement formal (centralized) sanctioning institutions in their group. We compare this environment to one in which only one group is exogenously endowed with sanctioning institutions. We find that subjects' ability to vote on institutions leads to (i) a more efficient partition of subjects between groups, (ii) a lower migration rate, (iii) an increase in overall payoffs, and (iv) a decrease in both inter- and intra-groups (payoff) inequality. Over time, subjects tend to vote for sanctioning institutions and contribute to the public good
Graphene zigzag ribbons, square lattice models and quantum spin chains
We present an extended study of finite-width zigzag graphene ribbons (ZGRs)
based on a tight-binding model with hard-wall boundary conditions. We provide
an exact analytic solution that clarifies the origin of the predicted width
dependence on the conductance through junctions of ribbons with different
widths. An analysis of the obtained solutions suggests a new description of
ZGRs in terms of coupled chains. We pursue these ideas further by introducing a
mapping between the ZGR model and the Hamiltonian for N-coupled quantum chains
as described in terms of 2N Majorana fermions. The proposed mapping preserves
the dependence of ribbon properties on its width thus rendering metallic
ribbons for N odd and zero-gap semiconductor ribbons for N even. Furthermore,
it reveals a close connection between the low-energy properties of the ZGR
model and a continuous family of square lattice model Hamiltonians with similar
width-dependent properties that includes the flux and the trivial square
lattice models. As a further extension, we show that this new description makes
it possible to identify various aspects of the physics of graphene ribbons with
those predicted by models of quantum spin chains (QSCs)
Se and Cu NMR studies of the electronic correlations in CuTiSe ()
We report Se and Cu nuclear magnetic resonance (NMR)
investigation on the charge-density-wave (CDW) superconductor CuTiSe
( and 0.07). At high magnetic fields where superconductivity is
suppressed, the temperature dependence of Se and Cu spin-lattice
relaxation rates 1/T_{1}^{77}1/T_{1}$
vs \emph{T} increases with the Cu doping. This can be described by a modified
Korringa relation which suggests the significance of electronic correlations
and the Se 4\emph{p}- and Ti 3\emph{d}-band contribution to the density of
states at the Fermi level in the studied compounds.Comment: Revised manuscript. Submitted to Journal of Physics: Condensed Matte
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