11,477 research outputs found
Evolution of magnetic states in frustrated diamond lattice antiferromagnetic Co(Al1-xCox)2O4 spinels
Using neutron powder diffraction and Monte-Carlo simulations we show that a
spin-liquid regime emerges at $all compositions in the diamond-lattice
antiferromagnets Co(Al1-xCox)2O4. This spin-liquid regime induced by
frustration due to the second-neighbour exchange coupling J2, is gradually
superseded by antiferromagnetic collinear long-range order (k=0) at low
temperatures. Upon substitution of Al3+ by Co3+ in the octahedral B-site the
temperature range occupied by the spin-liquid regime narrows and TN increases.
To explain the experimental observations we considered magnetic anisotropy D or
third-neighbour exchange coupling J3 as degeneracy-breaking perturbations. We
conclude that Co(Al1-xCox)2O4 is below the theoretical critical point
J2/J1=1/8, and that magnetic anisotropy assists in selecting a collinear
long-range ordered ground state, which becomes more stable with increasing x
due to a higher efficiency of O-Co3+-O as an interaction path compared to
O-Al3+-O
Simulating spin-3/2 particles at colliders
Support for interactions of spin-3/2 particles is implemented in the
FeynRules and ALOHA packages and tested with the MadGraph 5 and CalcHEP event
generators in the context of three phenomenological applications. In the first,
we implement a spin-3/2 Majorana gravitino field, as in local supersymmetric
models, and study gravitino and gluino pair-production. In the second, a
spin-3/2 Dirac top-quark excitation, inspired from compositness models, is
implemented. We then investigate both top-quark excitation and top-quark
pair-production. In the third, a general effective operator for a spin-3/2
Dirac quark excitation is implemented, followed by a calculation of the angular
distribution of the s-channel production mechanism.Comment: 20 pages, 7 figure
One-Dimensional Directed Sandpile Models and the Area under a Brownian Curve
We derive the steady state properties of a general directed ``sandpile''
model in one dimension. Using a central limit theorem for dependent random
variables we find the precise conditions for the model to belong to the
universality class of the Totally Asymmetric Oslo model, thereby identifying a
large universality class of directed sandpiles. We map the avalanche size to
the area under a Brownian curve with an absorbing boundary at the origin,
motivating us to solve this Brownian curve problem. Thus, we are able to
determine the moment generating function for the avalanche-size probability in
this universality class, explicitly calculating amplitudes of the leading order
terms.Comment: 24 pages, 5 figure
Butterfly diagram of a Sun-like star observed using asteroseismology
Stellar magnetic fields are poorly understood but are known to be important
for stellar evolution and exoplanet habitability. They drive stellar activity,
which is the main observational constraint on theoretical models for magnetic
field generation and evolution. Starspots are the main manifestation of the
magnetic fields at the stellar surface. In this study we measure the variation
of their latitude with time, called a butterfly diagram in the solar case, for
the solar analogue HD 173701 (KIC 8006161). To that effect, we use Kepler data,
to combine starspot rotation rates at different epochs and the
asteroseismically determined latitudinal variation of the stellar rotation
rates. We observe a clear variation of the latitude of the starspots. It is the
first time such a diagram is constructed using asteroseismic data.Comment: 8 pages, 4 figures, accepted in A&A Letter
Asteroseismic detection of latitudinal differential rotation in 13 Sun-like stars
The differentially rotating outer layers of stars are thought to play a role
in driving their magnetic activity, but the underlying mechanisms that generate
and sustain differential rotation are poorly understood. We report the
measurement of latitudinal differential rotation in the convection zones of 40
Sun-like stars using asteroseismology. For the most significant detections, the
stars' equators rotate approximately twice as fast as their mid-latitudes. The
latitudinal shear inferred from asteroseismology is much larger than
predictions from numerical simulations.Comment: 45 pages, 11 figures, 4 tables, published in Scienc
Entropy of Quantum Fields for Nonextreme Black Holes in the Extreme Limit
Nonextreme black hole in a cavity within the framework of the canonical or
grand canonical ensemble can approach the extreme limit with a finite
temperature measured on a boundary located at a finite proper distance from the
horizon. In spite of this finite temperature, it is shown that the one-loop
contribution of quantum fields to the thermodynamic entropy due
to equilibrium Hawking radiation vanishes in the limit under consideration. The
same is true for the finite temperature version of the Bertotti-Robinson
spacetime into which a classical Reissner-Nordstr\"{o}m black hole turns in the
extreme limit. The result is attributed to the nature of a horizon
for the Bertotti-Robinson spacetime.Comment: 11 pages, ReVTeX, no figures. New references added, discussion
expanded, presentation and English improved. Accepted for publication in
Phys. Rev.
Properties of oscillation modes in subgiant stars observed by Kepler
Mixed modes seen in evolved stars carry information on their deeper layers
that can place stringent constraints on their physics and on their global
properties (mass, age, etc...). In this study, we present a method to identify
and measure all oscillatory mode characteristics (frequency, height, width).
Analyzing four subgiants stars, we present the first measure of the effect of
the degree of mixture on the l=1 mixed modes characteristics. We also show that
some stars have measurable l=2 mixed modes and discuss the interest of their
measure to constrain the deeper layers of stars.Comment: Accepted to Ap
Critical Behavior of Non Order-Parameter Fields
We show that all of the relevant features of a phase transition can be
determined using a non order parameter field which is a physical state of the
theory. This fact allows us to understand the deconfining transition of the
pure Yang-Mills theory via the physical excitations rather than using the
Polyakov loop.Comment: RevTeX, 4-pages, 1 figur
- âŠ