2,525 research outputs found
Nondiagonal Coset Models and their Poincar\'E Polynomials
coset models of the type with nondiagonal
modular invariants for both and are considered. Poincar\'e
polynomials of the corresponding chiral rings of these algebras are
constructed. They are used to compute the number of chiral generations of the
associated string compactifications. Moddings by discrete symmetries are also
discussed.Comment: 22 pages, (RevTex), preprint GTCRG-92-1 and CNEA-CAB-039/92. % Minor
changes in the result
A short note on the nested-sweep polarized traces method for the 2D Helmholtz equation
We present a variant of the solver in Zepeda-N\'u\~nez and Demanet (2014),
for the 2D high-frequency Helmholtz equation in heterogeneous acoustic media.
By changing the domain decomposition from a layered to a grid-like partition,
this variant yields improved asymptotic online and offline runtimes and a lower
memory footprint. The solver has online parallel complexity that scales
\emph{sub linearly} as , where is
the number of volume unknowns, and is the number of processors, provided
that . The variant in Zepeda-N\'u\~nez and Demanet
(2014) only afforded . Algorithmic scalability is a
prime requirement for wave simulation in regimes of interest for geophysical
imaging.Comment: 5 pages, 5 figure
A ring in a shell: the large-scale 6D structure of the Vela OB2 complex
The Vela OB2 association is a group of 10 Myr stars exhibiting a complex
spatial and kinematic substructure. The all-sky Gaia DR2 catalogue contains
proper motions, parallaxes (a proxy for distance) and photometry that allow us
to separate the various components of Vela OB2. We characterise the
distribution of the Vela OB2 stars on a large spatial scale, and study its
internal kinematics and dynamic history. We make use of Gaia DR2 astrometry and
published Gaia-ESO Survey data. We apply an unsupervised classification
algorithm to determine groups of stars with common proper motions and
parallaxes. We find that the association is made up of a number of small
groups, with a total current mass over 2330 Msun. The three-dimensional
distribution of these young stars trace the edge of the gas and dust structure
known as the IRAS Vela Shell across 180 pc and shows clear signs of expansion.
We propose a common history for Vela OB2 and the IRAS Vela Shell. The event
that caused the expansion of the shell happened before the Vela OB2 stars
formed, imprinted the expansion in the gas the stars formed from, and most
likely triggered star formation.Comment: Accepted by A&A (02 November 2018), 13 pages, 9+2 figure
Evidence for polarons in iron pnictides of the Ln-1111 and AE-122 families
Examination of the electrical resistivities of iron pnictides shows that they
can be accounted by conduction by polarons. Their activation energies show a
linear behaviour with the critical temperatures of the spin density waves
(SDW), T*, as both vary with pressure. The slope matches the ratio SDW gap to
T*, while the intercept can be related to the transition temperature of the
lattice distortion, T0. An adapted Landau free energy predicts the observed
order of the transitions, according to which is higher, T* or T0. Simple
arguments favour combined Jahn-Teller antiferromagnetic bipolarons.Comment: 14 pages with 4 Figure
Pressure effects in the triangular layered cobaltites NaxCoO2
We have measured transport properties as a function of temperature and
pressure up to 30GPa in the NaxCoO2 system. For the x=0.5 sample the transition
temperature at 53K increases with pressure, while paradoxically the sample
passes from an insulating to a metallic ground state. A similar transition is
observed in the x=0.31 sample under pressure. Compression on the x=0.75 sample
transforms the sample from a metallic to an insulating state. We discuss our
results in terms of interactions between band structure effects and Na+ order.Comment: 18 pages, 5 figure
Effective t-J Hamiltonian for the Copper Oxides
Starting from the Emery model, which is assumed to describe the copper oxygen
planes, and including direct oxygen hopping matrix elements, we have been able
to derive the effective t-J Hamiltonian for the copper orbitals using the
Linked Cluster Expansion Method up to fourth order in the hybridization matrix
element.Comment: (ps version of the dvi file, resubmitted because previous
uucompressed version was corrupted), 9 page
N=1,2 supersymmetric vacua of IIA supergravity and SU(2) structures
We consider backgrounds of (massive) IIA supergravity of the form of a warped
product , where is a six-dimensional compact
manifold and is or a four-dimensional Minkowski space. We
analyse conditions for and supersymmetry on
manifolds of SU(2) structure. We prove the absence of solutions in certain
cases.Comment: 24 pages; v2: reference adde
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