7,346 research outputs found
Characters of graded parafermion conformal field theory
The graded parafermion conformal field theory at level k is a close cousin of
the much-studied Z_k parafermion model. Three character formulas for the graded
parafermion theory are presented, one bosonic, one fermionic (both previously
known) and one of spinon type (which is new). The main result of this paper is
a proof of the equivalence of these three forms using q-series methods combined
with the combinatorics of lattice paths. The pivotal step in our approach is
the observation that the graded parafermion theory -- which is equivalent to
the coset osp(1,2)_k/ u(1) -- can be factored as (osp(1,2)_k/ su(2)_k) x
(su(2)_k/ u(1)), with the two cosets on the right equivalent to the minimal
model M(k+2,2k+3) and the Z_k parafermion model, respectively. This
factorisation allows for a new combinatorial description of the graded
parafermion characters in terms of the one-dimensional configuration sums of
the (k+1)-state Andrews--Baxter--Forrester model.Comment: 36 page
From RE-211 to RE-123. How to control the final microstructure of superconducting single-domains
This paper reviews the usual techniques for producing YBCO-type
single-domains and the microstructure of the as-obtained samples. The problems
of seed dissolution and parasite nucleations are discussed in details.
Formation of microstructural defects, such as pores and cracks, are examined.
An important part of this review is devoted to the study of the influence of
RE-211 particles [RE2BaCuO5 where RE denotes Y, Yb, Nd, Sm, Dy, Gd, Eu or a
mixture of them. Generally Nd4Ba2Cu2O10 is preferred to Nd2BaCuO5] on the
microstructure and properties of RE-Ba-Cu-O single-domains. Trapping/Pushing
theory is described in order to explain the spatial distribution of RE-211
particles in the RE-123 [(RE)Ba2Cu3O7-d] monoliths. Formation of RE-211-free
regions is discussed. Different ways to limit the RE-211 coarsening are
reviewed. Microstructural defects in the RE-123 matrix caused by the RE-211
particles are presented. It is also shown that RE-211 particles play a
significant role on the mechanical properties of single-domain samples. We
finish this review by discussing the Infiltration and Growth process as a good
technique to control the microstructure.Comment: review paper to be published in Supercond. Sci. Technol.; 19 figures;
137 references; 37 page
SM(2,4k) fermionic characters and restricted jagged partitions
A derivation of the basis of states for the superconformal minimal
models is presented. It relies on a general hypothesis concerning the role of
the null field of dimension . The basis is expressed solely in terms of
modes and it takes the form of simple exclusion conditions (being thus a
quasi-particle-type basis). Its elements are in correspondence with
-restricted jagged partitions. The generating functions of the latter
provide novel fermionic forms for the characters of the irreducible
representations in both Ramond and Neveu-Schwarz sectors.Comment: 12 page
Explicit formulas for the generalized Hermite polynomials in superspace
We provide explicit formulas for the orthogonal eigenfunctions of the
supersymmetric extension of the rational Calogero-Moser-Sutherland model with
harmonic confinement, i.e., the generalized Hermite (or Hi-Jack) polynomials in
superspace. The construction relies on the triangular action of the Hamiltonian
on the supermonomial basis. This translates into determinantal expressions for
the Hamiltonian's eigenfunctions.Comment: 19 pages. This is a recasting of the second part of the first version
of hep-th/0305038 which has been splitted in two articles. In this revised
version, the introduction has been rewritten and a new appendix has been
added. To appear in JP
Ferromagnetism and interlayer exchange coupling in short period (Ga,Mn)As/GaAs superlattices
Magnetic properties of (Ga,Mn)As/GaAs superlattices are investigated. The
structures contain magnetic (Ga,Mn)As layers, separated by thin layers of
non-magnetic GaAs spacer. The short period GaMnAs/GaAs
superlattices exhibit a paramagnetic-to-ferromagnetic phase transition close to
60K, for thicknesses of (Ga,Mn)As down to 23 \AA. For
GaMnAs/GaAs superlattices of similar dimensions, the Curie
temperature associated with the ferromagnetic transition is found to oscillate
with the thickness of non magnetic spacer. The observed oscillations are
related to an interlayer exchange interaction mediated by the polarized holes
of the (Ga,Mn)As layers.Comment: REVTeX 4 style; 4 pages, 2 figure
A Canonical Form for Positive Definite Matrices
We exhibit an explicit, deterministic algorithm for finding a canonical form
for a positive definite matrix under unimodular integral transformations. We
use characteristic sets of short vectors and partition-backtracking graph
software. The algorithm runs in a number of arithmetic operations that is
exponential in the dimension , but it is practical and more efficient than
canonical forms based on Minkowski reduction
New path description for the M(k+1,2k+3) models and the dual Z_k graded parafermions
We present a new path description for the states of the non-unitary
M(k+1,2k+3) models. This description differs from the one induced by the
Forrester-Baxter solution, in terms of configuration sums, of their
restricted-solid-on-solid model. The proposed path representation is actually
very similar to the one underlying the unitary minimal models M(k+1,k+2), with
an analogous Fermi-gas interpretation. This interpretation leads to fermionic
expressions for the finitized M(k+1,2k+3) characters, whose infinite-length
limit represent new fermionic characters for the irreducible modules. The
M(k+1,2k+3) models are also shown to be related to the Z_k graded parafermions
via a (q to 1/q) duality transformation.Comment: 43 pages (minor typo corrected and minor rewording in the
introduction
Large-eddy simulation of the flow in a lid-driven cubical cavity
Large-eddy simulations of the turbulent flow in a lid-driven cubical cavity
have been carried out at a Reynolds number of 12000 using spectral element
methods. Two distinct subgrid-scales models, namely a dynamic Smagorinsky model
and a dynamic mixed model, have been both implemented and used to perform
long-lasting simulations required by the relevant time scales of the flow. All
filtering levels make use of explicit filters applied in the physical space (on
an element-by-element approach) and spectral (modal) spaces. The two
subgrid-scales models are validated and compared to available experimental and
numerical reference results, showing very good agreement. Specific features of
lid-driven cavity flow in the turbulent regime, such as inhomogeneity of
turbulence, turbulence production near the downstream corner eddy, small-scales
localization and helical properties are investigated and discussed in the
large-eddy simulation framework. Time histories of quantities such as the total
energy, total turbulent kinetic energy or helicity exhibit different evolutions
but only after a relatively long transient period. However, the average values
remain extremely close
The nanoscale phase separation in hole-doped manganites
A macroscopic phase separation, in which ferromagnetic clusters are observed
in an insulating matrix, is sometimes observed, and believed to be essential to
the colossal magnetoresistive (CMR) properties of manganese oxides. The
application of a magnetic field may indeed trigger large magnetoresistance
effects due to the percolation between clusters allowing the movement of the
charge carriers. However, this macroscopic phase separation is mainly related
to extrinsic defects or impurities, which hinder the long-ranged charge-orbital
order of the system. We show in the present article that rather than the
macroscopic phase separation, an homogeneous short-ranged charge-orbital order
accompanied by a spin glass state occurs, as an intrinsic result of the
uniformity of the random potential perturbation induced by the solid solution
of the cations on the -sites of the structure of these materials. Hence the
phase separation does occur, but in a more subtle and interesting nanoscopic
form, here referred as ``homogeneous''. Remarkably, this ``nanoscale phase
separation'' alone is able to bring forth the colossal magnetoresistance in the
perovskite manganites, and is potentially relevant to a wide variety of other
magnetic and/or electrical properties of manganites, as well as many other
transition metal oxides, in bulk or thin film form as we exemplify throughout
the article.Comment: jpsj2 TeX style (J. Phys. Soc. Jpn); 18 pages, 7 figure
Phase transition in a super superspin glass
We here confirm the occurrence of spin glass phase transition and extract
estimates of associated critical exponents of a highly monodisperse and densely
compacted system of bare maghemite nanoparticles. This system has earlier been
found to behave like an archetypal spin glass, with e.g. a sharp transition
from paramagnetic to non-equilibrium behavior, suggesting that this system
undergoes a spin-glass phase transition at a relatively high temperature,
140 K.Comment: 4 pages, 3 figure
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