660 research outputs found
Carrier dynamics and infrared-active phonons in c-axis oriented RuSrGdCuO film
The conductivity spectra of c-axis oriented thin RuSrGdCuO film
on SrTiO substrate, prepared by pulsed-laser deposition, are obtained from
the analysis of the reflectivity spectra over broad frequency range and
temperatures between 10 and 300 K. The free charge carriers are found to be
strongly overdamped with their scattering rate (1.0 eV at room temperature)
exceeding the plasma frequency (0.55 eV). Four phonon lines are identified in
the experimental spectra and assigned to the specific oxygen related in-plane
polarized vibrations based on the comparison with the results of a lattice
dynamics shell model calculations.Comment: 3 pages, 4 figure
Scalar Field Theory on Fuzzy S^4
Scalar fields are studied on fuzzy and a solution is found for the
elimination of the unwanted degrees of freedom that occur in the model. The
resulting theory can be interpreted as a Kaluza-Klein reduction of CP^3 to S^4
in the fuzzy context.Comment: 16 pages, LaTe
The Fuzzy Disc
We introduce a finite dimensional matrix model approximation to the algebra
of functions on a disc based on noncommutative geometry. The algebra is a
subalgebra of the one characterizing the noncommutative plane with a * product
and depends on two parameters N and theta. It is composed of functions which
decay exponentially outside a disc. In the limit in which the size of the
matrices goes to infinity and the noncommutativity parameter goes to zero the
disc becomes sharper. We introduce a Laplacian defined on the whole algebra and
calculate its eigenvalues. We also calculate the two--points correlation
function for a free massless theory (Green's function). In both cases the
agreement with the exact result on the disc is very good already for relatively
small matrices. This opens up the possibility for the study of field theories
on the disc with nonperturbative methods. The model contains edge states, a
fact studied in a similar matrix model independently introduced by
Balachandran, Gupta and Kurkcuoglu.Comment: 17 pages, 8 figures, references added and correcte
Noncommutative Chiral Anomaly and the Dirac-Ginsparg-Wilson Operator
It is shown that the local axial anomaly in dimensions emerges naturally
if one postulates an underlying noncommutative fuzzy structure of spacetime .
In particular the Dirac-Ginsparg-Wilson relation on is shown to
contain an edge effect which corresponds precisely to the ``fuzzy''
axial anomaly on the fuzzy sphere . We also derive a novel gauge-covariant
expansion of the quark propagator in the form where
is the lattice spacing on , is
the covariant noncommutative chirality and is an effective
Dirac operator which has essentially the same IR spectrum as
but differes from it on the UV modes. Most remarkably is the fact that both
operators share the same limit and thus the above covariant expansion is not
available in the continuum theory . The first bit in this expansion
although it vanishes as it stands in the continuum
limit, its contribution to the anomaly is exactly the canonical theta term. The
contribution of the propagator is on the other hand
equal to the toplogical Chern-Simons action which in two dimensions vanishes
identically .Comment: 26 pages, latex fil
Dynamical generation of fuzzy extra dimensions, dimensional reduction and symmetry breaking
We present a renormalizable 4-dimensional SU(N) gauge theory with a suitable
multiplet of scalar fields, which dynamically develops extra dimensions in the
form of a fuzzy sphere S^2. We explicitly find the tower of massive
Kaluza-Klein modes consistent with an interpretation as gauge theory on M^4 x
S^2, the scalars being interpreted as gauge fields on S^2. The gauge group is
broken dynamically, and the low-energy content of the model is determined.
Depending on the parameters of the model the low-energy gauge group can be
SU(n), or broken further to SU(n_1) x SU(n_2) x U(1), with mass scale
determined by the size of the extra dimension.Comment: 27 pages. V2: discussion and references added, published versio
Raman Phonons and Ageing-Related Surface Disorder in NaxCoO2
The polarized Raman spectra from ab and ac surfaces of single crystal NaxCoO2
(x~0.7), parent compound of recently discovered superconductor NaxCoO2.yH2O,
are reported and discussed. The crystals were hexagon platelets of typical size
3x3x0.1 mm. Three of the five (A1g+E1g+3E2g) Raman active phonons were
unambiguously identified at 458 (E1g), 494(E2g) and 574 (A1g) cm-1. The spectra
from ab and ac surfaces differ significantly and provide evidence that within
hours after preparation the ac surface, unlike the ab one, is strongly
disordered. Within several days the disorder extends over the ab surface too.Comment: 4 pages, 1 table, 4 figures. Physica C (accepted 26 September 2003
Long-range attraction between particles in dusty plasma and partial surface tension of dusty phase boundary
Effective potential of a charged dusty particle moving in homogeneous plasma
has a negative part that provides attraction between similarly charged dusty
particles. A depth of this potential well is great enough to ensure both
stability of crystal structure of dusty plasma and sizable value of surface
tension of a boundary surface of dusty region. The latter depends on the
orientation of the surface relative to the counter-ion flow, namely, it is
maximal and positive for the surface normal to the flow and minimal and
negative for the surface along the flow. For the most cases of dusty plasma in
a gas discharge, a value of the first of them is more than sufficient to ensure
stability of lenticular dusty phase void oriented across the counter-ion flow.Comment: LATEX, REVTEX4, 7 pages, 6 figure
Gauge Theory on Fuzzy S^2 x S^2 and Regularization on Noncommutative R^4
We define U(n) gauge theory on fuzzy S^2_N x S^2_N as a multi-matrix model,
which reduces to ordinary Yang-Mills theory on S^2 x S^2 in the commutative
limit N -> infinity. The model can be used as a regularization of gauge theory
on noncommutative R^4_\theta in a particular scaling limit, which is studied in
detail. We also find topologically non-trivial U(1) solutions, which reduce to
the known "fluxon" solutions in the limit of R^4_\theta, reproducing their full
moduli space. Other solutions which can be interpreted as 2-dimensional branes
are also found. The quantization of the model is defined non-perturbatively in
terms of a path integral which is finite. A gauge-fixed BRST-invariant action
is given as well. Fermions in the fundamental representation of the gauge group
are included using a formulation based on SO(6), by defining a fuzzy Dirac
operator which reduces to the standard Dirac operator on S^2 x S^2 in the
commutative limit. The chirality operator and Weyl spinors are also introduced.Comment: 39 pages. V2-4: References added, typos fixe
Matrix Models
Matrix models and their connections to String Theory and noncommutative
geometry are discussed. Various types of matrix models are reviewed. Most of
interest are IKKT and BFSS models. They are introduced as 0+0 and 1+0
dimensional reduction of Yang--Mills model respectively. They are obtained via
the deformations of string/membrane worldsheet/worldvolume. Classical solutions
leading to noncommutative gauge models are considered.Comment: Lectures given at the Winter School on Modern Trends in
Supersymmetric Mechanics, March 2005 Frascati; 38p
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