4,077 research outputs found
Noncommutative gauge fields coupled to noncommutative gravity
We present a noncommutative (NC) version of the action for vielbein gravity
coupled to gauge fields. Noncommutativity is encoded in a twisted star product
between forms, with a set of commuting background vector fields defining the
(abelian) twist. A first order action for the gauge fields avoids the use of
the Hodge dual. The NC action is invariant under diffeomorphisms and twisted
gauge transformations. The Seiberg-Witten map, adapted to our geometric setting
and generalized for an arbitrary abelian twist, allows to re-express the NC
action in terms of classical fields: the result is a deformed action, invariant
under diffeomorphisms and usual gauge transformations. This deformed action is
a particular higher derivative extension of the Einstein-Hilbert action coupled
to Yang-Mills fields, and to the background vector fields defining the twist.
Here noncommutativity of the original NC action dictates the precise form of
this extension. We explicitly compute the first order correction in the NC
parameter of the deformed action, and find that it is proportional to cubic
products of the gauge field strength and to the symmetric anomaly tensor
D_{IJK}.Comment: 18 pages, LaTe
Free Differential Algebras: Their Use in Field Theory and Dual Formulation
The gauging of free differential algebras (FDA's) produces gauge field
theories containing antisymmetric tensors. The FDA's extend the Cartan-Maurer
equations of ordinary Lie algebras by incorporating p-form potentials (). We study here the algebra of FDA transformations. To every p-form in the
FDA we associate an extended Lie derivative generating a corresponding
``gauge" transformation. The field theory based on the FDA is invariant under
these new transformations. This gives geometrical meaning to the antisymmetric
tensors. The algebra of Lie derivatives is shown to close and provides the dual
formulation of FDA's.Comment: 10 pages, latex, no figures. Talk presented at the 4-th Colloquium on
"Quantum Groups and Integrable Sysytems", Prague, June 199
R-Matrix Formulation of the Quantum Inhomogeneous Groups Iso_qr(N) and Isp_qr(N)
The quantum commutations and the orthogonal (symplectic) conditions
for the inhomogeneous multiparametric -groups of the type are
found in terms of the -matrix of . A consistent
Hopf structure on these inhomogeneous -groups is constructed by means of a
projection from . Real forms are discussed: in
particular we obtain the -groups , including the quantum
Poincar\'e group.Comment: 14 pages, latex, no figure
Inhomogeneous quantum groups IGL_{q,r}(N): Universal enveloping algebra and differential calculus
A review of the multiparametric linear quantum group GL_qr(N), its real
forms, its dual algebra U(gl_qr(N)) and its bicovariant differential calculus
is given in the first part of the paper.
We then construct the (multiparametric) linear inhomogeneous quantum group
IGL_qr(N) as a projection from GL_qr(N+1), or equivalently, as a quotient of
GL_qr(N+1) with respect to a suitable Hopf algebra ideal.
A bicovariant differential calculus on IGL_qr(N) is explicitly obtained as a
projection from the one on GL_qr(N+1). Our procedure unifies in a single
structure the quantum plane coordinates and the q-group matrix elements T^a_b,
and allows to deduce without effort the differential calculus on the q-plane
IGL_qr(N) / GL_qr(N).
The general theory is illustrated on the example of IGL_qr(2).Comment: 38 page
Generators of Local Supersymmetry Transformation from First Class Constraints
We show how the generator of local supersymmetry transformations can be found
from Fermionic first class constraints. This is done by adapting the approaches
of Henneaux, Teit- elboim and Zanelli and of Castellani that has been used to
find the generator of gauge trans- formations from Bosonic first class
constraints. We illustrate how a supersymmetric gauge generator can be found by
considering the spinning particle. The invariances that we find are not those
presented in the original discussion of the spinning particle.Comment: nine page
On the application of Mattis-Bardeen theory in strongly disordered superconductors
The low energy optical conductivity of conventional superconductors is
usually well described by Mattis-Bardeen (MB) theory which predicts the onset
of absorption above an energy corresponding to twice the superconducing (SC)
gap parameter Delta. Recent experiments on strongly disordered superconductors
have challenged the application of the MB formulas due to the occurrence of
additional spectral weight at low energies below 2Delta. Here we identify three
crucial items which have to be included in the analysis of optical-conductivity
data for these systems: (a) the correct identification of the optical threshold
in the Mattis-Bardeen theory, and its relation with the gap value extracted
from the measured density of states, (b) the gauge-invariant evaluation of the
current-current response function, needed to account for the optical absorption
by SC collective modes, and (c) the inclusion into the MB formula of the energy
dependence of the density of states present already above Tc. By computing the
optical conductvity in the disordered attractive Hubbard model we analyze the
relevance of all these items, and we provide a compelling scheme for the
analysis and interpretation of the optical data in real materials.Comment: 11 pages, 6 figure
Non-linear optical effects and third-harmonic generation in superconductors: Cooper-pairs vs Higgs mode contribution
The recent observation of a transmitted Thz pulse oscillating at three times
the frequency of the incident light paves the way to a new protocol to access
resonant excitations in a superconductor. Here we show that this non-linear
optical process is dominated by light-induced excitation of Cooper pairs, in
analogy with a standard Raman experiment. The collective amplitude (Higgs)
fluctuations of the superconducting order parameter give in general a smaller
contribution, unless one designs the experiment by combining properly the light
polarization with the lattice symmetry.Comment: Slightly revised introduction, to appear on Phys. Rev. B. as Rapid
Communicatio
Spectroscopic and thermodynamic properties in a four-band model for pnictides
In this paper we provide a comprehesive analysis of different properties of
pnictides both in the normal and superconducting state, with a particular focus
on the optimally-doped BaKFeAs system. We show that, by
using the band dispersions experimentally measured by ARPES, a four-band
Eliashberg model in the intermediate-coupling regime can account for both the
measured hierarchy of the gaps and for several spectroscopic and thermodynamic
signatures of low-energy renormalization. These include the kinks in the band
dispersion and the effective masses determined via specific-heat and
superfluid-density measurements. We also show that, although an
intermediate-coupling Eliashberg approach is needed to account for the
magnitude of the gaps, the temperature behavior of the thermodynamic quantities
does not show in this regime a significant deviation with respect to
weak-coupling BCS calculations. This can explain the apparent success of
two-band BCS fits of experimental data reported often in the literature.Comment: 12 pages, 6 figures, final versio
The Hot End of Evolutionary Horizontal Branches
In this paper we investigate the hot end of the HB, presenting evolutionary
constraints concerning the CM diagram location and the gravity of hot HB stars.
According to the adopted evolutionary scenario, we predict an upper limit for
HB temperatures of about logTe = 4.45, remarkably cooler than previous
estimates. We find that such a theoretical prescription appears in good
agreement with available observational data concerning both stellar
temperatures and gravities.Comment: postscript file of 10 pages plus 1 tables,rep.1 5 figures will be
added later as postscript file The tex file and the other two not postscript
figures are available upon request at [email protected], rep.
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