3,674 research outputs found
Lorentz-violating Yang-Mills theory: discussing the Chern-Simons-like term generation
We analyze the Chern-Simons-like term generation in the CPT-odd
Lorentz-violating Yang-Mills theory interacting with fermions. Moreover, we
study the anomalies of this model as well as its quantum stability. The whole
analysis is performed within the algebraic renormalization theory, which is
independent of the renormalization scheme. In addition, all results are valid
to all orders in perturbation theory. We find that the Chern-Simons-like term
is not generated by radiative corrections, just like its Abelian version.
Additionally, the model is also free of gauge anomalies and quantum stable.Comment: 16 pages. No figures. Final version to appear in the Eur.Phys.J.
Size and shape of Mott regions for fermionic atoms in a two-dimensional optical lattice
We investigate the harmonic-trap control of size and shape of Mott regions in
the Fermi Hubbard model on a square optical lattice. The use of Lanczos
diagonalization on clusters with twisted boundary conditions, followed by an
average over 50-80 samples, drastically reduce finite-size effects in some
ground state properties; calculations in the grand canonical ensemble together
with a local-density approximation (LDA) allow us to simulate the radial
density distribution. We have found that as the trap closes, the atomic cloud
goes from a metallic state, to a Mott core, and to a Mott ring; the coverage of
Mott atoms reaches a maximum at the core-ring transition. A `phase diagram' in
terms of an effective density and the on-site repulsion is proposed, as a guide
to maximize the Mott coverage. We also predict that the usual experimentally
accessible quantities, the global compressibility and the average double
occupancy (rather, its density derivative) display detectable signatures of the
core-ring transition. Some spin correlation functions are also calculated, and
predict the existence N\'eel ordering within Mott cores and rings.Comment: 5 pages, 6 figure
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Doping Nanocrystals And The Role Of Quantum Confinement
Recent progress in developing algorithms for solving the electronic structure problem for nanostructures is illustrated. Key ingredients in this approach include pseudopotentials implemented on a real space grid and the use of density functional theory. This procedure allows one to predict electronic properties for many materials across the nano-regime, i.e., from atoms to nanocrystals of sufficient size to replicate bulk properties. We will illustrate this method for doping silicon nanocrystals with phosphorous.Institute for Computational Engineering and Sciences (ICES
Electromagnetic response of high-Tc superconductors -- the slave-boson and doped-carrier theories
We evaluate the doping dependence of the quasiparticle current and low
temperature superfluid density in two slave-particle theories of the tt't''J
model -- the slave-boson theory and doped-carrier theory. In the slave-boson
theory, the nodal quasiparticle current renormalization factor
vanishes proportionally to the zero temperature superfluid density ;
however, we find that away from the limit displays a
much weaker doping dependence than . A similar conclusion applies to
the doped-carrier theory, which differentiates the nodal and antinodal regions
of momentum space. Due to its momentum space anisotropy, the doped-carrier
theory enhances the value of in the hole doped regime, bringing it to
quantitative agreement with experiments, and reproduces the asymmetry between
hole and electron doped cuprate superconductors. Finally, we use the
doped-carrier theory to predict a specific experimental signature of local
staggered spin correlations in doped Mott insulator superconductors which, we
propose, should be observed in STM measurements of underdoped high-Tc
compounds. This experimental signature distinguishes the doped-carrier theory
from other candidate mean-field theories of high-Tc superconductors, like the
slave-boson theory and the conventional BCS theory.Comment: 12 pages, RevTeX4, homepage http://dao.mit.edu/~we
Accounting for the Hidden Economy: Barriers to Legality and Legal Failures
This paper examines how much of the difference in the size of the informal sector and in per capita income across countries can be accounted by regulation costs (barriers to legality) and contractual imperfections in financial markets (legal failures). It constructs and solves numerically a general equilibrium model with credit constrained heterogeneous agents, occupational choices over formal and informal businesses, contractual imperfections and a government sector which imposes taxes and regulations on formal firms. The premium from formalization is better access to outside finance. differences in regulation costs and the degree of enforcement in financial contracts endogenously generate differences in the size of the informal sector and in total factor productivity (TFP). The numerical exercises suggest that: (i) regulation costs and not financial market imperfections account for the difference in the size of the informal sector between United States and Mediterranean Europe; (ii) this is not the case for countries with very weak enforcement systems, such as Peru, as both contractual imperfections and regulation costs account for the observed difference in the size of the informal sector. Regarding output per capita, regulation costs and the strength of enforcement explain roughly 60% of the difference in observed international incomes.
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