602 research outputs found
The Energy-Momentum Tensor in Noncommutative Gauge Field Models
We discuss the different possibilities of constructing the various
energy-momentum tensors for noncommutative gauge field models. We use Jackiw's
method in order to get symmetric and gauge invariant stress tensors--at least
for commutative gauge field theories. The noncommutative counterparts are
analyzed with the same methods. The issues for the noncommutative cases are
worked out.Comment: 11 pages, completed reference
Two stage cracking of metallic bi layers on polymer substrates under tension
Cu Nb nanoscale metallic multilayers have been extensively investigated to understand how their mechanical behavior is influenced by the individual layer thickness. The general observed trend is that the yield stress of the multilayer increases with decreasing layer thickness. Important mechanical behaviors that have not been studied in depth are the fracture of these multilayers and adhesion energy between the multilayer films and their substrate. Here, the influences of the layer thickness, layer order, and initial residual stresses of Cu Nb multilayers on polyimide were examined using in situ x ray diffraction and confocal laser scanning microscopy under tensile loading. With these techniques, it was possible to calculate the stresses developing in the individual materials and measure buckles that could be used to evaluate the interfacial adhesion. Layer thickness, deposition order, and the initial residual stresses were not shown to influence the initial fracture strains of the Cu Nb multilayer systems under tensile loading conditions. However, the adhesion energy between the multilayer and substrate was affected by the layer deposition order and by the initial residual stresse
Non-commutative U(1) Super-Yang-Mills Theory: Perturbative Self-Energy Corrections
The quantization of the non-commutative N=1, U(1) super-Yang-Mills action is
performed in the superfield formalism. We calculate the one-loop corrections to
the self-energy of the vector superfield. Although the power-counting theorem
predicts quadratic ultraviolet and infrared divergences, there are actually
only logarithmic UV and IR divergences, which is a crucial feature of
non-commutative supersymmetric field theories.Comment: 18 pages, latex, uses feynmf package; references added, Wess-Zumino
gauge remove
Charge pairing, superconducting transition and supersymmetry in high-temperature cuprate superconductors
We propose a model for high-T superconductors, valid for
, that includes both the spin fluctuations of the
Cu magnetic ions and of the O doped holes. Spin-charge separation
is taken into account with the charge of the doped holes being associated to
quantum skyrmion excitations (holons) of the Cu spin background. The
holon effective interaction potential is evaluated as a function of doping,
indicating that Cooper pair formation is determined by the competition between
the spin fluctuations of the Cu background and of spins of the O
doped holes (spinons). The superconducting transition occurs when the spinon
fluctuations dominate, thereby reversing the sign of the interaction. At this
point (), the theory is supersymmetric at short distances
and, as a consequence, the leading order results are not modified by radiative
corrections. The critical doping parameter for the onset of superconductivity
at T=0 is obtained and found to be a universal constant determined by the shape
of the Fermi surface. Our theoretical values for are in good
agreement with the experiment for both LSCO and YBCO.Comment: RevTex, 4 pages, no figure
Hydrodynamic Synchronisation of Model Microswimmers
We define a model microswimmer with a variable cycle time, thus allowing the
possibility of phase locking driven by hydrodynamic interactions between
swimmers. We find that, for extensile or contractile swimmers, phase locking
does occur, with the relative phase of the two swimmers being, in general,
close to 0 or pi, depending on their relative position and orientation. We show
that, as expected on grounds of symmetry, self T-dual swimmers, which are
time-reversal covariant, do not phase-lock. We also discuss the phase behaviour
of a line of tethered swimmers, or pumps. These show oscillations in their
relative phases reminiscent of the metachronal waves of cilia.Comment: 17 pages, 8 figure
Effects of Electronic Correlations on the Thermoelectric Power of the Cuprates
We show that important anomalous features of the normal-state thermoelectric
power S of high-Tc materials can be understood as being caused by doping
dependent short-range antiferromagnetic correlations. The theory is based on
the fluctuation-exchange approximation applied to Hubbard model in the
framework of the Kubo formalism. Firstly, the characteristic maximum of S as
function of temperature can be explained by the anomalous momentum dependence
of the single-particle scattering rate. Secondly, we discuss the role of the
actual Fermi surface shape for the occurrence of a sign change of S as a
function of temperature and doping.Comment: 4 pages, with eps figure
Magnetic Properties of YBa_2Cu_3O_{7-\delta} in a self-consistent approach: Comparison with Quantum-Monte-Carlo Simulations and Experiments
We analyze single-particle electronic and two-particle magnetic properties of
the Hubbard model in the underdoped and optimally-doped regime of \YBCO by
means of a modified version of the fluctuation-exchange approximation, which
only includes particle-hole fluctuations. Comparison of our results with
Quantum-Monte Carlo (QMC) calculations at relatively high temperatures () suggests to introduce a temperature renormalization in order to
improve the agreement between the two methods at intermediate and large values
of the interaction .
We evaluate the temperature dependence of the spin-lattice relaxation time
and of the spin-echo decay time and compare it with the results
of NMR measurements on an underdoped and an optimally doped \YBCO sample. For
it is possible to consistently adjust the parameters of the Hubbard
model in order to have a good {\it semi-quantitative} description of this
temperature dependence for temperatures larger than the spin gap as obtained
from NMR measurements. We also discuss the case , which is more
appropriate to describe magnetic and single-particle properties close to
half-filling. However, for this larger value of the agreement with QMC as
well as with experiments at finite doping is less satisfactory.Comment: Final version, to appear in Phys. Rev. B (sched. Feb. 99
Hole Doping Evolution of the Quasiparticle Band in Models of Strongly Correlated Electrons for the High-T_c Cuprates
Quantum Monte Carlo (QMC) and Maximum Entropy (ME) techniques are used to
study the spectral function of the one band Hubbard model
in strong coupling including a next-nearest-neighbor electronic hopping with
amplitude . These values of parameters are chosen to improve the
comparison of the Hubbard model with angle-resolved photoemission (ARPES) data
for . A narrow quasiparticle (q.p.) band is observed in the
QMC analysis at the temperature of the simulation , both at and away
from half-filling. Such a narrow band produces a large accumulation of weight
in the density of states at the top of the valence band. As the electronic
density decreases further away from half-filling, the chemical
potential travels through this energy window with a large number of states, and
by it has crossed it entirely. The region near momentum
and in the spectral function is more sensitive to doping
than momenta along the diagonal from to . The evolution with
hole density of the quasiparticle dispersion contains some of the features
observed in recent ARPES data in the underdoped regime. For sufficiently large
hole densities the ``flat'' bands at cross the Fermi energy, a
prediction that could be tested with ARPES techniques applied to overdoped
cuprates. The population of the q.p. band introduces a {\it hidden} density in
the system which produces interesting consequences when the quasiparticles are
assumed to interact through antiferromagnetic fluctuations and studied with the
BCS gap equation formalism. In particular, a region of extended s-wave is found
to compete with d-wave in the overdoped regime, i.e. when the chemical
potential has almost entirely crossed the q.p.Comment: 14 pages, Revtex, with 13 embedded ps figures, submitted to Phys.
Rev. B., minor modifications in the text and in figures 1b, 2b, 3b, 4b, and
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