2,030 research outputs found
Finite-Size Effects and Operator Product Expansions in a CFT for d>2
The large momentum expansion for the inverse propagator of the auxiliary
field in the conformally invariant O(N) vector model is calculated
to leading order in 1/N, in a strip-like geometry with one finite dimension of
length for . Its leading terms are identified as contributions from
itself and the energy momentum tensor, in agreement with a
previous calculation based on conformal operator product expansions. It is
found that a non-trivial cancellation takes place by virtue of the gap
equation. The leading coefficient of the energy momentum tensor contribution is
shown to be related to the free energy density.Comment: 10 pages LaTeX 2 eps figures, minor changes in text. Revised version
to be published in Phys.Lett. B. email: [email protected]
[email protected]
Covariant gaussian approximation in Ginzburg - Landau model
Condensed matter systems undergoing second order transition away from the
critical fluctuation region are usually described sufficiently well by the mean
field approximation. The critical fluctuation region, determined by the
Ginzburg criterion, , is narrow even
in high superconductors and has universal features well captured by the
renormalization group method. However recent experiments on magnetization,
conductivity and Nernst effect suggest that fluctuations effects are large in a
wider region both above and below . In particular some "pseudogap"
phenomena and strong renormalization of the mean field critical temperature
can be interpreted as strong fluctuations effects that are
nonperturbative (cannot be accounted for by "gaussian fluctuations"). The
physics in a broader region therefore requires more accurate approach. Self
consistent methods are generally "non - conserving" in the sense that the Ward
identities are not obeyed. This is especially detrimental in the symmetry
broken phase where, for example, Goldstone bosons become massive. Covariant
gaussian approximation remedies these problems. The Green's functions obey all
the Ward identities and describe the fluctuations much better. The results for
the order parameter correlator and magnetic penetration depth of the Ginzburg -
Landau model of superconductivity are compared with both Monte Carlo
simulations and experiments in high cuprates.Comment: 24 pages, 7 figure
Signature of Schwinger's pair creation rate via radiation generated in graphene by strong electric current
Electron - hole pairs are copuously created by an applied electric field near
the Dirac point in graphene or similar 2D electronic systems. It was shown
recently that for sufficiently large electric fields and ballistic times the
I-V characteristics become strongly nonlinear due to Schwinger's pair creation.
Since there is no energy gap the radiation from the pairs' annihilation is
enhanced. The spectrum of radiation is calculated. The angular and polarization
dependence of the emitted photons with respect to the graphene sheet is quite
distinctive. For very large currents the recombination rate becomes so large
that it leads to the second Ohmic regime due to radiation friction.Comment: 9 pages, 7 figure
On the Free-Energy of Three-Dimensional CFTs and Polylogarithms
We study the O(N) vector model and the U(N) Gross-Neveu model with fixed
total fermion number, in three dimensions. Using non-trivial polylogarithmic
identities, we calculate the large-N renormalized free-energy density of these
models, at their conformal points in a ``slab'' geometry with one finite
dimension of length L. We comment on the possible implications of our results.Comment: Latex, 13 pages, 2 eps figures; v2 typos corrected; v3 Expanded
discussion of the results, added reference
Phase diagram of the three-dimensional NJL model
With the exception of confinement the three-dimensional Nambu-Jona-Lasinio
(NJL_3) model incorporates many of the essential properties of QCD. We discuss
the critical properties of the model at nonzero temperature T and/or nonzero
chemical potential . We show that the universality class of the thermal
transition is that of the d=2 classical spin model with the same symmetry. We
provide evidence for the existence of a tricritical point in the
(,temperature) plane. We also discuss numerical results by Hands et al.
which showed that the system is critical for and the diquark
condensate is zero.Comment: 3 pages, To appear in the proceedings of Conference on Quarks and
Nuclear Physics (QNP 2002), Julich, Germany, 9-14 Jun 200
Ballistic transport, chiral anomaly and emergence of the neutral electron - hole plasma in graphene
The process of coherent creation of particle - hole excitations by an
electric field in graphene is quantitatively described using a dynamic "first
quantized" approach. We calculate the evolution of current density, number of
pairs and energy in ballistic regime using the tight binding model. The series
in electric field strength up to third order in both DC and AC are
calculated. We show how the physics far from the two Dirac points enters
various physical quantities in linear response and how it is related to the
chiral anomaly. The third harmonic generation and the imaginary part of
conductivity are obtained. It is shown that at certain time scale
the physical behaviour dramatically changes and the
perturbation theory breaks down. Beyond the linear response physics is explored
using an exact solution of the first quantized equations. While for small
electric fields the I-V curve is linear characterized by the universal minimal
resistivity %, at the conductivity grows
fast. The copious pair creation (with rate ), analogous to Schwinger's
electron - positron pair creation from vacuum in QED, leads to creation of the
electron - hole plasma at ballistic times of order . This process is
terminated by a relaxational recombination.Comment: 15 pages, 5 figures
Electrical conductivity beyond linear response in layered superconductors under magnetic field
The time-dependent Ginzburg-Landau approach is used to investigate nonlinear
response of a strongly type-II superconductor. The dissipation takes a form of
the flux flow which is quantitatively studied beyond linear response. Thermal
fluctuations, represented by the Langevin white noise, are assumed to be strong
enough to melt the Abrikosov vortex lattice created by the magnetic field into
a moving vortex liquid and marginalize the effects of the vortex pinning by
inhomogeneities. The layered structure of the superconductor is accounted for
by means of the Lawrence-Doniach model. The nonlinear interaction term in
dynamics is treated within Gaussian approximation and we go beyond the often
used lowest Landau level approximation to treat arbitrary magnetic fields. The
I-V curve is calculated for arbitrary temperature and the results are compared
to experimental data on high- superconductor
YBaCuO.Comment: 8 pages, 3 figure
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