19,903 research outputs found
Quantum Field Theories on Null Surfaces
We study the behaviour of quantum field theories defined on a surface as
it tends to a null surface . In the case of a real, free scalar field
theory the above limiting procedure reduces the system to one with a finite
number of degrees of freedom. This system is shown to admit a one parameter
family of inequivalent quantizations. A duality symmetry present in the model
can be used to remove the quantum ambiguity at the self-dual point . In the
case of the non-linear -model with the Wess-Zumino-Witten term a
similar limiting behaviour is obtained. The quantization ambiguity in this case
however cannot be removed by any means.Comment: 25 pages LaTex, no figures, new comments added to the
conclusion,corrected e-mail addres
Asymptotic Quasinormal Modes of d-Dimensional Schwarzschild Black Hole with Gauss-Bonnet Correction
We obtain an analytic expression for the highly damped asymptotic quasinormal
mode frequencies of the -dimensional Schwarzschild black hole modified
by the Gauss-Bonnet term, which appears in string derived models of gravity.
The analytic expression is obtained under the string inspired assumption that
there exists a minimum length scale in the system and in the limit when the
coupling in front of the Gauss-Bonnet term in the action is small. Although
there are several similarities of this geometry with that of the Schwarzschild
black hole, the asymptotic quasinormal mode frequencies are quite different. In
particular, the real part of the asymptotic quasinormal frequencies for this
class of single horizon black holes in not proportional to log(3).Comment: 10 pages, latex file, changes in several equations, changes in the
abstract qualitative nature of conclusions unaffecte
Waves on Noncommutative Spacetimes
Waves on ``commutative'' spacetimes like R^d are elements of the commutative
algebra C^0(R^d) of functions on R^d. When C^0(R^d) is deformed to a
noncommutative algebra {\cal A}_\theta (R^d) with deformation parameter \theta
({\cal A}_0 (R^d) = C^0(R^d)), waves being its elements, are no longer
complex-valued functions on R^d. Rules for their interpretation, such as
measurement of their intensity, and energy, thus need to be stated. We address
this task here. We then apply the rules to interference and diffraction for d
\leq 4 and with time-space noncommutativity. Novel phenomena are encountered.
Thus when the time of observation T is so brief that T \leq 2 \theta w, where w
is the frequency of incident waves, no interference can be observed. For larger
times, the interference pattern is deformed and depends on \frac{\theta w}{T}.
It approaches the commutative pattern only when \frac{\theta w}{T} goes to 0.
As an application, we discuss interference of star light due to cosmic strings.Comment: 19 pages, 5 figures, LaTeX, added references, corrected typo
Coulomb screening in graphene with topological defects
We analyze the screening of an external Coulomb charge in gapless graphene
cone, which is taken as a prototype of a topological defect. In the subcritical
regime, the induced charge is calculated using both the Green's function and
the Friedel sum rule. The dependence of the polarization charge on the Coulomb
strength obtained from the Green's function clearly shows the effect of the
conical defect and indicates that the critical charge itself depends on the
sample topology. Similar analysis using the Friedel sum rule indicates that the
two results agree for low values of the Coulomb charge but differ for the
higher strengths, especially in the presence of the conical defect. For a given
subcritical charge, the transport cross-section has a higher value in the
presence of the conical defect. In the supercritical regime we show that the
coefficient of the power law tail of polarization charge density can be
expressed as a summation of functions which vary log periodically with the
distance from the Coulomb impurity. The period of variation depends on the
conical defect. In the presence of the conical defect, the Fano resonances
begin to appear in the transport cross-section for a lower value of the Coulomb
charge. For both sub and supercritical regime we derive the dependence of LDOS
on the conical defect. The effects of generalized boundary condition on the
physical observables are also discussed.Comment: 14 pages, 11 figure
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