5,767 research outputs found
Perturbative tests of non-perturbative counting
We observe that a class of quarter-BPS dyons in N=4 theories with charge
vector (Q, P) and with nontrivial values of the arithmetic duality invariant I
:= gcd (Q wedge P) are nonperturbative in one frame but perturbative in another
frame. This observation suggests a test of the recently computed
nonperturbative partition functions for dyons with nontrivial values of the
arithmetic invariant. For all values of I, we show that the nonperturbative
counting yields vanishing indexed degeneracy for this class of states
everywhere in the moduli space in precise agreement with the perturbative
result.Comment: 10 pages, 0 figure
Entropy of the Kerr-Sen Black Hole
We study the entropy of Kerr-Sen black hole of heterotic string theory beyond
semiclassical approximations. Applying the properties of exact differentials
for three variables to the first law thermodynamics we derive the corrections
to the entropy of the black hole. The leading (logarithmic) and non leading
corrections to the area law are obtained.Comment: 8 pages. Corrected references
Global monopole in scalar tensor theory
The well known monopole solution of Barriola and Vilenkin (BV) resulting from
the breaking of a global SO(3) symmetry is extended in general relativity along
with a zero mass scalar field and also in Brans-Dicke(BD) theory of gravity.In
the case of BD theory, the behaviour of spacetime and other variables such as
BD scalar field and the monopole energy density have been studied
numerically.For monopole along with a zero mass scalar field, exact solutions
are obtained and depending upon the choice of arbitary parameters, the
solutions either reduce to the BV case or to a pure scalar field solution as
special cases.It is interesting to note that unlike the BV case the global
monopole in the BD theory does exert gravitational pull on a test particle
moving in its spacetime.Comment: 12 pages LaTex, 3 postscript figures, Communicated to
Class.Quant.Gra
Equation of motion approach to non-adiabatic quantum charge pumping
We use the equations of motion of non-interacting electrons in a
one-dimensional system to numerically study different aspects of charge
pumping. We study the effects of the pumping frequency, amplitude, band filling
and finite bias on the charge pumped per cycle, and the Fourier transforms of
the charge and energy currents in the leads. Our method works for all values of
parameters, and gives the complete time-dependences of the current and charge
at any site of the system. Our results agree with Floquet and adiabatic
scattering theory where these are applicable, and provides support for a
mechanism proposed elsewhere for charge pumping by a traveling potential wave.
For non-adiabatic and strong pumping, the charge and energy currents are found
to have a marked asymmetry between the two leads, and pumping can work even
against a substantial bias.Comment: 11 pages including 9 figures; expanded the paper to discuss
left-right asymmetry of charge and energy currents, and effect of finite bia
Negative discriminant states in N=4 supersymmetric string theories
Single centered BPS black hole solutions exist only when the charge carried
by the black hole has positive discriminant. On the other hand the exact dyon
spectrum in heterotic string theory compactified on T^6 is known to contain
states with negative discriminant. We show that all of these negative
discriminant states can be accounted for as two centered black holes. Thus
after the contribution to the index from the two centered black holes is
subtracted from the total microscopic index, the index for states with negative
discriminant vanishes even for finite values of charges, in agreement with the
results from the black hole side. Bound state metamorphosis -- which requires
us to identify certain apparently different two centered configurations
according to a specific set of rules -- plays a crucial role in this analysis.
We also generalize these results to a class of CHL string theories.Comment: LaTeX file, 32 pages; v2: reference added; v3: added new section 3.
Cosmology in scalar tensor theory and asymptotically de-Sitter Universe
We have investigated the cosmological scenarios with a four dimensional
effective action which is connected with multidimensional, supergravity and
string theories. The solution for the scale factor is such that initially
universe undergoes a decelerated expansion but in late times it enters into the
accelerated expansion phase. Infact, it asymptotically becomes a de-Sitter
universe. The dilaton field in our model is a decreasing function of time and
it becomes a constant in late time resulting the exit from the scalar tensor
theory to the standard Einstein's gravity. Also the dilaton field results the
existence of a positive cosmological constant in late times.Comment: 7 pages, Revtex Style, 6 Postscript figure
How Do Black Holes Predict the Sign of the Fourier Coefficients of Siegel Modular Forms?
Single centered supersymmetric black holes in four dimensions have
spherically symmetric horizon and hence carry zero angular momentum. This leads
to a specific sign of the helicity trace index associated with these black
holes. Since the latter are given by the Fourier expansion coefficients of
appropriate meromorphic modular forms of Sp(2,Z) or its subgroup, we are led to
a specific prediction for the signs of a subset of these Fourier coefficients
which represent contributions from single centered black holes only. We
explicitly test these predictions for the modular forms which compute the index
of quarter BPS black holes in heterotic string theory on T^6, as well as in Z_N
CHL models for N=2,3,5,7.Comment: LaTeX file, 17 pages, 1 figur
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