452 research outputs found
Fixed Scalars and Suppression of Hawking Evaporation
For an extreme charged black hole some scalars take on a fixed value at the
horizon determined by the charges alone. We call them fixed scalars. We find
the absorption cross section for a low frequency wave of a fixed scalar to be
proportional to the square of the frequency. This implies a strong suppression
of the Hawking radiation near extremality. We compute the coefficient of
proportionality for a specific model.Comment: 10 pages, late
Greybody Factors of Charged Dilaton Black Holes in 2 + 1 Dimensions
We have studied scalar perturbations of charged dilaton black holes in 2+1
dimensions. The black hole considered here is a solution to the low-energy
string theory in 2+1 dimensions. The exact decay rates and the grey body
factors for the massless minimally coupled scalar is computed for both the
charged and the uncharged dilaton black holes. The charged and the uncharged
black hole show similar behavior for grey body factors, reflection coefficients
and decay rates.Comment: The equation for the potential and figure:1 are changed. The changes
does not effect the result
On a coordinate independent description of string worldsheet theory
We study worldsheet conformal invariance for bosonic string propagating in a
curved background using the hamiltonian formalism. In order to formulate the
problem in a background independent manner we first rewrite the worldsheet
theory in a language where it describes a single particle moving in an
infinite-dimensional curved spacetime. This language is developed at a formal
level without regularizing the infinite-dimensional traces. Then we adopt
DeWitt's (Phys.Rev.85:653-661,1952) coordinate independent formulation of
quantum mechanics in the present context. Given the expressions for the
classical Virasoro generators, this procedure enables us to define the
coordinate invariant quantum analogues which we call DeWitt-Virasoro
generators. This framework also enables us to calculate the invariant matrix
elements of an arbitrary operator constructed out of the DeWitt-Virasoro
generators between two arbitrary scalar states. Using these tools we further
calculate the DeWitt-Virasoro algebra in spin-zero representation. The result
is given by the Witt algebra with additional anomalous terms that vanish for
Ricci-flat backgrounds. Further analysis need to be performed in order to
precisely relate this with the beta function computation of Friedan and others.
Finally, we explain how this analysis improves the understanding of showing
conformal invariance for certain pp-wave that has been recently discussed using
hamiltonian framework.Comment: 32 pages, some reorganization for more elaborate explanation, no
change in conclusio
Particle Production in Matrix Cosmology
We consider cosmological particle production in 1+1 dimensional string
theory. The process is described most efficiently in terms of anomalies, but we
also discuss the explicit mode expansions. In matrix cosmology the usual vacuum
ambiguity of quantum fields in time-dependent backgrounds is resolved by the
underlying matrix model. This leads to a finite energy density for the "in"
state which cancels the effect of anomalous particle production.Comment: 24 pages, 1 figure; v2: references added, minor change
Inelastic lifetimes of confined two-component electron systems in semiconductor quantum wire and quantum well structures
We calculate Coulomb scattering lifetimes of electrons in two-subband quantum
wires and in double-layer quantum wells by obtaining the quasiparticle
self-energy within the framework of the random-phase approximation for the
dynamical dielectric function. We show that, in contrast to a single-subband
quantum wire, the scattering rate in a two-subband quantum wire contains
contributions from both particle-hole excitations and plasmon excitations. For
double-layer quantum well structures, we examine individual contributions to
the scattering rate from quasiparticle as well as acoustic and optical plasmon
excitations at different electron densities and layer separations. We find that
the acoustic plasmon contribution in the two-component electron system does not
introduce any qualitatively new correction to the low energy inelastic
lifetime, and, in particular, does not produce the linear energy dependence of
carrier scattering rate as observed in the normal state of high-
superconductors.Comment: 16 pages, RevTeX, 7 figures. Also available at
http://www-cmg.physics.umd.edu/~lzheng
Geometric Entropy of Nonrelativistic Fermions and Two Dimensional Strings
We consider the geometric entropy of free nonrelativistic fermions in two
dimensions and show that it is ultraviolet finite for finite fermi energies,
but divergent in the infrared. In terms of the corresponding collective field
theory this is a {\em nonperturbative} effect and is related to the soft
behaviour of the usual thermodynamic entropy at high temperatures. We then show
that thermodynamic entropy of the singlet sector of the one dimensional matrix
model at high temperatures is governed by nonperturbative effects of the
underlying string theory. In the high temperature limit the ``exact''
expression for the entropy is regular but leads to a negative specific heat,
thus implying an instability. We speculate that in a properly defined two
dimensional string theory, the thermodynamic entropy could approach a constant
at high temperatures and lead to a geometric entropy which is finite in the
ultraviolet.Comment: LaTex, 19 pages, no figures. Some references adde
Thermal Partition Functions for S-branes
We calculate the thermal partition functions of open strings on the S-brane
backgrounds (the bouncing or rolling tachyon backgrounds) both in the bosonic
and superstring cases. According to hep-th/0302146, we consider the discretized
temperatures compatible with the pure imaginary periodicity of tachyon
profiles. The ``effective Hagedorn divergence'' is shown to appear no matter
how low temperature is chosen (including zero-temperature). This feature is
likely to be consistent with the large rate of open string pair production
discussed in hep-th/0209090 and also emission of closed string massive modes
hep-th/0303139. We also discuss the possibility to remove the divergence by
considering the space-like linear dilaton backgrounds as in hep-th/0306132.Comment: 33 pages, no figure; v2 typos corrected, a reference adde
Absorption of Fixed scalars and the D-brane Approach to Black Holes
We calculate the emission and absorption rates of fixed scalars by the
near-extremal five-dimensional black holes that have recently been modeled
using intersecting D-branes. We find agreement between the semi-classical and
D-brane computations. At low energies the fixed scalar absorption cross-section
is smaller than for ordinary scalars and depends on other properties of the
black hole than just the horizon area. In the D-brane description, fixed scalar
absorption is suppressed because these scalars must split into at least four,
rather than two, open strings running along the D-brane. Consequently, this
comparison provides a more sensitive test of the effective string picture of
the D-brane bound state than does the cross-section for ordinary scalars. In
particular, it allows us to read off the value of the effective string tension.
That value is precisely what is needed to reproduce the near-extremal 5-brane
entropy.Comment: 33 pages, harvmac, version to appear in Nucl. Phys.
The Study of Goldstone Modes in =2 Bilayer Quantum Hall Systems
At the filling factor =2, the bilayer quantum Hall system has three
phases, the spin-ferromagnet phase, the spin singlet phase and the canted
antiferromagnet (CAF) phase, depending on the relative strength between the
Zeeman energy and interlayer tunneling energy. We present a systematic method
to derive the effective Hamiltonian for the Goldstone modes in these three
phases. We then investigate the dispersion relations and the coherence lengths
of the Goldstone modes. To explore a possible emergence of the interlayer phase
coherence, we analyze the dispersion relations in the zero tunneling energy
limit. We find one gapless mode with the linear dispersion relation in the CAF
phase.Comment: 13 pages, no figures. One reference is added. Typos correcte
Polysulfide-triggered fluorescent indicator suitable for super-resolution microscopy and application in imaging
A new physiologically benign and cell membrane permeable BODIPY based molecular probe, MB-Sn, specifically senses intracellular hydrogen polysulfides (H2Sn, n > 1) localized in the endoplasmic reticulum. This reagent is suitable for mapping the intracellular distribution of H2Sn by wide-field as well as super-resolution Structured Illumination Microscopy (SIM)
- …