2,192 research outputs found
Controlling charge injection in organic field-effect transistors using self-assembled monolayers
We have studied charge injection across the metal/organic semiconductor
interface in bottom-contact poly(3-hexylthiophene) (P3HT) field-effect
transistors, with Au source and drain electrodes modified by self-assembled
monolayers (SAMs) prior to active polymer deposition. By using the SAM to
engineer the effective Au work function, we markedly affect the charge
injection process. We systematically examine the contact resistivity and
intrinsic channel mobility, and show that chemically increasing the injecting
electrode work function significantly improves hole injection relative to
untreated Au electrodes.Comment: 5 pages, 2 figures. Supplementary information available upon reques
Collapse of Kaluza-Klein Bubbles
Kaluza-Klein theory admits ``bubble" configurations, in which the
circumference of the fifth dimension shrinks to zero on some compact surface. A
three parameter family of such bubble initial data at a moment of time-symmetry
(some including a magnetic field) has been found by Brill and Horowitz,
generalizing the (zero-energy) ``Witten bubble" solution. Some of these data
have negative total energy. We show here that all the negative energy bubble
solutions start out expanding away from the moment of time symmetry, while the
positive energy bubbles can start out either expanding or contracting. Thus it
is unlikely that the negative energy bubbles would collapse and produce a naked
singularity.Comment: 6 pages, plain LaTeX, UMDGR-94-08
Generalization of the model of Hawking radiation with modified high frequency dispersion relation
The Hawking radiation is one of the most interesting phenomena predicted by
the theory of quantum field in curved space. The origin of Hawking radiation is
closely related to the fact that a particle which marginally escapes from
collapsing into a black hole is observed at the future infinity with infinitely
large redshift. In other words, such a particle had a very high frequency when
it was near the event horizon. Motivated by the possibility that the property
of Hawking radiation may be altered by some unknowned physics which may exist
beyond some critical scale, Unruh proposed a model which has higher order
spatial derivative terms. In his model, the effects of unknown physics are
modeled so as to be suppressed for the waves with a wavelength much longer than
the critical scale, . Surprisingly, it was shown that the thermal
spectrum is recovered for such modified models. To introduce such higher order
spatial derivative terms, the Lorentz invariance must be violated because one
special spatial direction needs to be chosen. In previous works, the rest frame
of freely-falling observers was employed as this special reference frame. Here
we give an extension by allowing a more general choice of the reference frame.
Developing the method taken by Corley, % and especially focusing on subluminal
case, we show that the resulting spectrum of created particles again becomes
the thermal one at the Hawking temperature even if the choice of the reference
frame is generalized. Using the technique of the matched asymptotic expansion,
we also show that the correction to the thermal radiation stays of order
or smaller when the spectrum of radiated particle around its peak is
concerned.Comment: 23 pages, 5 postscript figures, submitted to Physical Review
Instantons, supersymmetric vacua, and emergent geometries
We study instanton solutions and superpotentials for the large number of
vacua of the plane-wave matrix model and a 2+1 dimensional Super Yang-Mills
theory on with sixteen supercharges. We get the superpotential in
the weak coupling limit from the gauge theory description. We study the gravity
description of these instantons. Perturbatively with respect to a background,
they are Euclidean branes wrapping cycles in the dual gravity background.
Moreover, the superpotential can be given by the energy of the electric charge
system characterizing each vacuum. These charges are interpreted as the
eigenvalues of matrices from a reduction for the 1/8 BPS sector of the gauge
theories. We also discuss qualitatively the emergence of the extra spatial
dimensions appeared on the gravity side.Comment: 29 pages, 3 figures, latex. v2: references added, comments added. v3:
accepted version in PR
BPS Condensates, Matrix Models and Emergent String Theory
A prescription is given for computing anomalous dimensions of single trace
operators in SYM at strong coupling and large using a reduced model of
matrix quantum mechanics. The method involves treating some parts of the
operators as "BPS condensates" which, in certain limit, have a dual description
as null geodesics on the . In the gauge theory, the condensate is similar
to a representative of the chiral ring and it is described by a background of
commuting matrices. Excitations around these condensates correspond to
excitations around this background and take the form of "string bits" which are
dual to the "giant magnons" of Hofman and Maldacena. In fact, the matrix model
approach gives a {\it quantum} description of these string configurations and
explains why the infinite momentum limit suppresses the quantum effects. This
method allows, not only to derive part of the classical sigma model Hamiltonian
of the dual string (in the infinite momentum limit), but also its quantum
canonical structure. Therefore, it provides an alternative method of testing
the AdS/CFT correspondence without the need of integrability.Comment: 36 pages, 1 figure, 2 appendices, v2: references adde
Fate of Kaluza-Klein Bubble
We numerically study classical time evolutions of Kaluza-Klein bubble
space-time which has negative energy after a decay of vacuum. As the zero
energy Witten's bubble space-time, where the bubble expands infinitely, the
subsequent evolutions of Brill and Horowitz's momentarily static initial data
show that the bubble will expand in terms of the area. At first glance, this
result may support Corley and Jacobson's conjecture that the bubble will expand
forever as well as the Witten's bubble. The irregular signatures, however, can
be seen in the behavior of the lapse function in the maximal slicing gauge and
the divergence of the Kretchman invariant. Since there is no appearance of the
apparent horizon, we suspect an appearance of a naked singularity as the final
fate of this space-time.Comment: 13 pages including 10 figures, RevTeX, epsf.sty. CGPG-99/12-8,
RESCEU-6/00 and DAMTP-2000-30. To appear in Phys. Rev.
Strings on conifolds from strong coupling dynamics: quantitative results
Three quantitative features of string theory on AdS_5 x X_5, for any
(quasi)regular Sasaki-Einstein X_5, are recovered exactly from an expansion of
field theory at strong coupling around configurations in the moduli space of
vacua. These configurations can be thought of as a generalized matrix model of
(local) commuting matrices. First, we reproduce the spectrum of scalar
Kaluza-Klein modes on X_5. Secondly, we recover the precise spectrum of BMN
string states, including a nontrivial dependence on the volume of X_5. Finally,
we show how the radial direction in global AdS_5 emerges universally in these
theories by exhibiting states dual to AdS giant gravitons.Comment: 1+28 pages. 1 figur
Hawking radiation without black hole entropy
In this Letter I point out that Hawking radiation is a purely kinematic
effect that is generic to Lorentzian geometries. Hawking radiation arises for
any test field on any Lorentzian geometry containing an event horizon
regardless of whether or not the Lorentzian geometry satisfies the dynamical
Einstein equations of general relativity. On the other hand, the classical laws
of black hole mechanics are intrinsically linked to the Einstein equations of
general relativity (or their perturbative extension into either semiclassical
quantum gravity or string-inspired scenarios). In particular, the laws of black
hole thermodynamics, and the identification of the entropy of a black hole with
its area, are inextricably linked with the dynamical equations satisfied by the
Lorentzian geometry: entropy is proportional to area (plus corrections) if and
only if the dynamical equations are the Einstein equations (plus corrections).
It is quite possible to have Hawking radiation occur in physical situations in
which the laws of black hole mechanics do not apply, and in situations in which
the notion of black hole entropy does not even make any sense. This observation
has important implications for any derivation of black hole entropy that seeks
to deduce black hole entropy from the Hawking radiation.Comment: Uses ReV_TeX 3.0; Five pages in two-column forma
Relativistic Acoustic Geometry
Sound wave propagation in a relativistic perfect fluid with a non-homogeneous
isentropic flow is studied in terms of acoustic geometry. The sound wave
equation turns out to be equivalent to the equation of motion for a massless
scalar field propagating in a curved space-time geometry. The geometry is
described by the acoustic metric tensor that depends locally on the equation of
state and the four-velocity of the fluid. For a relativistic supersonic flow in
curved space-time the ergosphere and acoustic horizon may be defined in a way
analogous the non-relativistic case. A general-relativistic expression for the
acoustic analog of surface gravity has been found.Comment: 14 pages, LaTe
Bogoliubov theory of the Hawking effect in Bose-Einstein condensates
Artificial black holes may demonstrate some of the elusive quantum properties
of the event horizon, in particular Hawking radiation. One promising candidate
is a sonic hole in a Bose-Einstein condensate. We clarify why Hawking radiation
emerges from the condensate and how this condensed-matter analog reflects some
of the intriguing aspects of quantum black holes
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