13,304 research outputs found
Unforeseen high temperature and humidity stability of FeCl intercalated few layer graphene
We present the first systematic study of the stability of the structure and
electrical properties of FeCl intercalated few-layer graphene to high
levels of humidity and high temperature. Complementary experimental techniques
such as electrical transport, high resolution transmission electron microscopy
and Raman spectroscopy conclusively demonstrate the unforeseen stability of
this transparent conductor to a relative humidity up to at room
temperature for 25 days, to a temperature up to 150\,^\circC in atmosphere
and up to a temperature as high as 620\,^\circC in vacuum, that is more than
twice higher than the temperature at which the intercalation is conducted. The
stability of FeCl intercalated few-layer graphene together with its unique
values of low square resistance and high optical transparency, makes this
material an attractive transparent conductor in future flexible electronic
applications.Comment: Scientific Reports, volume 5, article no. 760
Two Dimensional Quantum Dilaton Gravity and the Positivity of Energy
Using an argument due to Regge and Teitelboim, an expression for the ADM mass
of 2d quantum dilaton gravity is obtained. By evaluating this expression we
establish that the quantum theories which can be written as a Liouville-like
theory, have a lower bound to energy, provided there is no critical boundary.
This fact is then reconciled with the observation made earlier that the Hawking
radiation does not appear to stop. The physical picture that emerges is that of
a black hole in a bath of quantum radiation. We also evaluate the ADM mass for
the models with RST boundary conditions and find that negative values are
allowed. The Bondi mass of these models goes to zero for large retarded times,
but becomes negative at intermediate times in a manner that is consistent with
the thunderpop of RST.Comment: 16 pages, phyzzx, COLO-HEP-309. (Confusing points in previous version
clarified, discussion of ADM and Bondi masses in RST case added.
Towards Quantum Cosmology without Singularities
In this paper we investigate the vanishing of cosmological singularities by
quantization. Starting from a 5d Kaluza--Klein approach we quantize, as a first
step, the non--spherical metric part and the dilaton field. These fields which
are classically singular become smooth after quantization. In addition, we
argue that the incorporation of non perturbative quantum corrections form a
dilaton potential. Technically, the procedure corresponds to the quantization
of 2d dilaton gravity and we discuss several models. From the 4d point of view
this procedure is a semiclassical approach where only the dilaton and moduli
matter fields are quantized.Comment: 9 pages, 2 figures, Latex, epsfig.sty, epsf.te
A theory of quantum black holes: non-perturbative corrections and no-veil conjecture
A common belief is that further quantum corrections near the singularity of a
large black hole should not substantially modify the semiclassical picture of
black hole evaporation; in particular, the outgoing spectrum of radiation
should be very close to the thermal spectrum predicted by Hawking. In this
paper we explore a possible counterexample: in the context of dilaton gravity,
we find that non-perturbative quantum corrections which are important in strong
coupling regions may completely alter the semiclassical picture, to the extent
that the presumptive space-like boundary becomes time-like, changing in this
way the causal structure of the semiclassical geometry. As a result, only a
small fraction of the total energy is radiated outside the fake event horizon;
most of the energy comes in fact at later retarded times and there is no
information loss problem. Thus we propose that this may constitute a general
characteristic of quantum black holes, that is, quantum gravity might be such
as to prevent the formation of global event horizons. We argue that this is not
unnatural from the viewpoint of quantum mechanics.Comment: 24 pages, 12 figures (not included, available by request), UTTG-22-9
Supersymmetry and Positive Energy in Classical and Quantum Two-Dimensional Dilaton Gravity
An supersymmetric version of two dimensional dilaton gravity coupled
to matter is considered. It is shown that the linear dilaton vacuum
spontaneously breaks half the supersymmetries, leaving broken a linear
combination of left and right supersymmetries which squares to time
translations. Supersymmetry suggests a spinorial expression for the ADM energy
, as found by Witten in four-dimensional general relativity. Using this
expression it is proven that is non-negative for smooth initial data
asymptotic (in both directions) to the linear dilaton vacuum, provided that the
(not necessarily supersymmetric) matter stress tensor obeys the dominant energy
condition. A {\it quantum} positive energy theorem is also proven for the
semiclassical large- equations, despite the indefiniteness of the quantum
stress tensor. For black hole spacetimes, it is shown that is bounded from
below by , where is the value of the dilaton at the
apparent horizon, provided only that the stress tensor is positive outside the
apparent horizon. This is the two-dimensional analogue of an unproven
conjecture due to Penrose. Finally, supersymmetry is used to prove positive
energy theorems for a large class of generalizations of dilaton gravity which
arise in consideration of the quantum theory.Comment: 21 page
Black Hole Formation by Sine-Gordon Solitons in Two-dimensional Dilaton Gravity
The CGHS model of two-dimensional dilaton gravity coupled to a sine-Gordon
matter field is considered. The theory is exactly solvable classically, and the
solutions of a kink and two-kink type solitons are studied in connection with
black hole formation.Comment: 11 pages, no figures, revte
The Stretched Horizon and Black Hole Complementarity
Three postulates asserting the validity of conventional quantum theory,
semi-classical general relativity and the statistical basis for thermodynamics
are introduced as a foundation for the study of black hole evolution. We
explain how these postulates may be implemented in a ``stretched horizon'' or
membrane description of the black hole, appropriate to a distant observer. The
technical analysis is illustrated in the simplified context of 1+1 dimensional
dilaton gravity. Our postulates imply that the dissipative properties of the
stretched horizon arise from a course graining of microphysical degrees of
freedom that the horizon must possess. A principle of black hole
complementarity is advocated. The overall viewpoint is similar to that
pioneered by 't~Hooft but the detailed implementation is different.Comment: (some misprints in equations have been fixed), 48 pages (including
figures), SU-ITP-93-1
Quantum Theories of Dilaton Gravity
Quantization of two-dimensional dilaton gravity coupled to conformal matter
is investigated. Working in conformal gauge about a fixed background metric,
the theory may be viewed as a sigma model whose target space is parameterized
by the dilaton and conformal factor . A precise connection is
given between the constraint that the theory be independent of the background
metric and conformal invariance of the resulting sigma model. Although the
action is renormalizable, new coupling constants must be specified at each
order in perturbation theory in order to determine the quantum theory. These
constants may be viewed as initial data for the beta function equations. It is
argued that not all choices of this data correspond to physically sensible
theories of gravity, and physically motivated constraints on the data are
discussed. In particular a recently constructed subclass of initial data which
reduces the full quantum theory to a soluble Liouville-like theory has energies
unbounded from below and thus is unphysical. Possibilities for modifying this
construction so as to avoid this difficulty are briefly discussed.Comment: 20 pages (Major additions made, including 5 pages on the relation
between conformal invariance and background independence.
Current status of laboratory and imaging diagnosis of neonatal necrotizing enterocolitis
Necrotizing enterocolitis continues to be a devastating disease process for very low birth weight infants in Neonatal Intensive Care Units. The aetiology and pathogenesis of necrotizing enterocolitis are not definitively understood. It is known that necrotizing enterocolitis is secondary to a complex interaction of multiple factors that results in mucosal damage, which leads to intestinal ischemia and necrosis. Advances in neonatal care, including resuscitation and ventilation support technology, have seen increased survival rates among premature neonates and a concomitant detection in the incidence of this intestinal disease.Diagnosis can be difficult, and identifying infants at the onset of disease remains a challenge. Early diagnosis, which relies on imaging findings, and initiation of prompt therapy are essential to limit morbidity and mortality. Moreover, early management is critical and life-saving.This review summarizes what is known on the laboratory and instrumental diagnostic strategies needed to improve neonatal outcomes and, possibily, to prevent the onset of an overt necrotizing enterocolitis
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