820 research outputs found
Regular Stringy Black Holes?
We study the first-order corrections to the singular 4-dimensional
massless stringy black holes studied in the nineties in the context of the
Heterotic Superstring. We show that the corrections not only induce a
non-vanishing mass and give rise to an event horizon, but also eliminate the
singularity giving rise to a regular spacetime whose global structure includes
further asymptotically flat regions in which the spacetime's mass is positive
or negative. We study the timelike and null geodesics and their effective
potential, showing that the spacetime is geodesically complete. We discuss the
validity of this solution, arguing that the very interesting and peculiar
properties of the solution are associated to the negative energy contributions
coming from the terms quadratic in the curvature. As a matter of fact, the
10-dimensional configuration is singular. We extract some general lessons on
attempts to eliminate black-hole singularities by introducing terms of higher
order in the curvature.Comment: 5 pages, 2 figure
A stable, single-photon emitter in a thin organic crystal for application to quantum-photonic devices
Single organic molecules offer great promise as bright, reliable sources of
identical single photons on demand, capable of integration into solid-state
devices. It has been proposed that such molecules in a crystalline organic
matrix might be placed close to an optical waveguide for this purpose, but so
far there have been no demonstrations of sufficiently thin crystals, with a
controlled concentration of suitable dopant molecules. Here we present a method
for growing very thin anthracene crystals from super-saturated vapour, which
produces crystals of extreme flatness and controlled thickness. We show how
this crystal can be doped with a widely adjustable concentration of
dibenzoterrylene (DBT) molecules and we examine the optical properties of these
molecules to demonstrate their suitability as quantum emitters in nanophotonic
devices. Our measurements show that the molecules are available in the crystal
as single quantum emitters, with a well-defined polarisation relative to the
crystal axes, making them amenable to alignment with optical nanostructures. We
find that the radiative lifetime and saturation intensity vary little within
the crystal and are not in any way compromised by the unusual matrix
environment. We show that a large fraction of these emitters are able to
deliver more than photons without photo-bleaching, making them
suitable for real applications.Comment: 12 pages, 10 figures, comments welcom
Magneto-elastic effects and magnetization plateaus in two dimensional systems
We show the importance of both strong frustration and spin-lattice coupling
for the stabilization of magnetization plateaus in translationally invariant
two-dimensional systems. We consider a frustrated spin-1/2 Heisenberg model
coupled to adiabatic phonons under an external magnetic field. At zero
magnetization, simple structures with two or at most four spins per unit cell
are stabilized, forming dimers or plaquettes, respectively. A much
richer scenario is found in the case of magnetization , where larger
unit cells are formed with non-trivial spin textures and an analogy with the
corresponding classical Ising model is detectable. Specific predictions on
lattice distortions and local spin values can be directly measured by X-rays
and Nuclear Magnetic Resonance experiments.Comment: 4 pages and 4 figure
Tuning the magnetic and structural phase transitions of PrFeAsO via Fe/Ru spin dilution
Neutron diffraction and muon spin relaxation measurements are used to obtain
a detailed phase diagram of Pr(Fe,Ru)AsO. The isoelectronic substitution of Ru
for Fe acts effectively as spin dilution, suppressing both the structural and
magnetic phase transitions. The temperature of the tetragonal-orthorhombic
structural phase transition decreases gradually as a function of x. Slightly
below the transition temperature coherent precessions of the muon spin are
observed corresponding to static magnetism, possibly reflecting a significant
magneto-elastic coupling in the FeAs layers. Short range order in both the Fe
and Pr moments persists for higher levels of x. The static magnetic moments
disappear at a concentration coincident with that expected for percolation of
the J1-J2 square lattice model
Super nucleation and orientation of poly (butylene terephthalate) crystals in nanocomposites containing highly reduced graphene oxide
The ring opening polymerization of cyclic butylene terephthalate into poly
(butylene terephthalate) (pCBT) in the presence of reduced graphene oxide (RGO)
is an effective method for the preparation of polymer nanocomposites. The
inclusion of RGO nanoflakes dramatically affects the crystallization of pCBT,
shifting crystallization peak temperature to higher temperatures and, overall,
increasing the crystallization rate. This was due to a super nucleating effect
caused by RGO, which is maximized by highly reduced graphene oxide.
Furthermore, combined analyses by differential scanning calorimetry (DSC)
experiments and wide angle X-ray diffraction (WAXS) showed the formation of a
thick {\alpha}-crystalline form pCBT lamellae with a melting point of ~250
{\deg}C, close to the equilibrium melting temperature of pCBT. WAXS also
demonstrated the pair orientation of pCBT crystals with RGO nanoflakes,
indicating a strong interfacial interaction between the aromatic rings of pCBT
and RGO planes, especially with highly reduced graphene oxide. Such surface
self-organization of the polymer onto the RGO nanoflakes may be exploited for
the enhancement of interfacial properties in their polymer nanocomposites
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