3,673 research outputs found
Cloud of strings in f(R) gravity
We derive the solution for a spherically symmetric string cloud configuration
in a d-dimensional spacetime in the framework of f(R) theories of gravity. We
also analyze some thermodynamic properties of the joint black hole - cloud of
strings solution. For its Hawking temperature, we found that the dependence of
the mass with the horizon is significantly different in both theories. For the
interaction of a black hole with thermal radiation, we found that the shapes of
the curves are similar, but shifted. Our analysis generalizes some known
results in the literature.Comment: Version accepted for CP
Quasinormal modes of a black hole with a cloud of strings in Einstein-Gauss-Bonnet gravity
The quasinormal modes for a scalar field in the background spacetime
corresponding to a black hole, with a cloud of strings, in
Einstein-Gauss-Bonnet gravity, and the tensor quasinormal modes corresponding
to perturbations in such spacetime, were both calculated using the WKB
approximation. In the obtained results we emphasize the role played by the
parameter associated with the string cloud, comparing them with the results
already obtained for the Boulware-Deser metric. We also study how the
Gauss-Bonnet correction to general relativity affects the results for the
quasinormal modes, comparing them with the same background in general
relativity.Comment: 15 pages, 7 figures; To appear in IJMP
Coupled quantum wires
We study a set of crossed 1D systems, which are coupled with each other via
tunnelling at the crossings. We begin with the simplest case with no
electron-electron interactions and find that besides the expected level
splitting, bound states can emerge. Next, we include an external potential and
electron-electron interactions, which are treated within the Hartree
approximation. Then, we write down a formal general solution to the problem,
giving additional details for the case of a symmetric external potential.
Concentrating on the case of a single crossing, we were able to explain recent
experinents on crossed metallic and semiconducting nanotubes [J. W. Janssen, S.
G. Lemay, L. P. Kouwenhoven, and C. Dekker, Phys. Rev. B 65, 115423 (2002)],
which showed the presence of localized states in the region of crossing.Comment: 11 pages, 10 figure
Value relevance of alternative methods of accounting for actuarial gains and losses
In 2010, IASB published an exposure draft about defined benefit plans where it proposed to eliminate the deferred recognition of actuarial gains and losses. IASB aimed to make fundamental improvements to the recognition, presentation and disclosures of defined benefit plans by mid-2011. To shed light on the debate on the recognition of actuarial gains and losses of defined benefit plans, this paper investigates the value relevance of financial information under three alternative methods of recognising actuarial gains and losses allowed by IAS 19: employee benefits (2004): the profit or loss method, the equity recognition method and the corridor method. Findings suggest that the equity recognition method, i.e., the recognition of all actuarial gains and losses in equity, best reflects the market’s valuation of actuarial gains and losses of defined benefit plans
Ultracold fermions in a one-dimensional bipartite optical lattice: metal-insulator transitions driven by shaking
We describe the behavior of a system of fermionic atoms loaded in a bipartite
one-dimensional optical lattice that is under the action of an external
time-periodic driving force. By using Floquet theory, an effective model with
renormalized hopping coefficients is derived. The insulating behavior
characterizing the system at half-filling in the absence of driving is
dynamically suppressed and for particular values of the driving parameter the
system becomes either a standard metal or an unconventional metal with four
Fermi points. We use the bosonization technique to investigate the effect of
on-site Hubbard interactions on the four Fermi-point metal-insulator phase
transition. Attractive interactions are expected to enlarge the regime of
parameters where the unconventional metallic phase arises, whereas repulsive
interactions reduce it. This metallic phase is known to be a Luther-Emery
liquid (spin gapped metal) for both, repulsive and attractive interactions,
contrarily to the usual Hubbard model which exhibits a Mott insulator phase for
repulsive interactions. Ultracold fermions in driven one-dimensional bipartite
optical lattices provide an interesting platform for the realization of this
long studied four Fermi-point unconventional metal.Comment: 11 pages, 6 figure
Spin- and band-ferromagnetism in trilayer graphene
We study the ground state properties of an ABA-stacked trilayer graphene. The
low energy band structure can be described by a combination of both a linear
and a quadratic particle-hole symmetric dispersions, reminiscent of monolayer-
and bilayer-graphene, respectively. The multi-band structure offers more
channels for instability towards ferromagnetism when the Coulomb interaction is
taken into account. Indeed, if one associates a pseudo-spin 1/2 degree of
freedom to the bands (parabolic/linear), it is possible to realize also a
band-ferromagnetic state, where there is a shift in the energy bands, since
they fill up differently. By using a variational procedure, we compute the
exchange energies for all possible variational ground states and identify the
parameter space for the occurrence of spin- and band-ferromagnetic
instabilities as a function of doping and interaction strength.Comment: 9 pages/ 8 figure
Quantum simulation of correlated-hopping models with fermions in optical lattices
By using a modulated magnetic field in a Feshbach resonance for ultracold
fermionic atoms in optical lattices, we show that it is possible to engineer a
class of models usually referred to as correlated-hopping models. These models
differ from the Hubbard model in exhibiting additional density-dependent
interaction terms that affect the hopping processes. In addition to the
spin-SU(2) symmetry, they also possess a charge-SU(2) symmetry, which opens the
possibility of investigating the -pairing mechanism for superconductivity
introduced by Yang for the Hubbard model. We discuss the known solution of the
model in 1D (where states have been found in the degenerate manifold of
the ground state) and show that, away from the integrable point, quantum Monte
Carlo simulations at half filling predict the emergence of a phase with
coexisting incommensurate spin and charge order.Comment: 10 pages, 9 figure
Kounis Syndrome Associated With Selective Anaphylaxis to Cefazolin.
info:eu-repo/semantics/publishedVersio
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