254 research outputs found
Extremal limit of the regular charged black holes in nonlinear electrodynamics
The near horizon limit of the extreme nonlinear black hole is investigated.
It is shown that resulting geometry belongs to the AdS2xS2 class with different
modules of curvatures of subspaces and could be described in terms of the
Lambert functions. It is demonstrated that the considered class of Lagrangians
does not admit solutions of the Bertotti-Robinson type
Phase transition between quantum and classical regimes for the escape rate of a biaxial spin system
Employing the method of mapping the spin problem onto a particle one, we have
derived the particle Hamiltonian for a biaxial spin system with a transverse or
longitudinal magnetic field. Using the Hamiltonian and introducing the
parameter where (U_{min})
corresponds to the top (bottom) of the potential and is the energy of the
particle, we have studied the first- or second-order transition around the
crossover temperature between thermal and quantum regimes for the escape rate,
depending on the anisotropy constant and the external magnetic field. It is
shown that the phase boundary separating the first- and second-order transition
and its crossover temperature are greatly influenced by the transverse
anisotropy constant as well as the transverse or longitudinal magnetic field.Comment: 5 pages + 3 figures, to be published in Phys. Rev.
Dirty rotating black holes: regularity conditions on stationary horizons
We consider generic, or "dirty" (surrounded by matter), stationary rotating
black holes with axial symmetry. The restrictions are found on the asymptotic
form of metric in the vicinity of non-extremal, extremal and ultra-extremal
horizons, imposed by the conditions of regularity of increasing strength:
boundedness on the horizon of the Ricci scalar, of scalar quadratic curvature
invariants, and of the components of the curvature tensor in the tetrad
attached to a falling observer. We show, in particular, that boundedness of the
Ricci scalar implies the "rigidity" of the horizon's rotation in all cases,
while the finiteness of quadratic invariants leads to the constancy of the
surface gravity. We discuss the role of quasiglobal coordinate r that is
emphasized by the conditions of regularity. Further restrictions on the metric
are formulated in terms of subsequent coefficients of expansion of metric
functions by r. The boundedness of the tetrad components of curvature tensor
for an observer crossing the horizon is shown to lead in the horizon limit to
diagonalization of Einstein tensor in the frame of zero angular momentum
observer on a circular orbit (ZAMO frame) for horizons of all degrees of
extremality.Comment: 31 pages. Misprints correcte
Circular orbits and acceleration of particles by near-extremal dirty rotating black holes: general approach
We study the effect of ultra-high energy particles collisions near the black
hole horizon (BSW effect) for two scenarios: when one of particle either (i)
moves on a circular orbit or (ii) plunges from it towards the horizon. It is
shown that such circular near-horizon orbits can exist for near-extremal black
holes only. This includes the innermost stable orbit (ISCO), marginally bound
orbit (MBO) and photon one (PhO). We consider generic "dirty" rotating black
holes not specifying the metric and show that the energy in the centre of mass
frame has the universal scaling dependence on the surface gravity .
Namely, where for the ISCO in case (i) or
in case (ii). For the MBO and PhCO in both scenarios that
agrees with recent calculations of Harada and Kimura for the Kerr metric. We
also generalize the Grib and Pavlov's observations made for the Kerr metric.
The magnitude of the BSW effect on the location of collision has a somewhat
paradoxical character: it is decreasing when approaching the horizon.Comment: 20 pages. Confusion between notations in Sec. 7 corrected. Version to
appear in CQ
Entropy of Quantum Fields for Nonextreme Black Holes in the Extreme Limit
Nonextreme black hole in a cavity within the framework of the canonical or
grand canonical ensemble can approach the extreme limit with a finite
temperature measured on a boundary located at a finite proper distance from the
horizon. In spite of this finite temperature, it is shown that the one-loop
contribution of quantum fields to the thermodynamic entropy due
to equilibrium Hawking radiation vanishes in the limit under consideration. The
same is true for the finite temperature version of the Bertotti-Robinson
spacetime into which a classical Reissner-Nordstr\"{o}m black hole turns in the
extreme limit. The result is attributed to the nature of a horizon
for the Bertotti-Robinson spacetime.Comment: 11 pages, ReVTeX, no figures. New references added, discussion
expanded, presentation and English improved. Accepted for publication in
Phys. Rev.
Coherent acceleration of material wavepackets in modulated optical fields
We study the quantum dynamics of a material wavepacket bouncing off a
modulated atomic mirror in the presence of a gravitational field. We find the
occurrence of coherent accelerated dynamics for atoms beyond the familiar
regime of dynamical localization. The acceleration takes place for certain
initial phase space data and within specific windows of modulation strengths.
The realization of the proposed acceleration scheme is within the range of
present day experimental possibilities
Zel'dovich states with very small mass and charge in nonlinear electrodynamics coupled to gravity
It is shown that in non-linear electrodynamics (in particular, Born-Infeld
one) in the framework of general relativity there exist "weakly singular"
configurations such that (i) the proper mass M is finite in spite of
divergences of the energy density, (ii) the electric charge q and Schwarzschild
mass m ~ q can be made as small as one likes, (iv) all field and energy
distributions are concentrated in the core region. This region has an almost
zero surface area but a finite longitudinal size L=2M. Such configurations can
be viewed as a new version of a classical analogue of an elementary particle.Comment: 11 pages. 1 reference added. To appear in Grav. Cosm
Entropy bound for a charged object from the Kerr-Newman black hole
We derive again the upper entropy bound for a charged object by employing
thermodynamics of the Kerr-Newman black hole linearised with respect to its
electric chargeComment: latex, 4 pages, no figures. In this version, the desired bound is
well obtained by varying correctly the entropy of the black hol
Matter sources for a Null Big Bang
We consider the properties of stress-energy tensors compatible with a Null
Big Bang, i.e., cosmological evolution starting from a Killing horizon rather
than a singularity. For Kantowski-Sachs cosmologies, it is shown that if matter
satisfies the Null Energy Condition (NEC), then (i) regular cosmological
evolution can only start from a Killing horizon, (ii) matter is absent at the
horizon, and (iii) matter can only appear in the cosmological region due to
interaction with vacuum. The latter is understood phenomenologically as a fluid
whose stress tensor is insensitive to boosts in a particular direction. We also
argue that matter is absent in a static region beyond the horizon. All this
generalizes the observations recently obtained for a mixture of dust and a
vacuum fluid. If, however, we admit the existence of phantom matter, its
certain special kinds (with the parameter ) are consistent with a
Null Big Bang without interaction with vacuum (or without vacuum fluid at all).
Then in the static region there is matter with . Alternatively, the
evolution can begin from a horizon in an infinitely remote past, leading to a
scenario combining the features of a Null Big Bang and an emergent universe.Comment: 5 two-column pages, revtex4, no figures. One reference corrected.
Final version accepted for publication in Class. Quantum Gra
Tunnelling series in terms of perturbation theory for quantum spin systems
Considered is quantum tunnelling in anisotropic spin systems in a magnetic
field perpendicular to the anisotropy axis. In the domain of small field the
problem of calculating tunnelling splitting of energy levels is reduced to
constructing the perturbatio n series with degeneracy, the order of degeneracy
being proportional to a spin value. Partial summation of this series taking
into account ''dangerous terms'' with small denominators is performed and the
value of tunnelling splitting is calculated with allowance for the first
correction with respect to a magnetic field.Comment: 7 pages, REVTeX 3.
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