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Sofia, Bulgaria

The "St. Kliment Ohridski" University of Sofia or Sofia University is the oldest higher education institution in Bulgaria, founded on 1 October 1888. The university's edifice was constructed between 1924 and 1934 with the financial support of the brothers Evlogi Georgiev and Hristo Georgiev, whose sculptures are now featured on its façade, and has an area of 18,624 m² and a total of 324 premises.Sofia University has 16 faculties and three departments, where 21,000 students receive their education. In addition, it also disposes of a university library, a university press, a computer centre, a sports centre and several other structures. The current rector is Ivan Ilchev.Sofia University is the best university in Bulgaria in National Rankings, and is constantly among the top 4 percent of World Universities according to QS World University Rankings. Wikipedia.

Wesselinowa J.M.,Sofia University
Journal of Magnetism and Magnetic Materials

Based on the Heisenberg model taking into account single-ion anisotropy and using a Green's function technique we have studied the influence of size and anisotropy effects on magnetization M, Neel temperature T N, coercive field H c and spin excitation energy of antiferromagnetic nanoparticles. The properties are compared with those of ferromagnetic nanoparticles. We have shown that the enhanced magnetization M and coercive field H c of antiferromagnetic nanoparticles is a surface effect, which is due to uncompensated surface spins. Moreover, the shape of the coercive field curve can be significantly influenced by surface magnetic anisotropy. © 2009 Elsevier B.V. All rights reserved. Source

Fiziev P.P.,Sofia University
Classical and Quantum Gravity

The Teukolsky master equation is the basic tool for the study of perturbations of the Kerr metric in linear approximation. It admits separation of variables, thus yielding the Teukolsky radial equation and the Teukolsky angular equation. We present here a unified description of all classes of exact solutions to these equations in terms of the confluent Heun functions. Large classes of new exact solutions are found and classified with respect to their characteristic properties. Special attention is paid to the polynomial solutions which are singular ones and introduce collimated one-way running waves. It is shown that a proper linear combination of such solutions can present bounded one-way running waves. This type of waves may be suitable as models of the observed astrophysical jets. © 2010 IOP Publishing Ltd. Source

Yazadjiev S.,Sofia University
Physical Review D - Particles, Fields, Gravitation and Cosmology

We derive inequalities between the area, the angular momentum, and the charges for axisymmetric closed outermost stably marginally outer trapped surfaces, embedded in dynamical and, in general, nonaxisymmetric spacetimes satisfying the Einstein-Maxwell-dilaton-matter equations. In proving the inequalities, we assume that the dilaton potential is non-negative and that the matter energy-momentum tensor satisfies the dominant energy condition. © 2013 American Physical Society. Source

Fiziev P.P.,Sofia University
Physical Review D - Particles, Fields, Gravitation and Cosmology

We study the relation between minimal dilatonic gravity (MDG) and f(R) theories of gravity and establish strict conditions for their global equivalence. Such equivalence takes place only for a certain class of cosmological potentials, dubbed here "withholding potentials," since they prevent change of the sign of the dilaton Φ. The withholding property ensures the attractive character of gravity, as well as absence of ghosts and a tachyon in the gravi-dilaton sector and yields certain asymptotic of the functions f(R). Large classes of withholding cosmological potentials and functions f(R) are found and described in detail. The particle content of the gravi-dilaton sector is found using perturbation theory around the de Sitter vacuum of MDG. Two phenomena, scalaron waves and induction of gravitational waves by the scalaron field, are discussed using the derived wave equations for MDG scalaron and graviton in the de Sitter background. Seemingly, the MDG and f(R) theories offer a unified description of dark energy and dark matter. © 2013 American Physical Society. Source

In this paper, we consider 5D spacetimes satisfying the Einstein-Maxwell-dilaton gravity equations which are U(1)2 axisymmetric but otherwise highly dynamical. We derive inequalities between the area, the angular momenta, the electric charge and the magnetic fluxes for any smooth stably outer marginally trapped surface. © 2013 IOP Publishing Ltd. Source

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