Sofia, Bulgaria
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.


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Wesselinowa J.M.,Sofia University
Journal of Magnetism and Magnetic Materials | Year: 2010

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.


Fiziev P.P.,Sofia University
Classical and Quantum Gravity | Year: 2010

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.


Yazadjiev S.,Sofia University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

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.


Yazadjiev S.S.,Sofia University
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2013

In the present Letter we study the long-standing problem for the thermodynamics of magnetized dilaton black holes. For this purpose we construct an exact solution describing a rotating charged dilaton black hole immersed in an external magnetic field and discuss its basic properties. We derive a Smarr-like relation and the thermodynamics first law for these magnetized black holes. The novelty in the thermodynamics of the magnetized black holes is the appearance of new terms proportional to the magnetic momentum of the black holes in the Smarr-like relation and the first law. © 2013 Elsevier B.V.


Yazadjiev S.,Sofia University
Classical and Quantum Gravity | Year: 2013

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.


Fiziev P.P.,Sofia University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

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.


Penchovsky R.,Sofia University
Biomacromolecules | Year: 2013

Here I describe accurate and time-efficient computational methods for designing small molecule-sensing allosteric ribozymes that serve as logic gates with NOT or YES Boolean logic functions. Theophylline-sensing ribozymes are engineered to have a high cleavage rate of 1.3 min-1 under physiologically relevant conditions. They are highly specific to theophylline and do not respond to caffeine, which differs in a single methyl group. These ribozymes are designed by fusing a theophylline aptamer with an extended version of the hammerhead ribozyme by modeling secondary structures. Purine-sensing ribozymes are designed by fusing the minimal version of the hammerhead ribozyme with bacterial guanine or adenine aptamers by modeling 3D interactions. I have developed high-throughput compatible arrays based on purine RNA sensors that can be used for antibacterial drug discovery. The ribozymes can be employed as molecular sensors in various applications, including exogenous control of gene expression, high-throughput screening arrays, and molecular computing. © 2013 American Chemical Society.


Penchovsky R.,Sofia University
Biotechnology Advances | Year: 2014

Nucleic acids have proven to be a very suitable medium for engineering various nanostructures and devices. While synthetic DNAs are commonly used for self-assembly of nanostructures and devices in vitro, functional RNAs, such as ribozymes, are employed both in vitro and in vivo. Allosteric ribozymes have applications in molecular computing, biosensoring, high-throughput screening arrays, exogenous control of gene expression, and others. They switch on and off their catalytic function as a result of a conformational change induced by ligand binding. Designer ribozymes are engineered to respond to different effectors by in vitro selection, rational and computational design methods. Here, I present diverse computational methods for designing allosteric ribozymes with various logic functions that sense oligonucleotides or small molecules. These methods yield the desired ribozyme sequences within minutes in contrast to the in vitro selection methods, which require weeks. Methods for synthesis and biochemical testing of ribozymes are also discussed. © 2014 Elsevier Inc.


Yazadjiev S.S.,Sofia University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

Adopting the phantom (ghost) scalar field description of dark energy, we construct a general class of exact interior solutions describing mixed relativistic stars containing both ordinary matter and dark energy in different proportions. The exterior solution that continuously matches the interior solutions is also found. Exact solutions describing extremal configurations with zero ordinary matter pressure are also constructed. © 2011 American Physical Society.


Penchovsky R.,Sofia University
Lab on a Chip - Miniaturisation for Chemistry and Biology | Year: 2013

Advances in modern genomic research depend heavily on applications of various devices for automated high- or ultra-throughput arrays. Micro- and nanofluidics offer possibilities for miniaturization and integration of many different arrays onto a single device. Therefore, such devices are becoming a platform of choice for developing analytical instruments for modern biotechnology. This paper presents an implementation of a bead-based microfluidic platform for fully automated and programmable DNA microarrays. The devices are designed to work under isothermal conditions as DNA immobilization and hybridization transfer are performed under steady temperature using reversible pH alterations of reaction solutions. This offers the possibility for integration of more selection modules onto a single chip compared to maintaining a temperature gradient. This novel technology allows integration of many modules on a single reusable chip reducing the application cost. The method takes advantage of demonstrated high-speed DNA hybridization kinetics and denaturation on beads under flow conditions, high-fidelity of DNA hybridization, and small sample volumes are needed. The microfluidic devices are applied for a single nucleotide polymorphism analysis and DNA sequencing by synthesis without the need for fluorescent removal step. Apart from that, the microfluidic platform presented is applicable to many areas of modern biotechnology, including biosensor devices, DNA hybridization microarrays, molecular computation, on-chip nucleic acid selection, high-throughput screening of chemical libraries for drug discovery. © 2013 The Royal Society of Chemistry.

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