Todor L. Boyadjiev

Antichlamydial and antisperm antibodies in patients with chlamydial infections.

Problem:  Establishing the correlation between antichlamydial antibodies (AchAbs) and antisperm antibodies (ASA) in patients with chlamydial infections.

ELECTRICALLY CHARGED EINSTEIN–BORN–INFELD BLACK HOLES WITH MASSIVE DILATON

We numerically construct static and spherically symmetric electrically charged black hole solutions in Einstein–Born–Infeld gravity with massive dilaton. The numerical solutions show that the dilaton potential allows many more black hole causal structures than the massless dilaton. We find that depending on the black hole mass and charge and the dilaton mass, the black holes can have either one, two, or three horizons. The extremal solutions are also found. As an interesting peculiarity we note that there are extremal black holes with an inner horizon and with triply degenerated horizon.

Mathematical Modeling of Boson-Fermion Stars in the Generalized Scalar-Tensor Theories of Gravity

A model of static boson–fermion stars with spherical symmetry based on the scalar–tensor theory of gravity with a massive dilaton field is investigated numerically. Since the radius of the star is a priori an unknown quantity, the corresponding boundary value problem is treated as a nonlinear spectral problem with a free internal boundary. The continuous analogue of Newton method is used to solve this problem. Information about basic geometric functions and the functions describing the matter fields which build the star is obtained. From a physical point of view the main result is that the structure and properties of the star in the presence of a massive dilaton field depend essentially on both its fermionic and bosonic comp-onents.

Boson stars in massive dilatonic gravity

We study equilibrium configurations of boson stars in the framework of a class scalar-tensor theories of gravity with massive gravitational scalar (dilaton). In particular we investigate the influence of the mass of the dilaton on the boson star structure. We find that the masses of the boson stars in presence of dilaton are close to those in general relativity and they are sensitive to the ratio of the boson mass to the dilaton mass within a typical few percent. It turns out also that the boson star structure is mainly sensitive to the mass term of the dilaton potential rather to the exact form of the potential.

Neutron star in the presence of a torsion-dilaton field

We develop the general theory of stars in Saa’s model of gravity with propagating torsion and study the basic stationary state of neutron star. Our numerical results show that the torsion force decreases the role of the gravity in the star configuration leading to significant changes in the neutron star masses depending on the equation of state of star matter. The inconsistency of the Saa’s model with Roll-Krotkov-Dicke and BraginskyPanov experiments is discussed. PACS number(s): 04.40.Dg,04.40.-b,04.50.+h

A numerical algorithm for modelling of boson-fermion stars in dilatonic gravity

We investigate numerically class of models of the static spherically symmetric boson-fermion stars in the scalar-tensor theory of gravity with massive dilaton field. The proper mathematical model of such stars is interpreted as a nonlinear two-parametric eigenvalue problem. The first of the parameters is the unknown internal boundary (the radius of the fermionic part of the star) Rs, and the second one represents the frequency Ω of the time oscillations of the bosonic field.To solve this problem, the whole space [0,∞) is splitted in two domains: internal [0,Rs] (inside the star) and external [Rs, ∞) (outside the star). In each domain the physical model leads to two nonlinear boundary value problems in respect of metric functions, the functions describing the fermionic and bosonic matter, and the dilaton field. These boundary value problems have different dimensions inside and outside the star, respectively. The solutions in these regions are obtained separately and matched using the necessary algebraic continuity conditions including Rs and Ω. The continuous analogue of Newton method for solving both the nonlinear differential and algebraic problems is used.The proposed method essentially differs from that one explained in our paper (J. Comput. Phys. 166 (2) (2001) 253) and ensures certain advantages. In this way, we obtain the behavior of the basic geometric quantities and functions describing a dilaton field and matter fields, which build the star.

Stability and Bifurcation of the Magnetic Flux Bound States in Stacked Josephson Junctions

The static distributions of the magnetic flux in stacked Josephson junctions are investigated numerically. To solve the nonlinear boundary value problem an iterative algorithm, based on the Continuous analog of Newton method is constructed. The linearized problems at every iteration step are solved by the Galerkin finite element method. In order to study the stability of possible distributions a Sturm-Liouville problem is generated. A minimal eigenvalue equal to zero means a bifurcation of the corresponding solution. The subspace iteration method is used to find the smallest eigenvalues and the corresponding eigenvectors.

Numerical Investigation of Josephson Junction Structures

Multilayered long Josephson Junction Structures form an interesting physical system where both nonlinearity and interaction between subsystems play an important role. Such systems allow to study physical effects that do not occur in single Josephson junction.The Sakai‐Bodin‐Pedersen model—a system of perturbed sine‐Gordon equations—is used to study the dynamic states of stacks of inductively coupled long Josephson Junctions (LJJs). The corresponding static problem is numerically investigated as well. In order to study the stability of possible static solutions a Sturm‐Liouville problem is generated and solved.The transitions from static to dynamic state and the scenario of these transitions are analyzed depending on the model parameters. Different physical characteristics—current‐voltage characteristics, individual instant voltages and internal magnetic fields, are calculated and interpreted.

Transitions from static to dynamic state in three stacked josephson junctions

Using the Sakai-Bodin-Pedersen model, a system of three perturbed sine-Gordon equations is numerically studied Effective numerical algorithms are proposed and realized to investigate the transitions from static to dynamic state in a stack of three Josephson junctions Critical currents for individual junctions are found for different values of the damping parameter at low magnetic field We show that the switching from static to dynamic state of the interior junction can trigger the switching of the exterior ones, and this process leads to current locking We find that the critical current of the individual junction depends on the damping parameter and on the static or dynamic states of the other junctions.

Numerical investigation of the magnetic flux static distributions in layered josephson junctions

Effective numerical algorithms are worked out for solving the nolinear system of ODE for finding the static distributions of the magnetic flux in N-stacked JJs, as well as the corresponding matrix Sturm-Liouville problem for studying their global stability. The particular case of three stacked JJs is investigated. A correspondence is made between loss of stability of a possible static distribution of the magnetic flux, obtained by solving the static problem, and the switching to dynamic state obtained by solving the dynamic problem. In this work we show by means of numerical simulation that the transient process of switching from static to dynamic state in symmetric three stacked JJs depends on the way of exceeding the external current.