Themes for PhD. students in the acad. year 2015/2016

Theme: Theoretical study of the frustrated spin model on a bipartite lattice.
Supervisor: prof. RNDr. Andrej Bobák, DrSc.
Anotácia: Generally, the frustration is generated by the competition of different kinds of interaction and/or by the lattice geometry. As a result, in the ground state all bonds are not fully satisfied. Well-known example is the Ising model on the antiferromagnetic triangular lattice. On the other hand, the honeycomb lattice is a derivative of the triangular lattice and is obtained by periodically removing 1/3 of the spins from a triangular lattice. Since a honeycomb lattice antiferromagnet with only nearest-neighbor exchange interactions is considered as a bipartite lattice, the ground state exhibits long-range ordering. The system becomes frustrated like the square lattice, if the next-nearest-neighbor exchange interactions are considered. Hence, it is interesting to study the magnetic properties on the honeycomb lattice under frustrating interactions, which are still not well understood at present.

Theme: Influence of an external pressure on magnetic properties of crystalline systems
Supervisor: prof. RNDr. Michal Jaščur, CSc.
Anotácia: The main aim of this investigation will be to analyze the influence of an external pressure on magnetic properties of crystalline systems. Using the methods of statistical physics, we intend to study in detail the behavior of the localized magnetic systems described by Ising and Heisenberg models in the presence of external pressure and magnetic field. Excepting of the analysis of phase diagrams of various physically interesting systems we will also investigate variations of all relevant physical quantities, such as the magnetization, internal energy, specific heat, free energy, susceptibility, entropy, thermal expansion coefficient, isothermal and adiabatic compressibilities.

Theme: Renormalization group study of quantum phase transitions in static and dynamic models with different symmetry types
Supervisor: prof. RNDr. Michal Hnatič, DrSc.
Anotácia: Quantum description of phase transitions is reduced to the quantum field theories for order parameter with various tensor structure. Recently, it has been found that the well-known renormalization group (RG) analysis does not cover all possible symmetry of physical models. Focusing on the description of quantum phase transitions, such as superconductivity and superfluidity, we plan to develop RG approach for new types of quantum field theories. It is planned to study various types of the phase transitions in two-dimensional and three-dimensional models, also main characteristics of phase transitions and the Goldstone singularities in them. The roles of the tensor structure and dimension of the order parameter on the critical behavior of the system will be analyzed. In the framework of the RG method the dynamic critical phenomena in these systems and stochastic equations with different symmetry will be considered. It is also planned to consider an influence of turbulent mixing of passive field on the critical behavior of models. We will study analytic properties of the quantum field expansions in static and dynamic models by instanton analysis.

Theme: Frustrated generalized planar rotator model
Supervisor: doc. RNDr. Milan Žukovič, PhD.
Anotácia: It is well known that the 2D XY model exhibits an unusual infinite order phase transition belonging to the Kosterlitz-Thouless (KT) universality class. Introduction of a nematic coupling into the XY Hamiltonian leads to an additional phase transition in the Ising universality class. It has been shown that the higher order harmonics lead to a qualitatively different phase diagram, with additional ordered phases originating from the competition between the ferromagnetic and pseudonematic couplings. The new phase transitions belong to the 2D Potts, Ising, or KT universality classes. The present study will investigate effects of geometrical frustration on the triangular lattice model with antiferromagnetic couplings, which shows an additional chiral phase above the KT line, as well as the presence of quenched nonmagnetic vacancies on the critical properties.​

Theme: Spin models with correlated disorder
Supervisor: doc. RNDr. Milan Žukovič, PhD.
Anotácia: The random-field and random-bond Ising magnets are prototypical model magnetic systems with quenched disorder. Typically, when such systems are studied, the quenched disorder is considered uncorrelated. Nevertheless, real systems are always emerging from physical processes, hence, correlations are present, which could play an important role in the models behavior. The present study will consider spatially correlated random field with a flexible Matern family of correlation functions and explore their influence on the models dynamics and their critical behavior.

Theme: Rigorous investigation of magnetization processes in classical and quantum spin models
Supervisor: doc. RNDr. Jozef Strečka, PhD.
Anotácia: Magnetization processes of low-dimensional classical and quantum spin models may exhibit several outstanding macroscopic features including fractional magnetization plateaus, magnetization jumps, discontinuous or continuous phase transitions at finite and zero temperature. The microscopic origin of those remarkable cooperative phenomena will be elucidated with the help of rigorously solved Ising, Ising-Heisenberg and Heisenberg spin models. It is intended that the obtained exact solutions will also serve for an interpretation of available experimental magnetization data of bimetallic coordination compounds, which contain trivalent rare-earth ions and divalent transition-metal ions.

Theme: Quantum entanglement in unconventional states of exactly solved models
Supervisor: doc. RNDr. Jozef Strečka, PhD.
Anotácia: Quantum entanglement belongs to the most intriguing consequences of the quantum mechanics, which is highly contra-intuitive and remains at a forefront of the current research interest. Among the most challenging issues related to this subject matter is to investigate the thermal entanglement through different measures (e.g. concurrence, fidelity, etc.), which quantify how the quantum entanglement evolves with rising temperature. The main focus of this dissertation thesis is to examine different measures of the thermal entanglement in the exactly solved lattice-statistical spin models. In particular, the considerable attention will be devoted to an open questions whether or not the thermal entanglement may coexist with a spontaneous long-range order and how it changes across quantum critical points.

Theme: Exactly solvable models of correlated spin and electron systems
Supervisor: doc. RNDr. Jozef Strečka, PhD.
Anotácia:The dissertation thesis is focused on theoretical investigation of various collective phenomena in the exactly solved correlated spin and electron systems. In particular, the main attention will be aimed at the exactly solvable spin-electron models, in which finite cluster of electrons are merely coupled to few localized Ising spins placed at lattice sites of one- or two-dimensional regular lattices. The results obtained by this study will be used for an explanation of the quantum origin of striking fractional magnetization plateaus and the enhanced magnetocaloric effect occurring during the adiabatic demagnetization.

Theme: Theoretical investigation of phase separation in binary and ternary liquid mixtures
Supervisor: doc. RNDr. Jozef Strečka, PhD.
Anotácia: The phase separation of binary and ternary liquid mixtures will be investigated within the framework of the lattice-statistical models such as the generalized Frenkel-Louis and Lin-Taylor models. In particular, the main emphasis will be laid by accounting for the multiparticle (e.g. three-body) interactions, which may play a crucial role in determining possible non-universal critical behavior. Besides, the dissertation thesis will focus on a problem of reentrant miscibility of some binary and ternary liquid mixtures with highly orientation-dependent forces (e.g. hydrogen bonding).

Theme: Circumbinary planets and brown dwarfs
Supervisor: doc. Mgr. Štefan Parimucha, PhD.
Anotácia: More than 60 double and multiple systems are known up to now, where extrasolar planets and brown dwarfs are detected. The architecture of these systems is various, from close binaries to wide systems. Analysis of (O-C) diagrams of eclipsing binaries can be used for their detection, when from periodic variations in diagrams we can reveal stellar and/or sub-stellar objects.

Theme: Štúdium vlastností interagujúcich dvojhviezd
Supervisor: doc. Mgr. Štefan Parimucha, PhD.
Anotácia: Interacting binaries are large group of objects, where strong interaction between the components is presented. This interaction is reflected in the changes of their brightness as well as in their spectra. Detailed study of these changes in different spectral regions, from the infrared to the x-ray reveals the physical nature of the changes of the stars themselves, but also of circumstellar matter.

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