Current research

The principal research objective of the condensed-matter theory group is a reliable and controllable microscopic description of electronic and atomic properties of solids, in particular of complex systems with nontrivial elementary cells, broken symmetries, in and out of thermodynamic equilibrium or in extreme conditions. Based on the research experience and expertise of the research personnel, advanced modern methods from quantum equilibrium and non-equilibrium statistical mechanics, Green-function formalism, electronic structure calculations, and numerical simulations are developed, improved and employed to determine qualitative model and quantitative realistic materials properties.

Electronic structure of complex materials with unusual properties is studied with TB-LMTO, FP-LAPW, and pseudopotential ab-initio density-functional methods. The calculations are used to predict new materials with potential applications. Relativistic effects and correlation-induced dynamical fluctuations in the electronic structure of alloys, surfaces, interfaces, interlayers and multilayers are taken into account. Real-space finite-element method applicable to nonperiodic structures is being developed.
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Dilute magnetic semiconductors are prototype materials with structural, magnetic, transport, and optical properties determined by combined effects of disorder, electron correlations, and hybridization. Our theoretical study, based on density-functional theory, aims at facilitating magnetic behavior of DMS by optimizing the electronic structure and by controlling native defects and intentional co-doping. Starting from (Ga,Mn)As, we are interested in non-traditional DMS based, e.g., on mixed III-V semiconductors and I-II-V compounds.
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A mapping of the total energy of electrons in magnetic materials and alloys on Heisenberg and Ising-like models is utilized to describe bulk and surface magnetic ordering. The theory is applied to dilute metallic and semiconducting magnetic materials, phase diagrams of layers of alloys, and theoretical explanation of ferromagnetism of thin films on a non-magnetic substrate.
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Electron correlations are studied in tight-binding models with a screened local interaction of the Hubbard type. Advanced summation methods of Feynman diagrams for two-particle Green functions are the core of our interest. Various simplified forms of parquet equations mixing self-consistently multiple electron-electron and electron-hole scatterings are used to describe peculiarities of the strong-coupling regime.
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Low-temperature quantum critical behavior where two or more noncommuting operators remain relevant are studied with many-body diagrammatic techniques. The objective is to understand and quantitatively describe admissible types of the critical behavior of vertex functions in interacting and disordered itinerant systems.
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Quantum coherence due to cooperative scatterings is studied. Of particular interest are effects of backscatterings on bulk transport properties of electrons in disordered media.
Effects of decoherence and dephasing of electrons interacting with optical phonons in quantum dots out of equilibrium are examined with non-equlibrium Green functions and transport equations.
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Time-resolved spectroscopy is another domain of quantum coherence. Interband excitation of electrons by strong femtosecond pulses and immediate electron response and relaxation are studied theoretically for the case of disordered semiconductors. The results, showing a competition between light and disorder scattering, represent not only a simple but representative simulation of the behavior of interacting systems, but also serve as a basic reference for testing transport equations.
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Structural properties and mechanism of formation of surface nanostructure of metals, semiconductors, and multi-component materials are studied by numerical simulations. Molecular dynamics and Monte-Carlo simulations are combined to achieve a multiscale description of the critical behavior of relevant statistical mechanical quantities.
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Theory of random systems, in particular spin glasses, is pursued. Microscopic approaches bases on the real-replica method combined with TAP equations are used to understand the mean-field theory of spin glasses.
In nonequilibrium systems dynamical fluctuations in irreversible processes, variational formulation of nonequilibrium steady states and nonequilibrium thermodynamics beyond linear response are investigated.
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Evolutionary games with particular emphasis on the Minority Game and its variants are investigated. Development of non-trivial spatial structures and dynamical spin-glass like phases there are studied with predominantly numerical simulations. Also analytical calculations using the replica method have been initiated. Beyond this, the so-called Maslov model for stock-market fluctuations is currently scrutinized.
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