Biomembranes, protein structure and folding

• Coarse-grained modeling of lipid bilayers
• Protein secondary structure
• Algorithms for global optimization of biomolecular systems

Within the research focusing on protein secondary structure formation, we have introduced the infinite helix as a valuable reference limiting case that can be used to critically assess, compare, and improve the parametrization of empirical force fields commonly used in biomolecular simulations. By comparing Density Functional Theory and force field results we also illustrate in a simple manner the importance of improving the description of the van der Waals interactions in both methodologies.

Physics of conventional and high-T_c superconductivity

• Pairing mechanisms
• Phenomenology of electrodynamic response
• Fluctuations

The discovery of high-temperature superconductivity in cuprates and the subsequent ``research rush'' have led to the appearance of about 100000 papers to date. Virtually every fundamental process known in condensed matter physics was probed as a possible mechanism of this phenomenon. Nevertheless, none of the theoretical efforts resulted in a coherent picture. For the conventional superconductors the mechanism was known to be the interaction between electrons and crystal-lattice vibrations, but the development of its theory lagged behind the experimental findings. The case of cuprate high-Tc superconductivity appears to be the opposite: we do not convincingly know which mechanism is to be incorporated in the traditional Bardeen-Cooper-Schrieffer (BCS) theory. Thus the path to high-Tc superconductivity in cuprates, perhaps carefully hidden or well-forgotten, has turned into one of the long-standing mysteries in physical science.

In contrast with all previous proposals, we have advanced the intra-atomic exchange of two electrons between the 4s and 3d_x^2-y^2 states of the Cu atom as the origin of high-Tc superconductivity in the layered cuprates and have shown that the basic spectroscopic and thermodynamic experiments can be explained by it.

Selected publications:

• Superconductivity of overdoped cuprates: the modern face of the ancestral two-electron exchange;
• Thermodynamics of anisotropic-gap and multiband clean BCS superconductors;
• Tight-binding modelling of the electronic band structure of layered superconducting perovskites

Semiconductor surfaces, crystal growth

• Self-assembled quantum dots
• Reconstructions of low-index III-V surfaces
• Surface chemical reactions

Self-organized quantum-dot heterostructures, or simply quantum dots (QDs), have left their “teens” behind. The high technological promises they have brought to optoelectronics have triggered an enormous scientific activity. As a representative example one can take the work on the InAs/GaAs lattice-mismatched heteroepitaxial system. In terms of the observed resultant morphology, the epitaxy of QDs [e.g., molecular beam epitaxy (MBE), or metal-organic chemical vapor deposition (MOCVD)] follows the Stranski-Krastanov growth mode. Though our knowledge on the nature of the Stranski- Krastanov regime has considerably increased, both experimentalists and theorists were still on the way to complete understanding the intricacies of QD growth kinetics. System-specific, microscopic information in this respect was a must. Important information comes from direct experimental probes, like in situ scanning tunnelling microscopy (STM), reflection high-energy electron diffraction (RHEED), or reflectance-difference spectroscopy (RDS). However, if one is aiming at a complete understanding of QD growth, the information accessible from experiments may not be sufficient. This is particularly true if one focuses on the characteristics of species dynamics at the surface, details of the surface atomic structure, thermodynamic quantities, etc.
We have addressed InAs heteroepitaxy on GaAs(001)—the III-V semiconductor system of ultimate importance for QD “self-fabrication”. An extensive track record of experimental studies has established a number of anomalies in the QD self-assembly for this system. It was also brought out clearly that any theory has to capture the subtleties of strain-dependent QD growth kinetics. Thus the first fundamental questions to be answered were how strain affects surface morphology, and its impact on surface adatom mobility. The characteristic length and time scales for the latter lie in the microscopic range, 0.1– 10 Angstroms and fs--ps, where the density-functional theory (DFT) is one of the most commonly used theoretical tools. We have developed an appropriate theoretical framework, building on DFT, to tackle the above mentioned problems in InAs/GaAs(001) heteroepitaxy.

Selected publications:

• First-principles study of InAs/GaAs(001) heteroepitaxy
• Atomic structure of the GaAs(001)-c(4x4) surface: First-principles evidence for diverstiy of heterodimer motifs
• Effect of strain on surface diffusion in semiconductor heteroepitaxy

Collaborations

• T. M. Mishonov Department of Physics, Sofia University, Bulgaria
• J. Ireta, Departamento de Quimica, Universidad Autonoma Metropolitana-Iztapalapa, Mexico

What others say about our research...

• "... A general approach to the tight-binding model for the electron spectrum is described in many works. For our purpose we will use a very clear description in Ref. [7] (it contains also the list of references to other works)."
  -- A. A. Abrikosov, Physica C 391,147-159 (2003)
• "... field dependent fluctuation susceptibility, was the subject of the intensive debates in early seventies [ 30--39 ] (see also the old but excellent review of W.J. Skocpol and M. Tinkham [ 40 ]) and after the discovery of HTS [ 41--44 ] ( see also very recent detailed essay of T.Mishonov and E.Penev [ 45 ] with references there)."
  -- A. I. Larkin and A. A. Varlamov, In Handbook on Superconductivity: Conventional and Unconventional Superconductors, edited by K.-H.Bennemann and J.B. Ketterson (Springer, 2002)