KOSEN 한인과학기술자네트워크

Research on condensed matter at KNU includes both "hard" and "soft" condensed matter physics. "Hard" matter refers to matter whose behavior is determined by interatomic forces, and quantum mechanics plays a prominent role. The study of crystals, metals, semiconductors, and superconductors falls in this category. "Soft" matter refers to materials whose behavior is intermediate between that of simple liquids and crystalline solids. Examples include polymers, colloids, liquid crystals, surfactants and most materials of biological origin. Living organisms are made of soft matter. These materials are literally "softer," exhibiting large responses to weak forces and being very sensitive to the geometry of the boundaries, thermal fluctuations, or the action of external fields. Contrary to "hard" matter where the macroscopic behavior is primarily determined by the energy of its elementary building blocks, in "soft" matter entropy plays an equally, if not more, important role as energy. In order to understand the structure and elasticity of soft/biological matter as a collective behavior on the basis of the geometry and interactions of its elementary constituents, we use the tools of statistical mechanics.

The current focus of research in my lab is the study of semiflexible polymers. Some important biopolymers, including DNA and the structural elements of the cytoskeleton (F-actin, microtubules, intermediate filaments) are semiflexible, that is, on a certain length scale, they behave as thermally fluctuating slender rods. This semiflexibility makes them markedly different from their flexible counterparts. The latter, which include most synthetic polymers (e.g., polystyrene), can be modeled as paths of random walkers (ideal coils). Semiflexibility, which is very important for their biological function, poses many challenges in their theoretical understanding. We study both single-molecule phenomena (e.g., force-extension relations) and the properties of assemblies of many polymers. An important part of our research is the theoretical analysis of networks of cross-linked polymers. We treat the constraints imposed by permanent random cross-links as quenched disorder (that is, disorder  frozen in time, which is markedly distinct from thermal disorder) using the tools of the statistical physics of disordered systems (replica field theory). Another topic of current interest is the emergence of order (self-organization) in brushes of semiflexible polymers with attractive interactions. The interplay between the opposing tendencies of the attraction to bring the chains closer together and of the permanent grafting to keep them in place results in the emergence of bundled states.

An area of of Physics where "soft" matter meets "hard" matter is the so-called vortex matter in type-II superconductors which here at KNU is studied experimentally in the "hard" condensed matter group. Vortex matter in high-Tc superconductors (flux-line states) can be viewed as soft matter embedded in hard matter. It is "soft" because it has rather small elastic moduli, it is very sensitive to thermal fluctuations (the Abrikosov lattice can undergo a melting transition), and quenched disorder can have a profound effect on its behavior yielding various glassy states. My background in condensed matter theory originated in the study of vortex matter and I still maintain an interest in it.