arXiv subjects
Publications and source records attributed to Kok-Kiong Loh.
We study a system with a weak, long-range repulsive Kac-type step-function interaction within the framework of a replicated effective $ϕ^4$-theory. The occurrence of extensive configurational entropy, or an exponentially large number of metastable minima in the free energy (characteristic of a glassy state), is demonstrated. The underlying mechanism of mesoscopic patterning and defect organizations is discussed.
We propose a model to understand periodic oscillations relevant to the origin of mesoscopic channels formed during a Langmuir-Blodgett deposition observed in recent experiments \{M. Gleiche, L.F. Chi, and H. Fuchs, Nature {\bf 403}, 173 (2000)\}. We numerically study one-dimensional flow of a van der Waals fluid near its discontinuous liquid-gas transition and find that steady-state flow becomes unstable in the vicinity of the phase transition. Instabilities leading to complex periodic density-oscillations are demonstrated at some suitably chosen sets of parameters.
The textures associated with a point defect centered in a circular domain of a thin film with XY-like ordering have been analyzed. The family of equilibrium textures, both stable and metastable, can be classified by a new radial topological number in addition to the winding number of the defect. Chiral textures are supported in an achiral system as a result of spontaneously broken chiral symmetry. Among these chiral textures, our theoretical analysis accurately describes two categories of recently discovered ``reversing spiral'' textures, ones that are energetically stable and metastable.
A numerical algorithm based on the finite element methods has been developed to accurately determine the shape of the boundary of a domain containing ``boojum'' textures. Within the context of the simple model we adopt, the effects of both bulk elastic anisotropy and line-tension anisotropy on the domain boundary can be examined. It is found that line-tension anisotropy must be present in order to account for domains with protruding features. Both elastic anisotropy and anisotropic line-tension can result in domains with indentations. The numerical algorithm has been extended to investigate the problem of a bubble in extended region ordered phase.
Domains and bubbles in tilted phases of Langmuir monolayers contain a class of textures knows as boojums. The boundaries of such domains and bubbles may display either cusp-like features or indentations. We derive analytic expressions for the textures within domains and surrounding bubbles, and for the shapes of the boundaries of these regions. The derivation is perturbative in the deviation of the bounding curve from a circle. This method is not expected to be accurate when the boundary suffers large distortions, but it does provide important clues with regard to the influence of various energetic terms on the order-parameter texture and the shape of the domain or bubble bounding curve. We also look into the effects of thermal fluctuations, which include a sample-size-dependent effective line tension.
Domains in Langmuir monolayers support a texture that is the two-dimensional version of the feature known as a boojum. Such a texture has a quantifiable effect on the shape of the domain with which it is associated. The most noticeable consequence is a cusp-like feature on the domain boundary. We report the results of an experimental and theoretical investigation of the shape of a domain in a Langmuir monolayer. A further aspect of the investigation is the study of the shape of a ``bubble'' of gas-like phase in such a monolayer. This structure supports a texture having the form of an inverse boojum. The distortion of a bubble resulting from this texture is also studied. The correspondence between theory and experiment, while not perfect, indicates that a qualitative understanding of the relationship between textures and domain shapes has been achieved.
Two-dimensional domains containing an XY-like order parameter exhibit non-trivial internal structure and take on shapes controlled by the configuration that the order parameter adopts. The textures exhibited by the order parameter in such domains are controlled by the interplay between bulk and surface contributions to the energy. We report calculations of the internal texture and the shape of such domains. These calculations lead to the determination of the equilibrium properties of two-dimensional domains, such as those observed in Langmuir monolayers. This allows for the unambiguous exploration of the implications of experimental findings.