Session NA: Condensed Matter Physics III

Influence of Confinement on Dynamical Heterogeneities in Dense Colloidal Samples

We study a colloidal suspension confined between two parallel walls as a model system for glass transitions in confined geometries. The suspension is a mixture of two particle sizes to prevent wall-induced crystallization. We use confocal microscopy to directly observe the motion of the colloidal particles. This motion is slower in confinement, thus producing glassy behavior in a sample which is a liquid in an unconfined geometry. Like particles in an unconfined near-glassy system, groups of particles in our confined system move together cooperatively. Normally these groups would be spatially isotropic. However, the confining boundaries induce a layering of the particles. We show that the layering modifies the shapes of the mobile groups within the sample so that they are planar. We investigate how the planar restriction of the shapes of the mobile groups may be the cause of the sample's glassy behavior.   

Deformation of Quasi-2D Oil-in-Water Emulsions

We create a quasi-2D nearly frictionless granular system, analogous to 2D granular systems of photoelastic disks but without static friction. To do this, we confine an oil-in-water emulsion between two glass plates such that the gap between the plates is smaller than the undeformed oil droplet diameter. For a range of droplet area fractions and plate separations, we observe the deformations the oil droplets experience due to contact with each other. The deformation of the droplet is correlated to the force its neighbors exert on it. As area fraction increases, the deformation of the droplets increases. By looking at the pattern of deformations throughout the system we can visualize the location of force networks due to droplet-droplet interactions.   

The equilibrium colloidal crystal/colloidal liquid interface

We use confocal microscopy to study an equilibrated crystal-liquid interface in a colloidal suspension. The surface shows spatial fluctuations due to capillary waves. Local measurements of the structure and dynamics near the rough surface reveal that the intrinsic surface, while meandering in space, is locally sharply defined. Examining different quantities finds slightly different widths of this intrinsic surface. In terms of the particle diameter $d$, this width is either $1.3d$ (based on structural information) or $2.4d$ (based on dynamics), both not much larger than the particle size.   

Random Close Packing of Disks and Spheres in Confined Geometries

We study the structure of many simulated random closing packings confined between two walls. Each packing consists of a binary mixture in equal number with a sizes ratio of 1.4. Our aim is to quantify how a confining boundary and the thickness between the boundaries alters the structure of randomly close packed disks in 2D and spheres in 3D. We find that confinement lowers the packing fraction, and induces heterogeneity in particle density where particles show strong layering near the wall. Both the particle density and the structure of the local packing show oscillations that decay outward from the wall. The decay in the oscillations is rapid, with a characteristic length scale on the order of the largest particle diameter. We invoke a simple model to describe the decrease in packing fraction with confinement.   

Shear Induced Structural Relaxation in a Supercooled Colloidal Liquid

Amorphous materials include many common products we use everyday, such as window glass, moisturizer, shaving cream and peanut butter. These materials have liquid-like disordered structure, but keep their shapes like a solid. The rheology of dense amorphous materials under large shear strain is not fully understood, partly due to the difficulty of directly viewing the microscopic details of such materials. We use a colloidal suspension to simulate amorphous materials, and study the shear- induced structural relaxation with fast confocal microscopy. We quantify the plastic rearrangements of the particles using standard analysis techniques based on the motion of the particles.   

Long-wavelength density fluctuations in random close packing

Long-wavelength density fluctuations in dense amorphous systems are both of scientific interest and also industrial relevance. These fluctuations relate to shear instabilities and the cracking of glassy materials. Some simulations investigated the random close packing of monodisperse hard spheres and suggested that there are few long-wavelength density fluctuations and thus the system is hyper-uniform. To check this suggestion, we study the random packing of colloidal particles using confocal microscopy. We take very large images (0.5 mm * 0.5 mm * 0.03 mm) at high resolution (less than 0.5 microns) by connecting overlapping microscope images of small regions, allowing us to investigate the density at large length scales. We find that the system is not hyper-uniform and long density fluctuations exist.   

Finite-temperature Quantum Monte Carlo study of a molecular spin ladder

Materials with \textit{ladder} crystal structures consist of two or more coupled one dimensional (1D) chains of atoms or molecules. An example is $(DTTTF)_2M(mnt)_2$, which is a two-leg molecular ladder material that is very similar to many of the molecular \textit{organic} superconductors. These ladder systems have several interesting properties that occur as the temperature(T) is lowered including a spin gap and structural distortions. Furthermore, small physical or chemical changes can lead to a large change in the T-dependence of their electronic properties. Two different theoretical models have been presented to understand these materials, the rectangular and zigzag lattices. To understand the magnetic and charge response functions of these two possible models, we have performed T-dependent Quantum Monte Carlo calculations that incorporate electron-electron interactions need to correctly generate the spin gap. Charge, spin and bond order susceptibilities of an extended Hubbard model with on-site Coulomb repulsion U at quarter filling were calculated for several different model lattice sizes. We investigate the charge ordering and bond distortion patterns within this model.   

A totally asymmetric exclusion process with hierarchical long range connections

A non-equilibrium particle transport model, the totally asymmetric exclusion process, is studied on a one-dimensional lattice with a hierarchy of fixed long-range connections\footnote{Journal of Statistical Mechanics, P07010 (2009).}. This model breaks the particle-hole symmetry observed on an ordinary one-dimensional lattice and results in a surprisingly simple phase diagram, without a maximum-current phase. Numerical simulations of the model with open boundary conditions reveal a number of dynamic features and suggest possible applications.   

The Peculiar Phase Transitions of the Ising Model on a Small-World Network

To describe many collective phenomena on networks, the Ising model again plays a fundamental role. Here, we study a new network with small-world properties that can be studied exactly with the renormalization group. The network is non-planar and has a recursive design combining a one-dimensional backbone with a hierarchy of long-range bonds. Varying the relative strength between nearest-neighbor and long-range bonds, we can define a one-parameter family of models that exhibits a rich variety of critical phenomena, quite distinct from those on lattice models. Exact results and numerical simulations reveal this behavior in great detail.   

How to use 100,000 PCs for studying magnetism

``Public resource computing'' refers to the use of volunteered processing time on (otherwise idle) computer processors at remote locations. We have recently incorporated the use of public resource computing into quantum mechanical calculations that are relevant to the study of magnetic molecules. Specifically, these calculations make use of a ``quantum Monte Carlo'' method, allowing the simulation of systems that are much more complex than those that can be studied using more conventional techniques. In this talk, we will introduce the basic principles of both the calculation method and the use of public resource computing.