Gotham-Metro Condensed Matter Meeting

Abstracts

Soft Condensed Matter Keynote Presentation

Colloids and Clusters: Watching Self-assembly in the Simplest Complex Systems
Vinothan Manoharan, PhD, Harvard University

Self-assembly refers to any thermodynamic process in which a bunch of particles (molecules, biomolecules, polymers, colloids) come together in solution to form an ordered structure. In living things it is a widely used and robust manufacturing method: DNA, RNA and proteins spontaneously form three dimensional structures with a high degree of order and specificity. By contrast, most synthetic systems in soft condensed matter do not assemble robustly. To better understand the physics of self-assembly, we use a variety of optical imaging techniques to directly observe the assembly of small clusters of spherical colloidal particles. When the particles interact through a non-specific depletion interaction, we find that the number of self-assembled configurations increases exponentially with the number of particles, while the most favorable states are those with the lowest symmetry. With specific DNA-mediated attractions, the number of states is sharply reduced. Experiments and theoretical calculations suggest that it is possible to robustly direct the assembly of specific structures through multiple competing DNA-mediated interactions.

 

Hard Condensed Matter Keynote Presentation

Topological Insulators and Topological Superconductors
Shoucheng Zhang, PhD, Stanford University

Recently, a new class of topological states has been theoretically predicted and experimentally observed. The topological insulators have an insulating gap in the bulk, but have topologically protected edge or surface states due to the time reversal symmetry. Similarly, topological superconductors or superfluids have novel edge or surface states consisting of Majorana fermions. In this talk, I shall review the recent theoretical and experimental progress in the field, and focus on a number of outstanding issues, including the quantized anomalous Hall effect, quantized magneto-electric effect, the topological Mott insulators and the search for topological superconductors.

 

Short Talks

Fluctuation Relations for Current Components in Open Electric Circuits
Sriram Ganeshan, Stony Brook University

We present a new class of fluctuation relations for currents through specific components of mesoscopic electric circuits, to which we will refer to as Fluctuation Relations for Current Components (FRCCs). FRCCs can be used to estimate system parameters when complete information about nonequilibrium many-body electron interactions is unavailable. We show that FRCCs are often robust in the sense that they do not depend on some basic types of electron interactions and some quantum coherence effects.
 

Janus Dumbbells Create High-strength Oil / Water Interfaces
Guy German, Department of Mechanical Engineering and Materials Science, Yale University

We describe a scalable high-yield bulk synthesis of sub-micron uniform polymer colloids with a dumbbell shape, where the wetting properties of each lobe can be tuned independently. We explore the contributions of shape and wetting asymmetry to the spreading of particles at neat oil–water interfaces and the strength of particle-laden interfaces. Using measured microscopic wetting properties and the geometry of the particles, we calculate the binding energy of individual particles at neat interfaces and the change in interfacial free energy due to densely packed adsorbed monolayers. For spherical particles and dumbbell particles that adsorb with their long-axis in the plane of the interface, we find that the interfacial strength is proportional to the change in the theoretically expected change in the interfacial free energy with particle adsorption. For dumbbell particles that pack densely with their long axis perpendicular to the interface, the interface strength is about three times higher. We hypothesize that this strengthening of the interface is due to a high bending rigidity created when particles have multiple contacts with each neighbour.
 
Coauthors:  Jin Nam², Jason D. Forster¹, T. Kyle Vanderlick², and Eric R. Dufresne^1,2,3,4.
 
1. Department of Mechanical Engineering and Materials Science, Yale University.
2. Department of Chemical and Environmental Engineering, Yale University.
3. Department of Physics, Yale University.
4. Department of Cell Biology, Yale University.
 

Encapsulation by Janus Oblate Spheroids
Wei Li, Lehigh University

The micro/nanoencapsulation technology has received considerable attention recently in the fields of drug delivery, biomaterial engineering, and material science. Based on recent advances in chemical particle synthesis, we propose a preliminary model of an encapsulation system based on the self-assembly of Janus oblate ellipsoids. The two semi-surfaces of these ellipsoids are assumed to have different chemical compositions, so as to be amphiphilic. In addition to these ellipsoids, the system consists of spherical particles representing the particles to be encapsulated. Using Monte Carlo simulation, we investigate the encapsulation process, focusing on the morphology and fraction of spherical particles that become encapsulated (the efficiency of encapsulation). We find relatively high encapsulation efficiency for our model and believe this method of encapsulation is of potential value in practical applications.
 
Coauthors: Ya Liu and J. D. Gunton, Lehigh University; Genevieve Brett, Skidmore College.
 

Switching Field Distributions for a Co-Ni Nanomagnet Under Spin-transfer Torques
Dan Gopman, New York University

Magnetization reversal in thin ferromagnetic films has been extensively studied for a better fundamental understanding of reversal and for optimization of the energy barriers for a new generation of robust magnetic information storage applications, such as magnetic random access memories (MRAM). Spin-valve nanopillars consisting of two ferromagnetic layers with perpendicular magnetization separated by a non-magnetic spacer are of particular importance to MRAM applications. The all-perpendicular geometry yields high thermal stability and permits switching the magnetization of a nanomagnet under spin-polarized electrical currents by the spin-transfer torque (STT) effect. A simple thermal activation model [1] can describe field-induced magnetization reversal, but its applicability in the presence of STTs is uncertain, because a direct current can drive a nanomagnet magnetization out of equilibrium [2]. Nonetheless, a basic model of spin-transfer induced switching predicts that the direct current will only modify the energy landscape for magnetization reversal, leading to a current-dependent effective energy barrier for thermally induced transitions [3]. I will present the Switching Field Distributions (SFD) for a Co/Ni uniaxial nanomagnet under the influence of thermal fluctuations and STT. SFDs are a robust method to examine the thermally activated magnetization reversal process and describe the rate of thermal activation of a nanomagnet's direction of magnetization away from an initial equilibrium state even in the presence of STT. Our results indicate that STT alters the energy landscape of a nanomagnet and could be utilized to stabilize a particular magnetic configuration of a spin-valve nanopillar, which should be considered in future MRAM technologies.
 

Thermopower Near the 2D Metal-Insulator Transition
Shiqi Li, City College of New York

Scaling theory of non-interacting, disordered electron gases, as well as theory for weakly interacting electrons, predict that no metal-insulator transition (MIT) occurs in two dimensions as the temperature T goes to 0. Based largely on transport measurements (resistivity, magnetoresistance, Hall coefficient), a MIT has been claimed in dilute 2D electron systems where the interactions are strong, a claim that continues to be debated. We have recently measured the thermoelectric power in the low-temperature diffusive regime in a strongly-interacting 2D electron system in silicon. We find that with decreasing electron density, the thermopower tends to infinitely at a density nt that is independent of disorder. The fact that the density nt at which it diverges does not depend on disorder implies that the thermopower is a more intrinsic property than the resistivity. Our results therefore provide stronger evidence for the occurrence of this much-debated transition.
 
Coauthors: Anish Mokashi and S. V. Kravchenko, Northeastern University, Boston; A. A. Shashkin, Institute of Solid State Physics, Chernogolovka, Russia; M. P. Sarachik, City College of New York.
 

Visualizing Individual Nitrogen Dopants in Monolayer Graphene
Liuyan Zhao, Columbia University

Substitutional doping with Nitrogen atoms in monolayer graphene is a promising way to alter its electronic properties. We introduced Nitrogen dopants into graphene matrix during CVD growth process and characterized individual Nitrogen dopant in monolayer graphene films with Scanning Tunneling Microscopy (STM). Individual Nitrogen dopant incorporates into graphene lattice mainly via graphitic form, and a fraction (~0.5) of the extra electron from a Nitrogen atom is delocalized into graphene lattice. Moreover, the electronic structure of N-doped graphene is strongly modified within only a few atomic spacings from Nitrogen sites. Thus, graphitic doping with Nitrogen dopants produces high-quality graphene with a large electron carrier concentration.
 
Coauthors: Rui He¹, KwangTaeg Rim1, Theanne Schiros¹, KeunSoo Kim^1,4, Hui Zhou¹, Christopher Gutiérrez¹, S. P. Chockalingam¹, Carlos J. Arguello¹, Lucia Pálová¹, Dennis Nordlund², Mark S. Hybertsen³, David R. Reichman¹, Tony F. Heinz¹, Philip Kim¹, Aron Pinczuk¹, George W. Flynn¹, and Abhay N. Pasupathy¹.
 
1. Columbia University.
2. Stanford Synchrotron Radiation Laboratory.
3. Center for Functional Nanomaterials, Brookhaven National Laboratory.
4. Sejong University, Seoul.