> College of Science > Physics Dept > Talks > Condensed Matter Seminars
Spring 2013 Condensed Matter Seminars

Seminars, held on Mondays, begin at 4:00 p.m. in the Robeson Room, 304
(unless otherwise indicated)

Refreshments are served at 3:30 p.m.


< Fall 2012 | January | February | March | April | May | Fall 2013 >

January
January 7
(poster)
Winter Break

January 14
(poster)

Special Seminar


Matthew D. Mower


University of Missouri, Columbia, Missouri

The effect of critical spin fluctuations on scattering and spin lifetimes in diluted magnetic semiconductors near the ferromagnetic transition

The s(p)-d spin exchange couplings between carriers and impurities in diluted magnetic semiconductors drive a ferromagnetic transition at the Curie temperature. The critical spin fluctuations near this transition lead to additional contributions to the resistivity of the material. We calculate the scattering rates of carriers in the presence of strong spin fluctuations on both the paramagnetic and ferromagnetic sides of the transition. Existing work on this topic typically focuses on the ferromagnetic side far from the transition.  We present a model of spin exchange mediated by dynamic spin fluctuations, calculated in the GW approximation. This produces a finite peak in the resistivity that is qualitatively accurate. We then use this model to calculate carrier spin lifetimes from the relevant spin relaxation mechanisms.

Host: Kyungwha Park

January 21
(poster)
Martin Luther King Holiday

January 28
(poster)

Ulrich Dobramysl


Department of Physics, Virginia Tech

Dynamics of Magnetic Flux Lines in Disordered Type-II Superconductors

Magnetic flux (vortex) lines in disordered type-II superconductors represent a complex non-equilibrium system, which is relevant for technological applications and comparatively easy to realize in experiments. We employ an elastic line model to investigate the steady-state properties and non-equilibrium relaxation kinetics of magnetic vortex lines using Langevin molecular dynamics. We study the influence of the type of disorder that is used for pinning vortex lines - either randomly distributed point-like or columnar attractive pinning centers. This allows us to distinguish the complex relaxation features of interacting flux lines subject to extended vs. uncorrelated disorder. Additionally, we find that our new Langevin molecular dynamics findings match earlier Monte Carlo simulation data well, verifying that these two microscopically quite distinct simulation methods lead to macroscopically very similar results for non-equilibrium vortex matter.

Host: Uwe Tauber

February

February 4
(poster)

Special Colloquium
4:00 P.M. in 210 Robeson

Dr. Rikkert Nap

Northwestern University

Surface Binding of Polymer Coated Nanoparticles: Coupling of Physical Interactions, Molecular Organization, and Chemical State

One of the key challenges in nanoscience/medicine is to design carrier system for drug delivery or imaging that selectively binds to target cells without binding to healthy cells. A common strategy is to end-functionalize the polymers coating of the delivery device with specific ligands that bind strongly to overexpressed receptors. However such devices are usually unable to discriminate between receptors found on benign and malignant cells. We demonstrate, theoretically, how one can achieve selective binding to target cells by using multiple physical and chemical interactions. We study the effective interactions between a polymer decorated nanosized micelle or solid nanoparticle with model lipid layers. The polymer coating contains a mixture of two polymers, one neutral (e.g., polyethylene glycol) for protection and the other a polybase with a functional end-group to optimize specific binding and electrostatic interactions with the charged lipid head-groups found on the lipid surface. The strength of the binding for the combined system is much larger than the sum of the independent electrostatic or specific ligand-receptor binding. Moreover, it is found that the molecular organization of the polymer coating is qualitatively different in the combined system. This is manifested in the strength of the binding and also in a regime of distances where the addition of two repulsions leads to an attraction.The search for optimal binding conditions lead to the finding of the non-trivial and highly non-additive coupling that exists in systems where chemical equilibrium, like acid-base reactions and ligand-receptor binding, molecular organization, and physical interactions are coupled together. The theoretical predictions not only provide practical guidelines for the design of polymeric carrier devices for targeted drug delivery, they also give fundamental physical insight in the competing and highly non-additive nature of the coupling between different forces found in inhomogeneous and constrained environments of many synthetic and biological systems.

Host: Michel Pleimling

February 11
(poster)

Special Colloquium
4:00 P.M. in 210 Robeson

Dr. Shengfeng Cheng

Sandia National Labs

Self-assembly of Model Microtubules

Self-assembly plays a central role in producing ordered superstructures. The crucial question is to identify the necessary features that a macromolecular monomer must have in order to drive self-assembly into a desired complex structure. In this talk I will present our recent work on the self-assembly of model microtubules. The model monomer has a wedge-shape with lateral and vertical binding sites. Using MD simulations, we calculated a diagram of the self-assembled structures from these monomers. A modified Flory-Huggins theory was developed to predict the boundaries between different structures that match well with simulation results. We found that to form tubules the interaction strengths must be in a limited range. In addition, helical tubes are frequently formed even though the monomer is nonchiral. The occurrence of the helical tubes is related to the large overlap of energy distributions for nonhelical and helical tubes. To enhance structural control of the self-assembly, we added chirality and a lock-and-key mechanism to the model. We could control both the pitch of the helicity and the twist deformation of the tube by modifying the locations of the binding sites and their interaction strengths. Our results shed new light on the structure of in vitro microtubules formed with various numbers of protofilaments of tubulins, which also exhibit similar twisted structures and various pitches, and have determined the fundamental features of macromolecular monomers for self-assembly into a tubular structure.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Host: Mark Pitt

February 18
(poster)

Special Colloquium
4:00 P.M. in 210 Robeson

Dr. Na Sai

University of Texas, Austin

Organic Materials and Interfaces for Electronics and Solar Energy Conversion: First-Principles Theory and Computation

Organic functional materials based on pi-conjugated molecules and polymers are candidates for the next generation flexible electronics and renewable energy technologies.  Their attractiveness is rooted in their flexibility and the promise of low-cost, large-scale fabrication. The diversity of molecular motifs ensures a broad configuration space for the design and synthesis of new materials. Organic materials exhibit unique behavior such as the presence of self-trapped localized charges (“polarons”) and the localization of electronic excitations (“excitons”). Organic-based interfaces, in particular, play a central role in enabling the exciton dissociation processes that is fundamentally important to organic photovoltaics. I will describe theoretical investigations of self-trapped localized hole polarons, optical excitations, interface energy level alignment, and interface dipoles in an array of organic assemblies that enlists molecular crystals, oligomers, inorganic/organic hybrids, and organic molecule/fullerene interfaces. I will highlight an investigation combining theory and experiment of the charge separation process at a donor/acceptor interface. I will discuss theoretical challenges in the first-principles electronic-structure treatment of localized excitations and interfacial electronic structures. I will outline future directions in the computational design of organic polymers for photovoltaic applications requiring high efficiency and photochemical stability.

Host: Uwe Tauber

February 25
(poster)

Special Colloquium
4:00 P.M. in 210 Robeson

Dr. Jian Qin

University of Chicago

Molecules that tangle: polymer entanglement and melt structure

Linear polymers are flexible chain molecules containing many chemical repeat units. The motion of individual polymers in a dense polymer liquid (molten plastic) is severely constrained by surrounding chains, and by the fact that chains cannot cut through one another. Effectively, polymers may be considered as being confined inside a tube-like region. The tube diameter, or the entanglement length, is the key parameter needed  by the standard molecular theory for polymer rheology. But a molecular understanding of the origin of the tube diameter is still lacking. We approach this problem by closing polymers into rings, in order to obtain a system  with well-defined, permanent topology, and using tools from the mathematical theory of knots to identify and count topological entanglements. For simulated polymer melts, this approach enables us to get a tube diameter value that is based on topological considerations alone, and that agrees with values obtained by more heuristic methods. We use this approach to study the effects of chain flexibility and addition of diluents upon the tube diameter.

Another consequence of the unique chain-like structure of polymers is the presence of composition fluctuations at the length scale of a polymer coil size. These composition fluctuations can be measured by small angle neutron and X-ray scattering. Understanding these fluctuation phenomena is important for understanding the structure and self-assembly behavior of multi-component polymeric systems. We show that composition fluctuations in systems of diblock copolymers can be accurately described by a statistical mechanical theory that we developed, by comparing its predictions with simulation results.

Host: Michel Pleimling

March
March 4
(poster)

Hyunhang Park

Department of Physics, Virginia Tech

Spin Systems far from Equilibrium: Aging and Dynamic Phase Transition

Among the many non-equilibrium processes that take place far from equilibrium, we deal with two different but related aspects. One is the non-equilibrium relaxation process that is called ’aging’, where we focus on the effects of disorder. The time-dependent dynamical correlation length L(t) is determined numerically and the scaling behavior of various two-time quantities is discussed as a function of L(t)/L(s) where t and s are two different times. For disordered Ising models deviations of L(t) from an algebraic growth law show up. The generalized scaling forms as a function of L(t) /L(s) reveal a generic simple aging scenario for disordered Ising ferromagnets as well as for Ising spin glasses. The other aspect is a non-equilibrium phase transition, called ’dynamic phase transition’, where we study how the presence of a surface affects this phenomenon. We examine layer-dependent quantities, such as the period-averaged magnetization per layer Q(z) and the layer susceptibility c(z), and determine local critical exponents through finite size scaling. Both for two and three dimensions, we find that the values of the surface exponents differ from those of the equilibrium critical surface. It is revealed that the surface phase diagram of the non-equilibrium system is not identical to that of the equilibrium system in three dimensions.

Host: Michel Pleimling

March 15
(poster)

Special Seminar

10 AM


Prof. Alfred Hucht and Martin P. Magiera

Institute for Theoretical Physics, University of Duisburg-Essen, Germany

Magnetic friction in spin systems

Magnetic contributions to friction due to spin correlations have attracted increasing interest in recent years. One interesting aspect is the energy dissipation due to spin waves in magnetic force microscopy, where magnetic structures are investigated by moving a magnetic tip over a surface. These friction forces can be constant at small driving velocities (Coulomb type) or proportional to the driving velocity (Stokes type).
On the other hand, magnetic friction is present in bulk magnetic systems which are moved in close proximity. In this context, Kadau et al. (Phys. Rev. Lett. 101, 137205 (2008)) proposed a simple model for magnetic friction mediated solely by spin degrees of freedom. In this model an Ising spin system is moved over a second spin system along a boundary, with constant velocity v. This permanent perturbation drives the system to a steady state far from equilibrium, leading to a permanent energy flow from the boundary to the heat bath.
The model exhibits a nonequilibrium phase transition, which has been investigated in several different geometries by means of analytical treatment as well as Monte Carlo (MC) simulations. The critical temperatures Tc of the considered models depend on the velocity v and has been calculated exactly for various geometries in the limit v → ∞. In this limit the class of models show mean-field-like critical behavior. We find many similarities to the driven lattice gas (DLG), where a Ising system is driven out of equilibrium by an applied field which favors the motion of particles in one direction.

Host: Uwe Tauber

March 15
(poster)

Special Seminar

2 PM

 

 

 

Prof. Benoit Hackens

NAPS/IMCN, UCL, Louvain-la-Neuve, Belgium

Imaging and manipulating quantum transport at the nanometer scale 

In the last thirty years, progresses in nanofabrication techniques have allowed to explore electron transport inside extremely small and "clean’" devices. Confined in such nano-devices electrons often reveal their true complex nature, and a rich spectrum of new phenomena emerges, with signatures of energy quantization, charge discreteness, and/or electron interferences and interactions. Characterization tools followed a similar downsizing trend, concomitant with the evolution of nanodevice fabrication. In particular, the Atomic Force Microscope (AFM) and Scanning Tunneling Microscope (STM) democratized nanoprobing down to the atomic scale. In this presentation, I will give a few examples illustrating how AFM-derived techniques can give access to real-space vizualisation of electron transport phenomena in nanodevices. I will show that, imaging the electron behaviour at the nanometer scale, one can even decrypt complex quantum transport phenomena in a very direct way. The technique also opens the door towards a more intimate manipulation of charges and quasiparticles, which is one of the key aspects of quantum computation.

Host: Jean Heremans

March 25
(poster)

Prof. J. Allam

University of Surrey, UK

Critical Crossovers and Universality Far from Equilibrium:Exciton-Exciton Reactions on Carbon Nanotubes

Understanding many-body systems that are far from equilibrium is one of the outstanding challenges in condensed-matter physics. However many-body theories of non-equilibrium kinetics are based on rather simple models of the interactions between particles, whose validity in describing real experimental systems has hardly been addressed. The state of experimental investigations themselves has been described as “deplorable”. We present experimental studies of the kinetics of exciton-exciton reactions on carbon nanotubes, which under particular experimental conditions represents a paradigm for reaction-diffusion systems that relax from an extreme non-equilibrium state. We demonstrate the existence of a crossover between classical and nonclassical kinetics, and the universality of the kinetics at long times. The crossover is non-monotonic and non-universal, with remarkable similarities to crossovers associated with phase transitions in complex liquids. We show that this behaviour can arise from competition between a finite interaction range and a finite interaction probability, and hence that similar complex crossovers are expected to be commonplace in real physical systems. 

Host: Uwe Tauber

April
April 8
(poster)

Prof. Raffaella De Vita

Engineering Science and Mechanics,Virginia Tech

Soapy Models for Lipid Bilayers

In this talk, I will present new modeling strategies for evaluating the deformations of lipid bilayers, which constitute the base component of cell membranes. I will put forth a new energy for the description of large distortions of lipid bilayers. This energy is formulated with mathematical rigor. It accounts for the soap-like liquid crystalline nature of lipid bilayers and the coupling between the deformations of lipid molecules and layers. The analogies between soap-like liquid crystals, with an infinite number of layers, and lipid bilayers, with only two layers, will be further discussed.  The proposed energy will then be utilized to study large deformations of planar lipid bilayers induced by cylindrical inclusions.

Host: Uwe Tauber

April 15
(poster)

Dr. Evan Glaser

Naval Research Laboratories

Magnetic Resonance Spectroscopy of p-type Dopants in Wide Band Gap Semiconductors:  GaN and ZnO

The quest to produce large hole concentrations (> 10^18 cm^-3) with high carrier mobilities in epitaxial and bulk GaN and ZnO wide bandgap semiconductors has been of high scientific and technological interest throughout the past 15-20 years.  The transport properties for several potential p-type dopants such as Mg and Be in GaN and Li, Na, and N impurities in ZnO have been investigated by many groups with varying degrees of success.  In this presentation I will highlight our research at NRL using defect-sensitive magnetic resonance techniques (including electron paramagnetic resonance and optically-detected magnetic resonance) that provide many insights on the nature of the electronic states associated with, in particular, Mg impurities in GaN and N in ZnO.  In addition, these studies reveal the presence of dopant-related and/or other defect complex centers that act as deep trap states and limit or significantly compromise the ability to achieve high hole densities in these materials.

Host: Giti Khodaparast

April 22
(poster)

Katharine Jensen 

Department of Physics, Harvard University

Structure formation and deformation dynamics in hard-sphere colloidal materials
Understanding how atomic-scale structures and dynamics give rise to the macroscopic properties of materials can be very difficult. In an ideal experiment, we would simply observe the three dimensional trajectories of every atom in the material over time and link these local motions to the bulk properties. Unfortunately, atoms are too small and move too quickly to do this. However, by using colloids consisting of micrometer-scale solid particles surrounded by a fluid as scale models of atomic crystals and glasses, we are able to use confocal microscopy to perform this ideal experiment. I will discuss the physics of forming crystals and glasses from simple, hard-sphere colloidal particles and how we've used these colloids to investigate the structure, defects, and dynamics of crystalline and amorphous materials, with particular focus on the mechanisms of deformation in glasses.

Host: Dick Zallen

April 29
(poster)

Prof. Sunghwan (Sunny) Jung

Engineering Science and Mechanics,Virginia Tech

Physics of drinking

Most living organisms cannot live without water. Hence, they have developed their own way to intake water into their bodies. Here, we present two drinking strategies exploited by different organisms. First is how cats drink water. Cats with incomplete cheeks use their tongue to drink water. We reveal that a cat laps by a subtle mechanism that transports water by wetting the dorsal side of the tongue. A combined experimental and theoretical analysis explaines that a cat exploits fluid inertia to defeat gravity and pull liquid into the mouth. Further, this prediction is extended to other felines in animal kingdoms.  The second example is Paramecium, one of the model systems in ciliates. In unstressed conditions, its locomotion presumably optimizes both swimming speed and food intake at a given metabolic power. Paramecium has a preferable swimming direction with a specific pattern (or stroke) of cilia. Both locomotion and feeding are often directed by fluid flows induced by cilia, and are not independent of each other. Here, we study the effect of body shape and swimming direction on locomotion and feeding efficiencies.

Host: Uwe Tauber

May
May 6
(poster)

Prof. Shane Ross

Engineering Science and Mechanics,Virginia Tech

Geometric and probabilistic descriptions of chaotic phase space transport

Several geometric and probabilistic methods for studying relatively low-dimensional chaotic phase space transport have been developed and fruitfully applied to diverse areas from celestial mechanics to weather models to biological invasions and beyond. Increasingly, systems of interest are determined not by analytically defined model systems, but by data from experiments or large-scale simulations. Emphasis on real-world systems sharpens our focus on those features of transport in phase space of finite-time systems which seem robust, leading to the consideration of not only invariant manifolds and invariant manifold-like objects, but also their connection with concepts such as symbolic dynamics, chaos, braids, coherent sets, Markov chain approximations, and optimal control. Transport in 2 dimensional time-varying systems will be discussed initially, primarily fluid motion. In higher dimensions, more exotic dynamics comes into play, e.g., in mechanical systems with 2 or more degrees of freedom (phase space of dimension 4 or more), stable and unstable invariant manifolds associated to hyperbolic bound orbits are the structures that govern phase space transport; so-called tube dynamics. We discuss interesting phase space transport scenarios arising from the intersection of these structures. We highlight some applications of the concepts discussed to areas such as stirring in hurricanes, interplanetary trajectories, microfluidic mixing, escape from multi-dimensional potential wells, and spread of plant diseases.

Host: Uwe Tauber

May 13
(poster)
Final Exam Week

May 20
(poster)
Summer Break

May 27
(poster)
Summer Break