> College of Science > Physics Dept > Talks > Condensed Matter Seminars
Fall 2011 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.


< Spring 2011 | August | September | October | November | December | Spring 2012 >

August
August 22

(Poster)
No Seminar

August 29

(Poster)
Dr. Brian Skinner (University of Minnesota)

Theory of Supercapacitors

Supercapacitors are electrical energy storage devices that are quickly replacing conventional batteries in high-power applications.  A number of these devices exhibit dramatically large capacitance, but the microscopic mechanisms that underlie their operation remain largely mysterious.  In this talk I discuss microscopic-level theories of the operation of supercapacitor devices.  I focus in particular on two types of supercapacitors: 1) those made from highly-porous, well-conducting electrodes and 2) those made from a stack of graphene sheets.  I show that in both cases the devices' surprising properties can be understood by considering the discrete, correlated nature of the capacitor charge.

Host: Beate Schmittmann

September
September 5

(Poster)
Dr. Kirill Korolev (Massachusetts Institute of Technology)

Space, Evolution, and the Petri dish

Spatial correlations and number fluctuations play an important role in evolution. Their effects become especially pronounced when organisms spread to new territories because the number of organisms at the front of the expansion is typically small. The interplay of number fluctuations and migration causes spatial genetic demixing, i.e., spatial separation of different genotypes. We formulated a simple phenomenological model that describes this genetic demixing and found a good agreement between the theory and experiments in bacterial colony biofilms. More importantly, genetic demixing affects many evolutionary processes. In particular, we showed that mutualism between two species can evolve only when it is sufficiently strong and fair, i.e., both species benefit about equally from the interactions. When the fitness advantage of mutualism is reduced, mutualism is generically lost via a directed percolation (DP) process, with the phase diagram strongly influenced by an exceptional DP2 transition.

Host: Uwe Tauber

September 12

(Poster)

Note: Special Location:
Surge Building 104B

Dr. Danielle S. Bassett
Department of Physics and Institute for Collaborative Biotechnologies, The University of California Santa Barbara

Networks of the Brain

Network theory has revolutionized the understanding of complex structures ranging from social groups to communication systems. Analytic tools developed in this field are particularly suited to address the difficult questions of neuroscience: How is the brain wired? How does it perform complex functions (e.g., learning or memory) using that wiring? In this talk, I will review the formalisms of network theory and describe recent models constructed to examine brain organization. Converging evidence suggests that human brain connectivity is organized in a hierarchically modular fashion shaped by spatial and energetic constraints. An understanding of these connection patterns and their sensitivity to cognitive function may facilitate important advances in neuropsychiatric disease diagnosis and rehabilitation treatments. More generally, the human brain serves as an extraordinary model system in which to probe temporal evolution, physical embedding, and functional properties of network systems.

Host: Beate Schmittmann

Special Seminar

September 13 (Tue)

3:30PM-4:30 PM
304 Robeson

(Poster)

Dr. Lee Bassett
Center for Spintronics and Quantum Computation University of California, Santa Barbara

Coupling Single Spins and Photons in Diamond

Nitrogen-vacancy (NV) centers in diamond are atomic-scale defects that trap single electrons with extraordinarily robust properties for quantum information applications. These individually-addressable spins have millisecond coherence times at room temperature, convenient optical spin-initialization and readout transitions, and can be manipulated at gigahertz frequencies with microwaves generated in micron-scale devices. We discuss recent efforts to coherently couple individual NV centers to light, towards the goal of integrating single spins within a scalable photonic network. Using the interaction between near-resonant light and an NV-center spin due to spin-dependent optical transitions, we demonstrate both non-destructive spin readout and all-optical spin control [1]. Furthermore, we are able to control these optical transition energies using DC electric fields, in order to compensate for crystal inhomogeneities and tune separate NV centers to optical indistinguishability [2]. Important applications of these techniques include the development of quantum repeaters for long-distance quantum communication, and the controlled entanglement of distant NV-center spins within photonic networks.

  1. B.B. Buckley, G.D. Fuchs, L.C. Bassett, & D.D. Awschalom, “Spin-Light Coherence for Single-Spin Measurement and Control in Diamond” Science, 330, 1212 (2010).
  2. L. C. Bassett, F. J. Heremans, C. G. Yale, B. B. Buckley, & D. D. Awschalom, “Electrical Tuning of Single Nitrogen-Vacancy Center Optical Transitions Enhanced by Photoinduced Fields” arXiv:1104.3878v1 [cond-mat.mes-hall].

Host: Beate Schmittmann

September 19

(Poster)
Prof. John Morris (Department of Chemistry, Virginia Tech)

Reaction Mechanisms at the Gas-Surface Interface : Why molecules bounce, break, and scatter

Our research objectives are to elucidate the atomic-scale mechanisms of interfacial bonding, diffusion, and reactions that govern gas-surface interactions. This challenge is particularly formidable for functionalized organic surfaces where the complicated nature of the interface can result in a broad array of reaction pathways. Our approach to this problem combines molecular beam scattering techniques with functionalized self-assembled monolayers. Together with surface analytical instrumentation, these methods reveal insight into many aspects of gas-surface interactions and help develop a detailed description of the transformation of reactants into products. This presentation will describe a wide range of systems from rare gas energy exchange, to the reaction mechanisms of highly oxidizing species, such as ozone. By building from chemically simple systems to more complex interfacial reactions, we are beginning to understand the rules for predicting why gas-phase molecules bounce, break, or scatter when they collide with a solid surface.

Host: Chenggang Tao

September 26

(Poster)
Dr. Olga Ivanova (Institute of Critical Technologies and Applied Science (ICTAS), Virginia Tech)

Incorporating Quantum Dots into Printing Media for Additive Manufacturing

Quantum Dots (QD) are semiconducting nanocrystals that have one or more physical dimensions between 1 and 100 nanometers. QDs possess unique optical properties that make them useful in several research areas; e.g., a successful application is the use of colloidal QDs as biological tags. Our research is focused on incorporating QDs into nanosolutions (nanomaterials and printing media) for use in Additive Manufacturing (AM). AM is a genre of processes for additively making objects from 3D model data, usually by forming layer upon layer. Additions of nanomaterials to 3D printing media generally improve and/or change the properties of final printed parts, but AM methods have their own inherent limitations when nanoparticles are applied. Our research goal is to create a wholly new class of nanocomposites via AM that will enable us to create objects not only with exceptional shapes, but also unique material properties.

Host: Beate Schmittmann

October
October 3

(Poster)
Qian He (Department of Physics, Virginia Tech)

Spatio-Temporal Patterns, Correlations, and Disorder in Evolutionary Game Theory

Evolutionary game theory originated from the application of mathematical game theory to biological studies. Well-known examples in evolutionary game theory are the prisoner’s dilemma, predator-prey models, the rock-paper-scissors game, etc. Recently, such well-known models have attracted increased interest in population dynamics to understand the emergence of biodiversity and species coexistence. Meanwhile, It has been realized that techniques from statistical physics can aid us to gain novel insights into this interdisciplinary field. In our research, we mainly employ individual-based Monte Carlo simulations to study emerging spatio-temporal patterns, spatial correlations, and the influence of quenched spatial disorder in rock-paper-scissors systems either with or without conserved total population number. In balanced rock-paper-scissors systems far away from the ``corner’’ of configuration space, it is shown that quenched spatial disorder in the reaction rates has only minor effects on the co-evolutionary dynamics. However, in model variants with strongly asymmetric rates (i.e., ``corner’’ rock-paper-scissors systems), we find that spatial rate variability can greatly enhance the fitness of both minor species in “corner” systems, a phenomenon already observed in two-species Lotka-Volterra predator-prey models. In addition, we numerically study the influence of either pure hopping processes or exchange processes on the emergence of spiral patterns in spatial rock-paper-scissors systems without conservation law (i.e., May-Leonard model). We also observe distinct extinction features for small spatial May-Leonard systems when the mobility rate crosses the critical threshold which separates the active coexistence state from an inactive absorbing state.

Host: Uwe Tauber

October 10

(Poster)
No Seminar

October 17

(Poster)
Dr. Thomas Maier (Oakridge National Lab)

How does the Hubbard model guide us in the search for higher-Tc superconductors?

Numerical calculations of the two-dimensional Hubbard model have found many phenomena that are also observed in unconventional superconductors, including antiferromagnetism, d-wave superconductivity as well as pseudogap and stripe behavior.This model is therefore believed to provide a basic framework for understanding the nature of the electronic pairing mechanism that leads to unconventional superconductivity. In this talk, I will present dynamic cluster quantum Monte Carlo simulations of this model as well as extensions that include a bi-layer and a dimer Hubbard model, and discuss what these calculations tell us about the factors that determine the transition temperature in unconventional superconductors. In particular, I will focus on the effects of correlations, quantum critical points and electronic inhomogeneities and the role of multiple Fermi surfaces and changes in the spin-fluctuation spectral weight. I will close with a summary of the optimal conditions for unconventional superconductivity and the guidance obtained from these simulations in the search for higher-Tc superconductors.

Host: Vito Scarola

October 24

(Poster)
Prof. Prabhakar Bandaru (Mechanical/Aerospace Eng. Dept. at the University of California San Diego)

Coiled carbon nanotubes and nanowires: Rational synthesis and applications to mechanical, electronic, and optical devices

The phenomenology and thermodynamic modeling involved in the experimental synthesis of helical morphologies of carbon nanotubes and nanowires, through chemical vapor deposition based techniques will be discussed. It will be shown how the use of non-wetting catalysts, such as In and Sn, may promote coiling of nanostructures. Coil formation is interesting in that helices abound in nature, e.g., DNA, proteins, etc. and a connection is now being made at the nanoscale between carbon based inorganic and organic structures. These structures can then be made practical for a wide variety of applications, e.g., energy absorbing mechanical springs, electrical inductors, optical frequency metamaterials, etc. and related experiments and possibilities will be outlined. 

Host: Hans Robinson

October 31

(Poster)
Dr. Fei Lin (Department of Physics, Virginia Tech)

Quantum Monte Carlo Study of Bose-Hubbard Model in the New Era: Opportunities and Challenges

Bose-Hubbard model, which is used to study interacting bosons on a lattice, has been around for more than 20 years. Recently there has been an increased interest in the model due to more efficient numerical simulation algorithms being developed on the theory side and the ability of contructing such a model in an optical lattice on the experiment side. However, because of the inhomogeneous nature of the trapped optical lattice, there arises a new problem: the system constructed has a mixture of different phases in different regions of the trap. In this talk I will discuss how quantum Monte Carlo (QMC) methods may be used to address such a problem by calculating both global (condensate fraction, superfluid density) and local physical quantities (local superfluid density, compressibility), which are useful in distinguishing different phases in the trap. I will also discuss some recent QMC simulations of disordered Bose-Hubbard model.

Host: Vito Scarola

November
November 7

(Poster)
Prof. Jeff Gore (Massachusetts Institute of Technology)

Evolution on rugged fitness landscapes

The effect of a genetic mutation may depend upon the presence of other mutations within an organism's genome. Such genetic interactions lead to ruggedness in the fitness landscape that may constrain the path of molecular evolution. We have used microbial experiments to gain quantitative insight into the nature and consequences of these interactions. In the first project, we consider five mutations in an antibiotic resistance gene to study the degree to which evolution is reversible in the face of two antibiotic treatments that exhibit a fitness trade-off. In the second project, we use a publicly available dataset of the fitness of ~ 5 million double gene-knockouts in yeast to quantify the strength of genetic interactions. We find a remarkable scaling of the strength of genetic interactions with the fitness effects of the individual mutations.

Host: Uwe Tauber

November 14

(Poster)
Dr. Ariel Amir (Harvard University)

Aging in glasses - full aging and beyond

Glassy systems are ubiquitous in nature, from window glasses, with enormous viscosities making their flow extremely slow, through the anomalous magnetic properties of spin-glasses, to memory effects observed in granular media. Among their key properties are slow relaxations to equilibrium without a typical timescale and aging, the dependence of relaxation on the system's age. Understanding these phenomena is a long-standing problem in physics. In this talk I will show that the particular example of electron glasses is useful as a test-case to understand the generic mechanisms involved, leading to aging. I will describe our approach to the problem, and show that it generally leads to a particular form of aging, which we found to agree well with data on electron glasses, as well as various other systems such as disordered semiconductors and structural glasses. I will also show results on the expected deviations from the universal form, and what we think can be learnt from them.

Host: Uwe Tauber

Special Seminar

November 15 (Tuesday)

3:00PM-4:00 PM
304 Robeson

(Poster)
Dr. Ariel Amir (Harvard University)

Dislocation dynamics and bacterial growth

Recent experiments have revealed remarkable phenomena in the growth mechanisms of rod-shaped bacteria: molecules associated with the cell wall growth move at constant velocity in circles oriented approximately along the cell circumference (Garner et al., Science 2011, Domínguez-Escobar et al., Science 2011, Deng et al., PNAS 2011). We view these dislocations in the partially ordered peptidoglycan structure, and study theoretically the dynamics of these interactingdislocations on the surface of a cylinder. The physics of the nucleation of these dislocations and the resulting dynamics within the model show surprising effects arising from the cylindrical geometry, which are predicted to have important implications on the growth mechanism. We also discuss how long range elastic interactions affect the dynamics of the fraction of active dislocations in the environment.

Host: Uwe Tauber

November 21

(Poster)
Thanksgiving Break

November 28

(Poster)
Prof. Daniel Dougherty (North Carolina State University)

Image Potential States of Epitaxial Graphene on SiC

Graphene is a single, atomically-thin, sheet of graphitic carbon that shows enormous promise as an electronic material due to high carrier mobilities, good mechanical and chemical robustness, and long spin diffusion lengths. Due to its quasi-two dimensional character and unique linear band dispersion, graphene exhibits electrostatic screening properties that are very different from metallic systems and that are likely to dominate its device properties due to the prevalence of charged impurities in gate dielectrics [1].

I'll describe scanning tunneling microscopy and spectroscopy experiments that probe the electrostatic surface potential outside of graphene layers grown on SiC(0001) surfaces by measuring transiently bound image-potential induced surface states [2]. The strong response of these states to the electric field of an STM tip points out some of the remarkable differences between metallic screening and graphene screening [3]. In addition, I'll show how spectroscopy of image potential states can be used with first principles electronic structure calculations to understand a number of complex structures that result from the intercalation of sodium at graphene interfaces.

[1] Das Sarma et al., Rev. Mod. Phys 83, 407 (2011).
[2] Silkin et al., Phys. Rev. B 80, 121408 (2009).
[3] Sandin et al., Appl. Phys. Lett. 97, 113104 (2010).

Host: Chenggang Tao

December
December 5

(Poster)
Pavel Kraikivski (Department of Biological Sciences, Virginia Tech)

Diffusion in Cytoplasm: Effects of Excluded Volume Due to Internal Membranes and Cytoskeletal Structures

The interior of the cell is filled with cytoskeletal networks and intracellular membranes with intricate geometry. Thus the space available for diffusion in cytoplasm is convoluted and macromolecule diffusivity is affected as a consequence of increase in a path length, sometimes termed as 'the effect of excluded volume'. I will present a systematic computational study of this effect by approximating intracellular structures as mixtures of random overlapping obstacles of various shapes. Threedimensional simulations of Brownian motion in the entangled filament networks reveal the anomalous time dependence of a particle mean squared displacement on the time and spatial scales determined by the ratio of a particle radius and the network mesh size. This is consistent with the experiments with tracer particles and with earlier modeling studies of diffusion on lattices. On a sufficiently large spatial scale, at which the medium can be considered as homogeneous, the diffusion becomes normal. Effective diffusion coefficients are computed using a fast homogenization technique. It is found that a simple two-parameter power law provides a remarkably accurate description of effective diffusion over the entire range of volume fractions and for any given composition of structures. This universality allows for quick estimation of diffusion coefficients, and also void percolation thresholds once the obstacle shapes and volume fractions are specified. It is estimated that the excluded volume effect alone can account for a four-to-six fold reduction in diffusive transport in cells, relative to diffusion in vitro. The study lays the foundation for an accurate coarse-grain formulation that would account for cytoplasm heterogeneity on a micron scale and binding of tracers to intracellular structures.

December 12

(Poster)
Final Exam Week