2018 Seminars

Dr. Kunal Mondal (North Carolina State University)

"Soft-Nano-materials, Interfaces, and Micro-Nano-fabrication to Build Tools and Functional Devices"

New competitive technologies should be developed to deal with the world’s emerging problems in healthcare, environmental, agriculture, energy and security sectors to benefit a broad spectrum of society while using minimal resources. Multifunctional interfaces of nanomaterials can be used to tackle such glitches by developing sensors and detection devices such as biosensors, explosives trace detectors, mechanical-stress sensors, wastewater management systems and energy storage devices owing to their nanoscopic surface properties. Considering this, several catalytic and photocatalytic metal/metal-oxide semiconductor nanostructures have been synthesized and used for environmental remediation, point-of-care diagnostics and energy storage applications. Several fabrication techniques including electrospinning, microfabrication, 3D printing etc. have been used to made functional nano/micro devices. Various physicochemical characterization techniques are used to study their properties in nanoscale. Furthermore, effort has been made on surface patterning and fabricating stretchable electronics by integration of conducting liquid metal in soft elastomers to explore ways to utilize these ‘softer than skin’ materials for bioelectronic applications. Finally, this concludes with an outlook and future challenges of these materials within this context.

Host: Rana Ashkar

Host:

Chengyuan Wen (Virginia Tech, Physics)

Host: Vinh Nguyen

Host:

Prof. David M. Leitner (University of Nevada, Reno )

“Watching energy transport in proteins: Identifying dynamics networks and thermodynamic properties”

Energy transport in a protein mediates protein function and represents the early events following a reaction or photoexcitation. New time-resolved measurements, and a variety of computational and theoretical methods allow us to map out and describe energy transport in great detail. I will describe some of our theoretical and computational work on the nature of energy transport in proteins, with focus on what we can learn about protein dynamics and thermodynamics by watching energy flow in proteins. By coarse graining energy transport dynamics from the all-atom to residue level, we have identified a relation between conformational dynamics at equilibrium and rates of energy transfer across non-bonded contacts. Measurements of rates of energy transfer thus provide a window into equilibrium dynamics of proteins and entropy associated with the dynamics of the contact.

Host:Vinh Nguyen

Jacob Carroll (Virginia Tech, Physics)

“The effects of inhibitory neuron fraction on the dynamics of an avalanching neural network”

The statistical analysis of the collective neural activity known as avalanches provides insight into the proper behavior of brains across many species. In this paper we present a neural network model based on the work of Lombardi, Herrmann, de Arcangelis et al. that captures the relevant dynamics of neural avalanches, and we show how tuning the fraction of inhibitory neurons in this model removes exponential cut-offs present in the distributions of avalanche strength and duration, and transitions the power spectral density of the network into an epileptic regime, as well as effecting the evolution of the network structure over time. We propose that the brain operates away from this regime of low inhibitory fraction to protect itself from the dominating avalanches present in these extended distributions.

Host: Uwe Tauber

Fall Break (No Colloquium)

Host:

Alex Grutter

"TBD"

Host: Satoru Emori

Shadi Esmaeili (Virginia Tech, Physics)

"From disorder to self-organization: A cyclic predator-prey system and a system of frustrated coupled oscillators"

Self-organization is the emergence of spontaneous order as a result of local interactions among the elements of a system. Systems far from equilibrium that are evolving toward their self-organized state show very interesting dynamic behaviors. We study the dynamic behavior of two systems: a cyclic predator-prey system with a complex spatiotemporal pattern, as well as a system of coupled oscillators with antagonistic coupling. In the predator-prey model, the response of the system to external perturbation is used as an approach to gain insights about its dynamic behavior. On the other hand, the breaking of time translation invariance was observed during the spontaneous relaxation of a system of coupled oscillators after a parameter quench in the absence of any stochastic fluctuation.

Host: Michel Pleimling

Prof. Michael Flatte' (University of Iowa)

"Quantum Coherent Electronic Technologies"

Electrons in most materials experience dramatic and frequent scattering from other electrons, phonons, and a variety of other excitations. Such scattering events often rapidly dissipate any memory the electron had of its quantum state, so the electrons can be described as an ensemble that is near local ther-mal equilibrium. If the electrons can retain a good memory of their quantum state, however, then they are quantum coherent and can be used for very unu-sual and exciting tasks such as quantum computing. Realizing these quantum technologies has traditionally been expected to require very special elements such as superconducting devices or very high mobility transistors, as well as very low temperatures, in order to avoid rapid loss of quantum coherence (decoherence). Over the past fifteen years we and others have identified re-markable examples of room-temperature quantum coherent behavior in con-densed matter electronic systems, usually involving spin coherence. Predicting the behavior of these spin coherent systems requires integrating theoretical techniques to cope with energy scales ranging from far smaller than the thermal energy to far larger. I will describe some examples of quantum coherent technol-ogies and identify some of the features they share.

Host: Giti Khodaparast

Dr. David Pappas (NIST)

"TBD"

Host: Sophia Economou

Thanksgiving Holiday No Seminars scheduled

Host:

Shannon Serrao (Physics, Virginia Tech)

"Fluctuation effects on a cyclic predator-prey system(May-Leonard model)"

Owing to close proximity with observed cyclic predator-prey dynamics in nature, we study the cyclic predator-prey model of May-Leonard with three species. The May-Leonard model is characterized by strong fluctuation induced effects to its non-equillibruim stationary state, notably the noise induced spatio-temporal spiral patterns on the two dimensional lattice; and the extinction of the long-lived coexistence state on account of large but rare fluctuations. We study both these stochastic effects by firstly, characterizing the size of the aforementioned spiral patterns to the lowest order using the Doi-Peliti coherent state path integral formalism and encoding the pattern quantitatively in the coefficients of the noisy complex Ginsburg-Landau equation. Secondly, on the well-mixed version of the model, we obtain the extinction times of all but one species driven by large fluctuations from a stable coexistence state and compare our results to Gillespie simulations across the transcritical bifurcation in the system.

Host: Uwe C. Tauber

Physics Faculty Search. No CSB Seminar Scheduled

Physics Faculty Search. No CSB Seminar Scheduled.

Physics Faculty Search. No CSB Seminar Scheduled.<./b>

Dr. Michael Cooney ( NASA Langley Research Center )

MEDLI2, ARCSTONE and Broadband Photodetectors for measuring radiative flux

Dr. Michael Cooney engineer in the Electronic Systems Branch, will discuss three projects he currently supports. The first is a space flight project, the Mars Entry Descent and Landing Instrumentation 2 (MEDLI2). MEDLI2 will extend and enhance the dataset from the MEDLI mission, which flew on the Mars Science Laboratory (MSL) in 2012 and was the first instrument to characterize Mars’ aero-thermal environment. MEDLI2 is scheduled to fly on the MARS 2020 mission. The second project is ARCSTONE, a lunar spectral reflectance instrument in response to the 2007 and 2017 Earth Science Decadal Surveys. Instrument intercalibration is a vital tool to maintain consistent datasets across various instruments and ensure historical continuity. The Moon is considered to be an excellent exoatmospheric calibration source, however the accuracy of the Moon as an absolute reference is limited to 5-10%. An orbiting spectrometer flying on a small satellite in low Earth orbit will provide lunar spectral reflectance with accuracy sufficient to establish an SI-traceable absolute lunar calibration standard for past, current, and future Earth weather and climate sensors. The final project is a research activity in partnership with Virginia Tech to develop broadband photodetectors for measuring radiative flux in response to the Earth Science Decadal survey. Existing Earth science radiation budget instruments rely on radiometers with relatively difficult custom manufacturing processes and slow readout speed. To provide a lower cost future mission options, photon based photosensors have the possibility to lower mission cost while enabling new mission architectures.

Host: Vinh Nguyen

Dr. Michael Cooney ( NASA Langley Research Center )

Job/Internship Possibilities at NASA Langley

Host: Vinh Nguyen

Prof. Sumanta Tewari (Clemson University)

"TBD"

Host: Ed Barnes

Weigang Liu (Department of Physics, Virginia Tech)

"A study of the complex Ginzburg-Landau equation: analytical and numerical results"

Rescheduled for March 14, 2018

Host: Uwe Tauber

Weigang Liu (Department of Physics, Virginia Tech)

"A study of the complex Ginzburg-Landau equation: analytical and numerical results"

The complex Ginzburg-Landau equation (CGLe) is a stochastic partial differential equation that describes a remarkably wide range of physical systems: coupled non-linear oscillators subject to external noise near a Hopf bifurcation instability; spontaneous structure formation in non-equilibrium systems, e.g., in cyclically competing populations; and driven-dissipative Bose-Einstein condensation, realized in open systems on the interface of quantum optics and many-body physics. We employ the perturbative field-theoretic renormalization group method to analytically investigate the universal critical behavior near the continuous non-equilibrium phase transition in the complex Ginzburg–Landau equation with additive white noise. We show that to first order in the dimensional expansion about the upper critical dimension, the initial-slip exponent in the complex Ginzburg–Landau equation is identical to its equilibrium model A counterpart. In our second project, we have employed a finite-difference method to numerically solve the noisy complex Ginzburg-Landau equation on a two-dimensional domain with the goal to investigate the coarsening dynamics following a quench from a strongly fluctuating defect turbulence regime to a long-range ordered phase. We study the spatio-temporal behavior characterized by the spontaneous creation and annihilation of topological defects (spiral waves). We check our simulation results against the known dynamical phase diagram in this non-equilibrium system, tentatively analyze the coarsening kinetics following sudden quenches between different phases, and have begun to characterize the ensuing aging scaling behavior. Moreover, we are currently extracting the activation energy barrier for the nucleation process of the stable spiral wave structures.

Host: Uwe Tauber

Chengyuan Wen (Department of Physics, Virginia Tech)

"Evaporation of Liquids and Solutions"

Evaporation of a liquid is a ubiquitous phenomenon. It drives the water cycle and can be used for cooling. It is also a useful tool for materials fabrication such as evaporation-induced self-assembly of colloidal particles and thin-film deposition via spin coating. In the first part of this talk, we will present million-atom scale molecular dynamics (MD) simulations of the evaporation process of water. An enhancement of water density near the liquid-vapor interface is found during fast evaporation. The temperature profiles based on both translational and rotational degrees of freedom are calculated at different stages of evaporation and evaporative cooling of the liquid-vapor interface is observed, which accounts for the higher water density at the interface. The velocity distribution of water molecules in the vapor phase during evaporation is also computed at various distances relative to the interface and fit to the Maxwell-Boltzmann distribution. Results indicate that local thermal equilibrium holds in the liquid phase, though the whole system is driven out of equilibrium. In the second part of this talk, we will focus on evaporating behavior of polymer solutions. In particular, polyelectrolyte solutions show rich physical behavior because of electrostatic interactions. We use MD simulations to study the evaporation of a solution of polyanionic chains (sodium polystyrene sulfonate). The polymers are represented by MARTINI-type bead-spring chains. Water is included as an explicit solvent and described with a model taking into account polarization effects. Counterions and salts are also explicitly included as mobile single beads. Our results show that the polyelectrolyte chains form layered structures with alternating polymer-rich and counterion-rich layers, indicating that one-pot evaporation technique may be developed to fabricate multilayer polyelectrolyte films that are currently mainly produced via a layer-by-layer deposition process. We will discuss the effects of polymer concentration, salt concentration, and evaporation rate on the structure of the resulting film. Finally, we will also briefly discuss our recent study of the evaporation of polymer solutions containing both polyanionic and polycationic chains.

Host: Shengfeng Cheng

Xiangwen Wang (Department of Physics, Virginia Tech)

"Data-driven modeling of heavy-tailed distributions and scaling laws in human dynamics"

Studying human behavior is of fundamental importance in many social applications. Yet, it remains a challenging problem due to the high complexity of human activities. In recent years, advances in information technology have resulted in the collection of vast amounts of human activity logs, thus enabling the quantitative modeling of human behavior. Using a variety of metrics like probability distributions, we prove the wide existence of heavy-tailed distributions and scaling laws in human behavior. In human online searches we describe the search behavior as a foraging process that takes place on the semi-infinite line. A pairwise power-law distribution respectively exponential distribution is reported for step-lengths in long-range respectively short-range displacements, indicating that the search process is a combination of Brownian-motion local phases and truncated-Levy-flight relocation phases. These results are confirmed through the analysis of mean squared displacements. In human online gambling, we view the net change of income of each player as a random walk and find that the win/loss distributions follow power laws with exponential cut-offs. The mean squared displacement of these net income random walks exhibits a transition between a super-diffusive and a diffusive regime. We present a model that allows to reproduce this behavior and identify the key features needed for observing this transition. For human movements in both real and virtual spaces, heavy-tailed step-lengths are also reported.

Host: Michel Pleimling

Mengsu Chen

Exploring quantum many-body systems via lattice model and exact diagonalization

The quantum many-body problem of solving Schrodinger equation of a large number of interacting microscopic particles is generally considered impossible to tackle analytically. Numerical simulations becomes the essential tools to study these systems, especially at the strongly correlated regime. We used exact diagonalization (ED), the only unbiased numeric method, to study the newly proposed interaction-induced states such as fractional Chern insulators (FCI), topological Mott insulators (TMI), emergent kinetics on various quantum lattice model describing many physic systems including 2D materials and optical lattices. We study quantum phase transitions between different parameter regimes of Hamiltonian, and exotic properties such as fractional charges, spontaneous time-reversal symmetry breaking in these phases.

Host: Vito Scarola

Professor Mark Dykman ( Physics, Michigan State University)

"Time-translation symmetry breaking in vibrational Floquet systems"

A periodically driven system has discrete time-translation symmetry with the period of the driving. Its quantum dynamics is described in terms of the Floquet states. Generally, if the system is in a Floquet state, its dynamical variables oscillate with the period of the driving. Recently much interest have attracted systems where the time symmetry is broken, the “time crystal” effect. Nonlinear oscillators, including Nano mechanical systems and modes in electromagnetic cavities, provide an ideal platform for studying this effect. We will show how the symmetry breaking occurs in an individual oscillator in the quantum coherent regime. We will then discuss the classical and quantum phase transitions to the broken-symmetry state in systems of coupled oscillators.

Host: Uwe Tauber

Sriram Ganeshan (Stony Brook, New York)

"Odd Surface waves in two-dimensional in-compressible fluids"

In everyday fluids, the viscosity is the measure of resistance to the fluid flow and has a dissipative character. Avron, Seiler, and Zograf showed that viscosity of a quantum Hall (QH) fluid at zero temperature is non-dissipative. This non-dissipative viscosity (also known as ‘odd’ or ‘Hall’ viscosity) is the antisymmetric component of the total viscosity tensor and can be non-zero for parity violating fluids. I will discuss free surface dynamics of a two-dimensional incompressible fluid with the odd viscosity (not quite quantum Hall hydro). For the case of incompressible fluids, the odd viscosity manifests itself through the free surface (no stress) boundary conditions. We first find the free surface wave solutions of hydrodynamics in the linear approximation and study the dispersion of such waves. As expected, the surface waves are chiral. In the limit of vanishing shear viscosity and gravity, we derive effective nonlinear Hamiltonian equations for the surface dynamics, generalizing the linear solutions to the weakly nonlinear case. In a small surface angle approximation, the equation of motion results in a new class of non-linear chiral dynamics which we dub as chiral Burgers equation. I will briefly discuss how this program can be extended to the free surface of quantum Hall hydrodynamics.

Host: Ed Barnes

Prof. Juan Vanegas (University of Vermont)

"Mechanics at the nanoscale: Local stress calculations in Biomolecular systems"

The microscopic or local stress field provides a unique connection between molecular simulations and mechanics of materials at the nanoscale. Lateral stress profiles are routinely used to understand the mechanical behavior of liquid interfaces such as lipid membranes from molecular dynamics (MD) simulations. However, the 1-dimensional stress profiles are not adequate to understand the multidimensional mechanical state in complex asymmetrical systems such as membrane proteins or other macromolecular structures. Furthermore, the fact that the microscopic stress from MD simulations is not uniquely defined is a theoretical consideration that is most often ignored, which has acute practical consequences when atomistic models are considered. I will present our recent work on the development of objective 3D local stress calculations by way of expressions that satisfy balance of linear and angular momentum for force-fields with arbitrarily high many-body interactions. I will show how some definitions of the microscopic stress violate mechanical equilibrium through various examples including defective graphene, lipid membranes, and fibrous proteins. I will also demonstrate the use of the traction vector, computed from the microscopic stress, as a powerful tool to visualize the local balance of forces at an interface. Focusing on the bacterial mechanosensitive channel MscL, I will show how the traction vector allows identification of a unique association pattern of lipids at specific sites on the MscL surface that may mediate gating of the bacterial channel by membrane tension or other stimuli.

Host: Shengfeng Cheng