Physics Department Colloquia

Fall 2019

Organizer: Vito Scarola

These meetings occur on Fridays from 2:30pm to 3:30pm in 130 Hahn Hall North.
Refreshments are served at 2:15pm (unless otherwise indicated)

August 2019
August 30

Friday 2:30pm
210 Robeson Hall
(poster)

Host:

September 2019
September 6

Friday 2:30pm
210 Robeson Hall
(poster)

Host:

September 13

Friday 2:30pm
210 Robeson Hall
(poster)

Prof. Shunsaku Horiuchi (Physics, Virginia Tech)

"The Hunt for Dark Matter"

Four fifths of the matter in the universe consists of something completely different from the "ordinary matter" we know and love. I will explain why this "dark matter" is an unavoidable ingredient to understand the universe as we observe it, and I will introduce the strategies to search for dark matter as a particle. I will then focus on an example of a possible hint of discovery, and describe the importance of astrophysics to elucidate its validity. I will finish by outlining ways forward in this exciting hunt.

Host: Mark Pitt

September 20

Friday 2:30pm
210 Robeson Hall
(poster)

Dr. Mark Stiles (NIST, Gaithersburg, MD)

"Energy-Efficient Neuromorphic Computing with Magnetic Tunnel Junctions"

Human brains can solve many problems with orders of magnitude more energy efficiency than traditional computers. As the importance of such problems, like image, voice, and video recognition increases, so does the drive to develop computers that approach the energy efficiency of the brain. Magnetic devices, especially tunnel junctions, have several properties that make them attractive for such applications. Their conductance depends on the state of the ferromagnets making it easy to read information that is stored in their magnetic state. In addition, the spin current can manipulate the magnetic state. Based on this electrical control of the magnetic state, magnetic tunnel junctions are actively being developed for integration into CMOS integrated circuits to provide non-volatile memory. This development makes it feasible to consider other geometries that have different properties. I describe computing primitives that have been constructed based on the different functionalities of magnetic tunnel junctions. The first group of these uses tunnel junctions in their superparamagnetic state for a population coding scheme or for stochastic computing. The second uses them as non-linear oscillators in the first nanoscale “reservoir” for reservoir computing.

Host: Satoru Emori

September 27

Friday 2:30pm
210 Robeson Hall

(poster)

Prof. Shengfeng Cheng (Physics, Virginia Tech)

"Watching Paint Dry is Surprisingly Interesting"

The drying process of a soft matter solution such as a colloidal suspension or a polymer solution provides an excellent playground to explore nonequilibrium physics. One famous example is the coffee ring effect, where a spill of coffee (a particle-laden drop) after drying leaves a ring-like deposit at its perimeter. As another example, novel stratification phenomena have recently been discovered in polydisperse particle suspension films that undergo rapid drying. In all these processes, the interplay of solvent evaporation, fluid dynamics, diffusion, phoresis, and capillarity leads to far-from-equilibrium settings where rich phenomena emerge. In this talk, I will describe our effort of using large scale molecular dynamics (MD) simulations to study various soft matter solutions undergoing drying, including solutions of colloids, polymers, and their mixtures. The solvent is first modeled explicitly as a Lennard-Jones liquid. For bidisperse particle suspensions, a state diagram of stratification outcome is determined and the counterintuitive “small-on-top'” stratification, with an enrichment of the smaller particles at the receding liquid-vapor interface during fast drying, is observed. The diagram is compared to the predictions of several theoretical models recently proposed on the basis of diffusiophoresis. An approach to control stratification via thermal gradients and associated thermophoresis is proposed and validated with MD simulations. The explicit solvent is further mapped to an implicit, uniform, viscous medium by matching the diffusion coefficients of the particles as well as their pair correlation functions. A consistent stratification behavior is observed in both explicit and implicit solvent models under the same drying condition. We also apply our models to investigate the drying process of solutions of polymers and particle-polymer mixtures. Our results reveal a new strategy of unfirmly dispersing nanoparticles into a polymer matrix using fast solvent evaporation, interesting stratification phenomena in drying polyelectrolyte solutions, the formation of a variety of dry structures in drying films or droplets, and ways to control these structures via tuning drying conditions.

Host: Mark Pitt

October 2019
October 4

Friday 2:30pm
210 Robeson Hall

(poster)

Fall Break

Host:

October 11

Friday 2:30pm
210 Robeson Hall

(poster)

Prof. Vinh Nguyen (Physics, Virginia Tech)

"Quantum Coherence in Single Erbium Optical Centers in Seimconductors"

The incorporation of rare earth into semiconductors is of significant interest for optoelectronic devices, ranging from emitting elements in solid-state lasers and displays to optical fiber telecommunications and to quantum information processing. In this regard, erbium has been the prime candidate as its optical emission occurs at the technologically important wavelength of 1.5 µm, which is important for telecommunications, matching the absorption minimum of silica-based optical fibers. In the talk, I review optical properties of Er-doped gallium nitride (GaN:Er) thin films on silicon by metal-organic chemical vapor deposition, progress in the formation of laser gain media, and coherent control of the optical centers for quantum information processing. Initially, a p-i-n light emitting diode based on GaN:Er was fabricated and emission at 1.5 µm and in the green spectral region was observed. Subsequently, optical gain in GaN:Er thin films in an optically pumped configuration was demonstrated. Experiment data indicated that optical gain was enhanced through synthesis of multi-quantum well (MQW) structures including GaN:Er layers. Recently, we have demonstrated stimulated emission through optical pumping of specific Er centers in the MQW structures. The material has recently emerged as one of the potential candidates for quantum information processing thanks to its combination of atomic-line properties, optical and electrical excitation, and solid-state host environment. A hybrid approach has been demonstrated in which optical excitation is used to change the charge state of the erbium optical centers, and this change is then detected electrically. In addition, I propose an experimental approach to coherently control impurity wavefunctions associated with single erbium optical centers in semiconductors.

Host:Mark Pitt

October 18

Friday 2:30pm
210 Robeson Hall

(poster)

Prof. Sophia Economou (Physics, Virginia Tech)

"Quantum Information Technologies: Quantum Networks, Near-term Processors and Algorithms"

Lately the field of Quantum Information Science and related technologies have attracted vigorous world-wide interest. These technologies include quantum computers that will be able to solve efficiently classes of important problems not accessible to classical computers, a new generation of sensors with improved sensitivity, and the ‘quantum internet’, an inherently secure network for communicating and accessing remote quantum computers securely. I will discuss the status of these technologies, showcase exciting advances, and present highlights from my group, including our work on quantum networks and on quantum algorithms for the simulation of chemistry problems with near-term quantum processors.

Host: Mark Pitt

October 25

Friday 2:30pm
210 Robeson Hall

(poster)

Prof. Michael Schull (University of CO and UNC-Chapel Hill)

"Where do Galaxies End?

I will review recent observations and theoretical estimates of the spatial extent of galaxies. Galaxies are defined as systems of stars and gas embedded in extended halos of dark matter and formed by the infall of smaller systems. Their sizes are determined by gravitational structures, gas dynamics, and chemical enrichment in heavy elements produced by stars and blown into extragalactic space by galactic winds. The full extent of galaxies remains poorly determined. The “virial radius” and “gravitational radius” provide estimates of the separation between collapsed structures in dynamical equilibrium and external infalling matter. Other measurements come from X-ray emission and ultraviolet absorption lines from metal-enriched gas in galactic halos. Astronomers have now identified large reservoirs of baryonic matter in the circumgalactic medium (CGM) and intergalactic medium (IGM) that contain 50-70% of the cosmological baryons formed in the Big Bang. The extent of the bound gas and dark matter around galaxies such as our Milky Way is approximately 200 kpc (650,000 light years). Investigations of physical processes at the “edge of galaxies” are crucial for interpreting new observations of the CGM and IGM, and their role in sustaining the star formation in galaxies.

Host: Nahum Arav

November 2019
November 1

Friday 2:30pm
210 Robeson Hall

(poster)

Prof. Ewelina Hankiewicz (Theoretical Physics, University of Wurzgburg)

"Unconventional Superconductivity in Topological Insulators and Rashba 2DEGS"

A topological insulator in the proximity to an s-wave superconductor is the prefect material to detect signatures of Majorana fermions. S-wave superconductor on the top of the surface states of 3D TI generates s-wave and p-wave pairing mixture in the surface state due to the spin-momentum locking [1,2]. We predict that in the Josephson junction setup, namely superconductor (S) /surface state of topological insulator/superconductor (S), existence of this p-wave component leads to novel features in transport like superconducting Klein tunneling i.e. the perfect transmission of hybridized Majorana states for normal incidence, the non-sinusoidal current phase relation [2] and unusual phase-dependent thermal conductance [3]. Further, we propose the experimental setups to observe signatures of Majorana fermions in the ac Josephson effect on TI hybrid structures [4] and in phase controlled Josephson junctions based on Rashba 2DEGs [5,6]. [1] L. Fu and C. L. Kane, Phys. Rev. Lett. 100, 096407 (2008). [2] G. Tkachov and E. M. Hankiewicz, Phys. Rev. B 88, 075401 (2013). [3] B. Sothmann and E. M. Hankiewicz Phys. Rev. B 94, 081407(R) (2016); B. Sothmann, F. Giazotto, and E. M. Hankiewicz New J. Phys. 19, 023056 (2017). [4] F. Dominguez, O. Kashuba, E. Bocquillon, J. Wiedenmann, R. S. Deacon, T. M. Klapwijk, G. Platero, L. W. Molenkamp, B. Trauzettel, and E. M. Hankiewicz Phys. Rev. B 95, 195430 (2017). [5] H. Ren, F. Pientka, S. Hart, A. T. Pierce, M. Kosowsky, L. Lunczer, R. Schlereth, B. Scharf, E. M. Hankiewicz, L. W. Molenkamp, B. I. Halperin and A. Yacoby, Nature 569, 93(2019). [6] B. Scharf, F. Pientka, H. Ren, A. Yacoby, and E. M. Hankiewicz Phys. Rev. B 99, 214503 (2019).

Host: Giti Khodaparast

November 8

Friday 2:30pm
210 Robeson Hall


(poster)

Prof. Rodney D. Van Meter Keio University

"The quantum Internet"

The Quantum Internet will transform how we communicate, compute and measure our universe. Using long-distance entanglement, we can execute cryptographic functions that strengthen the security of classical communications, measure physical phenomena more precisely than purely classical systems, and ultimately couple quantum computers together to extend the size of problems they can solve. But how can we build such a network? Experimental progress toward quantum repeaters -- the quantum equivalent of the Internet's switches and routers -- is moving at a dizzying rate, and theorists have proposed half a dozen approaches to managing errors to create high-fidelity entanglement along a chain of repeaters. The next frontier is extending from one-dimensional chains to complex topologies, and on to a network of networks -- a true Quantum Internet. In this talk, I will bridge the physics and the engineering to give a picture of the current status and open research problems.

Host: Sophia Economou

November 15

Friday 2:30pm
130 Hahn Hall North

(poster)

Prof. Arjun Yodh (University of Pennsylvania)

“Phase Transitions, Relaxation and Shape Transformations in Soft Materials”

I will describe experiments that probe phase transitions and relaxation phenomena in colloid solids and super-cooled liquids, and shape transformations in liquid crystal drops. The experiments with colloids capture the mechanisms that facilitate crystal-crystal phase transitions and elucidate the origin of non-exponential relaxation in super-cooled liquids. Experiments with drops containing liquid crystal oligomers exhibit remarkable shape transitions which depend on temperature and oligomer chain distribution in surprising ways.

Host: Shengfeng Cheng

November 22

Friday 2:30pm
130 Hahn Hall North

(poster)

Prof. Jennifer Ross (Physics, Syracuse University)

“Self-Assembly and Self-Propulsion of Active Biological Elements”

Abstract: The cell is a complex autonomous machine taking in information, performing computations, and responding to the environment. Many of the internal structures and architecture is transient and created through active processes. Recent advances in active matter physics with biological elements are opening new insights into the physics behind how cellular organizations are generated, maintained, and destroyed. I will present two recent stories on two different topics at the interface between biological and soft matter physics. The first will discuss self-organization of microtubules in the presence of “weakly interacting” crosslinkers. The second will discuss possible mechanisms for the cell to mix itself using self-propelled single molecule enzymes. These works illustrate the importance of the fundamental physics to build structures and propel matter inside living cells while informing on new physics we can learn from biological elements and materials.

Host: Rana Ashkar

November 29

Friday 2:30pm
210 Robeson Hall

(poster)

Thanksgiving Holiday (No Classes)

Host:

December 2019
December 6

Friday 2:30pm
210 Robeson Hall
(poster)

Prof. Bhuvana Srinivasan (Aerospace & Ocean Engineering, Virginia Tech)

“Numerical Studies of High-energy Density Fusion and Astrophysical Plasma's”

Experimental efforts to study nuclear fusion and astrophysical plasmas have produced significant scientific advances but remain challenged by diagnostic access and limitations.  Hence, there is a need for high-fidelity computational models in plasma physics to support experiments.  Plasma physics will be introduced in this talk along with a brief description of plasma models and their limitations. Recent advances in plasma fluid and kinetic modeling have supported development of sophisticated simulation tools. A hierarchy of models, ranging from magnetohydrodynamic (MHD) to fully kinetic, are developed and applied across a wide range of parameter regimes in the Plasma Dynamics Computational Laboratory at Virginia Tech. Some representative applications will be presented with a focus on high-energy-density plasmas for fusion and astrophysical studies.  Other research topics being pursued in the computational laboratory will be discussed briefly.

Host: Nahum Arav

December 13

Friday 2:30pm
210 Robeson Hall
(poster)

Exam Day (No Classes)

Host:

December 20

Friday 2:30pm
210 Robeson Hall
(poster)

University and Graduate Ceremonies (No Classes)

Host:
December 27

Friday 2:30pm
210 Robeson Hall
(poster)

Holiday (No Classes)

Host:
December 30

Friday 2:30pm
210 Robeson Hall
(poster)

Holiday (No Classes)

Host: