Unless indicated otherwise, Physics Colloquia are held on Fridays in Room 2310 at 701 W. Grace Street and begin at 4:00pm, with coffee at 3:50pm.



Spring 2017 Physics Colloquiua



Wednesday, April 26, 2017, 1:00 p.m.

Literature Seminar:
Iron Oxide Nanoparticles
as a Theranostic Agent

Patrick Woodworth

Department of Physics
Virginia Commonwealth University

   The development of nanoparticles with combined therapeutic, diagnostic and imaging capabilities would greatly impact how we treat many diseases today[1]. An all-in-one agent would allow doctors the ability to monitor, diagnose and treat diseases, such as cancer, quickly and on a case by case basis. This is the goal of nanotheranostics and why there is great interest in this area of research. Nanoparticles play a key role in nanomedicine, they can efficiently carry and deliver imaging probes, therapeutic agents, or biological materials to targeted sites. They also possess active functions that facilitate their use as nanoprobes for imaging/sensing or agents for therapies[2]. Many nanomaterials are already imaging agents and can be easily converted to theranostic agents by the addition of therapeutic functions on them[3].

 Inorganic nanoparticles as carriers offer the advantage of being very stable and highly resistant to degradation. However, nanotoxicity is a major concern with inorganic nanoparticles containing heavy metal atoms which requires a biocompatible surface coating[2]. Magnetic nanoparticles have been used in many biomedical applications, such as drug and gene delivery, where they are used as nanocarriers for magnetic-field-directed targeting of therapeutic agents to a biologic site of interest. They are also useful for cancer therapy, which involves localized heating produced by coupling these particles to an alternating magnetic field[4]. Specifically iron oxide nanoparticles are quickly becoming the preferred choice in biomedicine due to their biocompatiblity and cost. Superparamagnetic iron oxide nanoparticles have been extensively utilized for bioseparation, biosensing, and magnetic field assisted drug and gene delivery, as well as for magnetic hyperthermia, and have they have been shown to be excellent contrast agents for MRI’s[5]. These particles can be synthesized with a narrow size distribution, making them ideal for probing and manipulating, and can be easily removed from the body.

 In the article by Espinosa et al., they show that iron oxide nanoparticles have the dual capacity to act as both magnetic and photothermal agents. They chose iron oxide nanocubes for their high efficiency for the magnetic hyperthermia modality itself. Hyperthermia is used in cancer therapy in which body tissue is exposed to high temperatures, typically with other forms of cancer treatment, such as radiation or chemotherapy. Photothermal therapy is an experimental cancer treatment mediated by metallic nanoparticles or even semiconducting carbon nanotubes or graphene, which can be activated by near-infrared light, where the absorption of tissues is minimal. However, both magnetic and photothermal approaches have their disadvantages. Some of these include the fact that high doses of laser irradiation can damage normal tissue and that some of the nanoparticles might be biopersistant and potentially toxic. In contrast, iron oxide nanoparticles have already been approved for human use in anemia treatments, as magnetic resonance imaging contrast agents and hyperthermia. They also have excellent biodegradibility in vivo, and the iron ions they release upon dissolution can be assimilated by the body. Exposing the nanocubes to both an alternating magnetic field (hyperthermia) and near-infrared laser irradiation (photothermal therapy) was shown to amplify the heating effect, resulting in reduced tumor growth and in some cases complete tumor regression[1].

 REFERENCES:
[1] A. Espinosa, R. Di Corato, J. Kolosnjaj-Tabi, P. Flaud, T. Pellegrino, and C. Wilhelm, “Duality of Iron Oxide Nanoparticles in Cancer Therapy: Amplification of Heating Efficiency by Magnetic Hyperthermia and Photothermal Bimodal Treatment,” ACS Nano, vol. 10, no. 2, pp. 2436-2446, Feb. 2016.
[2] G. Chen, I. Roy, C. Yang, and P. N. Prasad, “Nanochemistry and Nanomedicine for Nanoparticle-based Diagnostics and Therapy,” Chem. Rev., vol. 116, no. 5, pp. 2826-2885, Mar. 2016.
[3] J. Xie, S. Lee, and X. Chen, “Nanoparticle-based theranostic agents,” Adv. Drug Deliv. Rev., vol. 62, no. 11, pp. 1064-1079, Aug. 2010.
[4] A. J. Giustini, A. A. Petryk, S. M. Cassim, J. A. Tate, I. Baker, and P. J. Hoopes, “MAGNETIC NANOPARTICLE HYPERTHERMIA IN CANCER TREATMENT,” Nano LIFE, vol. 01, no. 01n02, pp. 17-32, Mar. 2010.
[5] S. M. Janib, A. S. Moses, and J. A. MacKay, “Imaging and drug delivery using theranostic nanoparticles,” Adv. Drug Deliv. Rev., vol. 62, no. 11, pp. 1052-1063, Aug. 2010.

Acrobat .pdf file of abstract:

Woodworth Abstract



April 28, 2017

A Look at Selected Projects
in NASA’s
Planetary Science Portfolio

Bill Knopf

NASA Headquarters
Washington, D.C.





      Past Spring 2017 Colloquia

Friday, January 20, 2017, 4:00 p.m.

   Physics of the B Meson

Muruges Duraisamy

Department of Physics
Virginia Commonwealth University

duraisamypic

   The Standard Model (SM) of particle physics, even though very successful, is expected to break down at some energy scale and make way for a more complete theory. Exploration of what lies beyond the SM can be carried out at the energy frontier in colliders such as the LHC or at the intensity frontier at high luminosity experiments. In the intensity frontier, the B factories, BaBar and Belle, have produced an enormous quantity of data in the last decade. There is still a lot of data to be analyzed from both experiments. The B factories have firmly established the CKM mechanism as the leading order contributor to CP violating phenomena in the flavor sector involving quarks. New physics (NP) effects can add to the leading order term producing deviations from the SM predictions. In this talk, I will give a brief overview of the physics of B meson and CP violation, and discuss some of the current experimental results in various B meson decays.

Acrobat .pdf file of abstract:

Duraisamy Abstract



Friday, January 27, 2017, 4:00 p.m.

Using Physical Principles to Predict Product Shelf Life

David B. Kane

Altria Client Services

kanepic


Friday, February 3, 2017, 4:00 p.m.

   Defects in Semiconductors

Michael Reshchikov

Department of Physics
Virginia Commonwealth University

   In spite of many years of research, point defects with deep levels in semiconductors are still not well understood. The defects create unwanted paths of charge carriers recombination, which leads to premature breakdown in high-power electronic devices, reduces efficiency of the light-emitting devices and shortens their lifetime. Gallium Nitride (GaN) is a relatively new semiconductor, which is currently used in blue light emitting devices (LEDs, laser diodes), and is expected to transform all lighting technology in near future. Point defects in GaN can be studied by several techniques, among which photoluminescence (PL) appears to be the strongest tool.
In this presentation, the history of investigations into point defects in semiconductors will be reviewed, including showing interesting examples where incorrect theoretical predictions caused biased and incorrect explanations of experimental results and vice versa. A simple configuration coordinate model will be used to explain PL spectra from defects. The PL results will be compared with theoretical predictions and experimental results obtained by using other techniques, such as deep-level transient spectroscopy (DLTS) or positron annihilation spectroscopy (PAS).

Acrobat .pdf file of abstract:

Reshchikov Abstract


Friday, February 10, 2017, 4:00 p.m.

Fibrin Fiber Formation and Mechanics

Christine Helms

Department of Physics
University of Richmond

   Fibrin fibers are a major constituent of blood clots. They perform the mechanical task of stemming the flow of blood. The structure and strength of fibrin fibers relates to the medical outcome of an individual. Therefore, we measure the structure of fibrin clots and strength of individual fibrin fibers to understand better the mechanism(s) leading to poor clinical outcomes. Previous research showed that environmental factors, such as pH and ion concentration, affect clot structure and fiber diameter. Recently we added to that body of work by showing that high concentrations of nitric oxide also affect clot structure through the oxidation of important proteins. Conditions with altered clot structure often have an altered rate of fiber formation, altered fiber diameter and altered clinical responses, as well. This leads to the question, what is the mechanism responsible for the change? If we could understand the individual fibers, we may understand why changes to the clot are associated with heart attack and stroke. Therefore, we measured the modulus of an individual fibrin fiber using the atomic force microscope. Fibrin fibers have interesting mechanical properties from a materials standpoint because they form a regular structure but are extremely extensible. In addition, we found that the modulus of fibrin fibers is dependent on the diameter of the fiber, suggesting irregular density inside the individual fibers. In this talk, I will discuss the role of fibrin in health and try to convince you that the mechanism responsible for altered clot structure and stiffness is packing of the monomers inside the individual fibers. I will do this through the presentation of our recent data on the modulus of fibrin fibers.

Acrobat .pdf file of abstract:

Helms Abstract


Friday, February 17, 2017, 4:00 p.m.

What We Know So Far:
An Introduction to the Standard Model

Kevin Grizzard

Department of Medical Physics
Virginia Commonwealth University

   The modern understanding of nature at its most fundamental level is built on two foundations. One is general relativity, which describes gravity and the large scale structure of the universe. The other is commonly known as the Standard Model of particle physics (“the SM”), which incorporates quantum mechanics and special relativity in a description of the known elementary particles and their interactions via the electromagnetic force, the strong nuclear force, and the weak nuclear force. It also describes how elementary particles acquire mass via the Higgs mechanism, and the detection of a Higgs boson at the Large Hadron Collider in 2012, some fifty years after its proposal, was one of the greatest confirmations of a theoretical prediction in history. I will give an overview of the SM, noting some of its motivations and successes while emphasizing its essential conceptual features (e.g., the least-action principle; symmetries including Poincare invariance and gauge symmetry; the Higgs mechanism).

Acrobat .pdf file of abstract:

Grizzard Abstract


Friday, February 24, 2017, 4:00 p.m.

Highly Efficient Nanostructured
“Smart Coatings”
by Self-Assembly Fabrication

Antonio Checco

Soft & Biomolecular Materials
Brookhaven National Laboratory

   A current challenge in materials science is the fabrication of highly efficient “smart interfaces” with extreme and reconfigurable wetting, adhesion, and friction properties, or exquisite selectivity to target biomolecules. Here we demonstrate novel, large area superhydrophobic/anti-fogging silicon surfaces with ~20 nm feature size defined by block-copolymer self-assembly and plasma etching. We investigate by means of optical and scanning probe microscopies, and x-ray scattering how the nanoscale texture morphology influences macroscopic water wettability, resistance to water infiltration under (static and dynamic) pressure, dew formation, and hydrodynamic slippage. Our findings show that fine-tuning the texture size and morphology is crucial to optimal superhydrophobic, anti-fogging, and water slippage properties. Further, we illustrate strategies for further functionalization of the nanostructured silicon templates using graphene or membrane proteins for enabling the selective, tunable, and efficient translocation of water, or target ions and biomolecules.

Acrobat .pdf file of abstract:

Checco Abstract


Wednesday, March 1, 2017, 1:00 p.m.

Ultrafast Fluid Dynamics
and Cavitation Studies
with X-ray Lasers

Claudiu Stan

Soft & PULSE Institute
Stanford National Accelerator Facility
Meno Park, CA

   The extreme intensity of X-ray lasers, combined with their angstrom wavelengths and femtosecond pulse durations, enable scientists to observe the instantaneous structure of matter with atomic scale resolution. Another promising but less explored application of X-ray lasers is to drive rapid processes and transformations in materials. I will present our investigations on the dynamics of X-ray laser ablation in liquid microjets and microdrops. We found that the phenomena induced by X-ray lasers have unique features compared to the case of optical ablation. In particular, the X-ray laser produced highly symmetric liquid explosions, and we were able to model their basic fluid dynamics. In a following study, we used shock waves produced by X-ray lasers to induce and study cavitation in water on a few-nanosecond time scale. At these time scales, cavitation occurred in extremely metastable conditions, characterized by negative pressures that exceed significantly those achieved previously in bulk water. Our cavitation experiment enables the study of water under highly metastable conditions, and provides an avenue to understand the nucleation of cavitation in water. More generally, X-ray laser ablation has the potential to control with (sub)nanosecond precision the nanoscale dynamics of pressure-driven processes such as nucleation, phase transitions, and mechanical failure.

Acrobat .pdf file of abstract:

Stan Abstract


Friday, March 3, 2017, 4:00 p.m.

Designing Advanced Materials
for Energy and Nanoelectronic
Applications

Liping Yu

Department of Physics
Temple University
Philadelphia, PA

  Today, the needs for new or improved functional materials are greater than ever. In this talk, I will present our recent research advances in designing new materials for energy and nanoelectronic applications. I will focus on three examples: (i) designing super solar-light absorbing materials for nanoscale thin-film solar cell applications, (ii) designing highly conductive oxide interface materials for next-generation nanoelectronics, and (iii) designing functional layered two-dimensional materials for flexible electronics and energy applications. Some newly discovered functional materials, their experimental validation, as well as the underlying structure-property relationships (or design principles) will be presented. Along with these examples, I will show an inverse materials design approach powered by quantum-mechanical density functional theory and high-throughput first principles calculations. This approach places functionality first, searches for the material that has a set of physical properties optimized for such functionality, and aims to dramatically shorten the process of finding new materials. The research challenges and opportunities in the fields as exemplified above will also be briefly discussed.

 

Acrobat .pdf file of abstract:

Yu Abstract


Friday, March 17, 2017, 3:55 p.m.
Room 2310, 701 West Grace St. (Laurel St. Entrance)

Use the Force:
High-Bandwidth Nanosensing
for Materials Science and Healthcare

Loren Picco

H H Wills Physics Laboratory
University of Bristol, UK

  There is an unmet need for faster and more accurate measurements of nanoscale materials and devices. With compelling new applications in fields such as energy, electronics and healthcare there is an increasing drive to develop novel materials and scale-up from proof-of-concept demonstrations to practical implementations. Similarly, in industry, it is becoming apparent that understanding the nanoscale origins of metal fatigue and corrosion holds the key to more accurate lifetime estimations and the development of next generation coatings and components.

The high-speed atomic force microscope I have developed at the University of Bristol is an ideal diagnostic tool for the rapid characterisation of nanoscale surfaces. It is 1,000 times faster than a conventional atomic force microscope and enables the direct observation of molecular interactions and nanoscale processes with millisecond temporal resolution. In this talk I will discuss the development of the tool, including the underlying physics that empower it, key application areas enabled by these MHz measurements and future opportunities afforded by the technology.

Acrobat .pdf file of abstract:

Picco AbstractMarch2017



Friday, March 31, 2017, 4:00 p.m.

What Can Physics Tell Us
About Sickle Cell Disease?
(surprisingly, a lot)

Frank A. Ferrone

Department of Physics
Drexel University

  Sickle cell disease is a genetic disorder that affects the blood of about 100,000 Americans, and millions world-wide. It arises because a single point mutation allows the hemoglobin that fills the red cells to assemble into long, stiff, multistranded fibers. These fibers deny the red cell its necessary pliability, and thus clog the circulation, depriving the tissues of the oxygen they require. We now understand the mechanism by which this assembly proceeds in great detail. Remarkably, basic physical principles — entropic forces, Hooke’s law deformations, random barrier crossings, Brownian ratchets — play a significant role in understanding the nature of this disease, and in the strategies available to cure it.

Acrobat .pdf file of abstract:

Ferrone Abstract



Friday, April 7, 2017, 4:00 p.m.

Real Time Interrogation
of Surface Charge
to Investigate Surface
Chemistry in Solution

Julio Alvarez

Department of Chemistry
Virginia Commonwealth University

  The surface charge of a microchannel can be readily determined by measuring the voltage developed at two electrodes located at the outlets of the microchannel, when a liquid solution is pumped through by pressure driven flow. The spontaneous generation of this electrical potential difference, \Delta E, also known as streaming potential, is proportional to the liquid pressure and the surface charge. This is a result of the ionic and polar structure of the electrical double layer (EDL) at the surface-solution interface, which can extend up to ~30 nm into the solution. As the forward flow perturbs the local concentration of EDL counterions, a longitudinal gradient of charge is formed thus creating a \Delta E along the flow axis. Given that this phenomenon is surface driven and confined within the EDL, the effect is only detectable in micro-cells or conduits with small volume and large surface area. However, the polarity of \Delta E is a direct measure of the surface charge in contact with the solution. Values of \Delta E can range from a few mV up to several V depending on the pressure.

  This seminar is about the use of this principle to track real-time adsorption of biomolecules on surfaces and its application to biosensors. For the examples described, the pressure was maintained constant while changes on the surface charge induced by surface sensing and binding were monitored in real time. The most important aspect of this approach is that the analytical signal arises without the need of labeling the probes of interest and kinetic information about surface binding can be extracted. The extension of this technique to study other surface phenomena that rely on changes on the surface charge is briefly discussed.

Acrobat .pdf file of abstract:

Alvarez Abstract



April 14, 2017

Between topological strings
and topological phases

Jeffrey Teo

Department of Physics
University of Virginia

teopic

  Topological phases in two and three dimensions can be theoretically constructed by coupled-wire models whose fundamental constituents are electronic channels along strings. On the other hand, the collective topological phases support further fractionalized emergent quasi-string excitations or defects such as flux vortices. In this talk I will describe topological superconductors and Dirac (or Weyl) semimetals using coupled-wire models and discuss the fractional behavior of emergent topological strings.

Acrobat .pdf file of abstract:

Teo Abstract



Friday, April 21, 2017, 4:00 p.m.

Literature Seminar:
Combination
of Ionizing Radiation
and Gold Nanoparticles
in Cancer Therapy

Md Rezaul Karim Khan

Department of Chemistry
Virginia Commonwealth University

   Ionizing radiation is ubiquitous; we are constantly being exposed to the natural and artificial radiation. Exposure of high-energy ionizing radiation such as gamma rays or X-rays to living cellscan cause cancer, which is a leading cause of death worldwide and responsible for approximately 25 percent of all deaths in the USA and UK.1 Alternatively, this radiation can be used to destroy cancer cells using a procedure termed radiotherapy. Radiotherapy is a common primary treatment procedure for multiple malignancies, including cancers of the head and neck, breast, lung, and prostate.2 According to American National Cancer Institute around half of all cancer patients go through some type of radiotherapy during the course of their treatment. Depending on the type, size, and location of the cancer; total radiation dose varies in radiotherapy. To protect healthy cells, the total radiation dose is divided into several smaller doses over a period of several days known as fractionated radiotherapy. To achieve quality treatment, the doses are need to be properly measured using radiation sensors such as thermoluminescent detectors and scintillating detectors. Even though there are different kinds of sensors for radiation measurement, in many cases these dosimeters are not easy to handle and involve costly fabrication processes. Therefore there is a need for a simple visible sensor for fractionated radiotherapy. Nanotechnology combined with ionizing radiation can offer us an improved radiation sensor for better cancer treatment.3 In recent years, gold nanoparticles have attracted much attention of research for cancer treatment because of their simplistic synthesis and surface modification, strongly enhanced and tunable optical properties as well as excellent biocompatibility. A recent paper by Pushpavanam et al., addresses the fabrication of a simple and visible plasmonic nanosensor for radiation dose measurement during radiotherapy.2 Depending on the amount of ionizing radiation, colorless salt solutions of gold ions (Au^+) convert to different colors of plasmonic gold nanoparticles. A change in color can help ensure the ease of identifying the radiation dose with the bare eye. Another paper by Wolfe et al., reports about dose enhancement in cancerous cell in radiotherapy using gold nanoparticles. They show dose increment due to the interactions between ionizing radiation and gold nanoparticles via electron spin resonance dose measurement method in 2-Methyl-Alanine, a biological-equivalent sensitive material.4
References
1. Dreaden, E. C.; Austin, L. A.; Mackey, M. A.; El-Sayed, M. A. Size matters: gold nanoparticles in targeted cancer drug delivery. Therapeutic Delivery 2012, 3, 4, 457-478.
2. Pushpavanam, K., Narayanan, E., Chang, J., Sapareto, S., Rege. K. A Colorimetric Plasmonic Nanosensor for Dosimetry of Therapeutic Levels of Ionizing Radiation. ACS Nano 2015, 9, 12, 11540-11550.
3. Kwatra, D., Venugopal, A., Anant, S. Nanoparticles in radiation therapy: a summary of various approaches to enhance radiosensitization in cancer. Transl. Cancer Res. 2013, 2, 4, 330-342.
4. Wolfe, T., Guidelli, J.E., Gómez, A.J., Baffa, O., Nicolucci, P. oExperimental assessment of gold nanoparticle-mediated dose enhancement in radiation therapy beams using electron spin
resonance dosimetry. Phys. Med. Biol. 2015, 60, 4465-4480.

Acrobat .pdf file of abstract:

Khan Abstract




      Past Fall 2016 Colloquia

Friday, September 2, 2016, 4:00 p.m.

Physics Department Research Summaries

     Faculty gave descriptions of their research to the students, with five minutes for each faculty member. The presenters, in order, were:

Shiv Khanna
Yelena Prok
Tom McMullen
Joe Reiner
Puru Jena
Debian Ye
Denis Demchenko
Michael Reshchikov
Bob Gowdy
Jason Reed
Marilyn Bishop


Friday, September 9, 2016, 4:00 p.m.

Cluster-enhanced nanopore spectrometry

Joseph Reiner

Department of Physics
Virginia Commonwealth University

reinerpic

   Nanopore sensing is a powerful single molecule technique that utilizes Coulter-counting at the nanoscale. The principle of operation is straightforward. Individual molecules enter an isolated nanopore and block the flow of ions giving rise to current blockades that can be analyzed to learn about the size, charge and structure of a molecule. The technique is relatively easy to implement and it enables label-free, rapid and non-destructive detection of a wide variety of molecules. Recent interest in nanopore technology has grown with the commercial availability of a miniaturized DNA sequencer (Minion, Oxford Nanopore Technologies). This handheld sensor has demonstrated the potential to perform rapid genomic analysis in the field and motivates further study of the nanopore for detecting other molecules of interest. To further advance nanopore sensing, researchers have continued to focus on understanding the physical and chemical phenomenon that give rise to current blockades. This talk will describe my work in this area, which focuses on reducing blockade fluctuations and increasing analyte residence time with the use of metallic nanoclusters. The goal is to utilize cluster-based nanopore spectrometry to improve the selectivity of the pore and this will be demonstrated for a number of biologically relevant peptides.

Acrobat .pdf file of abstract:

Reiner Abstract



Friday, September 16, 2016, 4:00 p.m.

   Quantum Electrodynamics
and Condensed Matter:
How to Optimize the Number of Terms
in Perturbation Theory

Eugene Kolomeisky

Department of Physics
University of Virginia

kolomeiskypic

     In 1952 Dyson put forward a simple and powerful argument indicating that the perturbative expansions of QED are asymptotic. His argument can be related to Chandrasekhar’s limit on the mass of a star for stability against gravitational collapse. Combining these two arguments we estimate the optimal number of terms of the QED series to be (3.1)(137)^{3/2} \approx 5000. For condensed matter manifestations of QED in narrow band-gap semiconductors and Weyl semimetals, the optimal number of terms is around 80, while in graphene the utility of the perturbation theory is severely limited.

Acrobat .pdf file of abstract:

Kolomeisky Abstract


Friday, September 23, 2016, 4:00 p.m.

Watching Molecules Dissociate with Time-Resolved Spectroscopy

 Katharine Moore Tibbetts

 Department of Chemistry
Virginia Commonwealth University

tibbettspic

Ultrashort laser pulses make it possible to probe the dynamics of atoms and molecules on their natural timescales of femtoseconds to picoseconds through time-resolved “pump-probe” spectroscopic techniques. This talk will first discuss the properties of femtosecond laser pulses and how they can be measured using an interferometric technique called Frequency Resolved Optical Gating (FROG). Following this introduction, a brief overview of the principles, history, and techniques of pump-probe spectroscopy will be presented. The main portion of the talk will focus on using pump-probe measurements to study the dissociation of gas phase radical cations, including ionization mechanisms and detection of molecular dissociation products with time-of-flight mass spectrometry. Recent results of time-resolved pump-probe measurements on the dissociation of the acetophenone radical cation, along with supporting theoretical calculations of the relevant potential energy surfaces will be presented [1, 2]. From these theoretical and experimental results, the mechanisms of bond cleavage in acetophenone radical cation following excitation to electronic excited states are elucidated.
[1] T. Bohinski, K. M. Tibbetts, M. Tarazkar, D. A. Romanov, S. Matsika, and R. J. Levis,
The Journal of Physical Chemistry Letters 5, 4305 (2014).
[2] K. M. Tibbetts, M. Tarazkar, T. Bohinski, D. A. Romanov, S. Matsika, and R. J. Levis,
Journal of Physics B: Atomic, Molecular and Optical Physics 48, 164002 (2015).

Acrobat .pdf file of abstract:

Tibbetts Abstract


Friday, September 30, 2016, 4:00 p.m.

Nucleon Spin Structure: New results from Jefferson Lab

Yelena Prok

 Department of Physics
Virginia Commonwealth University

prokpic

     Understanding the spin structure of the nucleon remains an open challenge of particle physics, nearly 30 years after the initial discovery of the “spin crisis” by the European Muon Collaboration at CERN in 1980s. A vast set of polarization data has been accumulated over the next two decades at CERN, DESY and SLAC, with deep inelastic lepton-hadron scattering being a key tool in the investigation of the helicity structure of the nucleon. New data of unprecedented statistical precision and extensive kinematic coverage have become available more recently from the experiments conducted at Jefferson Lab. Together with the recent results from RHIC, COMPASS and HERMES, these new data allow us to constrain polarized parton distributions, test pQCD predictions in the valence region of high-x, and put limits on the gluon contribution to the nucleon spin. In this talk I will give a brief overview of the current knowledge of the nucleon spin structure, present most recent results from Jefferson Lab’s Hall B, and discuss the anticipated data from future experiments.

Acrobat .pdf file of abstract:

Prok Abstract


Friday, October 7, 2016, 4:00 p.m.

Understanding Patterns of Ferromagnetism in Three Dimensions

Anthony S. Arrott

Department of Physics
Simon Fraser University
and Virginia Commonwealth University
and Morgan State University

arrottpic


Spins carry magnetic moment, and it is the exchange interaction among electron spins that is responsible for ferromagnetism. The measurable quantity is the magnetization \vec{M}, which is the magnetic moment per unit volume. In an iron bar, there can be both an interior magnetic charge density from the divergence of \vec{M} (i.e., -\text{div} \vec{M}) and a surface charge density through \hat{n} \cdot \vec{M}, where \hat{n} is the surface normal. Similar to the way an electrical conductor acts against an external electric field to have the electrical field vanish in its interior, in the presence of an externally applied magnetic field, the magnetization pattern produces a magnetic surface charge density that creates a magnetic field that is almost equal and opposite to the applied field, although the cancellation is not as complete in the electric field case. In the absence of an applied field, the condition \hat{n} \cdot \vec{M} = 0 is a boundary conditions that results in, at least, a contribution to \text{curl} \vec{M}. If magnetostatics provided the entire explanation, \text{div} \vec{M} would vanish, but such a pattern does not minimize the exchange energy. This is because the exchange energy depends on the Laplacian of \vec{M}, which has contributions from both \text{curl} \vec{M} and \text{div} \vec{M}. By creating some \text{div} \vec{M}, the exchange energy is reduced. This results in a volume magnetic charge density, shown in the picture below, that in some regions is positive (red, \text{div} \vec{M} < 0) and in other regions negative (green, \text{div} \vec{M} > 0). The unlike charges attract, but they do not annihilate because that would increase the exchange energy. The patterns of \text{div} \vec{M} are the result of the rearrangement of spins so as to lower the magnetostatic interaction of the magnetic charges created by the exchange energy responding to the constraints of the presence or absence of surface charge density. (There is always some surface charge density in a singly connected body because topologically \hat{n} \cdot \vec{M} cannot be zero everywhere.) There are many ways that the magnetic charge can be arranged. The patterns are found solving 10^5 non-linear, integro-differential equations. They are nonlinear because the magnetization is a vector of constant magnitude, the integrals arise from the magnetostatics, and there are derivatives from the Laplacian describing the exchange energy. As can be seen in the picture below of surfaces of constant charge density for a ferromagnetic bar, in a cross section perpendicular to the axis of the bar, one pattern that occurs has the charge density arranged as extended quadrupoles with the positive charges on diagonally opposite corners of a square and negative charges as their nearest neighbors on the adjacent corners. Displacing the quadrupole center off the axis of the bar can further lower the magnetostatic energy. It can be lowered still further by having the quadrupole center trace out a helix from one end to the bar to the other. The period of the helix depends on the dimensions of the bar and the component of the applied magnetic field along the axis. In a long bar the period of the helix also can be modulated. These are new findings that were not anticipated in the 130 years since Ewing defined hysteresis. They do account for a mysterious observation unexplained for 40 years.

arrottabstractpic

Acrobat .pdf file of abstract:

Arrott Abstract


Friday, October 14, 2016, 4:00 p.m.

3D imaging Using Interference Microscopy

Jason C. Reed

Department of Physics
Virginia Commonwealth University

reedpic

Interferometric optical profilers deliver non-contact, fast, full-field measurements with vertical resolution down to a fraction of a nanometer. Over the last decade advancements to these instruments have allowed for the analysis of not only static but also dynamic objects, like cantilevers and other MEMS devices, moving or vibrating at up to 1 MHz frequencies. Special objectives and illumination allow for imaging and testing of objects enclosed in environmental or protective chambers or immersed in liquids — including biological samples.

Acrobat .pdf file of abstract:

Reed Abstract


Friday, October 28, 2016, 4:00 p.m.

Connections between Charge Density Wave Order in NbSe_2 and the Pseudo Gap Phase in Cuprate High Temperature Superconductors

Utpal Chatterjee

Department of Physics
University of Virginia

chatterjeepic

  Charge density waves (CDWs) and superconductivity are canonical examples of symmetry breaking in materials. Both are characterized by a complex order parameter – namely an amplitude and a phase. In the limit of weak coupling and in the absence of disorder, the formation of pairs (electron-electron for superconductivity, electron-hole for CDWs) and the establishment of macroscopic phase coherence both occur at the transition temperature T_c that marks the onset of long-range order. But, the situation may be drastically different at strong coupling or in the presence of disorder. We have performed extensive experimental investigations on pristine and intercalated samples of 2H-NbSe_2, a transition metal dichalcogenide CDW material with strong electron-phonon coupling, using a combination of structural (X-ray), spectroscopic (photoemission and tunnelling) and transport probes. We find that T_c(\delta) is suppressed as a function of the intercalation-concentration \delta and eventually vanishes at a critical value of \delta=\delta_c \;\;\;, leading to a quantum phase transition (QPT). Our integrated approach provides clear signatures that the phase of the order parameter becomes incoherent at the quantum/thermal phase transition, although the amplitude remains finite over an extensive region above T_c or beyond \delta_c. This leads to the persistence of a gap in the electronic spectra in the absence of long-range order, a phenomenon strikingly similar to the so-called pseudogap in completely different systems such as high temperature superconductors, disordered superconducting thin films and cold atoms.

Acrobat .pdf file of abstract:

Chatterjee Abstract


Friday, November 4, 2016, 4:00 p.m.

Investigation of Hydrogen Interactions with Metals and Carbon Nanostructures for Next Generation Energy Storage and Conversion Devices

Joseph Teprovich

Savannah River National Laboratory

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  Our experimental and theoretical investigation of the interaction of metal hydrides and complex metal hydrides with carbon nanostructure (C_{60}, CNT’s, etc.) has demonstrated that these composites reversibly interact with hydrogen. Through a series of spectroscopic analysis of these materials, the active hydrogen storage material resembles a metal-doped hydrogenated fullerene. Owing to our ability to judiciously control the metal doping and hydrogen content of these materials, we can fine-tune the properties of the materials for new applications. This led to the remarkable enhancement in lithium ion conduction in LiBH_4-C_{60} nanocomposites observed at room temperature. Experimental and theoretical work suggested a nanoionic mechanism is responsible for the enhanced ionic conduction due to the destabilization/breaking of the Li^+/(\text{BH}_4)^- ion pair by C_{60}. Our recent work has been focused on evaluating the photophysical properties of these carbon nanocomposites. The hydrogen content of these materials can be used fine-tune the emissive properties of the material with potential applications in luminescence down-shifting devices. This presentation will cover these findings in detail as well as on-going and future research on similar materials.

Acrobat .pdf file of abstract:

Teprovich Abstract


Friday, November 11, 2016, 4:00 p.m.

Can we play bowling with electrons?

Jongsoo Yoon

Department of Physics
University of Virginia

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  Much of the electronic properties of metals can be understood in the framework of the independent or free electron model, where the electron-electron interaction is ignored. In such a model electrons move freely within the metal, much like bowling balls rolling in a lane. Conventional theories expect that the ground state of the electron systems in two dimensions should be localized, or an electrically insulating state; in bowling analogy, the bowling balls rolled in a lane should never reach the pins.
Experiments, however, have shown a strong indication that the ground state is actually conducting. Although the mechanism behind the unexpected conducting ground state is still mysterious, the electron-electron interaction is believed to hold the key to the mechanism. Interestingly, unexpected conducting ground states with very similar characteristics are observed in systems where the electron-electron interaction is repulsive (electrons confined in the semiconductor interface) as well as the interaction is attractive (superconducting ultra-thin films where the superconductivity is suppressed).
In this talk, we concentrate on the experimental results of the mysterious conducting state observed on ultra-thin superconducting tantalum films. We will identify non-trivial electronic transport properties that are intrinsic to the mysterious conducting state, and map the three-dimensional phase diagram in magnetic field-temperature-disorder space.

Acrobat .pdf file of abstract:

Yoon Abstract


Friday, November 18, 2016, 4:00 p.m.

A glass transition in population genetics:
Emergence of clones in populations

Marija Vucelja

Department of Physics
University of Virginia

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  The fields of evolution and population genetics are undergoing a renaissance, due to the abundance of sequencing data. On the other hand, the existing theories are often unable to explain the experimental findings. It is not clear what sets the time scales of evolution, whether for antibiotic resistance, an emergence of new animal species, or the diversification of life. The emerging picture of genetic evolution is that of a strongly interacting stochastic system with large numbers of components far from equilibrium. In this talk, I plan to focus on the clone competition and discuss the diversity of a random population that undergoes selection and recombination (sexual reproduction). Recombination reshuffles genetic material while selection amplifies the fittest genotypes. If recombination is more rapid than selection, a population consists of a diverse mixture of many genotypes, as is observed in many populations. In the opposite regime, selection can amplify individual genotypes into large clones, and the population reaches the so-called “clonal condensation”. I hope to convince you that our work provides a qualitative explanation of clonal condensation. I will point out the similarity between clonal condensation and the freezing transition in the Random Energy Model of spin glasses. I will conclude with a summary of our present understanding of the clonal condensation phenomena and describe future directions and connections to statistical physics.

Acrobat .pdf file of abstract:

Vucelja Abstract


Wednesday, December 14, 2016, 4:00 p.m.
Room 2403, 701 West Grace St (Laurel Street Entrance)

Measuring and manipulating
2D and other nano materials
with high-speed atomic force microscopy

Loren Picco

H H Wills Physics Laboratory
University of Bristol, UK

  I will describe recent research highlights from my work at the University of Bristol. I will explain how our high-speed atomic force microscope can measure the sizes, shapes and material properties of tens of thousands of nanostructures with a speed and accuracy superior to typical electron microscopy techniques, providing unprecedented statistical confidence and insight into nanostructure distributions.
The tool has demonstrated the ability to distinguish between 2D materials such as TiS2, MoS, graphene, carbon nitride and black phosphorous as well as proving itself as a valuable tool for assessing critical industry challenges such as nanoscale corrosion and carburisation in metals.

Acrobat .pdf file of abstract:

Picco AbstractDec2016