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Research

Galactic Structure

DR. PETER FRINCHABOY

dr10.coverspec.300dpi (1) My primary research utilizes multi-wavelength observations (X-ray through mid-infrared) in the investigation of local stellar populations. Of particular interest is the use of star clusters and giant stars to probe the structure and evolution, chemical and dynamical, of the Milky Way and other Local Group galaxies. Related issues such as the role of environment and dark matter content of Miliky way satellite galaxies are also currently under study.

GALACTIC STRUCTURE & DYNAMICS

Understanding the structure of a galaxy from only one point of view is challenging, however being inside that galaxy makes it much more difficult. We understand more about other galaxies sturcture than we do for our own Milky Way. Using the Spitzer Space Telescope and Two Micron All-Sky Survey (2MASS) plus dynamical data, we are able to explore the Galaxy as never before. We are using Galactic stars clusters as dynamical probes to determine the structure and evolution of the Milky Way. TCU is also an Associate Member in the Sloan Digital Sky Survey (SDSS-IV) projects that provides faculty and students access to proprietary world class datasets.  

Galactic Chemical Evolution (SDSS-III/APOGEE)
In addition to the dynamical work on the Galaxy, I am also a leader of the Sloan Digital Sky Survey III/Apache Point Observatory Galactic Evolution Experiment (SDSS-III/APOGEE). SDSS-III/APOGEE will survey 100,000 stars in the galaxy to measure kinematics and provide detailed chemical analysis of 17 elements in these stars. Many of these stars will be in star clusters. Thus, we will have age-dated tracers with good distances and chemistry suitable for investigating the chemical evolution of the Galactic disk.

Stellar Populations
One way to recreate the history of a galaxy is by examining the stellar populations within that galaxy. The basis for understanding observations of other galaxies relies on a deep understanding of local stellar populations. The WIYN Open Cluster Study (WOCS), , aims to provide the most details stellar population study of key nearby star clusters.

My current WOCS research focuses on the infrared (near and mid-IR) properties of the WOCS clusters, which is being used to derive the lower end of the mass function, and binary star populations of the clusters. The primary facility for Mid-IR imaging is the Spitzer Space Telescope. Consisting of a 0.85-meter telescope and the cryogenically-cooled IRAC camera, Spitzer allows the WOCS clusters to be investigated by obtaining photometry at 3.6-8.0 microns. In addition to the WOCS optical data, we have obtained deep phtometry with the IRAC observations. Deep NIR (i.e., J,H,Ks) observation are being conducted with a number of ground bases facilities. Some of this work is done in collaboration with research groups at Texas A&M University and the University of Wisconsin.

Dark Matter and Local Group Galaxies
Cosmological Cold Dark Matter (CDM) models predict that a galaxy like the Milky Way should have 100’s of satelite gallaxies filled with dark matter. The reality is that the Milky Way has tens of satellites and how much dark matter they have is not well determined. A major complication in measuring this is that many of these galaxies are also being torn apart by the Milky Way. The dynamics of dwarf galaxies and stellar streams is the key to understand the evolution of dark matter on small scales. I have worked on exploring the Milky Way dwarf satellites, including Sagittarius, the Magellanic Clouds, GASS, Carina, Leo I and II, Ursa Minor, and Sculptor, as well as, investigating star clusters associated with the Galactic Anticenter Stellar Structure “GASS”, also know as the “Ring”, that may be associated with the proposed “Argo” or “Canis Major” dwarf galaxy. Much of this work is done in collaboration with research groups at the University of Virginia.

Galaxy Evolution

Dr. Kat Barger

Magellanic Clouds: Case study of inflows, outflows, and tidal bridges

Galaxy interactions displace the gas of the interacting galaxies. These interactions often lead to the formation of bridges, tails, and streams and also lead to the triggering of star formation across the individual systems. I exploit the gas surrounding the nearby Magellanic System to understand how galaxy interactions alter galaxy evolution and to understand the mechanics of gaseous inflows and outflows.

High Velocity Clouds

Galaxies evolve though a combination of internal and external process. Internally, galaxies are producing stars. As these stars evolve and as they die, they expel matter back into their surroundings, but not nearly enough to maintain star formation over long periods of time. Take the Milky Way for example, the Milky Way will run out of gas reservoirs in roughly 1 to 2 billion years (Larson et al. 1980). Since the the Milky Way has been forming stars for much longer than this and because the Milky Way will likely continue to form stars for quite a while, the Galaxy must be acquiring gas from an external source.

Surrounding the Milky Way are clouds of fast-moving hydrogen, with velocities inconsistent with galactic rotation, cover 40% of the sky in neutral hydrogen (e.g., Wakker & van Woerden 1991; Lockman et al. 2002). The gas around galaxies gas acts as a reservoir to replenish the gas in galaxies (e.g., Lehner & Howk 2011). The Milky Way acquires 20 to 40% of the Sun’s mass in neutral mass each year from these high velocity clouds (Wakker et al. 2008).

Fraction of Ionizing Radiation Escaping from Galaxies

At the epic of reionization, the gas around galaxies became ionized. When this happened, gas had a harder time flowing into galaxies, causing the build up of galaxies to slow. This phenomena might have also quenched galaxy formation at the smallest scales as gas had a harder time coalescing.

The ionizing radiation from galaxies might be the dominant source of the reionization of the universe (e.g., Madau et al. 1999; Bolton et al. 2005). However, high-mass galaxies alone are unable to reionize the universe (Fernandez & Shull 2011), while the contribution from low-mass galaxies is uncertain. The amount of ionizing radiation emitted by galaxies at both the present epoch and the epoch of reionization is poorly constrained.

Computational Biophysics

DR. HANA DOBROVOLNY

virus-rep-model My research uses mathematical models and computer simulations to understand and predict the behaviour of biological systems. I am particularly interested in studying disease processes and potential therapies or cures. The experiments and clinical trials used to study many diseases are very costly and time-consuming and the data we get are usually quite limited, so it’s often difficult to get a clear picture of which biological processes are important in causing a disease. This also makes it difficult to study different treatment regimens. By the time a drug makes it to a clinical trial, usually only a couple of different dose/timing regimens are tested in humans; not because they were found to be the optimal regimens after a thorough examination of all the possibilities, but typically based on the educated guess of the researchers heading the trial. An accurate computer model of the disease can not only help us understand the underlying dynamics of the disease but will be extremely helpful in assessing potential treatments. Computers can simulate thousands of different dose/timing regimens and will help doctors choose optimal regimens to test in patients.

Influenza

Influenza is a viral infection that affects millions of people every year. Most often, the illness is not serious and resolves on its own, but it has the potential to cause widespread illness and death during a pandemic. I use mathematical models of the infection process to study the causes of severe influenza, the emergence of drug resistance, the role of the immune response in clearing the infection, and antiviral treatment. The long-term goal of this research is to develop an accurate model of the infection in humans which can then be used to test a wide variety of drug treatment protocols and to simulate drug or vaccine treatment in high risk patients, reducing the risk to these patients.

Cardiac Arrhythmia

rpsIn a healthy heart an electrical pulse from the sino-atrial node causes a single electrical wave to propagate uniformly across the heart. This wave initiates the contraction that pumps the blood through your body. In a heart experiencing an arrhythmia, the electrical pulse generates waves that spread non-uniformly or break up into multiple waves. This causes different parts of the heart to contract at different times making it difficult for the heart to pump blood efficiently. I use mathematical models to try to understand how a heart becomes arrhythmic so that we can predict which patients should be given medication to prevent arrhythmias. I also use models to assess the effect of different anti-arrhythmic drugs on electrical activity in the heart.

Nanotechnology

Dr. Anton Naumov

My biophysics work is centered on the development and testing of drug delivery/imaging/sensing systems, based on carbon nanotubes and graphene. These nanomaterials do not only deliver molecular therapeutics in cells and tissues but also protect the normal tissue from possible adverse effects of their payload. Carbon nanotubes and graphene derivatives also possess a number of remarkable properties that allow them to serve as multimodal agents, providing simultaneous capabilities of drug transport, biological imaging via their intrinsic fluorescence, and biological sensing through to the change in their optical response. These models have a great promise for the advancement of molecular therapeutics of such complex conditions as cancer.

Another direction of my research is focused on controlled modification of the optical properties of graphene derivatives. Graphene is a novel nanomaterial that has multiple applications in modern electronics due to its high electrical conductivity, flexibility and transparency. In order to fully utilize outstanding properties of graphene in optoelectronics, we can induce optical response in graphene by controlled functionalization. This process also allows us to selectively modify optical properties of graphene derivatives in order to match the needs of a particular application. Such graphene-based structures can possess a broad range of optical characteristics and, thus, could serve as highly promising materials for a wide variety of modern optoelectronic device applications.

Fluorescence Spectroscopy

DR. KAROL GRYCZYNSKI

CaptureThe major goal of the Biophysics Group is to merge optics and fluorescence with nanotechnology in order to create new research and developmental frontiers for modern medical diagnostics, biotechnology, genomics, and proteomics. The scope of our research is to explore biologically relevant processes at cellular and molecular levels. The range of technologies we utilize is very broad and includes basic fluorescence (time-resolved fluorescence/fluorescence microscopy, practical applications of Forster resonance energy transfer (FRET) as well as advanced fluorescence that include multi-photon fluorescence, fluorescence of nanoparticles, fluorescence probe development, and plasmonic fluorescence (molecular fluorescence stimulated/controlled by metallic nanostructures).  To fully exploit biomedical opportunities, we closely work and share instrumentation with the Center for Commercialization of Fluorescence Technologies (CCFT) at the University of North Texas Health Science Center just 10 minutes from TCU Campus. This fosters a very productive and collaborative environment and combines skills in biomedical sciences, spectroscopy, microscopy, chemistry, engineering, and nanotechnology.  Goals of modern preventive medicine include the development of new technologies for efficient diagnosis of diseases in their early stages and the detection of risk factors for a specific disease in an individual patient (personalized medicine). Furthermore, new and successful medical treatments will often require technologies capable of monitoring therapy progress at the cellular level using non-invasive or minimally invasive approaches. These requirements call for extremely sensitive diagnostic technologies and ultrasensitive non-invasive imaging technologies. Advanced fluorescence today is the leading technology for ultrasensitive non-invasive detection with ultimate sensitivity in the single molecule level.  Our facilities are equipped with state-of-the-art fluorescence instrumentation that only very few laboratories in the world have. We developed highly collaborative research model and our collaborators come from leading institutions in US and the World.

Basic Spectroscopy:  UV-Vis-NIR, Absorption, Fluorescence, Time-resolved Fluorescence Spectroscopy, Fluorescence Lifetimes, Multi-Photon Fluorescence, Circular Dichroism, Linear Dichroism.
These are basic optical technologies that we develop and we utilize for biomedical applications.

Forster (Fluorescence) Resonance Energy Transfer (FRET). Macromolecular Structure/Dynamics.
FRET is probably the most utilized physical phenomenon to study molecular and cellular processes in-vivo and in-vitro.

Nanophotonics and Plasmonics.
Recent developments in nanotechnology open new possibilities for many optical technologies. Fluorescence takes advantage of quantum-photonic interactions of fluorophores with surface plasmons in nanometer thin metallic films and nanostructures promoting new concepts for developing modular early detection devices for fast and reliable biochemical and biomedical detection and sensing.

Fluorescence Microscopy and Single Molecule Detection. 
Fluorescence microscopy becomes a fundamental tool for studying molecular processes on cellular level. In combination with emerging advances in nontechnology and plasmonics we are developing new imaging methods to study biological processes.

Detecting Physiological Markers. Cancer Detection. 
Based on new technologies early reliable detection of physiological markers has the potential to significantly lower mortality related to ischemic heart disease, myocardial infarction, or cancer.

Photosynthetic Processes.
Understanding the energy pathway in photosynthetic processes is not only crucial element for understanding plant physiology, but also may help in developing new efficient technologies for solar energy conversion.

Optoelectronic and Structural Properties of ZnO-Based Devices and Systems

DR. YURI STRZHEMECHNY

Despite the fact that ZnO has been a widely and efficiently employed for centuries as a material of choice in many areas, its new promising applications became apparent only recently, with the advent of novel growth techniques producing bulk ZnO crystals of a very high quality. This translated into a high potential for optoelectronics, spintronics, and high-temperature, high-power microelectronics.

For many of these applications, and especially for the nanoscale-based, the condition of the surface and the subsurface region is a key performance-defining factor. Because of the large surface/volume ratio in ZnO nanostructures, device performance is determined essentially by the surface and near-surface properties. Current understanding of the relationship between the morphology of the ZnO nanostructures and their defect properties is still largely incomplete. The nature of the surface and sub-surface defect states is still ambiguous and only in a small number of studies in the past few years attempts were made to correlate properties of these states with the morphology of the nanocrystals themselves on the one hand and on the other hand to modify these states in a controllable fashion. In our studies we investigate a number of different ZnO systems, for which studies of surface/interface defect properties of ZnO may yield a significant outcome. In particular, a distinct class of surface/interface processes of interest for us is the influence of defects on the optolelectronic phenomena and structural properties in various ZnO nanostructures. We aim to extend our understanding of the fundamental mechanisms in such systems and their influence on the valuable applied properties.

Due to its potential optoelectronic applications (lasers and light emitters, planar waveguide devices, flat panel displays, etc.), the rare earth (Er, Tm, Tb, Dy) doping of semiconductors, and ZnO in particular, is an important field of study. In such materials, the shielded 4f levels of the rare earth (RE) ions produce narrow optical transitions. Despite the fact that the rear earth ions are excellent candidates for luminescent centers, their matrix environment often limits the luminescence efficiency. It is important to elucidate the optimum conditions for high-efficiency luminescence of those materials, and ZnO is one of the most important representatives. In addition, the rare earth ion dopant can serve as a sensitive probe of the chemistry and structure of its host. We are interested in addressing numerous problems and unresolved issues in this area such as difficulties with incorporating the RE ions into the host ZnO lattice, unknown mechanisms of energy transfer from the matrix to the RE species, almost unidentified relationship between their structural and optical properties. There is no clear model of the upconversion transitions in the RE-doped ZnO systems.

Experimental Facilities
Experimental instrumentation available at our lab includes: photoluminescence (PL) spectroscopy, Raman spectroscopy, and a high-vacuum (HV) analysis/processing multi-chamber setup. One of the main advantages of this multifunctional HV system is a versatile combination of in situ processing and characterization tools. It includes remote plasma treatment simultaneous with resistive annealing and ability to accommodate a number of subsurface-sensitive and surface-specific spectroscopic probes such as surface photovoltage (SPV) spectroscopy and Auger electron spectroscopy (AES).

We employ remote plasma as a tool for tailoring surface and sub-surface properties. Remote plasma processing refers to the arrangement, in which the surface-plasma contact occurs outside the plasma-generating region. The main advantage of using remote plasma follows from the fact that the chemically driven changes at the surface occur without significant temperature variations. This allows a separate control of the temperature at the surface of a specimen. In our research we demonstrated that well-defined remote-plasma treatment procedures allow control and qualitative improvement of key performance parameters of the studied surfaces.

SPV is known for its ability to detect surface states and distinguish their charge sates and donor- vs. acceptor-like nature. It is based on a vibrating Kevlin probe positioned near the surface of the sample of interest. Light of a variable frequency generates transitions from/to the gap states, modifies the population of the surface states, and induces changes in the surface barrier heights. This translates into the variation of the surface potential detected by the Kelvin probe. The SPV technique offers notable advantages: identification of conduction vs. valence band nature of the deep level transitions and the deep level positions within the band gap; ability to measure surface defect densities less than 1010 cm-2 as well as their cross sections. Additional information can be deduced from the SPV transient measurements. The adjacent AES probe provides surface-specific information about surface stiochiometry and contamination.

Complementary to the in vacuo spectroscopic probes, we have in our lab a multi-purpose optical bench setup allowing PL and Raman studies in a wide range of temperatures (6K – 320K), polarizations, and geometries, as well as several laser beams: ultraviolet and visible continuous wave HeCd and ultraviolet pulsed nitrogen. This facility provides information about optoelectronic, structural and chemical properties of the studied systems.

Statistical Physics

DR. BRUCE MILLER

Quantum Localization and Self-trapping
The average deBroglie wave length of an itinerant electron or positron equilibrated in a fluid is usually much greater than the mean inter-atomic spacing, so either may simultaneously interact with many atoms. Consequently self-trapping may occur, where the light particle “digs” a potential well for itself in the fluid and localizes in the self-induced ground state. The stability of the trapped state depends sensitively on the thermodynamic properties of the fluid and is appreciable in the neighborhood of the critical point. Experimentally, localization alters the decay rates of the positron and positronium, and the mobility of electrons.

Starting in 1990 we employed the Feynman-Kacs path integral to explore the relation between the quantum states of the light particle and local fluctuations in the fluid. We used path integral Monte Carlo (PIMC) to study positron annihilation in fluids, and to develop simulated data that provides a benchmark for testing the predictions of various theories. We also derived a quantum virial expansion for the average properties of the particle at low density, and used PIMC to evaluate the coefficients. The method was used to study the temperature dependence of the positron lifetime in a dilute gas. Recently we have used PIMC to study self-trapping at the liquid-vapor critical point of a Lennard-Jones fluid.  We have also adapted the path integral method to represent a quantum particle on a lattice where, in principle, we can study both critical phenomena and Anderson Localization. This results in rapid convergence and permits us to test theories of localization over a wide parameter range. We plan to use the lattice model to study the influence of critical point fluctuations on quantum localization.

Nonlinear Dynamics

The fundamental assumption of statistical mechanics is that dynamical systems are “chaotic”. For decades physicists assumed that chaos occurs when a stable dynamical system is perturbed. However, simulations using the first vacuum tube computers broke the proverbial bubble by showing that most of the phase space remains stable under small perturbation. A breakthrough came in 1954 when three mathematicians were able to explain the structure of phase space and its separation into stable and chaotic regions. By studying accelerated billiards with discontinuous boundaries, we have found new sources of orbital instability. We developed a simple model, the wedge billiard, which exhibits the complete range of Hamiltonian chaos. The model has been used successfully as a teaching tool and formed the basis for some deep theorems on the ergodic properties of a many body system. Recently it has been studied experimentally using cold atoms. We have extended our approach with a hyperbolic boundary that can be continuously deformed into either a parabola or wedge. This is also a rich system that shows how nearly integrable behavior can be connected by chaotic regimes. Currently we are investigating a quantum mechanical version of the wedge billiard.

Gravitational Evolution, Equilibrium, and Fractal Geometry
As a star ages, it radiates energy, heats up, and contracts, so its heat capacity is negative. In an ordinary physical system (e.g. a glass of water) this would not be possible. Because the gravitational force is purely attractive and of infinite range, the evolution and thermodynamic stability of astronomical objects such as galaxies and globular clusters is subtle and complex. In our group, we have used idealized models to study the nature of gravitational evolution in depth. Our studies of a system of parallel mass sheets have demonstrated weak chaos suggesting that stable regions in the phase space trap the system for long times. Relaxation to equilibrium was demonstrated for the first time in a planar system with two distinct populations. The evolution of a system of concentric mass shells approaches equilibrium much more rapidly than its planar counterpart. Mean field theory predicts that a phase transition occurs in this system at sufficiently low energy. This was verified for the first time by our group using dynamical simulation. We have also investigated how rotation influences core collapse. In other work, we have constructed a completely integrable cousin of an N-body gravitational system that, along with the energy, conserves an N dimensional generalization of angular momentum. Recently, by including the Hubble expansion in the evolution, we are studying the multifractal properties of clustering in models of a matter-dominated universe.

Theoretical Atomic and Molecular Physics

DR. MAGNUS RITTBY

The research in the group is focused on the development and application of quantum theoretical techniques for the study of atomic and molecular systems. Projects range from the study of the structure and properties of molecular clusters to the development of new theoretical and computational techniques as well as more fundamental questions regarding the interpretation of quantum theory.

Molecular Clusters
Pure and mixed molecular clusters of carbon, silicon, and germanium atoms provide an interesting and challenging group of molecular systems to a theorist. Sufficiently high-level theoretical methods have to be employed to provide accurate data to be used in conjunction with the analysis of experimental data.

In these theoretical studies we employ state-of-the-art computational techniques to solve the quantum mechanical many-body electron problem in the Born-Oppenheimer approximation. These techniques includes the so called coupled cluster methods where the electronic wave-function is essentially expressed in an infinite sum with certain constraints that lead to a finite computational scheme. Relatively recently, an alternative approach, the density functional method (DFT), has been developed for the description of electronic ground states. Here, instead of attempting to describe the electronic wave function, the focus is on calculating the electron density. Such DFT techniques can provide very accurate information at a very modest computational cost and enable us to study and describe large molecular clusters more accurately.

Although advanced software is available for electronic structure calculations an additional challenge is to provide results to experimentalists that are meaningful in that they come with some type of “error bars” to facilitate in the comparison with real experimental results. One of the major goals in the group is to develop and employ techniques in a way to facilitate the resolution of experimental spectra. As a result a number of new theoretical techniques that serve as interfaces between theory and experiment have been developed.

Electronic Structure Methods
Quantum theoretical and computational methods are developed and refined in order to perform efficient calculations of the electronic structure of atoms and molecules. Our main interest has been in coupled cluster methods and the closely related many-body perturbation theoretical techniques.

Fundamental Quantum Theory
Quantum theory is a well-established theory which provides a highly accurate description of microcosmic phenomena. Although developed in the early part of the 20th century several problems concerning the interpretation of quantum theory still remain. Ongoing projects involve the study of the structure of the theory in the complex energy plane using complex scaling techniques as well as investigations of new and alternative descriptions of the quantum theory measurement.