University of Wyoming Physics & Astronomy Colloquium Series

Fridays -- 4:10 PM -- Prowse Room 234

Pre-Colloquium tea served at 3:45 in the southeast corner of the 2nd floor

Fall 2010 & Spring 2011 Schedule

Aug 9 Weilie Zhou (U. New Orleans)
Metal Oxide Nanomaterials: Synthesis and Applications

Sep 10 Michael Zumwalt (Agilent Technologies)
"Life is What Happens to You While You're Busy Making Other Plans"

The study of physics is at the core of our understanding nature so it should not come as a surprise that establishing a background in this science provides a very good springboard for a variety of careers and lifelong pursuits. However, such pursuits are not necessarily planned, but can really be a matter of trying to be where you want to be and then going along for the ride. This has been my perspective in looking back at the past 30 years of my life. This talk will be autobiographical starting with my first realization of an interest in physics to how I came to be where I am now, working with chemists, while still finding opportunities to use my background.

Sep 24 Michael Johnson (U. Illinois-Chicago)
Biophysical approaches to the discovery and design of antimicrobial agents: application to SARS

Structure-based drug discovery and design uses fundamental physical principles to guide the design of molecules that bind tightly to, and inhibit the activity of critical enzymes in viruses or bacteria. I will discuss some of the approaches used, illustrated by our design of therapeutic agents against the virus that caused the Severe Acute Respiratory Syndrome (SARS) pandemic.

Oct 14 George Martins (Physics Dept., Oakland University)
Iron-pnictides: A brief introduction, effective low-energy models, and theoretical results

For more than 25 years, high-temperature superconductivity has fascinated physicists, and eluded all attempts for a theoretical understanding. These materials are characterized by two-dimensional copper-oxygen planes that have been thought to be the fundamental building blocks responsible for the unusual physics. In 2008, the discovery of a new class of superconducting materials was reported, which have iron and arsenic as fundamental constituents (referred to as iron pnictides -- elements of the nitrogen group). Iron pnictides have quite different properties than the cuprates, and have therefore challenged our understanding of superconductivity. In this talk, a brief introduction to experimental and theoretical results for the iron pnictides will be given, trying to compare its similarities and differences with the cuprates. The pnictides parent compound, in all the families of superconducting pnictides known, is a semi-metal with an Spin Density Wave (SDW) ground state, which under doping, for some families, and under pressure for others, becomes superconducting (max T_c ~ 55 K). The model of choice has been a 5-band Hubbard model, with longer than nearest-neighbor tight binding hoppings and several on-site correlation terms. An RPA solution to this model will be presented for the dynamic spin susceptibility and also for the pairing function (time permitting). It will be argued that the extreme richness of the Fermi surface (coming from all the hybridize d-states) results in RPA solutions which are very dependent on the tight binding hopping terms. This may be one of the important difference in relation to the cuprates.

Oct 22 Toshiya Ueta (University of Denver)
From Circumstellar Envelopes of AGB Stars to the ISM

IRAS and ISO observations indicated that the extended dust shells of asymptotic giant branch (AGB) stars could be explained by a simple model of a past constant mass outflow piling up at the interface with the interstellar medium (ISM). Recent observations made by Spitzer have shown that even outflows from AGB stars can induce shocks at the ISM-AGB wind interface. Recent AKARI survey of the circumstellar envelopes of evolved stars have shown similar far-IR signatures of the interface between the ISM and AGB winds in many objects, while observations made in other wavelengths (especially by GALEX in UV) have been providing supporting insights. New Herschel observations are expected to provide detailed views of these interface regions. Therefore, the extended dust shells of AGB stars would not only allow us to prove the mass loss history of the parent AGB stars but also permit us to glimpse how the ejecta eventually merge with the ISM at these interface regions with or without shock interactions. In this invited review, I will summarize development in the past (IRAS/ISO), recent (Spitzer/AKARI) and on-going (Herschel) on research at the interface regions between the circumstellar envelopes of AGB stars and ISM.

Nov 10 Luis Manuel (Instituto de Fisica Rosario)
Triangular Antiferromagnet: Bosonic Spinons and Kinetic Antiferromagnetism

We study two aspects of the triangular antiferromagnet. First, we show that the anomalous excitation spectrum of the spin 1/2 triangular Heisenberg model, recently found with series expansion, can be naturally interpreted in terms of bosonic spinon excitations using a Schwinger boson mean field theory. In particular, we find a qualitative and quantitative agreement of the strong renormalization of the high energy part of the spectrum with respect to spin wave results along with the appearance of roton like minima at the midpoints of faces of the hexagonal Brillouin zone. Second, we show that the ground state magnetic phase diagram of the triangular t-J model depends in a crucial way on the sign of the hopping integral t, due to the particle-hole asymmetry of the triangular lattice. For positive t, the motion of the doped holes gives rise to an enhancement of the 120 antiferromagnetic correlations present in the half-filled case. In particular, for small J/t we have found that an antiferromagnetic version of the Nagaoka phase is stabilized for a wide range of doping.

Nov 12 John Neumeier (Montana State)
Measuring the Thermal Expansion of Solids: How and Why?

We have developed a very sensitive instrument for measuring the thermal expansion of solid materials, with a resolution of about 0.1 angstroms. It has been used to measure very complex materials such as quasi-one-dimensional compounds, important magnetic materials, and some elements. The results reveal a number of surprises as well as important information connecting structural and physical properties. In this presentation, I'll describe the instrument in detail and highlight some results.

Nov 15 Stephan Mandt (University of Cologne)
Breakdown of diffusion, equilibration rates and negative absolute temperatures for ultracold atoms in optical lattices

Nov 15 Adam Myers (U. Illinois)
Quasars and Cosmology: Mapping the Dark Universe with Luminous Black Holes

Quasars are actively accreting black holes, with masses of more than a billion suns, at the hearts of distant galaxies. Over the last decade several advances have led to the increased use of quasars in a cosmological context. First, quasars are now understood to be a phase in the lives of normal galaxies, rather than a separate entity. Second, deep, wide-area optical imaging surveys have greatly inflated the numbers of known quasars. Because quasars are easy to observe over extremely large volumes back to almost the dawn of time, they are novel probes of geometry and structure throughout the history of our Universe. I will discuss how large maps of quasars have recently started to be used to probe dark matter, dark energy and the evolution of massive galaxies at early times. Over the next decade, quasars will augment many cosmological measurements. However, several improvements in the classification of quasars in imaging surveys will be necessary to fully realize these augmentations.

Nov 19 Jiajun Chen (U. New Orleans)
Highly Sensitive and Selective Gas Sensors Based on Vertically Aligned Metal Oxide Nanowires

Mimicking the biological olfactory systems that consist of olfactory receptor arrays with large surface area and massively-diversified chemical reactivity, three dimensional (3D) metal oxide nanowire arrays were used as the active materials for gas detection. Metal oxide nanowire arrays share similar 3D structures as the array of mammal\x{2019}s olfactory receptors and the chemical reactivity of nanowire array can be modified by surface coatings. In this dissertation, two standalone gas sensors based on metal oxide nanowire arrays prepared by microfabrication and in-situ micromanipulation, respectively, have been demonstrated. The sensors based on WO3nanowire arrays can detect 50 ppb NO2with a fast response; well-aligned CuO nanowire array present a new detection mechanism, which can identify H2S at a concentration of 500 ppb. To expand the material library of 3D metal oxide nanowire arrays for gas sensing, a general route to polycrystalline metal oxide nanowire array has been introduced by using ZnO nanowire arrays as structural templates. The effectiveness of this method for high performance gas sensing was first investigated by single-nanowire devices. The polycrystalline metal oxide coatings showed high performance for gas detection and their sensitivity could be further enhanced by catalytic noble metal decorations. To form electronic nose systems, different metal oxide coatings and catalytic decorations were employed to diversify the chemical reactivity of the sensors. The systems can detect low concentrated H2S and NO2at room temperature down to part-per-billion level. The system with different catalytic metal coatings is also capable of discriminating five different gases (H2S, NO2, NH3, H2and CO).

Nov 19 Julain Rincon (Centro Atomico Bariloche)
Transport properties through nanoscopic rings: interactions and interference effects

Many of the most intriguing quantum effects are observed or could be measured in transport experiments through nanoscopic systems such as quantum dots, wires and rings formed by large molecules or arrays of quantum dots. In particular, the separation of charge and spin degrees of freedom and interference effects have important consequences in the conductivity through these systems. Charge-spin separation was predicted theoretically in one-dimensional strongly interacting systems (Luttinger liquids) and, although observed indirectly in several materials formed by chains of correlated electrons, it still lacks direct observation. We present results on transport properties through Aharonov-Bohm rings (pierced by a magnetic flux) with one or more channels represented by paradigmatic strongly-correlated models. For a wide range of parameters we observe characteristic dips in the conductance as a function of magnetic flux which are a signature of spin and charge separation. Interference effects could also be controlled in certain molecules and interesting properties could be observed. We analyze transport properties of conjugated molecules, benzene in particular, and find that the conductance depends on the lead configuration. In molecules with translational symmetry, the conductance can be controlled by breaking or restoring this symmetry, e.g. by the application of a local external potential. Quantum interference effects show up in transport through conjugated annulene molecules producing important effects on the conductance for different source-drain configurations leading to the possibility of an interesting switching effect. These results open the possibility of observing these peculiar physical properties in anisotropic ladder systems and in real nanoscopic and molecular devices.

Nov 22 John Debes (NASA/Goddard)
The Link Between Dusty White Dwarfs and Planetary Systems

For most people who study exoplanets, all the interesting events occur within the first few tens of millions of years. In reality, a trove of information can be gleaned by sifting for planetary systems that have survived the death of their central star. In particular, much can be learned about metal polluted and dusty white dwarfs, two populations that have signatures of planetary systems. To date, 18 dusty white dwarfs and a few dozen metal polluted white dwarfs are known, a small enough sample that it is difficult to achieve a statistical understanding of how these objects are related to planetary systems that have survived post main sequence evolution. I will discuss a new dynamical mechanism for linking planetary systems to dusty white dwarfs and present a path forward for gaining a larger sample of dusty white dwarfs through NASA's WISE mission.

Nov 30 Hannah Jang-Condell (U. Maryland)
Gaps in Protoplanetary Disks as Probes of Planet Formation

Hundreds of exoplanets have been discovered to date, most with orbital parameters entirely unlike our own Solar System. To understand the origins of these planetary systems, I investigate how planets interact with the circumstellar disks from which they form. Embryonic planets dynamically perturb the protoplanetary disks in which they form. Sufficiently massive planets can open annular gaps in the disk along their orbital paths. I will discuss consequences for the formation of planets embedded in these gaps as well as the observable features of these gaps. Current instrumentation is already able to detect sub-Saturn mass planets embedded in known protoplanetary disks. Comparison of theoretical models of gap shadowing to observations of real disks puts important constraints on where, when, and how planets form.

Dec 2 John Wisniewski (U. Washington)
Young Circumstellar Disks: Exoplanet Diagnostics

Circumstellar disks provide a useful astrophysical diagnostic of the formation and early evolution of exoplanets. It is commonly believed that young protoplanetary disks and transitional disks serve as the birthplace of planets, while older debris disks can provide insight into the architecture of exoplanetary systems. Spectacular ground-truth of this disk-exoplanet connection was provided by the recent successful direct imaging of exoplanets in the Fomalhaut, HR 8799, and Beta Pic systems. In this talk, I will discuss how one can use high contrast imaging techniques to spatially resolve circumstellar disk systems. I will focus on the initial results and future prospects from the Subaru Strategic Exploration of Exoplanets and Disks (SEEDS) project, a 5-yr program guaranteed 120 nights on the 8.2m Subaru Telescope to spatially resolve ~200 nearby circumstellar disks and investigate ~300 nearby stars in a direct imaging exoplanet survey.

Feb 8 Axel Hoffman (Argonne National Lab)
Pure Spin Currents: Discharging Spintronics

As semiconducting electronic devices are miniaturized to ever-smaller dimensions, power dissipation becomes an ever-increasing problem due to leakage charge currents. Spintronics may help addressing some of these issues by utilizing besides the charge degree of freedom also the electron spin. Conventional spintronics approaches are used for non-volatile devices, such as magnetic random access memory, where spin currents are mainly considered as spin-polarized charge currents and as a result the spin and charge currents are in parallel and directly coupled. Looking further into the future, the question arises whether eliminating charge currents altogether could provide additional benefits for applications. Towards addressing this question, non-local device geometries allow for separating spin and charge currents, which in turn enables the investigation and use of pure spin currents. This approach opens up new opportunities to study spin-dependent physics and gives rise to novel approaches for generating and controlling angular momentum flow. In this lecture, I will discuss different approaches for generating pure spin currents, such as non-local electrical injection from a ferromagnet, charge-to-spin current conversion via spin Hall effects, and spin pumping from ferromagnetic resonance.

Feb 11 Rodrigo Nemmen (NASA Goddard)
Probing Massive Black Holes in Nearby Galaxies: What's their Astrophysical Diet?

There is compelling evidence that a significant fraction of nearby galaxies - including our own Milky Way - host supermassive black holes at their centers. Thanks to advances in astronomical observations combined with theoretical models, astronomers are now in a position to understand in detail the behavior of these black holes. In this talk, I will present how observations of the nuclei of nearby galaxies can be used to understand the astrophysical diet of massive black holes: how much gas is attracted and trapped inside the event horizon over time, how the gas "swirls down the drain" and what fraction of the "diet" is rejected producing powerful cosmic exhausts of particles moving near the speed of light! I will present a simple analogy to understand the behavior of these black holes: they behave like massive cosmic engines which in many ways are similar to car engines! But are they more like Ferraris or Corollas?

Feb 18 Wouter Monfrooij (U. of Missouri-Columbia)
Percolation and quantum criticality: who wins?

When magnetic ions are embedded in metals three things can happen when such a compoud is cooled down. The magnetic moments of these ions can line up with each other, the moments can be shielded, or the compound does not know what to do, not even at zero Kelvin. The latter case is referred to as a quantum critical point because the system is on the verge of ordering, but something prevents it from doing so. And since this happens at zero Kelvin it cannot be anything to do with thermal fluctuations; quantum fluctuations must be involved. So what exactly is going on in these metals at the quantum critical point, a state of matter where electrons become so correlated with each other that they move as if they weigh thousands of times as much? This talk is aimed at figuring out whether percolation effects- that randomly remove magnetic moments- might be at the heart of this puzzling state of matter. This talk does not require any extensive background in sold state physics; it will be given at the upper undergraduate level.

Mar 11 Jeff Shield (University of Nebraska)
Welcome to the Small Time: Structure and Properties of Nanoclusters

March 25 Thomas Kodenkandath (Ascent Solar Technologies)
Development of Functional Materials for Enabling Technologies: The case of PV and HTS.

April 28 Matt Povich (Penn State)

May 25 Charles Kerton (Iowa State)
IRAS 01202+6133: A Possible Case of Protostellar Collapse Triggered by a Small HII Region

The molecular gas surrounding an HII region is thought to be a place where star formation can be induced. This talk will present the results of an infrared and submm imaging and spectroscopic survey of three small galactic HII regions that has characterized the star-formation activity in their surroundings. Particularly interesting are spectroscopic observations of the submm/infrared source IRAS 01202+6133 which show a classic profile indicative of infall motions and are consistent with the idea that the object is a rare example of a masssive young protostellar object triggered by the nearby HII region.

Previous colloquia series: Fall 2002 Spring 2003 Fall 2003 Spring 2004 Fall 2004 Spring 2005 Fall 2005 Spring 2006 Fall 2006 Spring 2007 Fall 2007 Spring 2008 2008-09 2009-10
Contact for program information: Chip Kobulnicky