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University of Wyoming

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 2009 & Spring 2010 Schedule

Sep 11 Zbigniew Celinski (University of Colorado, Colorado Springs)
High Frequency Microwave Devices Based on Ferromagnetic Metals

We will present results for tunable microwave devices based on a microstrip and co-planar waveguide geometries. These structures, prepared by sputtering on GaAs or Si substrates, are compatible in size and growth process with on-chip high-frequency electronics. We will discuss on-wafer notch filters, nonlinear devices such as signal to noise enhancers and nonreciprocal devices such as isolators. For the notch filters, we observed power attenuation up to 100 dB/cm and an insertion loss on the order of 2-3 dB for both Permalloy- and Fe-based structures. The operational frequency ranges from 5 to 35 GHz for external fields below 5 kOe. We discuss methods to increase operational frequency and reduce device linewidth. Using these techniques we are able, for example, to obtain an operational frequency of 11 GHz at zero applied field and to narrow the device linewidth from 3 GHz to 330 MHz. The operational frequency, which can be obtained from the ferromagnetic resonance condition, is set by material properties such as saturation magnetization, anisotropy fields, the gyromagnetic ratio, and the magnitude of an applied field . Thus, by using different materials and external fields we can create devices which function over a wide range of frequencies. A tunable microstrip signal-to-noise enhancer based on NiFe films operates due to the first- and second-order Suhl instability. The enhancement factor is 10 dB for a structure involving only the second order instability and an enhancement factor of 30 dB is found for a structure where the first order instability is allowed. We use Ni nanowires to build microwave isolators (non-reciprocal devices). The attenuation of the wave in forward and reverse direction shows a difference in transmission coefficients. The isolation is ~ 6 dB/cm at 23 GHz. The bandwidth of the device is relatively large (5-7 GHz) in comparison to ferrite-based devices.<-->

Sep 18 Greg Sloan (Cornell)
Dwarfs, globulars, and the early Universe: Infrared spectroscopy and the role of dust through time

Dust is a critical component of galaxies. Without it, normal stars cannot form. And yet, our understanding of how dust is produced in the early Universe and what form it would take remains limited. Using the Infrared Spectrograph on the Spitzer Space Telescope, we have observed sources of dust in Galactic globular clusters and nearby dwarf galaxies, including systems with elemental abundances almost as primitive as the highest redshift objects yet observed. These samples probe the relation between the initial elemental abundances of a star and the quantity of dust it can inject into its environment before it dies. We have found that carbon stars can produce significant amounts of dust, even in the most primitive systems we have observed. The influence of carbon stars in the early Universe may be understated.

Oct 2 Barb Whitney (Space Science Institute/U. Wisconsin)
The GLIMPSE survey and Star Formation in our Galaxy

I will overview the GLIMPSE surveys of our Galactic Plane, done with the Spitzer Space Telescope over the last 6 years, and continuing on for the next 2 years. I will highlight some of the scientific results from the survey, and then go into more details about my area of interest, star formation. I will describe a program we have designed to determine the star formation rate of our Galaxy by producing a population synthesis model of forming stars and comparing to the GLIMPSE catalogs. This independent method can be compared to the standard Kennicutt-Schmidt law that is used on all the other galaxies in the universe.

Oct 9 Tina Aragona (Lehigh University)
Optical Spectroscopy of LS 5039 and HD 259440

High mass X-ray binaries that also produce very high energy radiation on the order of MeV-TeV (\x{03B3}-ray binaries) are a rare and interesting class of objects. Only five of these objects have been discovered to date, and I am currently involved in a multiwavelength study of two of these systems. I will present the latest orbital solution for LS 5039 and newly determined stellar parameters for HD 259440, the optical counterpart of HESS J0632+057.

Oct 30 David McKinnon (Charles Sturt University)
Space to Grow: The Faulkes Telescopes and improving science engagement in schools

Australian universities are experiencing declining enrollments in Physics and Astronomy courses. Senior high school students are not electing to study these subjects in their final two years of school. This project is a collaboration between the educational sector and two universities designed to investigate the impact of inquiry-based astronomy on redressing the increasingly negative attitudes of students as they progress through high school science. The use of the Faulkes Telescopes is designed as an engagement mechanism to allow students and their teachers to undertake real science investigations while addressing other key curriculum areas (engineering, mathematics, physics and technology).

Nov 6 Mingzhong Wu (Colorado State)
Excitation of chaotic spin waves in magnetic thin film-based feedback rings

Nonlinear spin waves in magnetic systems are of both fundamental and technological interest. This presentation will cover two topics on the chaotic excitation of nonlinear spin waves. One will be on the excitation of chaotic surface spin waves through four-wave modulational instability, and the other will be on the excitation of chaotic backward volume spin waves through three-wave nonlinear processes. The high order nonlinear Schrödinger equation predicts that the modulational instability of nonlinear waves could lead to the chaotic behaviors of the waves. Such chaotic excitations have been observed experimentally for the waves that have attractive nonlinearity. For the waves with repulsive nonlinearity, however, there have been no experimental observations so far. The first half of this talk will present our recent work on the excitation of chaotic surface spin waves through the four-wave modulational instability processes. The work was conducted in a magnetic film strip-based active feedback ring. The magnetic film/field configuration was set to allow for the propagation of surface spin waves that have repulsive nonlinearity. The self-generation of chaotic spin waves in the ring was observed. The chaotic nature of the signals was confirmed by irregular waveforms, broad spectra, and finite fractal dimensions. Previous work has demonstrated the three-wave interactions between the surface and backward volume spin waves as well as the excitation of chaotic spin waves through those interactions. In principle, the three-wave interactions of pure backward volume spin waves are also possible at low magnetic fields. In practice, these interactions have never been observed experimentally. The second half of this talk will present our recent experimental observation on the three-wave interactions of backward volume spin waves and the excitation of chaotic spin waves through the three-wave processes. As in the four-wave process experiments, the three-wave interaction experiments were also carried out with a feedback ring structure, and the chaotic natures of the signals were confirmed by the broad power-frequency spectra and the finite fractal dimensions. <-->

Feb 5 Carlos Busser (U. Wyoming)
Transport properties of carbon nanotube Quantum dots

Since their discovery, carbon nanotubes (CNT) have been the subject of intense activity due their possible application in a new generation of electronic devices. For that reason the study of the electronic transport properties of CNT-based devices has become an interesting issue for the condensed matter community. In particular Quantum dots (QDs) fabricated with CNTs have attracted considerable attention in recent years as potential single electron transistors. Experiment with such devices, where the electrons are trapped in a small region of the CNT, have shown good agreement with the theory, in which the strong interactions between electrons play an essential role and cannot be ignored. In this talk I will explain the basic properties of CNTs, and how to use them as QD systems. In particular, I will discuss how the degeneracy of the chiral states of the CNTs can affect the Kondo and Dicke regimes and how this is reflected in the transport properties of CNT quantum dots. Finally, I will comment on the competition between the SU(2) and SU(4) symmetries.

Feb 12 Kyle Dawson (U. Utah)
Probing Cosmological Expansion with Distant Supernovae

Observations of Type Ia supernovae (SNe Ia) in the mid-to-late 90's led to the surprising discovery of dark energy. Not satisfied with a mere discovery of dark energy, the last decade has seen an increasing number of large SN surveys dedicated to uncovering the fundamental nature of this strange energy that dominates the expansion of the universe. In this talk, I will describe our current understanding of SNe Ia and how the earliest measurements uncovered the existence of dark energy. I will then describe several SN surveys that have taken place over the last five years using large, ground-based telescopes. Finally, I will describe our program with the Hubble Space Telescope to study the most distant SNe and I will show some preliminary results from this analysis.

Feb 26 Victor Gurarie (University of Colorado)
Doing condensed matter physics with ultracold atomic gases

A few years ago several experimental breakthroughs created a new interdisciplinary field. It combines atomic physics with condensed matter physics, and promises to manufacture new states of matter with constituents which are custom-picked atoms whose interactions can be manipulated at will. Since then the field has witnessed a number of remarkable successes, such as creating an artificial high temperature superconductor, observing an artificially prepared superconductor-insulator transition, and creating an artificial magnet. At the same time, a number of far fetching predictions about the future successes in the field so far failed to materialize. I will discuss the achievements and setbacks of the field, and describe how future progress can only be achieved by combining the vision coming from condensed matter physics and realistic limitations imposed by atomic physics.

Mar 1 Leszek Malkinski (U. New Orleans)
Voltage Control of Magnetism in Multiferroic Nanocomposites

Multiferroic composites, which consist of ferroelectric and magnetic materials have mutual coupling between their magnetic and electric properties. If the ferroelectric exhibits piezoelectricity and the magnetic material is magnetostrictive, the transfer of electric to magnetic energy (and vice versa) occurs through stresses transferred from one material to another. Because the description of these composites involves convoluted magnetic, electric and mechanical parameters they can be classified as smart or multifunctional materials. Magnetoelectric coefficient is a measure of efficiency of conversion of magnetic to electric energy. Applications of magnetoelectric effect include magnetic field sensors and high frequency transformers. On the other hand the converse magnetoelectric effect can be used in spintronic and microwave devices to control magnetization by the means of electric field (or voltage). Large, heavy and power-inefficient inductive circuits can be replaced by miniature multiferroic devices working at higher frequencies. It is believed that the composites made of nanoparticles will exhibit significant magnetoelectric effect because of large ratio of interface area between the magnetic and ferroelectric phases to the volume of the nanoparticles, which provides efficient transfer of stresses between the phases. In particular composites of core-shell nanoparticles are expected to be good candidates for numerous applications. The fundamentals of multiferroics as well as technology, measuring methods and selected applications of multiferroic nanocomposites will be discussed.

Mar 5 Michael DiPompeo (U. Wyoming)

Mar 26 Michael Brotherton (U. Wyoming)
Quasar Bolometric Luminosity Corrections: Theoretical Considerations

Mar 29 Young Song (Colorado State U.)
Electric Filed Tuning of Ferromagnetic Resonance Responses in Ferromagnetic and Ferroelectric Heterostructures

Electric field tuning of magnetic responses and magnetic field tuning of electric responses have attracted tremendous attention in recent years due to their possible innovative device applications. Ferromagnetic/ferroelectric (FM/FE) heterostructures with large magnetic and electric polarization are a good candidate for multi-functional device applications. The possibility of simultaneous magnetic and electric tunability in these heterostructures can be especially useful for microwave and millimeter wave planar devices such as tunable phase shifters, resonators, and delay lines. In this work, a remarkably large tuning of the resonance frequency of hybrid spin-electromagnetic waves with an electric field has been achieved for FM/FE heterostructures. Two heterostructures were fabricated. One consisted of yttrium iron garnet (YIG)/barium strontium titanate (BST) thin films, and the other consisted of in-plane c-axis oriented hexagonal barium ferrite (BaM) and BST thin films. The YIG/BST monolithic heterostructure shows an incremental voltage tunable frequency shift rate of 2 MHz/V at 25 V for a ferromagnetic resonance at 9.5 GHz. This is nearly one order of magnitude greater than that reported previously. The BaM/BST structure shows a tunability of about 1.1 MHz/V with a maximum shift of 32 MHz at a bias voltage of 30 V at 60 GHz. This is over 200 times greater than previously reported for the thick BaM-PZT pasted structure.

Apr 16 John Xiao (U. Delaware)
Nonlocal Magnetization Dynamics in Magnetic Heterostructures

Spin torque transfer refers to a phenomenon that a spin polarized current can reorient magnetization in multilayer structures. The reorientation takes place through spin procession caused by the torque from the spin polarized current. On the equal footing, a processing ferromagnet excited by an rf external magnetic field can inject spins into a neighboring layer, termed as spin pumping or spin battery effect. Such spin pumping can be used to create a pure spin current, which is heavily investigated and debated in the field of spintronics. Spin pumping can be indirectly investigated through Gilbert damping enhancement, and directly measured by electrical measurements in magnetic heterostructures with ohmic interfaces and tunneling barrier. While the former have been well described within the standard spin pumping theory, the voltage generation by magnetization precession of a ferromagnetic layer in tunneling structures is very intriguing. In this talk, we will present a systematic experimental investigation of spin pumping effects in tunneling structures, as well as possible mechanisms.

June 14 George Neil (Jefferson National Lab)
Free Electron Lasers and Other Accelerator Driven Light Sources

The success of third generation synchrotrons has inspired the desire for more advanced capabilities in photon performance. Two specific approaches within a continuum of design choices are under development to meet the challenging needs of tomorrow's research: free electron lasers and energy recovering linacs. Free electron lasers take advantage of collective effects and amplification to achieve very high peak brightness while energy recovering linacs make use of the injection of fresh electron bunches at higher brightness and shorter pulse width than can be achieved in synchrotrons. Initial demonstrations of each approach are presently underway to guide further development. The Jefferson Laboratory Free Electron Laser has achieved 14 kW of average power in the 1 to 10 micron region and is presently undergoing commissioning of a kW average power ultraviolet system. This talk will review the approaches to, and technical status of, 4th Generation Light Sources.

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
Contact for program information: Chip Kobulnicky