Pre-Colloquium tea served at 3:45 in the Prowse Room
|September 12|| Atomic and Ionized Hydrogen Gas at the Disk-Halo Interface in Spiral
Rene Walterbos (New Mexico State University)
Observations over the past two decades have provided ample evidence for thick layers of HI and diffuse ionized gas extending sometimes far above the midplane of galaxies into their halos. A few nearby galaxies, including the Milky Way also show evidence for a distribution of HI clouds and filaments further into the halo. Some of these features may be explained by galactic fountains, a cyclical process of gas outflow and inflow driven by star formation in the disk. Accretion of gas from the IGM and from tidal interactions is also called for to explain some of the larger scale phenomena. The kinematic signature of the gas at the disk-halo interface may provide information on the origin of the gas. I will describe some recent results on the kinematics of HI from deep observations with the WSRT and VLA radio telescopes of a sample of nearby disk galaxies, as part of the HALOGAS project, and from deep multi-slit spectroscopy of ionized gas in edge-on galaxies with the ARC 3.5m telescope. A general result is a decrease in rotational velocity of gas with height above the disk. The sense of the lag is as predicted by galactic fountain models, though its magnitude is far larger than simple models would predict. In general, few galaxies show strong evidence for accretion of cool gas, and the bulk of IGM inflow predicted to exist remains as of yet undetected.
|September 19|| Title, TBD
Paul Butler (Carnegie Institute of Washington)
Modern science began with Copernicus speculating that the Earth is a planet and that all the planets orbit the Sun. Bruno followed up by speculating that the Sun is a star, that other stars have planets, and other planets are inhabited by life. For this and other heresies, Bruno was burned at the stake in a public square in Rome in 1600. Astronomy and extrasolar planets were a really hot field at the time. Over the past 20 years more than a thousand extrasolar planets have been found, first from ground-based precision Doppler and photometric transit surveys, and more recently by the Kepler space mission. We have concentrated on building precise Doppler systems to survey the nearest stars. Our systems at Lick, Keck, AAT, and Magellan have found hundreds of planets, including 5 of the first six planets, the first saturn-mass planet, the first neptune-mass planet, the first terrestrial mass planet, and the first multiple planet system. We are currently focusing our attention on new custom built "R4" echelle spectrometers designed for Iodine cells, which are yielding 1 m/s precision. These spectrometers have a footprint about the size of a ping pong table, allowing for temperature stabilization, yet deliver higher resolution and dispersion than the much larger classic echelle spectometers, such as the Lick Hamilton, the AAT UCLES, and the Keck HIRES. The two working examples, PFS on Magellan, and the Levy spectrometer on the 2.4-m APF, cost about US$2M each. They do not use fibers or scrambling, and have throughput of 20 to 30%, a factor of 2 to 4 better than classic echelles. These spectrometers will lead to the discovery of many terrestrial mass and potentially habitable planets over the next decade.
|September 23|| Magnetic Materials in Medicine: Applications in Diagnosis, Management, and the Treatment of Disease
Tim St Pierre (University of Western Australia)
Scientists working in the field of magnetic materials are increasingly focusing their attention on new applications of magnetic detection and magnetic transduction techniques in the biomedical sciences. Iron is a key functional element in the human body and surpasses all other naturally occurring elements in the body in terms of both the variety and magnitudes of its magnetic states. In many diseases, the quantity and the magnetic state of iron are altered by the disease. Hence, detecting and measuring the magnetic properties of the iron in vivo or in samples of body fluids can give insights into the state of health of a human subject. Example applications include assessing the risk of organ damage in hereditary hemochromatosis , determining the dose of iron chelator drugs required for patients with thalassemia , and identifying infectious forms of the malarial parasite in finger-prick blood samples . Scientists are also working on the development of synthetic magnetic particles that can be injected into the human body for the diagnosis and treatment of disease. The particles used are generally in the size range of 10 to 100 nm. They can be used to enhance the contrast in magnetic resonance images to help identify tumors in tissue , to act as local heat sources to treat cancer , and to carry, concentrate, and release drugs more specifically than drugs without a magnetic carrier . In this presentation, the physical and chemical principles behind these biomedical applications and their impact on medicine will presented at a level suitable for a generalist audience.
|October 17|| WIRO OPEN HOUSE, NO ASTRO COLLOQUIUM
||October 31||High redshift quasars in VIKING; Progress and Science
Joseph Findlay (University of Wyoming)
Luminous high redshift quasars are versatile probes of the early Universe, setting constraints on the seeding and growth of the first population of massive black holes, gas phase metallicity evolution, cosmic reionization and much more besides! Quasars at the highest redshifts are extremely rare, searches must be conducted over extremely wide fields and moderate depths. Since 2012 we have been conducting a followup campaign on z>6.5 quasar candidates selected in the ESO public VIKING survey. In this talk I will discuss selection techniques, present the first spectroscopically confirmed sources and discuss ongoing science with the sample.
|November 14||Evolution of quasars at high redshift from large optical surveys
Ian McGreer (Unversity of Arizona/Steward Observatory)
A wealth of data indicates that massive galaxies universally harbor supermassive black holes at their centers. Most of the mass in these black holes accumulated during relatively brief quasar phases, where the luminosity generated by the black hole accretion exceeds that of the total starlight from the galaxy. Surveys of quasars at optical wavelengths are especially efficient at tracing the growth of supermassive black holes; we are now approaching a million quasars with spectroscopic redshifts, and observations reach to systems formed within the first billion years of the universe. The improved statistics provided by recent surveys show that quasars sharply decline in number density at z>3, as known for decades, but more surprisingly that the characteristic luminosity (the break in the luminosity function) evolves to continually higher luminosities out to z~6. This steady shift to more extreme systems suggests that the most massive black holes formed early and grew rapidly. I will discuss some limitations of current studies of quasar demographics and how these can be addressed with simulations of quasar properties and with observations that break degeneracies in theoretical models of quasar activity.
|November 21||Generation of Pure Spin Currents using Magnetic Insulators
Mingzhong Wu (Colorado State University)
Pure spin currents are very promising for information transfer and processing for future low-power "green" electronics. There is a strong interest in using yttrium iron garnet (YIG) to generate pure spin currents, either via spin pumping or the spin Seebeck effect (SSE). This interest originates from the fact that the damping in YIG materials is lower than in any other known magnetic materials. The development of YIG-based, nanoscale spin batteries, however, demands YIG films that have a thickness in the nanometer range and at the same time exhibit low damping similar to single-crystal YIG materials. The growth of such YIG films will be discussed in the first part of this presentation. The second part will report on the exploration of optimal regimes for spin-wave spin pumping using YIG thin films, which will provide significant implication for the future development of high-efficient spin batteries. There are three main types of magnetic insulators: spinels, garnets, and hexagonal ferrites, with the first two being usually classified as soft magnetic materials and the third as hard. Previous work has demonstrated the SSE in "soft" spinels and garnets, but not in "hard" hexagonal ferrites. In the third part of this presentation, the generation of pure spin currents via the SSE in a self-biased hexagonal ferrite thin film will be reported. This work demonstrates a spin battery that requires neither a magnetic field nor a microwave source nor a heating/cooling device, but light only.
|November 24|| Strongly Correlated Materials, Growth and Characterization
Neel Haldolaarachchige (Princeton U.)
|November 28|| THANKSGIVING, NO COLLOQUIUM
|December 8|| Controlling Spins with Light: Optically Induced Magnetization in Strontium Titanate and Spin-Exchange Interaction in Colloidal Quantum Dots
William Rice (Los Alamos National Lab)
The ability to fully control spin degrees of freedom using electric fields has been a long-sought goal in condensed matter physics. Here, we explore two different material systems that show photon-based control over spin magnetization. In the first project, using magnetic circular dichroism (MCD) spectroscopy and SQUID magnetometry, we find that bulk oxygen-deficient SrTiO3-delta exhibits long-lived, optically induced magnetization. We observe that when oxygen vacancies are introduced via thermal annealing in UHV, we are able to optically induce magnetization at zero magnetic field using sub-bandgap (400-500 nm), circularly polarized light; this magnetization inverts when the circular polarization is reversed. Interestingly, we observe that the relaxation dynamics strongly depend on temperature: the induced magnetization exhibits hours-long decay times when the temperature is below 10 K, but only persists for seconds near 18 K, the temperature where the magneto-optical effects first appear. Utilizing the remarkably long relaxation times and all-optical magnetization control, we demonstrate that SrTiO3-delta can be used as optical magnetic memory. In the second project, we synthesis and create optical-quality films out of two different nanocrystal systems: CuInS2 and Mn2+-doped CdSe. For both nanocrystal materials, we find evidence of spin-exchange interactions, analogous to those seen in diluted magnetic semiconductors. In particular, Mn:CdSe shows strong temperature- and magnetic-field dependent changes in both MCD and circular-polarization-resolved photoluminescence, following a Brillouin-function behavior. We find that the strong spin-exchange interactions in Mn:CdSe produce an effective internal magnetic field that causes (1) fast precession of the exciton when excited with circularly polarized light and (2) a long-lasting "tipping" of the Mn2+ moments. This latter behavior demonstrates that light can be used to modify spin degrees of freedom in colloidal nanocrystals.
|December 12|| Atomic-level ordering of complex oxide half-metals for high-performance spin-based electronics
Adam Hauser (UCSB)
High spin polarization materials invite the possibility of room-temperature spin injection, and relieve the resistance mismatches that make spin injection from a ferromagnetic metal to a semiconductor interface problematic. To improve upon currently utilized spin polarizer materials, we must now investigate more complex material systems. Although complexity promises tailor-made solutions, a major challenge lies in lowering defect and disorder levels to the point that the intrinsic material physics appears. In this talk, we will discuss the methods required to create a highly ordered double perovskite thin film, and show current research into a promising material system, Sr2CrReO6. Sr2CrReO6 is a predicted pseudo-half-metal of high interest for spin injection applications due to its ferromagnetic ordering with Curie temperature exceeding 500K, when grown with high Cr/Re ordering. An improved understanding of the magnetic structure of Sr2CrReO6 is necessary to understand and develop technologies that rely on the spin polarization of the material. We have analyzed the magnetic configuration for highly ordered Sr2CrReO6 films as a function of epitaxial strain using magnetometry and x-ray magnetic circular dichroism (XMCD) measurements of Cr, Re, and Osites. Interestingly, OK-edge XMCD indicates that the oxygen sites carry at least a portion of the bulk magnetization. Spin moment values measured for Cr match calculations incorporating spin-orbit effects, while both spin and orbital moments measured for Re sites are slightly higher than previously predicted.
|December 15|| Catalyst-free Synthesis of Millimeter-long Topological Insulator Bi2Se3 Nanoribbons and the Observation of Hugh Critical Current Density in Iron-based Superconductor SmFeAsO0.8F0.15 Crystals
Lei Fang (Northwestern U.)
Topological Insulator is a novel state of matter featuring insulating bulk and conducting edges or surfaces. This unconventional electronic state is protected by spin-orbital coupling and time reversal symmetry of a material. Novel properties have been observed in topological insulators, such as back-scattering immunity, Dirac-cone bands dispersion, and spin-momentum locking of surface electrons. These rich physical properties give rise to numerous application opportunities for spintronics and quantum computation techniques. A compelling mission for topological insulator research is the scalable synthesis of high quality thin films or nanowires which can be directly used for nano device fabrication. In this talk, I firstly introduce our recent progress on a catalyst-free synthesis of millimeter-long topological insulator Bi2Se3 nanoribbons. The as-grown nanoribbons are single crystalline and have excellent surface quality with 2 nm surface roughness. Two-dimensional Shubnikov-de Hass quantum oscillations with a pi-Berry's phase arising from the topological state were observed. Surface electron's mobility up to 4000 cm^2/Vs can be determined from our quantum oscillation measurements. The ultra-long nanoribbons with high quality surface state may have potential for fabrication of multiple electronic devices onto individual nanoribbon. The second part of my talk focuses on the newly discovered iron-based superconductors. Due to the relatively high superconducting transition temperature (T_C) and critical current density (J_C), iron-based superconductors have prompted large enthusiasm for electricity distribution and superconducting magnet applications. SmFeAsO1-xFx is of particular interest as it has the highest T_C (57 K) in iron-based superconductors. However, the application potential of SmFeAsO1-xFx remains unknown because of the lack of sizeable single crystals for study. To meet this challenge, we fabricated micro Hall magnetometer array and micro calorimeter to characterize micrometer-sized SmFeAsO0.8F0.15 crystals which were incorporated columnar nano defects by high energy, heavy ion irradiation. A record high J_C of 2X10^7 A/cm2 was observed at 5 K and self-fields. We also observed a notable reduction in the thermodynamic superconducting anisotropy, from 8 to 4 upon irradiation. The huge J_C, low anisotropy and high TC of SmFeAsO0.8F0.15 strongly indicate that this material may be the best iron-based superconductor for application.
|February 6|| Title, TBD
Kevin France (CU Boulder)
|February 20|| Title, TBD
Kris Beckwith (Tech-X Corporation)
|March 20|| SPRING BREAK, NO COLLOQUIUM
||April 3|| EASTER, NO COLLOQUIUM
|April 10|| Title, TBD
Greg Rudnick (Kansas)