Fall 2014

The CMS experiment and the activities of the UA CMS group

The CMS Experiment at the CERN Large Hadron Collider (LHC) in Switzerland is finalizing the analysis of the Run-1 data, and preparing the restart of the data taking in Spring 2015 at a higher LHC energy. The new data will be used to continue the study of the Higgs Boson, to determine if its properties are consistent with expectation from the Standard Model, and explore another uncharted energy range to search for new physics. A brief overview of the CMS experiment and the activities of the CMS group at the University of Alabama is given.

How supernova explosions are influenced by what the star is made of, and why it is important for measuring the makeup of the universe

The material ejected from a supernova resulting from the explosion of a white dwarf star is expected to depend on the composition of that star. I will discuss how we are using simulations to compute this relationship and some of the physics behind the nuclear combustion processes at work. The resulting changes are important for predicting how the brightness of distant supernovae might depend on the stars they are made from, which change with cosmic time. This makes it possible to better constrain the expansion history of the universe, and therefore hopefully infer whether a major component of the universe, Dark Energy, is equivalent to a cosmological constant or is more complex.

Studying Galaxy Evolution with the Sloan Digital Sky Survey

I will give a broad overview of my different areas of research in the field of galaxy evolution and describe the MaNGA survey, a project which is part of Sloan Digital Sky Survey 4. I will also discuss some recent results on the HI gas content in merging galaxies in SDSS.

Phystec and FCI

There is a critical shortage of qualified high school physics teachers in the U.S., especially in Alabama. The Physics Teacher Education Coalition (PhysTEC) consists of more than 260 colleges and universities that are committed to address this problem. The Department of Physics & Astronomy has a 3-year grant from PhysTEC to increase the number and quality of certified physics teachers graduating from UA. Major components of the program are a Teacher-in-Residence (TiR), a Learning Assistant (LA) program, and a partnership with Alabama Science in Motion (ASIM) to provide early teaching experiences. In addition to the PhysTEC program, we will discuss aspects of the more general issue of student and program assessment, including in particular the use and analysis of concept tests.

The stellar mass morphology of nearby galaxies

I will describe the results of a morphological study of 2418 galaxies in the Spitzer Survey of Stellar Structure in Galaxies, which is currently the largest available database of galaxy images taken at middle infrared wavelengths. The study is based on classical morphological analysis, where the images are visually inspected and the galaxies are classified in a well-defined system called the Comprehensive de Vaucouleurs revised Hubble-Sandage (CVRHS) system. The main filter used for the analysis has a wavelength of 3.6 microns. The effects of internal extinction are greatly reduced at this wavelength, allowing us to see the distribution of older stars which trace the stellar mass distribution of galaxies. I will present histograms of various aspects of the stellar mass morphology of galaxies, and show some of the images.

The E-Nova Project: A Multi-Wavelength Initiative to Probe the Ejecta and Environments of Novae

When imagining a nuclear explosion, we often picture strong, spherical shock waves, like a bomb or supernova; however, nature’s most common thermonuclear explosions look nothing like this, showing delayed and multiple phases of mass ejection that can last for months after the nuclear fuel is ignited. These most common explosions are novae—thermonuclear runaways on the surfaces of accreting white dwarfs—and their complexities are best revealed with an intensive multi-wavelength observational program highlighting radio and X-ray data—our E-Nova Project. I will discuss our recent results, featuring observations from the newly-upgraded Karl G. Jansky Very Large Array, and spotlighting sources like recent novae with giant companions and the four novae that have been detected in gamma rays to date (an emission process that was not predicted and remains an intriguing mystery). The implications for Type Ia supernova progenitors will also be discussed.

The Science and Technology of Spin Angular Momentum Transfer

Since its original discovery in the late-’90s, spin angular momentum transfer torque (STT, or spin-torque) has played an increasingly important role in magnetotransport and magnetodynamics in nanomagnets. In small structures starting below ~ 100nm in size, STT is effective for manipulation and control of magnetic states, leading to new technology interests such as solid-state memories (STT MRAM), compact microwave oscillators, and potentials for spin-based low power high performance logic devices. In this talk, I will review the basic scientific findings of spin-torque, relating experimental observations to simple quantitative analysis. Issues discussed include the physics of a spin-current-induced torque, and device concepts such as STT switches and oscillators, using as examples the familiar two-terminal spin-valves and magnetic tunnel-junctions. I will review the progress of various technology efforts in applications of spin-torque, with an emphasis on STT MRAM. From these discussions I will highlight the remaining challenges, both scientific and technological, in the drive for a consumer-electronics future of these STT-based technologies and beyond.

Graphene for Enhancement of Raman Effect

Results are presented showing enhancement of the Raman spectral signal of molecules by their placement on a graphene substrate. The conditions for observation of signal enhancement are identified as well as conditions when no enhancement is observed. A decrease in the enhancement effect is observed with decreasing number of graphene layers in few-layer graphene. Although an enhancement effect can also be seen for the same molecules on transition metal dichalcogenides, the magnitude of the enhancement effect is greatly decreased relative to monolayer graphene.

Star Formation and Stellar Populations of Ultra-Luminous Galaxies at High Redshifts

The Herschel space mission has generated, for the first time, a rich data set that allow us to explore the far-IR (FIR) universe at z > 1 on a large scale that is comparable to those in optical/near-IR. As the mission is now over, these are the best – and all – that we can have before the next generation FIR facilities arrive. We are undertaking a program to study the extreme, dust-enshrouded star formation in galaxies at z >1, targeting the Ultra-Luminous InfraRed Galaxies (ULIRGs) revealed by the Herschel very wide-field surveys. The most serious challenge is the persisting problem of poor angular resolution in FIR/sub-mm that the Herschel still suffered. We are developing a method to extract the major component(s) of Herschel sources using optical data directly as the position priors, which is appropriate to the study of ULIRGs. We will construct the largest, well-defined sample of high-z ULIRG, whose total IR luminosity are directly measured based on multiple FIR bands, and to enable a slew of follow-up studies. Some preliminary results will be presented here.

The Beauty of the Many-Body Strong Interaction

Dew drops on a leaf illuminated at dawn are beautiful. The physics that results in such beauty is dominantly electromagnetic, the theory of which is oft-purported to be the best understood by man in the guise of Quantum Electro-Dynamics or QED, However, one would be a fool to describe dew drops using Feynmann diagrams! It is the many-body electromagnetic interaction that results in the beauty of the dew drop just as it is the many-body gravitational interaction that results in the beauty of the cosmos. Quantum Chromo-Dynamics (QCD) also posses beautiful structure in its many-body problem, the study of which is only in its infancy. Nature provides a cold QCD (ground state) medium in the interior of every hadron and mankind can now generate a “hot QCD” medium called Quark-Gluon Plasma through collisions of heavy ions. This talk will provide some background in bulk QCD physics and an overview of the achieved discoveries within the landscape of bulk QCD and those yet to come.

Supermassive black hole binaries: the search continues

Gravitationally bound supermassive black hole binaries are thought to be a natural product of galactic mergers and growth of the large scale structure in the universe. They however remain observationally elusive, thus raising a question about characteristic observational signatures associated with these systems. In my talk I will discuss current theoretical understanding and latest advances made in observational searches for supermassive black hole binaries.

Spring 2015

Dark Matter Detection Results, LUX, LZ and the State of the Field

Particle dark matter is thought to be the overwhelming majority of the matter in the Universe. Its gravitational contribution overwhelms that from the ordinary matter that we, the earth and the stars, are composed of. However, direct evidence for the existence of particle dark matter remains controversial. I will discuss the general motivations for the hunt for dark matter, and review the technologies that have been used to pursue it. I will discuss some of the latest results in the field of direct detection, and look at the best techniques that may help us to definitively detect the illusive dark matter particles. My discussions will include the LUX Experiment which has recently reported world leading results in the search for WIMPs (weakly interacting massive particles). LUX is a 350 kg liquid Xe time projection chamber, and is operating underground at the Sanford Lab, Homestake, SD. I will review future LUX running, and also the follow on experiment: LZ (LUX-ZEPLIN), a 7 tonne liquid Xe detector which is proposed to be constructed at Sanford Lab in 2016+.

http://luxdarkmatter.org
http://particleastro.brown.edu

Assembly of Massive Galaxies: Emerging Insights and Outstanding Challenges

Hierarchical Cold Dark Matter models of structure formation provide one of the most successful paradigms for the growth of dark matter on large scales, but central challenges remain in elucidating how massive galaxies grow over cosmic time and across different environments. I will discuss techniques to set constraints on the merger history of galaxies and outline the powerful use of structural archaeology. While violent galaxy majors were traditionally thought to play a dominant role in galaxy growth, I will present a wide array of evidence that more quiescent processes (minor mergers, gas accretion, and secular processes) play a central role. The evidence stems from galaxy surveys conducted with ground-based facilities and the Hubble Space Telescope (e.g., GEMS, GOODS, GNS, the ACS Treasury Survey of the Coma cluster), and includes the structural properties of nearby field spiral galaxies; the cosmic star formation history over half of the age of the Universe, and the structure and activity of the most massive galaxies at redshifts of 2 to 4, when the Universe was less than one third of its present age. I will outline outstanding challenges that some of these results present for hierarchical simulations of galaxy evolution. I will end by discussing future prospects and collaborative opportunities with the VENGA integral field spectroscopic survey of nearby spirals, and the HETDEX/SHELA 28 square degree survey, which explores galaxygrowth across different environments over a huge volume at the epochs where proto-clusters collapse, and the cosmic star formation and black hole accretion activity peak.

Atomic-level ordering of complex oxide half-metals for high-performance spin-based electronics

High spin polarization materials (P>80%) are of high interest for room-temperature spin injection applications. However, much of the investigation space for simple alloys and binary oxides is explored, and continued progress requires us to 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 double perovskite system, Sr2CrReO6. Sr2CrReO6 is a predicted pseudo-half-metal of high interest for spin injection applications due to its ferrimagnetic Curie temperature exceeding 500 K, when grown with high Cr/Re ordering [1]. The material is of particular interest as it is also found to be semiconducting (Eg ? 0.2 eV), making it a promising candidate to relieve resistance mismatches that interfere with spin injection [2]. An improved understanding of the magnetic structure of Sr2CrReO6 is necessary to 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 O sites [3]. Interestingly, O K-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.

[1] Hauser et al., Phys.Rev. B 85, 161201(R) (2012).
[2] Schmidt et al., Phys Rev B 62, R4790 (2000).
[3] Hauser et al., Phys. Rev. B 89, 180402(R) (2014).

Casimir Energies and Forces: An Accelerating Subject

There has been an acceleration of interest in quantum vacuum forces, including Casimir-Polder and Casimir forces. Dramatic improvements in theory and experiment have taken place in the past few years. Formerly, the subject of quantum fluctuation energies and forces was a rather esoteric one, but recently there have been various special funding mechanisms to explore this field in the United States and Europe. Many international conferences and workshops are held on the subject, some of them occurring simultaneously! Among the potential applications of zero-point energy is understanding the accelerating expansion of the universe. Indeed, Dark Energy, which makes up some 70% of the energy of the universe, likely results from quantum fluctuations! The subject is truly interdisciplinary,with important contributions from and applications to condensed matter, atomic, and high energy physics, mathematics, chemistry, engineering, and nano science. This talk will give a general overview of the subject of quantum vacuum energy, including discussion of recent developments, such as Casimir torques, systematics of self-energies, Casimir repulsion, and negative Casimir energies.

Cosmological Highlights from the Sloan Digital Sky Survey

I will describe some of the scientific highlights from the Sloan Digital Sky Survey (SDSS), concentrating on those connected to cosmology and galaxy formation. In the three phases to date,SDSS-I, II, and III, the Sloan collaboration has carried out severalof the largest and most ambitious surveys of the distant universeand the Milky Way galaxy, with deep digital imaging over one thirdof the sky and spectroscopy of more than 2 million galaxies, 200,000 quasars, and half a million stars. Cosmological achievements include: probing the epoch of reionization with the most distantknown quasars; comprehensively characterizing the properties ofgalaxies and the relations between galaxies and their parent darkmatter halos; discovering ubiquitous substructure in the outerMilky Way and more than a dozen new companion satellite galaxies;mapping cosmic expansion over the last four billion years withmore than 500 Type Ia supernovae; and, through its precision measurements of structure on very large scales, providing a central pillar of the standard cosmological model based on inflation, cold dark matter, and dark energy. I will review these highlights,with particular attention to recent progress in measuring theproperties of dark energy through baryon acoustic oscillations.I will summarize plans and prospects for SDSS-IV, which began in July 2014.

Dynamics, Damping, and Spin Transport in Spintronic Thin Film Heterostructures

Spintronics is an emerging set of technologies in which devices rely on manipulation of the electrons’ spin degrees of freedom, which is directly related to the collective magnetic moment in solid-state materials. Development of spintronic devices thus requires knowledge of the dynamics of magnetic moments in ferromagnetic metals, plus the couplings of spin and charge degrees of freedom. Recent theoretical and experimental developments have identified some useful effects that link charge and spin and produce possibilities for new devices. In this talk I will discuss some basic applications for spintronics, I will define some key parameters necessary for design and I will introduce phenomena such as spin pumping and the spin Hall effect and discuss their usefulness. I will review the experimental techniques useful in measurements, will show some recent experimental results,and discuss future work and unknowns. At NIST we have recently developed new methods to measure and manipulate ferromagnetic damping and measure key spin transport parameters, such as the spin diffusion length, in metals at room temperature. Our results are precise enough to probe previously inaccessible regimes and show that extensions to current theories are necessary to explain what we observe.

Dark Energy and the Progenitors of Type Ia Supernovae

I’ll show the best current constraints on Dark Energy from a joint analysis between the Sloan Digital Sky Survey II and the Supernova Legacy Survey(SNLS). The SNLS was a 5 year program on CFHT to discover and follow more than 450 Type Ia supernovae for use as standard candles. The progenitors of SNe Ia have been uncertain for more than 80 years, but recently we’ve used clever tricks to either identify or rule out progenitors in some cases. One method is to look for signs of the supernova ejecta being shocked as it slams into a companion star. I’ll show some new, surprising results regarding this effect, which until now has only been theoretically postulated. I’ll also talk about the Las Cumbres Observatory Global Telescope Network — a robotic network of 11 telescopes placed around the world specifically to study time variable phenomena such as extrasolar planets, supernovae, AGN, GRBs, and variable stars.

A Magnetic Majorana Fermion Factory

Superconductivity reorganizes electrons into a condensate of Cooper pairs. The most common excitations in a superconductor are weakly-interacting quasiparticles that mix particle (electron) and antiparticle (hole) character, and satisfy Fermi statistics. In the special case of spinless p-wave superconductors in one and two dimensions, however, there are also zero-energy excitations that are their own antiparticles, and are in this sense called Majorana fermions, an exotic real fermion predicted by Majorana in 1937. Majorana fermions in condensed matter have unusual quantum statistics and potential advantages for quantum computation that have motivated efforts to find a practical strategy for experimental realization. I will discuss the possibility of generating Majorana fermions “on demand” by placing a chain of magnetic adatoms on the surface of a superconductor, focusing on the case of Fe chains on Pb addressed in a recent experiment and emphasizing the interplay between traditional surface physics and chemistry and the hunt for exotic quasiparticles.

Light-Matter Interaction and Transport in Quantum Materials: A Spintronics Perspective

Spintronics is a discipline that concerns the role of electron spins in transport and magnetic phenomena. Recently discovered quantum materials, including atomically thin two-dimensional crystals and topological insulators, have interesting optical and transport properties because their electrons behave in a quasi-relativistic fashion governed by the Dirac equation. In this talk I will discuss a number of light-matter interaction and transport properties in these emerging quantum materials from the perspective of spintronics. Because of considerable spin-orbit coupling and emergence of the valley pseudospin degrees of freedom, they exhibit interesting spin/valley dynamics and strong magneto-optical response. I will also discuss the outlook of realizing spintronics and light-matter coupling in complex heterostructures based on these quantum materials.

Atomically thin semiconductors: new devices and new physics

The discovery of two-dimensional crystals—materials only one to a few atoms thick—continues to drive exciting developments in condensed matter physics, more than 10 years after atomically thin graphene was first peeled from graphite. The techniques used to isolate graphene have now been generalized to other materials with layered structures including a nearly perfect insulator hexagonal boron nitride, an entire family of atomically thin semiconductors such as MoS2 and WSe2, and many more. These materials can be picked up and stacked together to make a wide variety of electronic devices composed entirely of atomically thin, transparent, and flexible materials. In this talk I will present an overview of these developments and describe our contributions to the field, including the demonstration of a photovoltaic device and light-emitting diode made from a three-atom thick sheet of WSe2. Finally, I will give an outlook for how continued improvements in materials and device fabrication are opening up a playground for new devices and new physics in this area.

IceCube: The Beginning of Neutrino Astronomy

The IceCube Neutrino Observatory is the largest neutrino detector in the world, instrumenting one cubic kilometer of ice at the geographic South Pole. IceCube was designed to detect high energy neutrinos from possible cosmic ray acceleration sites such as active galactic nuclei, gamma ray bursts and supernovae. In 2013, IceCube announced the first detection of a diffuse flux of high energy neutrinos whose characteristics are consistent with astrophysical origin. The IceCube signal includes the highest energy neutrinos ever detected. I will discuss the latest results from IceCube and future prospects for the next generation of neutrino detection at the South Pole.

Black Holes in Horava Gravity

In 2009 Horava introduced an alternate theory of gravity which is inherently non-relativistic. Much of our intuition based on Einstein’s general relativity is therefore inapplicable when exploring this new theory. We will discuss the notions needed to make sense of black holes in Horava gravity, justify their existence, and explore their properties. Applications to non-relativistic holographic duality will be presented.

Building the Black Hole in Our Own Backyard

Astronomers now know that supermassive black holes are a natural part of nearly every galaxy, but how these black holes form, grow, and interact within the galactic center is still a mystery. In theory, gas-rich major galaxy mergers can easily generate the central stockpile of fuel needed for a low mass central black hole ‘seed’ to grow quickly and efficiently into a supermassive one. Because of the clear theoretical link between gas-rich major mergers and supermassive black hole growth, this major merger paradigm has become a well-accepted way to form the billion solar mass black holes that power bright quasars in the early universe. It’s much less clear, though, how well this paradigm works for growing the ‘lightest’ supermassive black holes; these million solar mass black holes tend to lie in galaxies like our own Milky Way, where the supermassive black hole is currently quiescent and major mergers were few and far between. This talk will touch on some current and ongoing work on refining our theories of black hole growth for this lightest supermassive class.