Forum Schedule: Fridays 3:45pm - 4:45pm
| Date | Speaker | Title & Abstract |
| Aug 22 | Prof. Haozhe Liu (Harbin Institute of Technology, China) (host: HiPSEC) | Pressure Induced Phase Transitions in Selected A2B3 Topological Insulators The 'simple' topological insulator A2B3, where A = Sb, Bi, B = Te, Se, attracted lots of attention in last couple of years. High-pressure phase transitions in these systems were long standing puzzle even before the topological insulator concept become popular. In this presentation, theoretically and experimentally high-pressure researches on these topological insulator A2B3 compounds will be reviewed, and complicated phase transition sequence for various compounds will be presented. The similarity to related element's high-pressure behavior will be discussed. In addition, synchrotron x-ray tomography technique will be briefly introduced for the density measurement on amorphous and melt samples under high-pressure conditions using diamond anvil cell and large volume press techniques. |
| Sep 13 | ||
| Sep 20 | Dr. Costel Rotundu (UCLA - Department of Physics and Astronomy) (host: Dr. Valentin Iota) | High-Pressure Measurements on Single Crystals of Electron-Doped Cuprate Superconductor Pr2-xCexCuO4 Physical properties of the electron-doped cuprate showed no (or extremely small) changes when compressed to relatively low pressures, i.e. in the 2-2.5 GPa range [1]. This behavior of the electron-doped cuprates contrast with that of the hole-doped cuprates for which Tc is first substantially enhanced with application of pressure [2]. I present high-pressure diamond-anvil cell (DAC) synchrotron x-ray, resistivity, and ac-susceptibility measurements on the electron-doped HTSc cuprate Pr2-xCexCuO4 to much higher pressures than previously reported under quasi-hydrostatic conditions of pressure by using Ne gas as pressure transmitting medium [3]. At 2.72 GPa between 88 and 98% of the superconducting T' phase of the optimally doped Pr1.85Ce0.15CuO4 transforms into the insulating phase T. With application of pressure, the T phase becomes more insulating, so we present here an example of electron doping in the T structure. The results have implications for the search for ambipolar high-Tc cuprate superconductors. For the DAC resistivity measurements we developed a method to "wire" and attach the sample using focused ion beam ultrathin lithography [4]. This method shows promise for resistivity measurements at multi-Mbar pressures. |
| Sep 24 (Tuesday 12:15-1:30) | Dr. Hui Li (Los Alamos National Lab) (host: Prof. Bing Zhang) | A Model for Astrophysical Jet and Lobe System Powerful jets from supermassive black holes have fascinated astronomers and physicists for more than half a century. As more observations become vailable, it is generally recognized that plasma physics holds the key to understanding the formation and propagation of jets as well as the dissipation processes within jets (sometimes with extreme parameters). We will describe the key observations that help us to determine the physical parameters and what we believe the key processes are. We will present relativistic magnetohydrodynamic (MHD) simulations of jet propagation, emphasizing the role of current-driven instabilities and energy conversion between magnetic fields and kinetic+internal energies. These studies are yielding new insights on understanding the jet stabilities and their morphologies in multi-wavelength observations. Progress has benefited from extensive knowledge in plasma physics. Furthermore, dedicated laboratory plasma experiments to study jets have begun to offer new insights into various aspects of jet research. |
| Sep 27 | ||
| Oct 4 | ||
| Oct 11 | Dr. Raffaella Margutti (Harvard CfA) (host: Prof. Bing Zhang) | Eruptions, disruptions and (repeated) explosions: massive stars at the end of their life. The role of central engines and sustained mass-loss. Observations are drawing a complex picture of the latest stages of massive stars evolution and their explosions. In this seminar I concentrate on two among the least understood aspects, adopting an observational perspective: (i) How do massive stars loose a significant fraction of their mass in the years preceding the explosion? (ii) What is the role of central-engines in explosions originating from progenitors that managed to loose their entire hydrogen envelope? I address these questions taking advantage from panchromatic observations of two remarkable explosions: the puzzling, double explosion of SN2009ip in 2012, and the mildly relativistic, engine-driven SN2010bh, that bridges the gap between ordinary explosions and fully relativistic gamma-ray bursts. |
| Oct 18 | ||
| Oct 25 | Nevada Day Recess | |
| Nov 1 | ||
| Nov 8 | Dr. Dave Besson (The University of Kansas - Physics and Astronomy) (host: Prof. Bing Zhang) | Radar Detection of Cosmic Rays The Telescope Array Radar Array experiment seeks measurement of ultra-high energy cosmic rays, with energies of order 1 Joule by detection of the forward radar echo reflected from the core of a down-coming atmospheric air shower. The recent commissioning of a 40 kW transmitter in Delta, UT has initiated the data-taking phase of this experiment. We discuss progress to date and future plans. |
| Nov 15 | ||
| Nov 22 | ||
| Nov 29 | Thanksgiving Recess | |
| Dec 5 (3:45PM BPB-217 refreshments @ 3:30PM) | Indrani Banerjee (Indian Institute of Science, Bangalore, India) (host: Prof. Bing Zhang) | Correlation Between Mass and Spin of Black Holes and Nucleosynthesis in the Flows Around Black Holes This talk will comprise of two parts. In the first part I will establish a correlation between the mass and spin of stellar mass black holes. Stellar mass black holes, formed from the core collapse of very massive, rapidly rotating stars, are expected to exhibit a high density accretion disk around them developed from the spinning mantle of the collapsing star. A wide class of such disks, due to their high density and temperature, are effective emitters of neutrinos and hence called neutrino cooled disks and they are often associated with gamma-ray bursts (GRBs). Tracking the physics relating the observed (neutrino) luminosity to the mass, spin of black holes and the accretion rate of such disks, we establish a correlation between the spin and mass of stellar mass black holes at their formation stage. In the second part of my talk, I will discuss the various results we have obtained while studying the nucleosynthesis in the aforesaid accretion disks and outflows associated with them. We report the synthesis of several unusual nuclei like P31, K39, Sc43, Cl35 and various isotopes of titanium, vanadium, chromium, manganese and copper in the disk. Much of these heavy elements are ejected from the disk via winds and outflows. Ni56 is abundantly synthesized in most of the cases in the outflow which implies that the outflows from these disks in a majority of cases will lead to an observable supernova explosion. Although, emission lines of many of these heavy elements have been observed in the X-ray afterglows of several GRBs by Chandra, BeppoSAX, XMM-Newton etc., Swift is yet to detect these lines. |
| Dec 6 (10:00AM BPB-217 at Astronomy Coffee) | Prof. Banibrata Mukhopadhyay (Indian Institute of Science, Bangalore, India) (host: Prof. Bing Zhang) | New Mass Limit for Magnetized White Dwarfs and Its Consequences I will show that highly magnetized white dwarfs have a mass limit which is significantly larger than that proposed by Chandrasekhar. This immediately explains several puzzling observations of over-luminous, peculiar type Ia supernovae, which are supposed to have significantly super-Chandrasekhar white dwarfs as their progenitors. This new limit has been arrived by exploiting electromagnetism, quantum mechanics, magnetohydrodynamics and statistical mechanics in observed, accreting, magnetized white dwarfs. This also predicts for the possible existence of a second standard candle which has many far reaching implications, including that related to the expansion history of the universe. |
| Dec 6 (3:45PM BPB-217 refreshments @ 3:30PM) | Dr. Joel Meyers (Canadian Institute for Theoretical Astrophysics) (host: Prof. George Rhee) | Planck and Inflation The results from the Planck satellite have provided excellent constraints on many cosmological parameters allowing us to probe the physics of inflation. While all observations are currently consistent with the simplest models of inflation, many more complicated scenarios are also consistent with the data. In this talk, I will focus on the theoretical aspects of inflation in light of the Planck data. I will highlight a few observables which are measured by Planck and discuss how future observations will give us non-trivial information about the physics of the early universe. |
| Dec 13 |