May 1, 2015 10:30AM TBE A-120. (HiPSEC thesis defense) Niup Bandaru, STRUCTURE AND OPTICAL PROPERTIES OF TRANSITION METAL DICHALCOGENIDES (TMDs) – MX2 (M = MO, W & X = S, Se) UNDER HIGH PRESSURE AND HIGH TEMPERATURE CONDITIONS.

Layered structured materials such as transition metal dichalcogenides (TMDs) have gained immense interest in recent times due to their exceptional structural, electrical and optical properties. Recent studies show semiconducting TMDs such as MX2 (M= Mo, W & X = S, Se) could be used as potential shock absorbing material, which has resulted in extensive studies on structural stability of these materials under the influence of high pressure. Understanding the structural stability of transition metal dichalcogenides (TMDs) such as MoS2, MoSe2, WS2, and WSe2 under high pressure has been very challenging due to contradicting observations and interpretations reported in the past. Hence, the main objective of this work is to study the crystal structure and optical properties of bulk MX2 at high hydrostatic pressures up to 51 GPa using a diamond anvil cell with synchrotron radiation in addition to high pressure Raman spectroscopic and high temperature X-ray diffraction (XRD) experiments. Crystal structures of MX2 materials are observed to be stable up to 500C with nonlinear thermal coefficients of expansion. Results of high pressure experiments show a pressure induced isostructural hexagonal distortion to a 2Ha-hexagonal P63/mmc phase in MoS2 around 26 GPa as predicted by theoretical calculations reported earlier. No pressure induced phase transformation is observed in other MX2 (MoSe2, WS2, WSe2) compounds. A semi empirical model based on the energy of interaction of bond electrons is proposed to explain the observed inconsistency between MoS2 and other TMDs studied. Using this model, it is shown that except MoS2, no other MX2 within the scope of this study undergoes pressure induced phase transition in the pressure range 0 - 50 GPa. High pressure Raman results show continuous red shifts in dominant vibrational modes with increase in pressure in MX2. Additionally, emergence of a new peak, namely 'd-band' associated with 2Ha structure in MoS2 supports the observation of a isostructural phase transition in high pressure X-ray diffraction experiments. In addition to the studies on bulk MoS2 material, thin film (approximately 100 nm thicknesses) is successfully fabricated via DC magnetron sputtering system and sulfurization technique.

April 24, 2015 3:30PM BPB-217. Jason Rhodes, Exploring The Dark Sector with Euclid and WFIRST-AFTA.

Dark energy, the name given to the cause of the accelerating expansion of the Universe, is one of the most profound mysteries in modern science. Current cosmological models hold that dark energy is currently the dominant component of the Universe, but the exact nature of dark energy remains poorly understood. There are ambitious ground-based surveys underway that seek to understand dark energy and NASA is participating in the development of significantly more ambitious space-based surveys planned for the next decade. NASA is providing mission-enabling technology to the European Space Agency's (ESA) Euclid mission in exchange for US scientists to participate in the Euclid mission. NASA is also developing the Wide Field Infrared Survey Telescope-Astrophysics Focused Telescope Asset (WFIRST-AFTA) mission for possible launch in 2023. WFIRST was the highest ranked space mission in the Astro2010 Decadal Survey and the AFTA incarnation of the WFIRST design uses a 2.4m space telescope to go beyond what the Decadal Survey envisioned for WFIRST. Understanding dark energy is one of the primary science goals of WFIRST-AFTA. I'll discuss the status of Euclid and WFIRST and comment on the complementarity of the two missions. I'll also briefly discuss other, exciting science goals for WFIRST, including a search for exoplanets using both microlensing and a dedicated coronagraph for exoplanet imaging.

April 10, 2015 3:30PM BPB-217. Joshua C. H. Lui, Shedding light on two-dimensional electrons in graphene and beyond.

Graphene, a single layer of carbon atoms, has stimulated intense scientific interest due to its distinctive electronic and mechanical properties. Graphene exhibits strong interactions with light over a broad spectral range. This enables us to examine its electronic and vibrational properties through optical spectroscopy. In addition to gaining understanding of the properties of single-layer graphene, we can also probe the behavior of electrons in few-layer graphene. This reveals the unique electronic and vibrational properties for graphene of each layer thickness and stacking order, as well as their distinct capability to induce an electrically tunable band gap. I will also highlight recent development of 2D materials beyond graphene.

April 9, 2015 9:00AM BPB-217. PhD. Defense: Houjun Lu, Constraining the Progenitor and Central Engine of Gamma-Ray Bursts with Observational Data.

Gamma-ray bursts (GRBs) are extremely energetic explosions at cosmological distances. We have made great progress in understanding the mysteries of these events since they were discovered more than forty years ago. However, some open questions still remain, e.g. how many classes of GRBs are there, what are the progenitors of these classes, and what is the central engine powering these huge explosions? Thanks to the NASA missions Swift and Fermi, which are used to detect the multi-wavelength emission from these transients, our understanding of GRBs has been greatly advanced. In this dissertation, I use multi-wavelength data to constrain the progenitor and central engine of GRBs. My dissertation consists of three parts: (1) By adding the third dimension ``amplitude'' as a complementary criterion in classifying GRBs, we test whether some short GRBs are ``tip-of-iceberg'' of long GRBs, and explain why some high redshift long GRBs have short durations in the rest frame. (2) Using Swift data, we investigate whether the data are consistent with the hypothesis that there exist millisecond magnetar central engines in some long GRBs. We reach the conclusion that at least some long GRBs have a magnetar central engine. (3) We test how well the data are consistent with the magnetar central engine model for short GRBs. We identify that a good fraction of short GRBs have a supra-massive magnetar central engine, which collapses to a black hole after hundreds of seconds. We use the data to constrain the neutron star equation of state.

April 7, 2015 3:30 BPB-217. LiDong Pan, How to weigh a monopole – studying complex material response with THz spectroscopy.

An important theme of modern condensed matter physics is the realization of novel excitations in materials (e.g. quasiparticles). Although they are not fundamental particles, such quasiparticles do constitute the most basic description of the excited states of the "vacuum" they reside in. In this regard the magnetic textures of the excited states of spin ices, magnetic pyrochlore oxides with dominant Ising interactions, can be modeled as effective magnetic monopoles. Utilizing the unique phase sensitive capabilities of time domain terahertz spectroscopy and microwave cavity techniques, we study the complex dynamic magnetic susceptibility of quantum spin ice Yb2Ti2O7. We find strong evidence of inertial effects in the monopoles dynamics. From the spectral weight, an effective mass of the monopoles is also obtained. Our results establish the magnetic monopoles as true coherently propagating quasiparticles in quantum spin ice.