Physics Seminar

Probing and Manipulating Interfaces of Low Dimensional Nanostructures

Speaker: Dr. Judy Wu (University of Kansas)
Location: OPB 234, 2:50pm
Abstract: In low dimension nanostructures, the quantum confinement is manifested in the much enhanced Coulomb interaction and much reduced dielectric screening effect. Consequently, the photo-generated excitons (a pair of photo-generated electron and hole) have a large binding energy on the order of sub-eVs characteristic to low-dimensional nanostructures such as quantum dots, carbon nanotubes (CNTs), graphene and the emerging 2D materials. This not only results in fascinating physical properties in low-dimensional nanostructures but also opportunities in exploration of novel devices taking advantages of the excellent electronic, optoelectronic and mechanical properties. This work presents a few examples in probing and manipulating exciton dissociation and charge transfer at the interfaces of low-dimensional materials. We show nanohybrids low-dimensional nanostructures with atomically designed electronic structures at the interface can provide a promising pathway towards extraordinary quantum efficiency in optoelectronics including photovoltaics, photodetectors, sensors, displays etc.

Exploring the Exciton Landscape of 2D Atomically Thin Semiconductors in Energy, Space, and Time

Speaker: Dr. Nicholas J. Borys (Lawrence Berkeley National Lab)
Location: OPB 234, 2:50pm
Abstract: In molecules and semiconductors, the absorption of a photon creates an excited state composed of multiple valence electrons. Remarkably, this complex many-body state behaves as a negatively charged electron bound to a positively charged hole—an “exciton”—exhibiting characteristics that are analogous to those of a hydrogen atom. In semiconductor nanostructures, excitons govern light-matter interactions and form fundamental packets of energy that can be manipulated, transported and harvested for next-generation technologies ranging from photovoltaics to quantum computing. Transition metal dichalcogenide semiconductors, such as monolayer MoS2, are an emergent class of ultrathin thin semiconductors that are only three atomic layers thick and host a rich suite of two-dimensional excitonic phenomena. While it is tempting to assume that these atomic sheets are analogous to conventional quantum wells, their atomically thin width, tunable many-body interactions and intricate band structure combine to produce novel phenomena that defy such simplifications, creating new opportunities for fundamental investigations of many-body physics to the development of new optoelectronic devices. Our work concentrates on probing excitonic light-matter interactions in two-dimensional monolayer semiconductors on timescales from femtoseconds to the steady-state, and at length scales from the micron to the nanoscale. Using optical excitation spectroscopy, we have demonstrated that although the optical transitions of the excitons are concentrated into a narrow spectral band, the excitons can be efficiently excited over a significantly broader spectral range. We can also discriminate between direct versus indirect excitation of excitons which has led to new understandings of energetic renormalization effects and exciton formation dynamics. Finally, using nano-optics for sub-diffraction imaging and spectroscopy, we have uncovered a striking diversity of optoelectronic regions, including “nanobubbles,” charge puddles, disordered edges and grain boundaries, that dramatically influence excitonic photophysics on nanoscale dimensions. From these studies, we can now begin to envision how to pattern two-dimensional monolayer semiconductors with excitonic circuitry for model optoelectronic and quantum excitonic devices.

Probing the Plasma Physics and Kinematics of the Intracluster Medium with X-ray Observations and Simulations

Speaker: Dr. John ZuHone (Harvard-Smithsonian CfA)
Location: OPB 234, 2:50pm
Abstract: Since its launch in 1999, the Chandra X-ray Observatory has provided unprecedented views of the hot plasma of galaxy clusters, revealing structures such as shocks, cold fronts, and indications of gas turbulence. Before its unfortunate demise, the Hitomi mission provided the first direct measurements of gas motions in the Perseus cluster. All of these observations indicate the intracluster medium (ICM) has interesting plasma properties and is continuously stirred by gas motions large and small driven by mergers and AGN feedback. In this talk, I will present the results of a number of hydrodynamical simulations of galaxy cluster mergers and compare them to observations with a view towards constraining the plasma and kinematic properties of the ICM. I will finish my talk with a view toward future X-ray missions and what may be revealed in the cluster plasma by the combination of high spectral and spatial resolution.

Cold gas in Giant ellipticals: Life cycle of cold gas in hot atmospheres

Speaker: Dr. Kiran Lakhchaura
Location: OPB 234, 2:50pm
Abstract: Elliptical galaxies, long thought to be devoid of any gas and dust, are now known to host a complex multiphase interstellar medium covering an impressive temperature range of ~50-10^7 K. I have analysed the Chandra X-ray observations of a nearly complete sample of nearby X-ray and optically bright elliptical galaxies, in order to understand the connection between the hot gas and cold gas properties, their interplay and their role of in the AGN feedback cycles. In my talk, I will be discussing some of the interesting results from this work.

Nanotechnology Goes Atomic Scale Where Every Atom Matters

Speaker: Dr. Garnett W. Bryant (NIST)
Location: OPB 234, 2:50pm
Abstract: Optical nanostructures, such as semiconductor quantum dots and metallic nanoparticles, have been intensely studied for the last three decades. Quantum dots are often referred to as artificial atoms, with intense, discrete electron-hole transitions, also known as excitons, which are ideal for such applications as nanoscale lasing, sensing, single photon and entangled photon generation, and quantum information processing. On the other hand, plasmons in metallic nanoparticles provide a way to confine optical fields to nanoscale dimensions, serving as nanoantennas to direct light into and away from quantum emitters. Both quantum dots and metallic nanoparticles are made from tens of thousands of atoms. In my group at NIST, we are interested in pushing such nanostructures down to the atomic scale where every atom matters. In this talk I will describe some of the theoretical and experimental work being done to understand these nanostructures as they are pushed to the atomic limit. I will highlight some of the ways that the fundamental physics of quantum dots and plasmonic nanoparticles must be rethought in this atomic limit. At the same time, I will describe some of the experimental challenges faced in making atomically precise structures and some of the reasons to go there.

X-Ray Polarimetry of Astronomical Sources: Introducing a NASA Mission with a New Window on the X-ray Universe

Speaker: Dr. Martin Weisskopf (NASA MSFC)
Location: OPB 234, 2:50pm
Abstract: NASA has recently selected the Imaging X-ray Polarimetry Mission (YXPE) as the next astrophysics Small Explorer Mission. We briefly review the history of astronomical X-ray polarimetry, beginning with early sounding-rocket experiments by Robert Novick at Columbia University and his team, of which I was a member. After describing various techniques for measuring X-ray polarization, we discuss the polarimeter aboard Orbiting Solar Observatory 8 (OSO-8) and its scientific results. Next, we consider the X-ray polarimeter aboard the ill-fated original Spectrum-X mission, which provided important lessons on polarimeter design, systematic effects, and programmatics of a shared focal plane. Finally conclude with a description of the Imaging X-ray Polarimetry Explorer (IXPE) and its prospective scientific return. IXPE is a partnership between NASA and the Italian Space Agency and is scheduled to launch in early 2021.

Gravitational Lensing of Gravitational Waves and Precision Cosmology

Speaker: Dr. Zong-Hong Zhu (Beijing Normal University)
Location: OPB 234, 2:50pm
Abstract: The standard siren approach of gravitational wave cosmology appeals to the direct luminosity distance estimation through the waveform signals from inspiralling double compact binaries, especially those with electromagnetic counterparts providing redshifts. It is limited by the calibration uncertainties in strain amplitude and relies on the fine details of the waveform. We will show the next generation detector, e.g., the Einstein Telescope, is expected to produce 10^4 −10^5 gravitational wave detections per year, 50−100 of which will be lensed. Then we report a waveform-independent strategy to achieve precise cosmography by combining the accurately measured time delays from strongly lensed gravitational wave signals with the images and redshifts observed in the electromagnetic domain. We demonstrate that just 10 such lensing systems can provide a Hubble constant uncertainty of 0.68% for a flat Lambda Cold Dark Matter universe in the era of third generation ground-based detectors.

Majestic Lightening

Speaker: Dr. Themis Chronis (UAH Physics & Astronomy)
Location: OPB 234, 2:50pm
Abstract: This presentation will encompass the research results of a team effort that address head on long standing questions on atmospheric electricity and more specifically lightning. These results have revealed a quite novel diurnal lightning behavior on a global scale, land-sea storm energetics contrast, terrestrial gamma ray production and the value of lightning in severe weather nowcasting. I will also present some of my current research initiatives that include the possible role of chemistry in atmospheric electricity.

What More do we Know about Gamma-Ray Bursts in the Fermi Era?

Speakers: Dr. Narayana Bhat (UAH CSPAR)
Location: OPB 234, 2:50pm
Abstract: Gamma-ray bursts (GRB) are the most energetic events in the Universe emitting photons in a very wide energy range from radio to GeV gamma-rays. Their peak luminosity is in the 50-300 keV range. Due to their cosmological nature, the total energy emitted by a GRB source is in excess of 1054 ergs. As a result, during their peak, they emit more energy than all the stars and galaxies in the Universe combined. More than three decades since their serendipitous discovery, followed by several breakthroughs from space-borne and ground-based observations, they remain one of the most interesting astrophysical phenomena yet to be completely understood. The current gamma-ray burst mission is the Fermi Gamma-ray Space Telescope that was launched in June 2008, detects about 2 GRBs in 3 days. One of the unique features of Fermi is its broad energy band-width spanning 7 orders of magnitude from 10 keV to 300 GeV. Because of the unique energy band-width we are able to investigate some unique characteristics relating to both spectral and temporal nature of GRBs which were hitherto unknown. Among these are the multi-component spectral features, energy dependent durations, spectral lags and variability time scales. Starting with an extended introduction tracing the history of GRBs, we will briefly describe how these studies of Fermi GRBs lead to a better understanding of the gamma-ray production processes at various energies.

Department Mini-Seminars

Speakers: Drs. Lieu, Nishikawa, R Miller
Location: OPB 234, 2:50pm
Abstract: Three short presentations highlighting some current research in the department.

Department Mini-Seminars

Speakers: Drs. J. Miller, Sun, and Bonamente (UAH Physics & Astronomy)
Location: OPB 234, 2:50pm
Abstract: Three short presentations highlighting some current research in the department.

Department Mini-Seminars

Speakers: Drs. Duan, Sedeghi, and Gregory (UAH Physics & Astronomy)
Location: OPB 234, 2:50pm
Abstract: Three short presentations highlighting some current research in the department

Total Solar Eclipses: Probing the Structure of our Nearest Star

Speaker: Dr. Gordon Emslie (WKU Physics & Astronomy)
Location: MSB 100, 3:30pm 
Part of UAH's Eclipse Week. 

The Science and Spectacle of Solar Eclipses

Speaker: Dr. Gordon Emslie (WKU Physics & Astronomy)
Location: MSB 100, 12:00noon
Part of UAH's Eclipse Week.