Spring 2023 Colloquium Schedule
Time: Tuesdays 3 pm to 4 pm. Refreshment starts at 2:45 pm
Venue: 25 Park Place Room 223 and Room 2608
Colloquium contact: Yang-Ting Chien ([email protected]) and Viacheslav Sadykov ([email protected])
*Thursday
Date |
Speaker |
Title and Abstract |
Jan 10 | First Week of Semester | |
Jan 17 | Second Week of Semester after MLK Day | |
Jan 24 |
Shan Wu (Lawrence Berkeley National Lab) Host: Mike Crenshaw Room 2608 |
Magnetic switching resistance materials
Quantum materials are complex systems in which electrons interact strongly and collaboratively. As such, quantum mechanics is dominant in the versatile materials that allow us to explore emergent quantum phenomena and their potential applications in future technologies. Emergent quantum materials embrace material properties that demonstrate the significance of interplay among multiple dimensions in the system. These exotic properties can be harvested into applications for next-generation quantum technologies. Antiferromagnetic (AFM) spintronics is among the newest of quantum technologies, which realize the manipulation of spin-transport in antiferromagnets for memory devices. Recently, the intercalated transition metal dichalcogenide (TMD) antiferromagnet Fe1/3+δNbS2 has shown relevant spintronic features, namely, resistance switching. The original explanation of AFM switching hypothesized that it was driven by a coexisting spin glass phase. Here, I will present our discovery of highly tunable antiferromagnetic orders and discuss how they relate to the observed resistance switching in this material. The rapid change of the magnetic states underlies the sensitive switching behaviors, providing crucial insights on the magnetic ground states that form the basis for understanding the fascinating spintronic behavior. I will also talk about my recent work on this system and open opportunities in the intercalated TMDs as part of the exploration of the spintronic materials. |
Jan 26* |
Kun Wang (Mississippi State University) Host: Mike Crenshaw Room 223 |
Probing Quantum Transport and Energy Conversion at the Molecular Scale
Molecules – the smallest unit of matter – have been playing a pivotal role in today’s materials science, nanotechnology, and quantum science. The capability to manipulate physical and chemical behaviors of single molecules and understand how they respond to external stimuli represents important opportunities for optoelectronics, energy, and quantum applications. In this colloquium, I will talk about my recent studies on developing experimental tools to probe quantum transport in molecular-scale systems and further leveraging them to address challenges in molecular electronics, energy conversion, and nanosensing. First, I will introduce strategies to construct molecular-scale electronic devices, such as diodes and transistors. Second, I will discuss my work on developing new experimental approaches to interrogate thermoelectric energy conversion in molecular junction devices, which points to new opportunities for energy harvesting and refrigeration. Third, I will describe how single molecules can be used as nanoscopic quantum sensors to directly access plasmonic hot carriers and how the experimental approach developed in my work will enable systematic study of plasmon-driven processes in many plasmonic and nanophotonic systems. Finally, I will give an overview for my future research directions. |
Jan 31 |
Jay Mathews (University of Dayton) Host: Mike Crenshaw Room 223 |
At the lasing threshold: Germanium-tin as a gain medium for silicon-based lasers
Silicon is the basis for a multi-billion dollar industry, but its optical properties have limited its use in optoelectronics for infrared (IR) applications. There are a number of applications that could benefit from low-cost Si integrated photonics including photovoltaics, infrared detection and imaging, optical interconnects (OICs), and photonic integrated circuits (PICs). In particular, Si-based OICs and PICs require efficient laser sources, modulators, low loss waveguides, optical switches, and photodetectors, all of which must be integrated into a single Si chip using complementary metal-oxide-semiconductor (CMOS) processing. The biggest gap in technology at the moment is the development of an on-chip light source. One possible solution is to use GeSn alloys as a gain medium for laser devices. These thin films are grown on Si substrates and compatible with standard Si processing techniques, they have band gaps in the infrared, and the band structure of GeSn yields more efficient optical absorption and emission than that in Si. These unique properties have prompted a recent effort to develop optoelectronics from GeSn films, and this research has resulted in a number of prototype photonic devices such as photodetectors that cover all telecommunications wavelengths, infrared light-emitting diodes, and optically-pumped waveguide lasers. However, GeSn waveguide devices have only demonstrated lasing at cryogenic temperatures without complicated strain-inducing structures. In this talk, I will present recent work on the fabrication and characterization of waveguide devices made from GeSn alloys and subjected to optical pumping at room temperature. Based on the results, an emission model was developed to predict the optical emission characteristics based on the material properties, waveguide geometry, and experimental conditions. The model was used to explain the emission data from the GeSn waveguides, and it was compared to other recent GeSn waveguide experiments. I will present details of the modeling, along with some predictions on how room temperature lasing could be achieved in GeSn waveguides. |
Feb 2* |
Yulia Maximenko (National Institute of Standards and Technology) Host: Mike Crenshaw Room 2608 |
Flatland quantum simulation and visualization with atomic resolution
Quantum computing and simulation promise to revolutionize fundamental physics, technology, and quantum chemistry. Simulating quantum systems using analog platforms was first proposed in the 1980s, but recent technological advances have brought this idea to new heights. Trapped atoms and ions, superconducting circuits, and advanced solid-state platforms have achieved an unprecedented level of quantum control and are able to model increasingly complex Hamiltonians. Quantum simulation in 2D solid platforms has proved to be incredibly versatile, while also being compatible with the existing semiconductor technology. In this colloquium, I will showcase the exciting recent developments in the field of 2D quantum simulators, highlighting twisted moiré systems and atomic manipulation. Scanning tunneling microscopy (STM) has proved crucial for the progress of this field. My focus will be on revealing the topological and strongly correlated physics in twisted layered graphene and on the surprising insights gained through the use of STM. Through high-resolution magnetic field scanning tunneling spectroscopy, we have demonstrated the importance of the fine details of quantum geometry in these novel 2D platforms. Specifically, I will report on the discovery of an emergent anomalously large magnetic susceptibility caused by the band topology in twisted double bilayer graphene. |
Feb 7 | No colloquium | |
Feb 14 | No colloquium | |
Feb 21 |
Chen Shi (University of California, Los Angeles) Host: Petrus Martens Room 2608 |
Understanding the solar corona and wind in the epoch of Parker Solar Probe
The outflow of hot plasma from the solar corona, including the more continuous solar wind and the bursty plasma expulsions called Coronal Mass Ejections (CMEs) are the main drivers of space weather. The solar magnetic field, driving the cavity in the interstellar medium in which all planets are embedded, stores and releases a significant amount of energy into the interplanetary space. Understanding the origin and dynamical processes associated with the generation of the solar corona, the acceleration of the solar wind and its ongoing turbulent dynamics are some of the most important tasks of space physics. After decades of exploration fundamental questions concerning the physical processes underlying the origin of the solar corona and solar wind acceleration remain. Two of the most outstanding topics are magnetic reconnection in the solar corona and turbulence in the solar wind. As Parker Solar Probe lowers its orbit into regions close to the Sun never observed directly previously, many novel observations have been made. In this seminar, I will review recent progress on the theory, modeling, and observations of the solar wind. I will discuss opportunities and challenges within the context of current and future missions, including the presently flying, from Parker Solar Probe to Solar Orbiter as well as future in situ exploratory missions such as HelioSwarm. |
Feb 28 |
Luiz Santos (Emory University) Host: Yang-Ting Chien Room 2608 |
Interplay of Interactions and Topology in Electronic Fractals Rapid theoretical and experimental progress in characterizing phases of matter has brought to light a plethora of new topological phases. A remarkable example arises in two-dimensional crystals, where electrons are subject to an external magnetic field. This iconic system supports a rich structure of fractal electronic bands with topological properties that are traditionally linked to the quantum Hall effect, but only recently have been realized in the burgeoning class of moiré crystals. While the non-interacting properties of fractal bands have long been classified, the interplay of interactions and band topology remains a much uncharted territory. In this colloquium I will discuss new quantum behavior that emerges from the interplay of interactions and the self-similar character of fractal bands formed in the presence of large magnetic fluxes. I will present universal results concerning quantum phase transitions and interaction driven electronic orders in fractal bands. Among these are new classes of unconventional superconductors (including topological ones) that fall outside the paradigm of phonon-mediated superconductivity. Fractal electronic bands thus represent rich platforms to explore novel quantum phases of matter. |
Mar 7 |
Lulu Zhao (University of Michigan) Host: Petrus Martens Room 2608 |
Space Radiation Environment
Space Weather is the physical and phenomenological state of natural space environments. One important component of the space weather is the space radiation. Space radiation refers to high-energy ionized particles of different origin present in space are of great concern for aviation, satellites and biological systems. It covers mainly the galactic cosmic ray (GCR) background, solar energetic particle (SEPs) and the Earth’s trapped radiation belts. SEPs are not only a major component of space radiation, but their intensity and energy spectra dramatically change in short periods of time. In fact, the space radiation hazard is one of the major unsolved problems (“tall poles”) hindering space travel beyond low-Earth orbit.SEPs are suggested to be accelerated to high-energy either by magnetic reconnection-driven processes in solar flares or by shocks driven by coronal mass ejections. SEPs can be accelerated up to tens of GeV, and the flux of >10 MeV protons could exceed their background level by several orders of magnitude. Protons of >150 MeV are very difficult to shield, and the sparsity and large variation of the SEP events make them difficult to predict. Many efforts have been made to predict the SEPs using both physics-based approaches and machine learning methods. In this presentation, I will discuss the current status and future visions of the space radiation prediction. |
Mar 9* |
Xiaocan Li (Dartmouth University) Host: Petrus Martens Room 2608 |
Modeling Particle Acceleration and Transport in Solar Flares
Solar flares are among the most explosive and dynamic events in our solar system. These phenomena release massive amounts of energy and can accelerate a large number of particles to very high energies. As these particles can significantly impact our technology and human activities, it is crucial to understand how they are accelerated during solar flares. However, this task is challenging due to the vast and complex nature of solar flares. In recent years, an emerging framework combining theory, numerical modeling, and observations has made significant progress toward understanding particle acceleration during solar flares. In this talk, I will summarize those made by modeling, including kinetic particle-in-cell simulations to study small-scale physics and macroscopic modeling to investigate large-scale processes. By combining these results with observations, we have gained important insights into particle acceleration and transport during solar flares. I will also highlight the challenges in understanding complex physics and interpreting modern high-resolution observations using massive numerical simulations. I will discuss current and future efforts aimed at addressing these challenges. This research area is highly active and exciting, offering numerous opportunities for discovery and impact. |
Mar 14 | Spring break | |
Mar 21 |
Student flash talks Host: Sebastien Lepine Room 2608 |
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Mar 28 |
Ian Ferguson (Kennesaw State University) Host: Unil Perera (Rescheduled to Fall 2023) |
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Apr 4 |
Paul Charbonneau (Université de Montréal), Nelson Speaker Host: Petrus Martens Room 223 |
Avalanches, earthquakes and solar flares
Avalanches, earthquakes and solar flares; beside posing a threat to human societies and infrastuctures, what could these three natural phenomena possibly have in common? They unfold on widely different spatial scales, from hundreds of meters for a snowslope, hundreds of kilometers for a seismic fault, up to tens of thousands kilometer for a solar active region. Moreover, their internal dynamics relies on very different physics: friction between snowflakes, elastic deformation or rock, and dissipation of electrical currents. One key commonality, however, is that all three are natural systems that accumulate energy very slowly, but release it in a strongly intermittent and scale-invariant manner. In this talk I will discuss a physical modelling paradigm whereby such natural complex systems and phenomena emerge from a great many simple dynamical elements interacting locally with one another at scales much smaller than the global scale of the system. Complexity thus emerges from simplicity! In illustrating and explaining these ideas, the emphasis will be placed on solar flares, these being one of the major drivers of space weather. |
Apr 11 |
Sonny Mantry (University of North Georgia) Host: Yang-Ting Chien Room 223 |
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Apr 18 |
Misty Bentz (Georgia State University, Physics and Astronomy) Host: Sebastien Lepine Room 223 |
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Apr 25 | Final Exam Week Starts / No Colloquium |
Fall 2022 Colloquium Schedule
Time: Tuesdays 3 pm to 4 pm. Refreshment starts at 2:45 pm
Venue: 25 Park Place Room 223
Colloquium contact: Yang-Ting Chien ([email protected]) and Viacheslav Sadykov ([email protected])
*Thursday