06/03/2021 - Frustrated Self-Assembly of Non-Euclidean Crystals of Nanoparticles
Xiaoming Mao, Professor, Department of Physics
Self-organized complex structures in nature, e.g., viral capsids, hierarchical biopolymers, and bacterial flagella, offer efficiency, adaptability, robustness, and multi-functionality. Can we program the self-assembly of three-dimensional (3D) complex structures using simple building blocks, and reach similar or higher level of sophistication in engineered materials? Here we present an analytic theory for the self-assembly of polyhedral nanoparticles (NPs) based on their crystal structures in non-Euclidean space. We show that the unavoidable geometrical frustration of these particle shapes, combined with competing attractive and repulsive interparticle interactions, lead to controllable self-assembly of structures of complex order. Applying this theory to tetrahedral NPs, we find high-yield and enantiopure self-assembly of helicoidal ribbons, exhibiting qualitative agreement with experimental observations. We expect that this theory will offer a general framework for the self-assembly of simple polyhedral building blocks into rich complex morphologies with new material capabilities such as tunable optical activity, essential for multiple emerging technologies.
06/10/2021 - MOCVD-grown AlSiO gate dielectric on (001) β-Ga2O3
Zhe Ashley Jian, 3rd Year, Department of Electrical Engineering and Computer Science
β-Ga2O3 has become a promising semiconductor for high power applications. High-quality dielectrics are crucial for enabling high performance β-Ga2O3 field-effect transistors (FETs) especially because achieving p-type doping does not seem feasible for this material system. The material properties of high-quality gate dielectric include high dielectric constant, negligible gate leakage, low density of interface and bulk traps, and large breakdown field. Recently, aluminum silicon oxide (AlSiO) has been proposed as a high-performance and reliable gate dielectric for GaN-based devices. Previous studies showed that the alloying of Al2O3 with silicon to form AlSiO has the potential to combine the merits of both SiO2 and Al2O3 thus realizing low density of interface traps (Dit), high conduction/valence band offset and high breakdown strength. The promising results of AlSiO as a dielectric for GaN-based devices motivated us to expand its applications to Ga2O3. In this work, the interface and bulk properties of AlSiO dielectric grown by metal-organic chemical vapor deposition (MOCVD) on (001) β-Ga2O3 were investigated systematically using a deep UV-assisted capacitance-voltage methodology. The surface preparation with a combination of UV-ozone and wet chemical treatment reduced near-interface traps resulting in a negligible hysteresis. The interface states density Dit was extracted accurately by accounting for dielectric bulk traps employing improved physical models. An average interface state density of 6.63 ×1011 cm-2 eV-1 and AlSiO bulk trap density of 4.65×1017 cm-3 eV-1 were quantified, which is half of that for Al2O3 deposited by atomic layer deposition (ALD). In addition, a high dielectric breakdown field of ~7.8 MV/cm and more effective suppression of gate leakage were achieved on these devices compared with ALD-Al2O3 on similar metal-oxide-semiconductor (MOS) structures. The negligible hysteresis, lower interfacial trap density, low leakage current and high breakdown electric field achieved on AlSiO/Ga2O3 MOSCAPs reveals MOCVD AlSiO as a promising gate dielectric for high-performance Ga2O3 devices.
06/17/2021 - Holography and Chern-Simons-matter Theories
Robert Saskowski, 3rd Year, Department of Physics
Despite the electromagnetic, weak, and strong forces having a thorough quantum description, gravity remains the missing piece of the quantum puzzle. A key tool for investigating gravity is gauge/gravity duality, which allows us to recast gravitational questions as field-theoretic ones. In this talk, I will begin with a pedagogical introduction to gauge/gravity duality, predominantly focusing on the AdS/CFT correspondence, covering the general story of where it comes from, what it means, and how to get interesting information out of it. Specifically, I will focus on the field theory side of this correspondence, which can be dealt with by turning the theory into a matrix model via supersymmetric localization. The second half of the talk will focus on a specific class of three-dimensional theories known as Chern-Simons-matter theories, as well as a discussion of their gravitational duals and the resolvent approach to investigating the free energy. In particular, I will present some recent results regarding subleading corrections in Gaiotto-Tomasiello theory that I derived with Jim Liu (arXiv: 2106.03901).
06/24/2021 - Long-range Rydberg-atom-ion molecules of Rubidium and Cesium
Alisher Duspayev, 3rd Year, Department of Physics
Rydberg atoms and molecules have become an active field of research within atomic, molecular and optical physics. In this talk, a novel type of Rydberg molecule proposed by our group is discussed. It is formed through long-range electric-multipole interaction between the Rydberg atom and the pointlike ion. After an overview on the current status of research with Rydberg atoms and molecules as well as their applications, the theoretical framework for the Rydberg-atom-ion molecule is discussed. The results are presented as the calculated potential energy curves that are asymptotically connected with Rydberg nP or nD states of rubidium or cesium. These curves exhibit deep, long-range wells that support many vibrational states of the proposed molecule. Vibrational structure and photoassociation of the molecule are further discussed. The talk concludes with the prospects for future experimental realizations and additional studies.