Colloquium Calendar

First-principles modeling of the properties of Ga2O3

Abstract: β-Ga2O3 is a wide-band-gap semiconductor with promising applications in high-power electronics and photodetectors that are transparent to visible light. In this talk, I will show how first-principles calculations, based on density functional theory with hybrid functionals, can be used to predict and explain the properties of Ga2O3.

We first focus on modifying Ga2O3’s properties for electronic applications through doping. While n-type doping is straightforward, p-type doping is elusive, with only deep acceptors available. We explore the properties of possible acceptors, and discuss the viability of obtaining semi-insulating material [1]. All dopants we considered lead to deep acceptor levels that are more than 1.3 eV above the valence-band maximum. N and Mg were identified as the most promising deep acceptors. We evaluated incorporation in different configurations, and considered the effect of native defects as well as complexes. We also predict diffusion activation energies, finding that Mg is significantly more mobile, explaining experimental observations.

Alloying allows to modify the lattice constants, band gaps, and conduction-band offsets. We will provide quantitative results for alloys with In2O3 and Al2O3 for the ground state [2,3] and for the orthorhombic kappa structure [4,5], which has attracted significant attention due to its large predicted spontaneous polarization.

When Ga2O3 is used as a transparent conducting oxides (TCO), two conflicting properties have to be balanced: transparency and conductivity. The requirement of transparency is typically tied to the band gap of the material being sufficiently large to prevent absorption of visible photons. This is a necessary but not sufficient condition: indeed, the high concentration of free carriers, required for conductivity, can also lead to optical absorption. This absorption can occur through direct absorption to higher-lying conduction band states, or by an indirect process, for example mediated by phonons or charged impurities. We will elucidate the fundamental limitations of optical absorption in Ga2O3 and shed light on experimental observations [6-9].

[1] H. Peelaers, J. L. Lyons, J. B. Varley, and C. G. Van de Walle, APL Mater. 7, 022519 (2019).

[2] H. Peelaers, D. Steiauf, J. B. Varley, A. Janotti, and C. G. Van de Walle, Phys. Rev. B 92, 085206 (2015).

[3] H. Peelaers, J. B. Varley, J. S. Speck, and C. G. Van de Walle, Appl. Phys. Lett. 112, 242101 (2018).

[4] S. Seacat, J. L. Lyons, and H. Peelaers, Appl. Phys. Lett. 116, 232102 (2020).

[5] S. Seacat, J. L. Lyons, and H. Peelaers, Appl. Phys. Lett. 119, 042104 (2021).

[6] H. Peelaers and C.G. Van de Walle, Appl. Phys. Lett. 111, 182104 (2017).

[7] H. Peelaers and C.G. Van de Walle, Phys. Rev. B 100, 081202(R) (2019).

[8] A. Singh, O. Koksal, N. Tanen, J. McCandless, D. Jena, H. Xing, H. Peelaers, and F. Rana, Appl. Phys. Lett. 117, 072103 (2020).

[9] O. Koksal, N. Tanen, J. McCandless, D. Jena, H. Xing, H. Peelaers, F. Rana, and A. Singh., Phys. Rev. Research 3, 023154 (2021).

Particle Physics Monte Carlo Event Generators for Present and Future Colliders

Abstract: Monte Carlo event generators are the mighty workhorses of collider-based particle physics, for signal simulation and event reconstruction. The main goal of the talk is to elucidate  the inner structures of these versatile "black boxes" of particle physics. After a short motivation of collider physics, the different components of MC event generators will be discussed: phase space and event sampling, perturbative evaluation of matrix elements at fixed order, parton showers and hadronization and new physics simulations. A special focus will be given to future lepton colliders (e+e- and muon colliders) regarding beam simulation, lepton parton distribution functions and photon radiation.

Probing Black Holes from the Boring to the Breathtaking

Abstract: All galaxies host a supermassive black hole at their centers, at least a million times the mass of the Sun. Material falling onto these monsters can be as bright as the galaxy itself, or it may be lurking unseen behind thick blankets of dust. Some of these monsters go through growth spurts and feeding frenzies that can greatly impact their host galaxies, possibly even terminating all nearby star formation. Other supermassive black holes seem to have no impact on their hosts, passively growing and evolving while galaxies take no notice. In this talk, I will explore the breathtaking Cold Quasars, which are some of the most luminous accreting black holes in the universe, and yet, surprisingly, their host galaxies have star formation rates of 1000 Msun/yr, casting doubt on whether black hole feedback impacts star formation at all. I will discuss how Cold Quasars are an anomaly in the current understanding of quasar formation. On the other hand, I will examine the boring black holes, what kinds of galaxies they live in, and what JWST is revealing about their life cycle.

Nature and fate of photoexcitations in energy materials

Abstract: The ability to synthesize and probe new classes of photoactive materials with tunable structure and composition – such as halide perovskites, transition metal oxides and nitrides, van der Waals heterostructures of 2d materials, organic crystals, and more – has driven the development of new theory, computational methods, and intuition for predicting their photophysics. In these novel semiconductors, photoexcited correlated electron-hole pairs, or excitons, can be strongly bound and do not conform to simple models, and new understanding is needed to interpret and predict their behavior. Here, I will discuss recent advances of ab initio calculations – based on density functional theory and field-theoretic many-electron Green's function formalisms – of excitons in these complex crystals, focusing on how the properties of these quasiparticles are influenced by lattice structure and dynamics, temperature, dielectric screening, and carrier concentration. I will compare with experiments and discuss how new theoretical methods and intuition can lead to the discovery of new physical behavior and the development of optoelectronic devices for energy applications.

Crafting Generative Models & Unraveling High Energy Physics with Parameterized Quantum Circuits

Abstract: The advent of Noisy Intermediate-Scale Quantum (NISQ) computing has spotlighted the significance of parameterized quantum circuits (PQCs) as essential tools in quantum technology. Particularly in the realm of quantum generative models, PQCs have shown promise in encapsulating complex data distributions, reproducing the statistics of the training data, and detecting anomalous instances.

This seminar will weave a narrative around the confluence of PQCs in both quantum machine learning and the intricate world of high energy physics. Through a fusion of case studies and in-depth discussions, I will highlight the promise and potential of PQCs, standing at the crossroads of data-driven innovation and fundamental scientific discovery.

Abstract: The Office of Civil Rights and Title IX will present on KU’s civil rights grievance procedures, mandatory reporting requirements, and the parameters of KU civil rights policies. We will cover addressing microaggressions, University-wide and local resources, reporting trends, and options for reporting incidents to OCRTIX. We will also provide time for questions and answers from the group!