Long-range charge transfer processes - transmission of electrons and protons over long distances - play a fundamental role in cellular metabolism. In my research, I employ multi-scale computational methods to track charge transfer processes in complex molecular machines, such as respiratory complexes and bacterial hydrogenases. Using the combination of classical molecular dynamics, quantum chemistry and hybrid quantum mechanics/molecular mechanics (QM/MM) simulations, I am able to identify specific protein residues that are key to function. These sites can then be targeted with mutations to inhibit enzyme activity or enhance their efficiency.  Overall, this research is essential for advancing medical and industrial applications, including disease treatment and biocatalysis.

At relatively low polymer concentrations, granular hydrogel particles, commonly called microgels, undergo a jamming transition and behave like a soft elastic solid with reversible yielding. Over the past decade, these microgel packings have been utilized as sacrificial scaffolds to enable studies of cell behavior in a controlled 3D-environment. Here, cells can either be randomly dispersed within the microgel packings or precisely structured into well-controlled geometric shapes using 3D-printing to systematically study cell migration and invasion, differentiation pathways, collective cell behavior, and tissue/organoid development in 3D. Similar to 2D cell assays, the behavior of cells in 3D depends on the material properties of their microenvironment. Thus, controlling the macroscopic elastic behavior and the interstitial pore structures of microgel packings is critical for their application as a 3D scaffolds. Here, we present our recent progress in understanding the origins of elasticity in granular microgel packings. We prepare charged polyelectrolyte microgels with varying charge density to investigate the effects salts have on elasticity of microgel packings and relate their behavior to classic polymer physics scaling laws. In addition, we investigate the effects crosslinking concentrations have on the bulk rheological properties and the interstitial pore structure of concentrated packings of uncharged polyacrylamide microgel particles at the onset of jamming. Finally, we share our recent efforts in designing microgel particles as a transparent soil medium to enable spatial temporal studies of plant roots and plant-microbe interactions within rhizosphere. 

Quantum sensing enables precision measurements beyond classical limits by exploiting quantum properties such as entanglement and squeezing. These resources allow us to detect extremely weak signals—such as minute magnetic fields, subtle forces, or small phase shifts—that are inaccessible with conventional sensors. In this presentation, I will introduce quantum sensing techniques based on two types of squeezed light: two-mode squeezed light and single-mode squeezed light. I will discuss the quantum advantages they offer in applications such as absorption measurements and quantum-enhanced stimulated Brillouin scattering (SBS) spectroscopy and imaging. In addition to quantum light sources, I will highlight the role of Whispering-Gallery Mode (WGM) optical sensors, which act as amplifiers to enhance light–matter interactions and further boost sensitivity. A proof-of-concept experiment will be presented to demonstrate the effectiveness of this approach in detecting nanoparticles under varying temperature conditions. This method underscores the versatility of quantum sensing and its integration potential with various optical sensor platforms, paving the way for high-sensitivity, scalable solutions based on integrated photonic technologies.

Ga2O3, a wide-band gap semiconductor, is of interest for high-power devices and deep-UV photodetectors. Many of these applications require the formation of heterostructures to create a conduction-band offset to confine charge carriers. This is commonly achieved through alloying with Al2O3. However, Al2O3 has a significantly smaller lattice constant than Ga2O3, which can introduce strain on the heterostructure. Experimentally, it has been difficult to grow Al2O3-Ga2O3 alloys with high Al content, so that the maximum achieved conduction-band offset is only around 0.33eV. High Al containing alloys are also difficult to n-type dope.
 

We use hybrid density functional theory simulations to design a heterostructure which closely matches the lattice constant of Ga2O3, while maintaining a conduction-band offset. We found that alloys of In2O3 and Al2O3 form a lattice-matched monoclinic structure with a 1 eV conduction-band offset [1]. Moreover, we show that this alloy can readily be n-type doped using Si [2]. We will also discuss the role of charge-carrier compensation by cation vacancies in Al2O3-Ga2O3 and In2O3-Al2O3 alloys.

[1] S. Seacat, J.L. Lyons, and H. Peelaers, Phys. Rev. Materials 8, 014601 (2024).
[2] S. Seacat and H. Peelaers, J. Appl. Phys. 135, 235705 (2024).

Abstract: The calculation of the ground-state electronic energy of a molecule is one of the primary tasks of quantum chemistry. In this talk, we survey recent progress for this task using benzene, the chromium dimer, and the FeMo cofactor as representative systems. We focus on computing the full configuration interaction (CI) energy, which for a given one-particle basis set is the exact non-relativistic electronic energy. Unfortunately, the computational expense of a full CI calculation scales exponentially with respect to system size, which prevents its use for almost all chemically relevant systems.

As an alternative, the last decade has seen the introduction of selected full CI methods that mitigate the exponential scaling of full CI by only including a subset of terms in the full CI wave function based on a selection criterion or rule. In this talk, we introduce selected CI and demonstrate how it has enabled computations of nearly exact electronic energies for larger chemical systems. We also compare it to other methods for approximating the full CI solution such as the density matrix renormalization group (DMRG).

Finally, we briefly discuss the application of quantum computing in quantum chemistry. Quantum chemistry is often referred to as a “killer app” for quantum computing in the mainstream media. We explore what the selected CI and DMRG methods reveal about the prospects of quantum computing in the field of quantum chemistry.

Bio: Kurt R. Brorsen has been an Assistant Professor in the Department of Chemistry at the University of Missouri since 2018. He was a postdoctoral researcher at the University of Illinois at Urbana-Champaign from 2014-2018 and received his Ph.D. in Physical Chemistry from Iowa State University in 2014. His research focuses on the development of new ab initio quantum chemistry methods to include nuclear quantum effects in computational chemistry calculations.

Accelerating the development and deployment of advanced energy systems is crucial in an increasingly “energy-hunger” world, especially with the advent of AI era. In today’s U.S. energy market, nuclear power accounts for 20% of the total electricity generation capacity and half of its clean energy production. Therefore, further enhancing the safety and economic efficiency of both existing and future nuclear systems is pivotal for achieving our national energy demand. The swift advancement of advanced nuclear technologies has heightened the need for precise, spatiotemporally resolved, multi-scale multi-physics coupled fundamental modeling of fluid flow and heat transfer, along with numerically robust, reliable, and efficient analysis to support the lifecycle of nuclear power plants – both for the legacy and new constructions. The more in-depth understanding of the phenomenological behavior of fluid flow and heat transfer allows for a more cost-effective implementation of new innovations. 

In the present work, leveraging the high-resolution two-phase flow experimental data uniquely obtained from the RBHT reflood test facility in collaboration with U.S. Nuclear Regulatory Commission (NRC), detailed physics-based theoretical modeling, comprehensive numerical analyses and code validations are performed to understand the fuel-to-coolant thermal-hydraulic responses during quenching transients. Valuable insights obtained from these analyses are further enhanced through high-resolution visualization study and physics-based modeling of two-phase flow interface behavior under natural-circulation film boiling conditions, utilizing a small-scale quench test facility as well as extensive system-scale numerical simulations. The use of advanced image processing techniques allows an in-depth investigation to probe the boundary-layer scale as well as to facilitate the development of sophisticated theoretical flow/heat transfer models for code applications. The present work presents a thorough investigation of the transient fluid flow and heat transfer behaviors, as well as reactor thermal-hydraulic responses, thus extending the boundaries of our current knowledge. The experimental and modeling techniques that we are developing and demonstrating provide an exciting avenue for high-spaciotemporal resolution flow and heat transfer characterization. The results obtained are particularly valuable for the future development of thermal-fluid models and for numerical code validation, ultimately supporting the sustainability of nuclear energy. 

In recent years Choudhury lab has designed a series of 2D quaternary chalcogenides involving a variety of building units, including simple tetrahedral, supertetrahedral, and ethane-like moieties. These 2D materials are synthesized by a building-block approach, in which preformed building blocks containing alkali ions react with metal (transition, rare-earth, or main-group) chlorides in so-called solid-state metathetic reactions. In these reactions, alkali ions are partially or fully replaced by the metal from the metal chloride, generating alkali halides by-products that drives the reaction forward. In all these compounds, 2D layers are formed through various connectivities of metal polyhedra and main-group building units, with alkali (Li or Na) ions occupying the interlayer spaces. However, crystal structure determination of these materials is often challenging due to stacking faults within the layers.

In this presentation, we will discuss a range of properties exhibited by these 2D materials, including spontaneous water and ammonia adsorption, ion conduction, and their potential use as cathode materials in lithium-ion batteries.

Finally, we will present our ongoing efforts to develop new 2D materials in a more rational manner, including our work to understand their formation through in situ X-ray diffraction techniques using synchrotron radiation.

Abstract:

The surface state of a topological material is widely regarded as a promising platform for next-generation information technology, such as a topological quantum computer utilizing the Majorana zero mode. Materials exhibiting such topological surface states are rare. Moreover, there has not been a known compound with the topological normal state that transitions into a topological superconductor. Such compounds offer a unique opportunity to study the interplay between topological normal and superconducting states. In this talk, I present the angle-dependent magnetic field response of electrical transport properties of half-Heusler YPtBi in both normal and superconducting states. The angle dependence of both magnetoresistance and the superconducting upper critical field breaks the rotational symmetry of the cubic crystal structure, and the angle between the applied magnetic field and the current-carrying plane of a plate-like sample prevails. Furthermore, the measured upper critical field is notably higher than the bulk response for an in-plane magnetic field configuration, suggesting the presence of quasi-2D superconductivity. These results imply the transport properties cannot be explained solely by the bulk carrier response, requiring robust normal and superconducting surface states to flourish in YPtBi. Therefore, half-Heusler YPtBi stands out as a unique system with both topological normal and superconducting surface states.

Abstract: Moiré superlattices in two-dimensional (2D) materials represent a highly promising platform for uncovering and controlling novel quantum phenomena. The robust excitonic responses observed in transition metal dichalcogenides (TMDs) provide a powerful means to optically probe and manipulate these interactions. In this presentation, I will demonstrate the intricate interplay between excitons and charge carriers confined within moiré potentials, showcasing how the precise engineering of 2D materials into supermoiré lattices can systematically control these interactions. This control leads to the emergence of exotic quasiparticles. Specifically, we reveal the behavior of interlayer valley excitons in bichromatic TMD moiré trilayers, where periodic superlattices create tunable multiple orbital configurations. The synergy between the tunable potential landscape and the layer degree of freedom enables the formation of interlayer quadrupolar moiré trions. These findings establish a foundation for the development of electrically tunable, multi-orbital moiré potentials, unlocking new avenues for the discovery and exploration of new quantum phases in 2D materials.

Bio: Xi Wang earned her BS in Physics from the University of Science and Technology of China (USTC). She completed her Ph.D. at Florida State University. As an EFRC Postdoctoral Fellow in the Physics Department at the University of Washington, Seattle, she worked with Prof. Xiaodong Xu and Prof. Daniel Gamelin. Xi joined the Department of Physics at Washington University in St. Louis in January 2024. Her research focused on designing, fabricating, and characterizing high-quality heterostructures made from two-dimensional materials, with a particular emphasis on excitonic-related quantum many-body interactions in moiré superlattices. She is a recipient of the 2024 Ralph E. Powe Junior Faculty Enhancement Award.

Abstract: Advances in supercomputing capabilities and electronic structure calculations based on density-functional theory (DFT) now make it possible to design materials with new properties starting from the atomic scale and guide their experimental synthesis and characterization. Concurrent advances in scanning transmission electron microscopy (STEM) enable imaging and spectroscopy of materials with unprecedent spatial and energy resolution. Naturally then, the combination of theory and microscopy provides an unparalleled probe to unravel the atomic-scale structure-property correlations in complex materials with defects and disorder. In this presentation, I will discuss my group’s efforts to develop new ferroic materials with defects and disorder for energy and optical applications. Examples will include the discovery of chalcogenide perovskites with colossal optical anisotropy [1,2], unraveling the origin of ferroelectricity in nanoscale hafnia [3]; and the realization of a relatively understudied class of multiferroic that combines ferroelectricity and chirality [4]. 

Bio: Rohan Mishra is an Associate Professor of Mechanical Engineering & Materials Science, and Physics (by courtesy) at Washington University in St. Louis. He is also an affiliate faculty at the Institute of Materials Science & Engineering at Washington University, where he serves as the Director of Graduate Studies. From 2012-2015, he was a postdoctoral researcher in the Scanning Transmission Electron Microscopy group at Oak Ridge National Laboratory with a joint-affiliation from the Department of Physics at Vanderbilt University. He has a Bachelor in Technology in Metallurgical and Materials Engineering from National Institute of Technology Karnataka in India (2008) and a PhD in Materials Science and Engineering from The Ohio State University (2012). He leads the Materials Modeling and Microscopy group (mcube.wustl.edu) that works on establishing quantitative structure-property correlations in materials using a synergistic combination of electronic structure theory and electron microscopy. Their end goal is the rational design of materials with properties tailored for various energy applications. Mishra has coauthored over 100 journal articles. He received the NSF CAREER award in 2022.

References:

[1]          B. Zhao et al., Advanced Materials 36, e2311559 (2024).

[2]          H. Mei et al., Advanced Materials 35, e2303588 (2023).

[3]          X. Li et al., 2024), p. arXiv:2408.01830.

[4]          G. Ren et al., Advanced Functional Materials  (2024).

Abstract:

"Mixed-dimensional heterostructures are a unique class of material combining components of distinct dimensionalities through van der Waals interactions, e.g., zero-dimensional organic molecules self-assembled on two-dimensional (2D) materials. In addition to the versatile tunability of organic molecules and advantageous properties of 2D materials, the interface coupling has demonstrated exotic electronic and excitonic properties beyond those of the individual component, promising for next-generation electronic and optoelectronic applications. Here I will introduce some intriguing interfacial phenomena in mixed-dimensional heterostructures, our understanding on the physics and their potential applications. I will show that organic molecules and interlayer coupling provides an incredible avenue to engineer the band structure and interlayer exciton dynamics in mixed-dimensional heterostructures, such as that consisting of titanyl phthalocyanine molecules and monolayer MoS2. Finally, I will introduce a computational approach for the heterogeneous and incomplete screening at the interface for mixed-dimensional heterostructures, advancing our understanding and capabilities in this emerging field."

TBA

Somewhat to my surprise, two independent research projects we are pursuing take place in or with suspended atomically thin membranes. In the first case, we will review thermal transport measurements inspired by a search for physics of the Kitaev quantum spin liquid; and for the second, we'll show how suspended membranes can lead to quantum sensing of rotations with remarkable precision, sufficient for lab-scale sensing of gravitation and variations in the length of a day. 

In contemporary condensed matter physics and photonics, four length scales are fundamentally interesting and intertwined: 1) Polaritonic wavelength λ in infrared (IR) and terahertz (THz) frequencies ω (e.g. plasmon, phonon, exciton, or magnon polaritons), which defines the scale of the light confinement and light-matter interaction; 2) Magnetic lengths  l_B =√(ℏ/eB)=257Å/√(B[T]), (with B the magnetic field), which defines the restricted electron motion in a B field; 3) Diffusion length D of the hot carriers at interfaces and the edges, which defines the scale of energy relaxation, and 4) Periodicities of superlattices induced by moiré engineering, which defines the energy scale of emerging quantum phases. In this talk, I will report 1) A new type of optical near-field nanoscopy technique (Landau level nanoscopy) to tackle all four above-mentioned ‘lengths’ simultaneously; 2) A new type of infrared polaritons that can be tuned via magnetic field; 3) A nanoscale probe of the many-body physics through the excitations of magnetoexcitons in graphene across the allowed and forbidden optical transitions. Our approach establishes the Landau-level nanoscopy as a versatile platform for exploring magneto-optical effects at the nanoscale. Our preliminary research also sets the stage for future spectroscopic investigations of the topological and chiral photonic phenomena in complex quantum materials using low-energy photons.