Physics and Astronomy

In this talk, I will present our two recent studies on realizing topological properties via Rashba and Dresselhaus spin-orbit coupling (SOC) in exotic quantum materials. First, I will introduce twisted type-II Rashba homobilayers as a new platform for achieving topological moiré flat electronic bands. Using bismuth telluride iodide (BiTeI) as an example, we demonstrate the formation of narrow flat bands and transitions from valley Hall to quantum spin Hall insulators by varying twist angles. The tunability in SOC, interlayer interactions, and twist angles enhances the potential of Rashba materials for observing correlated topological phenomena. In the second study, we extend SOC to emerging Lieb-lattice altermagnetic materials, predicting the axial Hall effect, a Berry-curvature-driven anomalous Hall response. We identify the axial direction as a hidden topological degree of freedom, and first-principles calculations confirm the emergence of this effect in strained altermagnets, particularly in ternary transition-metal dichalcogenides. The axial Hall effect originates from the interplay between Dresselhaus SOC and the intrinsic piezomagnetic response of Lieb-lattice altermagnets. These findings emphasize the critical role of SOC and noncollinear spin textures in altermagnets and open new pathways for exploring intrinsic Hall phenomena in topological magnetic systems.

Concept of diffusion is widely used to describe propagation of light through multiple scattering media such as clouds, interstellar gas, colloids, paints, biological tissue, etc. Such media are often called random. This terminology is, however, misleading. Notwithstanding its complexity, the process of wave propagation is entirely deterministic – uniquely defined by the exact positions of scattering centers and the shape of the incident wavefront – making it possible to deduce the precise pattern of wave field throughout the system. Technological advances over the last decades enabled one to synthesize arbitrary wavefields opening new frontier in light control inside strongly scattering media.

Coherent excitation of the multiple-scattering medium enabled by wavefront shaping (WFS) requires altogether new approach to predicting and understanding the ultimate limit for a targeted energy delivery into and through a diffusive system, as well as sensitivity of the remitted field to localized perturbations. Indeed, while diffusion-based descriptions have long served as the gold-standard for interpreting e.g. diffuse optical tomography (DOT) and functional near-infrared spectroscopy (fNIRS) measurements, they break down for sample-specific illuminations produced via WFS. An integration of WFS into DOT promises significant improvements in signal strength and, consequently, penetration depth, yet it has lacked a rigorous theoretical foundation. By combining theoretical modeling and physical insight, I will present the groundwork for integrating wavefront shaping into the next generation of optical imaging and sensing technologies – extending their reach and precision in ways not possible within the traditional diffusion-based approaches.

Speaker bio: Alexey Yamilov is a professor of Physics at Missouri S&T. His research activities are in the areas of theoretical and computational condensed matter physics and optics; he employs a variety of analytical and numerical techniques to study transport of the electromagnetic, electronic and other types of waves in the inhomogeneous media, where a line-of-sight propagation is hindered by scattering. The purpose of the research is to uncover and exploit physical phenomena caused by wave interference to: (i) understand behaviors originating not only from the fundamental laws of physics but also from complexity of the system itself; (ii) develop optimal techniques for coherent control of wave propagation; (iii) design structures/systems with a set of desired properties

Abstract: One of the enduring questions in the study of strongly correlated electrons is how to read the universal, long-distance physics directly from a microscopic Hamiltonian. In fractional quantum Hall systems, certain solvable models achieve this through generalized Pauli principles—local rules that organize their zero-energy excitations and encode the essential topological information of the phase, much like a strand of DNA encodes the blueprint of an organism.

I will discuss how this organizing principle can be extended beyond the familiar holomorphic setting to more complex composite-fermion and parton states, where entangled Pauli principles emerge. These developments point toward a broader and more constructive framework for understanding and building solvable models of topological matter.

Magnetic skyrmions are topologically protected spin configurations that have gained enormous interest as building blocks for next-generation spintronic devices. A distinctive feature of skyrmion systems is the topological Hall effect (THE): when electrons traverse a skyrmion, their trajectories bend in a spin-dependent fashion, generating a transverse Hall current. In a skyrmion crystal, this leads to electronic bands with non-trivial Chern numbers. In this talk, I will show how an externally applied electric field—via the Rashba spin–orbit interaction—provides a powerful tuning knob for the topological band properties. Specifically, we find that the Chern numbers of individual subbands can be modified by the field, driving a transition between an ordinary insulator and a Chern insulator. The Chern insulating state leads to an edge current in finite samples that can be manipulated by changing the topology of the bands.  Remarkably, for partially filled subbands, the Hall current can be reversed in sign at a critical electric-field strength, so that the current direction is flipped. These results suggest new strategies for manipulating charge transport in skyrmionic systems.

Multiferroics refers to compounds where more than one ferroic order such as (anti)ferromagnetism, ferroelectricity, ferroelasticity etc. coexist. The coupling between the various ferroic orders allows for cross-control of the related order parameters, the most important of which is the manipulation of the magnetization or electric polarization by electric or magnetic fields which are relevant in technological applications such as MERAMs or MESO logic. This colloquially makes the term multiferroics synonymous to the coexistence of these ferroic orders. However, the mechanisms leading to the coupling are still debated. One way to describe this coupling is to classify them according to their coupling strength of the orders from independent origins (type I, eg. BiFeO3) to the strong coupling case (type II, eg. TbMnO3) wherein the electric polarization emerges via symmetry-breaking by the magnetic ordering. Recently a new class of multiferroics, the polar magnets (eg. Fe2Mo3O8), is described to have a polar structure in the paramagnetic phase like type I, but the electric polarization emerges only below magnetic ordering like type II, therefore not requiring a particular magnetic structure for the symmetry-breaking and confining both the ferroic order parameters to the same ion. In this presentation, I combined neutron scattering, bulk property measurements and theoretical methods to determine the crystal/magnetic structures, dynamics and electronic structure of several in-house synthesized polar magnets, which are essential for the elucidation of the emergent multiferroicity in these compounds utilizing symmetry concepts. These results will also be compared to other multiferroic “non-polar magnets”.

This work was supported by the University of Missouri Research Council award URC-23-037, "Emergent multiferrocity in novel polar magnets".

Speaker bio: Naren Narayanan studied Materials Science at TU Darmstadt and received his Ph.D. (Dr. rer. nat.) in Condensed Matter Physics and Materials Science at TU Darmstadt and IFW Dresden, Germany in 2010. He was a joint postdoctoral research fellow / postdoctoral researcher at ANSTO, UNSW and ANU, Canberra, Australia before joining MURR as a staff scientist in Aug. 2022. His research interests are broadly in the area of oxides, elucidating the composition-property relationships by investigating their structure and dynamics (excitations). They can be categorized into three parts: Preparation of materials: Synthesis of highly correlated compounds (both single crystals and polycrystalline oxides, especially multiferroics, frustrated spin systems), experimental characterization of them (Neutron and Synchrotron X-ray scattering, bulk properties measurements) and the theoretical modeling and interpretation of the obtained experimental results. 

Moiré patterns are well known in the visual arts and textile industries—the term comes from the textured patterns seen in mohair silk fabrics. It arises whenever two periodic structures are superimposed, giving rise to new periodicities. Remarkably, applying these ideas to atomically thin materials, the coupling between the layers enables designer materials where properties like bandwidth, electron velocity, and band topology can be controllably altered. With more than 1,000 possible “easily exfoliatable” materials to play with, this opens the possibility of billions of designer band structures with applications to both fundamental science and technology.

In this talk, I will focus on two recent developments. First, I will show that when the relative rotation between two sheets becomes small, lattice relaxation and electrostatic interactions are not small corrections but decisive ingredients that reshape the electronic structure of moiré materials [1,2].  For twisted bilayer graphene, these effects create a “magic window” of twist angles which stabilize a Lifshitz transition to an ultraflat, heavy-fermion–like Fermi surface pinned to the Fermi energy. Second, I will discuss superconductivity and electron hydrodynamics—two directions that push beyond conventional paradigms. Superconductivity in twisted bilayer graphene may arise from purely electronic collective modes, while new evidence shows its extreme sensitivity to Coulomb screening, consistent with unconventional pairing [3].  I will also discuss the hydrodynamic regime in which electrons flow like a viscous liquid. Our combined theory–experiment program shows that in this regime the conductivity is given by the sum of universal and dissipative Drude-like terms, validated by measurements of ambipolar hydrodynamic transport [4].  Both these directions emphasize that moiré materials are not only playgrounds for strong correlations, but also testing grounds for broader concepts in quantum matter.

References
[1] M.M. Al Ezzi, G.N. Pallewela, C. De Beule, E.J. Mele, and S. Adam, Analytical Model for Atomic Relaxation in Twisted Moiré Materials, Phys. Rev. Lett. 133, 266201 (2024).

[2] D. R. Klein, U. Zondiner, A. Keren, J. Birkbeck, A. Inbar, J. Xiao, M. Sidorova, M. M. Al Ezzi, L. Peng, K. Watanabe, T. Taniguchi, S. Adam, and S. Ilani,  “Imaging the Sub-Moiré Potential Landscape using an Atomic Single Electron Transistor”, arXiv:2410.22277 (2024).

[3] J. Barrier, L. Peng, S. Xu, V.I. Fal’ko, K. Watanabe, T. Taniguchi, A.K. Geim, S. Adam, and A.I. Berdyugin, Coulomb screening of superconductivity in magic-angle twisted bilayer graphene, arXiv:2412.01577 (2024).

[4] C. Tan, D.Y.H. Ho, L. Wang, J.I.A. Li, I. Yudhistira, D.A. Rhodes, T. Taniguchi, K. Watanabe, K. Shepard, P.L. McEuen, C.R. Dean, S. Adam, and J. Hone, Dissipation-enabled hydrodynamic conductivity in a tunable bandgap semiconductor, Sci. Adv. 8, eabi8481 (2022).

Transient absorption microscopy (TAM), based on femtosecond lasers, has emerged as a powerful tool for investigating the ultrafast excitation dynamics of photoexcited carriers, particularly their real-space transport properties, in solid-state materials. In this seminar, I will introduce the fundamental principles, experimental implementation, and applications of TAM in studying various transport regimes, including classical diffusion, super-diffusion, and ballistic transport, in a range of systems such as 3D and 2D semiconductors, van der Waals heterostructures, and Mott insulators.

 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.