Physics and Astronomy

Abstract: Electrons in many solid materials, despite of their Coulomb interactions, can be qualitatively treated as non-interacting quasiparticles, a concept central to Fermi-liquid theory, established by Lev Landau in the 1950s. However, an increasing number of novel quantum phenomena, especially those related to unconventional superconductivity, fractional quantum Hall effects and Luttinger liquid physics, cannot be captured in this Fermi-liquid framework because of the strong electron correlations. Albeit with research over decades, our understanding of non-Fermi liquid physics in quantum materials remains far from complete. In this talk, I will introduce and discuss fascinating new opportunities allowed by two-dimensional (2D) materials and moiré systems for investigating non-Fermi liquid behaviors in strongly correlated electronic systems. I will talk about our recent observation of moiré Luttinger liquids in small-angle twisted tungsten ditelluride (WTe2) and novel moiré excitons in a strained moiré heterostructure. The prior result demonstrates that the Luttinger liquid physics, often expected for interacting one-dimensional systems, can also survive in an anisotropic 2D system. The latter one suggests a new formation for excitons. Our results encourages the search for novel fractionalized quasiparticles, such as spinons and holons, in the 2D anisotropic Moiré systems. The talk will also discuss exciting opportunities in 2D materials further our understanding of strongly correlated quantum phenomena in 2D materials.

Speaker bio: Pengjie Wang is an assistant professor in physics at the University of Illinois Urbana-Champaign, where he just started this January. His current research interests focused on the interplay of strong correlations and topology in 2D crystals and twisted stacks, and ultra-low temperature technique developments. Prior to his current position, he was a postdoctoral researcher working with Prof. Sanfeng Wu at Princeton University from 2019 to 2023. He got his Ph.D. under the supervision of Prof. Xi Lin in 2018, in the International Center for Quantum Materials, Peking University.

This talk examines the use of deposition processes involving molecular vapors in precision control of materials for semiconductor applications. Recent work from our group has employed (1) oxidative molecular layer deposition (oMLD) to synthesize sequence-controlled semiconducting polymers as well as (2) functional group lithography to provide patterned deposition on 2D materials. Both these areas provide tremendous opportunities for further development using different molecular vapors, but the vastness of chemical space makes these opportunities daunting. We discuss the use of machine learning tools to reduce the size of chemical space and enable more efficient navigation of chemical space. The future vision for this work includes establishing robust continuously variable parameterization of chemical space, efficient navigation of chemical space through multi-objective Bayesian optimization, and the acceleration of experimental measurements using autonomous labs enabled by robotics and agentic control. These opportunities align with the goals and vision of the MU NRT program on Accelerating Materials Frontiers through Creativity and Data Science, and offer an exciting outlook for the future of experimental materials research at the University of Missouri. 

One hundred and forty seven years after Edwin Hall's discovery of the classic effect in which an electric potential difference is generated across an electric current, different versions of the Hall effect continue to fascinate and are at the very heart of contemporary theories of electronic transport. The anomalous Hall effect, the spin Hall effect, and the orbital/valley Hall effect in their classical and quantum realizations have forced us to reconsider long-held distinctions between conductors and insulators, ushering a conceptual revolution in solid state theory. This talk tells the story of this revolution including the most recent discoveries concerning the role of quantum geometry in the nonlinear Hall effect in normal metals and superconductors.

Abstract:
The concept of strongly correlated topological insulators is extremely attractive, not only because their surface states host massless helical carriers protected from backscattering, but because in 5f-electron systems these surface states can become more correlated, more renormalized, and more exotic than the bulk states themselves. This leads to surface electronic structures with no analog in conventional topological insulators. In 5f systems, Coulomb interactions, spin–orbit coupling, and hybridization occur on similar energy scales, placing these materials in a regime where competing interactions can readily drive new quantum phases. Moreover, many 5f compounds are close to the intermediate-valence regime, where enhanced hybridization and renormalization could promote topologically nontrivial electronic structures. These systems are therefore prime candidates for realizing heavy-fermion topological states, Kondo-insulating topological phases, and intermediate-valence-driven topologies relevant for next-generation quantum applications. In this talk, we will present recent advances in correlated topological materials, with a focus on emergent 5f-electron systems displaying topological behavior.

Bio:
Krzysztof Gofryk is a condensed matter physicist in the Nuclear Fuels and Materials Division at Idaho National Laboratory, where he leads the Center for Quantum Actinide Science and Technology (C-QAST). He received his Ph.D. in 2006 from the Institute of Low Temperature and Structure Research of the Polish Academy of Sciences and the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany. Before joining INL, he held research positions at the Institute for Transuranium Elements in Karlsruhe, as well as at Los Alamos and Oak Ridge National Laboratories. Dr. Gofryk is a recipient of the DOE Early Career Award, the Presidential Early Career Award for Scientists and Engineers (PECASE), and INL’s Exceptional Achievement Award. His research focuses on emergent quantum phenomena in strongly correlated electron systems under extreme conditions of low temperature, pressure, and high magnetic fields. His work spans f-electron materials, heavy-fermion physics, quantum criticality, magnetism, and the interplay of strong electronic correlations, spin–orbit coupling, and topology in actinide compounds.

High harmonic generation (HHG) is an extreme non-linear phenomenon where strong laser-field pulses interact with a medium to produce coherent and high-frequency harmonics of the incident light.  Since its first observation in solids in 2011, pioneering theoretical studies have clarified some of the details of the microscopic mechanism behind this phenomenon, like the role of intra- and inter-band transitions, the contribution of the transition dipole moments to the even and odd harmonic peaks, effects of broken symmetry, etc. However, the role of electron correlation effects in the HHG in strongly correlated materials is much less understood. This talk will discuss the role of these effects in the high-harmonic (HH) spectra of solids, using time-dependent density-functional theory and dynamical mean-field theory, for the examples of semiconductor ZnO, perovskites BaTiO3 and BiFeO3 and transition-metal oxide VO2. It is found that correlation effects significantly modify the HH spectrum of all systems, in particular through the ultrafast modification of the electronic spectrum in ZnO. In the case of BaTiO3, correlation effects generate "super-harmonics" – periodic enhancements and suppressions of specific harmonic orders that depend on the correlation strength. Memory effects in HHG were found to lead to a further extension of the harmonic cutoff. For the HH spectrum of VO2,  we find correlation-induced higher harmonics, in good agreement with experimental data. The obtained results shed light on the role of electron correlations in the HH spectrum in complex materials and may help pave the way for future advancements in the field of ultrafast science and attosecond physics.

Quantum materials such as topological, 2D and nanostructured materials have attracted tremendous attention due to their exotic quantum properties. In this presentation, we will talk about our findings of chemical doping effect in inducing magnetism and superconductivity in chalcogenide compounds. Several interesting physical properties have been observed such as the anomalous Hall effect in Cr-doped Sb2Te3, metamagnetic behavior in Fe-doped Bi2Se3, and the coexistence of ferromagnetism and superconductivity in Nb-doped Bi2Se3 single crystals. We will also present results of the synthesis and characterization of superconducting TaS2 nanowires. Our approach includes the synthesis of 1D charge-density-wave (CDW) TaS3 nanostructure precursors followed by the nondestructive and controlled adjustment of the S composition. TaS3 nanowires show the canonical CDW behavior, but the converted TaS2 nanowires show superconductivity and vortex avalanche behaviors at temperatures below its Tc ~ 3.8 K, which is about three times higher than that of 2D bulk TaS2 crystals. Physical properties of polycrystalline Zn1-xCrxTe samples will also be presented. The samples show metallicity and ferromagnetic behavior for higher Cr doping concentration. Furthermore, optical transparency in the visible light range of these polycrystalline ferromagnetic Zn1-xCrxTe was found to be 40% - 85% for Cr doping concentration up to x = 0.18.