Condensed Matter Seminar

Abstract: In many problems spanning transition-metal catalysis and quantum materials, quantitative prediction hinges on treating electron correlation and electron–phonon coupling beyond standard mean-field and perturbative approaches. In this talk, I will describe our efforts to advance the auxiliary field quantum Monte Carlo (AFQMC) method, enabling first-principles electronic structure calculations in challenging correlated systems with accuracy beyond state of the art coupled cluster methods and more favorable cost scaling.

Abstract: Strong electron correlations can transform simple chemical building blocks into quantum materials with striking macroscopic signatures, such as high-Tc superconductivity and charge density waves. Yet a central challenge remains: how do we quantitatively connect atomic-scale chemical composition and crystal structure to emergent quantum behavior, without relying on empirical parameters?  In this seminar, I will describe ab initio many-body theories to uncover microscopic “design rules” for correlated quantum materials.

Abstract: Recent experiments on rhombohedral multilayer graphene (RMG) with a substrate-induced moire potential have identified both Chern insulators and fractional Quantum Hall states at zero magnetic field, whose origin is presently mysterious. The operative degrees of freedom are in the valence band minima that feature strong correlations and non-trivial quantum geometry. The first part of this talk will study a microscopic model of RMG.

Abstract: Controlling phases of matter with light is a central goal in condensed matter physics. I will present new theories of light–matter interaction that show how light shapes and drives instabilities in quantum materials across electronic, structural, and collective-mode sectors. First, I show that resonant optical absorption can drive layer sliding in van der Waals flakes (e.g. in CrI3, MoTe2, MoSe2) via nonlinear light–matter coupling.

Abstract: Semiconductor moiré lattices provide a flexible platform to study flat, topological bands that host a variety of closely competing many-body ground states. In this talk, I will present single-electron transistor microscopy of WSe2 bilayers at low twist angles, which reveals rich interplay between magnetism, correlations, and topology. At zero magnetic field, we observe a series of quantum anomalous Hall states and demonstrate topological phase transitions as a function of twist angle and electric field.

Abstract: Crystals with layered structures have recently attracted tremendous research interest. In these materials, individual layers are held together by relatively weak van der Waals interactions and can be exfoliated into atomically thin sheets. A variety of metastable stacking configurations can emerge in such systems, which can be further exploited to tune their physical properties through twistronics and slidetronics.

(Pizza will be served.)

Abstract: Nontrivial topological defects such as knotted solitons called hopfions have been observed in a variety of materials including chiral magnets, nematic liquid crystals and even in ferroelectrics as well as studied in other physical contexts such as Bose-Einstein condensates.  These topological entities can be modeled using the relevant physical variable, e.g., magnetization, polarization or the director field.  Specifically, we find exact static soliton solutions for the unit spin vector field of an inhomogeneous, anisotropic three-dimensional (3D) Heisenberg ferro

Abstract: Photocathodes—materials that convert photons into electrons through the photoelectric effect (explained by Einstein)—are important for many modern technologies that rely on light detection or electron-beam generation. However, existing photocathode materials have become increasingly difficult to meet the performance requirements of related cutting-edge technology upgrades. Most of these materials, and the theory (Spicer’s three-step model) to understand their photoemission properties, were discovered and established more than 60 years ago.

Abstract: Time-reversed pairs of topological bands with opposite Chern numbers are commonly found in moiré superlattices based on transition metal dichalcogenides. Recent experimental breakthroughs have demonstrated that these topological bands provide a rich platform for realizing fractional quantum matter at zero external magnetic fields.