Session G04: New Properties in Moiré Materials I

Many-body effects on exciton dynamics in layered heterostructures

Optical excitations in semiconductors have the potential of forming long-lived excitons, coupled electron-hole pairs, serving as key ingredients in efficient light harvesting and optical information science. The stability and coherence of these excitations is strongly coupled to the underlying material structure and can thus be largely designed. Such design principles are of great interest in layered heterostructures, with multiple emerging experiments showing controllability of exciton relaxation lifetimes and mechanisms as a function of layer composition and interlayer commensuration. In this talk I will describe our theoretical research on the underlying interactions responsible for these relaxation processes from first principles. Using the example of transition metal dichalcogenide (TMD) hetero-bilayers, I will discuss structural dependencies of the exciton optical and spin selection rules from a many-body perspective and their huge implications on light absorption. I will further present the case of TMD-Graphene interfaces, where large exciton hybridization occurs due to strain-induced symmetry breaking which is tunable through interlayer twisting. Finally, I will present our new first-principles approach to exciton interaction dynamics resulting from these properties, supplying new understanding of the relation between the underlying material structure and exciton coherence.   

Direct visualization of hybridized excitons in twisted WS2/MoSe2 heterobilayers

Twisted heterobilayers of transition metal dichalcogenides (TMD) provide a unique and tunable platform to study properties of excitons. Particularly, in WS₂/MoSe₂ heterostructure systems, the conduction band minimum of two layers are nearly degenerate, leading to hybridization between intralayer excitons and interlayer excitons. Optical measurements including photoluminescence and reflectance spectra have shown emerging temperature- and twist-angle-dependence. To further understand the formation and evolution of hybridized excitons, we performed time-resolved angle-resolved photoemission spectroscopy (TR-ARPES) to directly visualize energetics and dynamics of exciton states in momentum space. We also characterized dynamics of samples with different twist angles at various temperature and fluence conditions. Our study has shown that moiré superlattice and band alignment play significant roles in the ultrafast exciton dynamics.   

Strain and Exciton Renormalized Local Density of States in a Monolayer MoS2-Au (111) Moiré Heterostructure at Sub Nanometer Scale

For the first time, the effects of strain and optical excitations are studied in tandem at the sub nanometer (nm) scale in a monolayer MoS₂-Au (111) heterostructure. The imperfect topography of the Au (111) surface at its domain edges induces non-uniform, sub nm scale strain in the MoS₂ monolayer above the Au (111) substrate. The experimental local density of states (LDOS) of the strained region is significantly enhanced between 400 meV and 1200 meV and exhibits an overall energy downshift relative to that of the unstrained region. These effects are explained by density functional theory and molecular dynamics simulations of strained MoS₂ configurations that demonstrate energy downshifts near the high-symmetry momentum K and Q points in the first Brillouin zone along with a decrease in the effective mass at each valley for tensile strain. Next, measurements under 515 nm continuous wave light are conducted to investigate the non- equilibrium behavior of the sample. While strong charge transfer effects between the Au (111) and MoS₂ quench any optical effect in unstrained areas, the LDOS in the strained regions exhibits significant light-induced modification: an effective upward energy shift. This can be explained by a local accumulation of excitons near the K and Q regions due to the strain renormalized electronic bandstructure. These results pave the path forward for fine tuning the LDOS of 2D materials by strain and optical excitations.   

Quantum interference and magnetotransport in moiré superlattices: quantum criticality and 1D localization

Moiré materials provide a highly tunable platform in which novel electronic phenomena can emerge. We study strained moiré materials in a uniform magnetic field and predict a sequence of transitions in the direction of electrical conductivity as magnetic field or strain is varied. This dramatic anisotropy reflects the emergence of nematic, one-dimensional physics at quantum Hall plateau transitions in strained moiré materials, along a direction which switches. We provide two complementary understandings of this phenomenon: in the strong field limit, the transitions map onto localization-delocalization transitions of Aubry-André-Harper chains; in weak field, the transitions are captured by Fabry-Pérot-like quantum interference in a semiclassical network model. These transitions should be observable in strained moir'e materials at realistic fields and low strain disorder, as well as unstrained systems with anisotropic Fermi surfaces.   

Moiré modification of graphitic thin films

Moiré patterns formed by stacking atomically thin van der Waals crystals can give rise to dramatic new physical properties, in select cases generating flat bands that host a variety of strongly correlated and topological states of matter. I will focus on the evolution of moiré effects in graphitic structures as their thickness varies from just a few layers into the bulk limit. In particular, I will overview the properties of a family of Bernal-stacked graphene structures that feature a single twisted interface residing within the crystal. In the atomically thin limit of less than seven total graphene layers, we observe striking commonalities in the emergent correlated and topological states arising from the moiré flat bands across many different layer-number constructions. Theoretical modeling reveals that this effect originates from the localization of low-energy states to the twisted interface within the structure. We further find that moiré band formation persists even in bulk graphite structures with a single rotational fault. When we expose twisted graphene-graphite samples to a large magnetic field, standing waves form along the c-axis of the crystal and hybridize the 2D moiré states with the bulk states of the 3D graphite thin film. Our results demonstrate that creating a small twist within graphite can modify the properties of the entire bulk crystal.   

Density Functional Theory-Derived General-Purpose Machine Learning Interatomic Potential for Monolayer and Bilayer Transition-Metal Dichalcogenides

Small-twist angle bilayer graphene and transition metal dichalcogenides (TMDs) lead to the formation of large-area moiré superlattices. The flat electronic bands and novel moiré excitons in TMD moiré superlattices are closely linked to the structural rearrangement of the constituent atoms.  However, due to the large number of atoms in small-twist angle unit cells, atomic relaxations using density functional theory (DFT) are computationally prohibitive.  Recently, Kolmogorov−Crespi (KC) interatomic potentials have been developed that can efficiently distinguish different stacking interlayer interactions. However, this approach requires refitting KC parameters for each distinct bilayer TMD. Here, we develop a robust general-purpose neural network potential for a wide range of monolayer and bilayer TMDs derived from DFT calculations. This NNP reaches the DFT accuracy for atomic relaxations of large-scale moiré superlattices, and we further use it to compute vibrational and thermal properties of a variety of twisted bilayer TMDs.

* This work is supported by DOE, and computational resources are provided by NERSC   

Atomistic calculation of structural and phononic properties of twisted two-dimensional transition-metal dichalcogenide

Two-dimensional (2D) materials show a wide range of interesting physical phenomena that can be tuned by the twist angle between layers. Here we present atomic structures and phononic properties of various twisted 2D transition-metal dichalcogenides in the range of the twist angle from θ = 0˚ (3R-stacking) to θ = 60˚ (2H-stacking). To study large moiré supercells efficiently, we use an atomistic approach based on intralayer elastic energy from first-principles density functional perturbation theory and the Kolmogorov-Crespi interlayer energy. With this approach, we investigate the effects of twisting on the atomic structure and low-frequency interlayer phonon modes, and compare the results of various stacking combinations.   

Insight into the Stacking Effect on Shifted Moiré Patterns of Bilayer Phosphorene: A comprehensive First-principles Study

A deeper understanding of the role of interlayer interaction in controlling the structural and electronic properties of twisted bilayer phosphorene is very crucial. We employed a first-principles approach, to comprehensively study bilayer phosphorene through relative translation along different directions to reveal a direct correlation between the potential energy surface and the interlayer equilibrium distance. The high symmetric moiré patterns with the most stable, metastable, and transition states were found associated with AB, Aδ, and TS stacking configurations, respectively. The transition state (TS) configuration between the AB and Adelta configurations was found with an energy barrier of ∼1.253 meV/atom. The character of the electronic bandgap with respect to shifting shows an anisotropic behavior ranging between 0.62 – 1.22 eV with transitions from indirect to direct bandgap nature under shifting, implying a tunable bandgap by stacking engineering. Orbital hybridization at the interfacial region induces a redistribution of the net charge (∼0.002 – 0.01e) leading to a strong polarization with stripe-like electron depletion near the out-of-plane lone pairs and accumulation in the middle of the interfacial region. It is expected that such interesting findings will make moiré patterns of shifted bilayer phosphorene promising as a versatile shiftronics material for nanoelectronics applications.