Phonon Renormalization in Reconstructed MoS2 Moire Superlattices

In moiré crystals formed by stacking van der Waals (vdW) materials, surprisingly diverse correlated electronic phases and optical properties can be realized by a subtle change in the twist angle. Here, we discover that phonon spectra are also renormalized in MoS2 twisted bilayers, adding a new perspective to moiré physics. Over a range of small twist angles, the phonon spectra evolve rapidly due to ultra-strong coupling between different phonon modes and atomic reconstructions of the moiré pattern. We develop a new low-energy continuum model for phonons that overcomes the outstanding challenge of calculating properties of large moiré supercells and successfully captures essential experimental observations. Remarkably, simple optical spectroscopy experiments can provide information on strain and lattice distortions in moiré crystals with nanometer-size supercells. The newly developed theory promotes a comprehensive and unified understanding of structural, optical, and electronic properties of moiré superlattices.The spectroscopy experiments at UT-Austin (J.Q.) were primarily funded by the U.S. Department of Energy, Office of Basic Energy Sciences under grant DE-SC0019398 and a grant from the University of Texas. Material preparation was funded by the Welch Foundation via grant F-1662. The collaboration between the X.L., C.S., K.L., and M.A. groups is facilitated by the NSF-MRSEC under DMR- 1720595, which funded J.C. and J.E. partially. L.L. and F.L. acknowledge support by the TU-D doctoral program of TU Wien, as well as from the Austrian Science Fund (FWF), project I-3827. The authors ac- knowledge discussions with S. Reichardt and the use of facilities and instrumentation supported by the National Science Foundation through the Center for Dynamics and Control of Materials: an NSF MRSEC under Cooperative Agreement No. DMR-1720595. P.T. acknowledges support from the National Natural Science Foundation of China (Grant No.11874350) and CAS Key Research Program of Frontier Sciences (Grant No. ZDBS-LY-SLH004). M.L. acknowledge the support from the Project funded by China Post- doctoral Science Foundation (Grant No. 2019TQ0317). The PFM work (D.L. and K.L.) was supported by NSF DMR-2004536 and Welch Foundation Grant F-1814. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, Grant Number JPMXP0112101001, JSPS KAKENHI Grant Numbers JP20H00354 and the CREST(JPMJCR15F3), JST.Center for Dynamics and Control of Material

Spatiotemporal dynamics of excitons and electron-hole plasma in atomically thin semiconductors with a moiré potential

Transition metal dichalcogenides (TMDs) are layered semiconductor materials that can be exfoliated into atomically thin monolayers and deliberately stacked into vertical heterostructures to engineer artificial 2D materials with unique optical and electronic properties. When two such monolayers are stacked together with a small twist angle, the resulting heterostructure contains a moiré potential that can localize or impede the transport of optically generated excitons. In a similar way, thin layers of hexagonal boron nitride (hBN) can be exfoliated, twisted, stacked, and employed as a substrate to provide an externally sourced moiré potential to a TMD monolayer and thereby modify its exciton diffusion. In this dissertation, I present novel experimental results to demonstrate the above phenomena and explore the spatiotemporal dynamics of both excitons and electron-hole plasma that can form through optical excitation in TMD semiconductor structures with a moiré potential. Specifically, we stacked two TMD monolayers, MoSe₂ and WSe₂, into a vertical heterostructure with a small relative twist angle (near 60° or H-stacking) and hBN encapsulation. By performing spatiotemporally resolved pump probe measurements with different pump powers and repetition rates, we probed diffusion in this heterostructure across three orders of magnitude of excitation densities. While the moiré potential impedes the diffusion of interlayer excitons at low excitation densities, high excitation densities above the Mott density allow for the formation of an electron-hole plasma that undergoes a sub-picosecond rapid expansion driven primarily by coulomb repulsion and Fermi pressure. In a separate sample, we explore a WSe₂ monolayer that is placed on top of an hBN substrate with two distinct regions. One region of this substrate contains only a single thin layer of hBN, while the other substrate region has an additional thin hBN layer that is stacked above the first layer with a small relative twist angle. The WSe₂ monolayer that resides above the hBN layers only feels a moiré potential on top of the twisted substrate. Spatiotemporally resolved pump probe measurements show that the WSe₂ monolayer has reduced exciton diffusion on top of the twisted hBN substrate.Physic

Fermi Pressure and Coulomb Repulsion Driven Rapid Hot Plasma Expansion in a van der Waals Heterostructure

Transition metal dichalcogenide heterostructures provide a versatile platform to explore electronic and excitonic phases. As the excitation density exceeds the critical Mott density, interlayer excitons are ionized into an electron-hole plasma phase. The transport of the highly non-equilibrium plasma is relevant for high-power optoelectronic devices but has not been carefully investigated previously. Here, we employ spatially resolved pump-probe microscopy to investigate the spatial-temporal dynamics of interlayer excitons and hot-plasma phase in a MoSe2/WSe2 twisted bilayer. At the excitation density of ∼1014 cm-2, well exceeding the Mott density, we find a surprisingly rapid initial expansion of hot plasma to a few microns away from the excitation source within ∼0.2 ps. Microscopic theory reveals that this rapid expansion is mainly driven by Fermi pressure and Coulomb repulsion, while the hot carrier effect has only a minor effect in the plasma phase

Mitochondria, hydrogenosomes and mitosomes: products of evolutionary tinkering!

Contains fulltext : 35651.pdf (publisher's version ) (Closed access