Center for Functional Nanomaterials Seminar

First part: Strongly interacting electronic systems possess rich phase diagrams resulting from the competition between different quantum ground states. A universal mechanism that relieves this frustration is the emergence of microemulsion phases, where regions of different phases self-organize across multiple length scales. Unfortunately, the experimental characterization of these phases poses significant challenges. I will talk about the signatures of the mixture phase between an electronic "Wigner crystal" and an electron liquid in a MoSe2 monolayer across a large region of the density-temperature phase diagram by using cryogenic reflectance and magneto-optical spectroscopy. The transit into this region is marked by anomalies in exciton reflectance, spin susceptibility, and umklapp scattering, establishing it as a distinct phase of electronic matter. Our study establishes the electronic microemulsion phase as an important contender in the physics of correlated electrons [1]. Second part: The structural engineering of van der Waals (vdW) heterostructures via stacking and twisting has been used to create moiré superlattices, enabling engineering of the optical and electronic properties of solid-state systems. However, due to the nanoscale size of typical moiré domains (on the order of 1-10 nanometers and thus much smaller than the optical diffraction limit), the effects of domain crystal structure on the excitonic properties have not been systematically investigated. I will discuss the impacts of the local crystal structure on interlayer excitons (an electron and a hole sitting in different layers) by fabricating near zero twist angle TMD bilayers. Small twist angles create domains that are so large that they can be resolved with far-field optics, enabling the investigation of how each individual domain affects interlayer exciton properties [2]. Lastly, I will present that a suspended MoSe2 monolayer hosts spatially homogeneous, lifetime-broadened excitons, and demonstrate how exciton-photon coupling can be controlled by electromechanically changing the distance between the suspended MoSe2 monolayer and an underlying metallic mirror [3]. Additionally, I will briefly outline the future vision of establishing reliable Ohmic contacts from room to cryogenic temperatures by integration of TMDs with high-k dielectrics. References [1] "Observation of an electronic microemulsion phase emerging from a quantum crystal-to-liquid transition", Jiho Sung, Jue Wang, Ilya Esterlis, Pavel A. Volkov, Giovanni Scuri, You Zhou, Elise Brutschea, Takashi Taniguchi, Kenji Watanabe, Yubo Yang, Miguel A. Morales, Shiwei Zhang, Andrew J. Millis, Mikhail D. Lukin, Philip Kim, Eugene Demler, Hongkun Park, https://doi.org/10.48550/arXiv.2311.18069 (accepted for publication in Nature Physics) [2] "Broken mirror symmetry in excitonic response of reconstructed domains in twisted MoSe2/MoSe2 bilayers", Jiho Sung, You Zhou, Giovanni Scuri, Viktor Zólyomi, Trond I. Andersen, Hyobin Yoo, Dominik S. Wild, Andrew Y. Joe, Ryan J. Gelly, Hoseok Heo, Samuel J. Magorrian, Damien Berube, Andres M. Mier Valdivia, Takashi Taniguchi, Kenji Watanabe, Mikhail D. Lukin, Philip Kim, Vladimir I. Fal'ko and Hongkun Park, Nature Nanotechnology, 15, 750 (2020). [3] "Controlling excitons in an atomically thin Membrane with a mirror", You Zhou, Giovanni Scuri, Jiho Sung, Ryan J. Gelly, Dominik S. Wild, Kristiaan De Greve, Andres Y. Joe, Takashi Taniguchi, Kenji Watanabe, Philip Kim, Mikhail D. Lukin and Hongkun Park, Physical Review Letters, 124, 027401 (2020).