CAREER: Correlated Excitons in Semiconducting Moire Superlattices

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

Non-technical abstract: Fundamental particles can be classified into fermions and bosons, which have distinct collective behaviors. Interactions between electrons, a fermion, results in now well known emergent properties. On the other hand, creation of unconventional bosonic matter has been challenging since bosons in natural materials are often weakly-interacting and/or short-lived. This project bridges this gap by creating unconventional bosonic matter using excitons in semiconducting moiré superlattices. Compared to existing platforms such as cold atoms and quantum wells, this platform offers orders-of-magnitude higher temperature scale (tens of Kelvin vs. <1 Kelvin) and scalability (>10000 particles vs. ~100 particles). Research is incorporated into a broad outreach plan under the theme of “Science as a Lego game”. Leveraging the Lego-like nature of optical setups, integrated outreach activities are targeted at students at many educational levels to demystify quantum science and introduce them to potential career options. Specific activities include designing a high-school lab-based class series on building optical modules and a Saturday class on optics in quantum science; initiating a summer research intern program for undergraduate students; and organizing a quantum science summer school for graduate students and postdocs. Technical abstract: Condensed matter physics aims to understand how interactions endow simple elementary particle building blocks with complex emergent phenomena. Exotic quantum phases of fermionic electrons have been intensively studied in solid-state systems, including recently in moiré superlattices. However, collective bosonic states have remained the purview of ultra-cold atoms due to the lack of suitable condensed matter platforms and experimental probes. This project aims to overcome both challenges and establishe excitons in semiconducting moiré superlattices -- bosons composed of tightly bound electron-hole pairs -- as a platform for engineering exotic collective states of interacting bosons. The principal investigator is developing a set of novel pump- probe spectroscopies that enables direct measurement of exciton compressibility, isolating low energy “charge” and “spin” excitations of excitons, as well as investigating their dynamics. This allows identification and investigation of a plethora of exotic bosonic phases such as bosonic Wigner crystal, Mott-superfluid transition, valley pseudospin exchange interactions and pseudospin orders, etc. This project forges a path to a new class of “quantum exitonic materials” where exotic properties and novel device concepts arise from strongly correlated bosons, thereby establishing a new fundamental pillar of condensed matter alongside the extensively studied quantum electronic materials. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.