[NCTS Lecture Series] Novel quantum phases in atomically thin 1D/2D materials: Two recent discoveries
Title: [NCTS Lecture Series] Novel quantum phases in atomically thin 1D/2D materials: Two recent discoveries
Speaker: Prof. Steven G. Louie (UC Berkeley & Lawrence Berkeley National Lab)
Time: 2025/2/24 (Mon.) 14:20-15:20
Place: NCTS Physics Lecture Hall, 4F, Cosmology Hall, NTU
Abstract
Atomically thin quasi 1D and 2D materials exhibit many fascinating properties and phenomena. Study of these systems constitutes one of the most exciting and rapidly growing areas of condensed matter physics. Here, we present two recent discoveries – discussing their theoretical prediction and physical understanding. First, based on topological classification theory, we have discovered a new class of graphene nanoribbons (GNRs) - called Janus GNRs (JGNRs) – that hosts different quantum magnetic phases, ranging from antiferromagnetism to ferrimagnetism to ferromagnetism by tuning its edge structure. Our design principle and theoretical predications for the JGNRS have been validated through bottom-up synthesis and scanning probe measurements. Second, we investigate the excitonic insulator (EI) phase in 2D materials, a many-body state of spontaneous electron-hole pairs (excitons) that was theorized since the 1960s but has been elusive of definitive experimental identification in intrinsic real materials. We address the problem by developing a parameter-free ab initio methodology, enabling the computation of the electron-hole pairing order parameters and excitation spectra at finite temperature within the framework of GW and GW-BSE calculations together with a Bardeen-Cooper-Schrieffer (BCS) like formalism. For monolayer MoS2 in the 1T' structure, a multi-band 2D EI candidate, our results reveal that its ground state is an unconventional EI phase, with spontaneous broken symmetries. We identify telltale spectroscopic signatures that distinguish it from the high temperature band insulator phase.
