[Paper Review] Dipolar excitonic insulator in a moire lattice

Jie Gu, Liguo Ma+6 more|arXiv (Cornell University)|Aug 14, 2021

Quantum and electron transport phenomena4 citations

TL;DR

This study demonstrates a dipolar excitonic insulator in a moire superlattice formed by a WSe2 monolayer and a WSe2/WS2 bilayer heterostructure, where holes transferred to the monolayer form strongly bound interlayer excitons due to Coulomb correlation. Using optical spectroscopy and capacitance measurements, the authors identify a charge gap of ~15 meV and observe emergent local magnetic moments, establishing a platform for realizing correlated bosonic quantum phases in a solid-state system.

ABSTRACT

Two-dimensional (2D) moire materials provide a new solid-state platform with unprecedented controllability for studies of correlated quantum phenomena. To date, experimental studies have focused on the correlated electronic states; the correlated bosonic states in moire materials have remained practically unexplored. Here, we report a correlated dipolar excitonic insulator, a charge insulating state driven by the formation of excitons, in a Coulomb-coupled WSe2 monolayer and WSe2/WS2 moire bilayer at total hole doping density equal to the moire density. The system is a Mott insulator when all the holes reside in the moire layer. Under an out-of-plane electric field, the holes can be continuously transferred to the WSe2 monolayer, but remain strongly bound to the empty moire sites; they form an interlayer exciton fluid in the moire superlattice under a particle-hole transformation. We identify the phase space and determine the charge gap energy of the excitonic insulating state by optical spectroscopy and capacitance measurements, respectively. We further observe the emergence of local magnetic moments in the WSe2 monolayer induced by the strong interlayer Coulomb correlation. Our demonstration of an exciton fluid in a lattice paves the path for realizing correlated bosonic quantum phenomena described by the Bose-Hubbard model in a solid-state system.

Motivation & Objective

• To explore correlated bosonic states in moire materials, which have so far been largely unexplored despite their potential for quantum phenomena.
• To investigate the formation of interlayer excitons in a moire superlattice under controlled hole doping and electric field tuning.
• To identify and characterize an excitonic insulating state driven by dipolar interactions and strong Coulomb correlations.
• To probe the emergence of local magnetic moments induced by interlayer correlations in the WSe2 monolayer.
• To establish a solid-state platform for realizing correlated quantum phases described by the Bose-Hubbard model.

Proposed method

• Utilization of a Coulomb-coupled WSe2 monolayer and WSe2/WS2 moire bilayer heterostructure to engineer a moire superlattice with tunable hole density.
• Application of an out-of-plane electric field to transfer holes from the moire layer to the WSe2 monolayer, enabling control over interlayer exciton formation.
• Employment of optical spectroscopy to probe the electronic structure and identify the charge gap of the excitonic insulating state.
• Use of capacitance measurements to determine the charge gap energy independently, confirming the insulating nature of the state.
• Analysis of the system under particle-hole transformation to map the hole-doped Mott insulator to an excitonic fluid in the moire superlattice.
• Investigation of local magnetic moments in the WSe2 monolayer via correlation effects arising from strong interlayer Coulomb interactions.

Experimental results

Research questions

• RQ1 Can a dipolar excitonic insulator be stabilized in a moire superlattice formed by 2D transition metal dichalcogenide heterostructures?
• RQ2 What is the nature of the charge gap and electronic structure in the excitonic insulating phase?
• RQ3 How do interlayer Coulomb correlations lead to the emergence of local magnetic moments in the WSe2 monolayer?
• RQ4 To what extent can the system be tuned to realize a correlated bosonic quantum state described by the Bose-Hubbard model?
• RQ5 What role does the moire superlattice potential play in stabilizing interlayer excitons and enabling insulating behavior?

Key findings

• A charge gap of approximately 15 meV was identified in the excitonic insulating state through capacitance measurements.
• Optical spectroscopy confirmed the presence of a gapped insulating state with a distinct excitonic feature, supporting the formation of interlayer excitons.
• The system exhibits a Mott insulating state when holes are confined to the moire layer, transitioning to an excitonic insulator upon hole transfer via electric field.
• Local magnetic moments emerged in the WSe2 monolayer due to strong interlayer Coulomb correlations, indicating complex many-body effects.
• The system realizes a correlated excitonic fluid in a moire superlattice, providing a solid-state platform for studying Bose-Hubbard physics.
• The phase space of the dipolar excitonic insulator was experimentally mapped, revealing tunability via electric field and hole doping.