Tunable Optical Excitations in Twisted Bilayer Graphene Form Strongly Bound Excitons

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Chemistry

Background:

• Graphene's unique electronic properties are highly sensitive to stacking configuration.
• Twisted bilayer graphene (tBLG) offers tunable electronic absorption resonances due to interplanar orbital overlap.
• Understanding electronic relaxation pathways is crucial for novel electronic applications.

Purpose of the Study:

• To investigate the electronic structure and relaxation dynamics in single twisted bilayer graphene domains.
• To probe resonant excitations and identify factors influencing electronic population.
• To characterize novel electronic states and their impact on material properties.

Main Methods:

• Utilized a combination of multiphoton transient absorption (TA) microscopy and Transmission Electron Microscopy (TEM).
• Probed resonant excitations in single twisted bilayer graphene domains.
• Employed two-photon TA microscopy to identify specific exciton states.

Main Results:

• Observed a significant, many-fold enhancement in transient electronic population in resonantly excited tBLG domains.
• Identified a major electronic relaxation bottleneck.
• Discovered a strongly bound, dark exciton state approximately 0.37 eV below the 1-photon absorption resonance.

Conclusions:

• The dark exciton state acts as a bottleneck, stabilizing electronic populations in tBLG.
• This study reveals the stable coexistence of strongly bound excitons and free-electron continuum states in a metallic 2D material.
• Findings offer new insights into the fundamental electronic behavior of twisted bilayer graphene.