[Paper Review] Asymmetry-enriched electronic and optical properties of bilayer graphene

Bor-Luen Huang, Chih‐Piao Chuu+1 more|arXiv (Cornell University)|May 28, 2018

Graphene research and applicationsMaterials Science2 references4 citations

TL;DR

This study investigates how geometry modulation and gate voltage induce asymmetric electronic and optical responses in Bernal-stacked bilayer graphene, revealing topological protected states, complex wavefunction node structures, and dramatically altered optical absorption spectra. Key findings include forbidden optical transitions, asymmetric absorption peaks, and tunable subband structures, enabling potential control of electronic and optical properties for future 2D device applications.

ABSTRACT

The electronic and optical response of Bernal stacked bilayer graphene with geometry modulation and gate voltage are studied. The broken symmetry in sublattices, one dimensional periodicity perpendicular to the domain wall and out-of-plane axis introduces substantial changes of wavefunctions, such as gapless topological protected states, standing waves with bonding and anti-bonding characteristics, rich structures in density of states and optical spectra. The wavefunctions present well-behaved standing waves in pure system and complicated node structures in geometry-modulated system. The optical absorption spectra show forbidden optical excitation channels, prominent asymmetric absorption peaks, and dramatic variations in absorption structures. These results provide that the geometry-modulated structure with tunable gate voltage could be used for electronic and optical manipulation in future graphene-based devices.

Motivation & Objective

• To understand the impact of sublattice symmetry breaking and one-dimensional periodicity on electronic and optical properties in bilayer graphene.
• To investigate how domain walls and gate voltages modify wavefunctions, density of states, and optical response in AB-stacked bilayer graphene.
• To identify experimentally verifiable signatures of topological protected states and asymmetric optical excitation in geometry-modulated bilayer graphene systems.
• To explore the potential of using geometric and electrostatic control for manipulating electronic and optical behavior in 2D materials.

Proposed method

• Employing a tight-binding model with exact diagonalization to compute energy subbands and sublattice wavefunctions in bilayer graphene.
• Applying a gate voltage to break layer symmetry and induce tunable band gaps and metallic behavior via domain wall states.
• Using the Kubo formula within the gradient approximation to calculate optical absorption spectra.
• Analyzing the joint density of states and its relation to optical transitions and van Hove singularities.
• Introducing geometry modulation to break spatial symmetry and generate complex node structures in wavefunctions.
• Examining the interplay between domain wall width, gate voltage, and optical response to identify tunable spectral features.

Experimental results

Research questions

• RQ1 How does geometry modulation in bilayer graphene affect the wavefunction structure and topological protection of domain wall states?
• RQ2 What are the optical absorption characteristics under broken sublattice symmetry and gate voltage in bilayer graphene?
• RQ3 How do the optical selection rules and excitation channels change in the presence of asymmetric sublattice coupling?
• RQ4 What role does the domain wall width play in modifying the optical absorption spectrum and density of states?
• RQ5 Can the predicted asymmetric and forbidden optical transitions be experimentally verified using STS or optical spectroscopy?

Key findings

• Geometry modulation leads to complex node structures in wavefunctions, disrupting simple standing wave patterns observed in pristine systems.
• The system exhibits forbidden optical excitation channels under specific linear relations between (A1, A2) and (B1, B2) sublattices.
• Optical absorption spectra display prominent asymmetric peaks even in the absence of selection rules, indicating strong sublattice asymmetry effects.
• Gate voltage induces a pair of strong, symmetric absorption peaks and a plateau in the density of states across the Fermi energy.
• The number, frequency, and intensity of absorption peaks are strongly dependent on both the modulation period and gate voltage strength.
• Wavefunctions of topological protected domain wall states show unusual spatial distributions, with enhanced complexity under combined geometry and electrostatic modulation.