Tuning d-orbitals to control spin-orbit coupling in terminated MXenes

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Chemistry

Background:

• Spin-orbit coupling (SOC) is crucial for topological insulators (TIs), typically linked to relativistic effects.
• The role of orbital effects in modulating SOC and bandgaps in TIs has been largely overlooked.
• Designing materials with large nontrivial bandgaps is essential for advanced TI devices.

Purpose of the Study:

• To demonstrate a novel method for controlling effective SOC and nontrivial bandgaps in 2D materials.
• To investigate the influence of orbital effects on SOC in MXene-based topological insulators.
• To reduce the element-dependency in tuning SOC for TI applications.

Main Methods:

• First-principles calculations were employed to study 5d W2CO2 and 4d Mo2CO2 MXenes.
• The hybridization strength between transition metal atoms (W, Mo) and ligand elements (C, O) was systematically varied.
• Electronic band structures and spin-orbit coupling effects were analyzed.

Main Results:

• Decreasing hybridization between W atoms and C/O in 2D W2CO2 was found to enhance the nontrivial bandgaps at the Γ-point.
• Weak hybridization promotes electron localization in d-orbitals, increasing on-site Coulomb repulsion and orbital polarization.
• Enhanced orbital polarization directly boosts the SOC effect, leading to larger bandgaps.
• Similar trends were observed in 4d Mo2CO2 MXenes.

Conclusions:

• Orbital effects provide an efficient and direct pathway to control nontrivial bandgaps in 2D MXene topological insulators.
• Tuning hybridization strength is a viable strategy to engineer SOC and bandgaps, offering an alternative to element selection.
• This research opens new possibilities for designing high-performance TI devices by manipulating orbital interactions.