Axial-Bonding-Driven Dimensionality Effect on the Charge-Density Wave in NbSe₂

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Materials

Background:

• 2H-Niobium Diselenide (NbSe2) is a well-known material exhibiting charge-density-wave (CDW) states.
• The dependence of CDW states on material dimensionality and the underlying mechanisms have been debated.
• Understanding these effects is crucial for exploring quantum phenomena in layered materials.

Purpose of the Study:

• To experimentally investigate the charge-density-wave (CDW) phase in bilayer NbSe2.
• To elucidate the role of dimensionality and chemical bonding in governing CDW behavior.
• To explore the influence of selenium (Se) ions and their bonding on CDW formation.

Main Methods:

• Experimental demonstration of a robust 3x3 CDW phase in freestanding and supported bilayer NbSe2.
• Temperature and thickness-dependent Raman spectroscopy focusing on the A1g phonon mode involving Se ions.
• First-principles calculations to analyze intra- and interlayer bonding related to Se-p orbitals.

Main Results:

• A robust 3x3 CDW phase was observed in bilayer NbSe2 significantly above the bulk transition temperature.
• Raman intensity of the A1g phonon mode showed a strong dependence on temperature and thickness.
• Calculations revealed a competition between intra- and interlayer bonding of Se-p orbitals drives the CDW.

Conclusions:

• Selenium (Se) out-of-plane displacement plays a critical role in driving the CDW distortion in NbSe2.
• The observed effects highlight a Se-dominated dimensionality effect in layered CDW systems.
• This study offers new insights into chemical bonding and mechanical stability in layered CDW materials.