Effect of dimensionality on the charge-density-wave in few-layers 2H-NbSe$_2$

By M. Calandra, I. Mazin, F. Mauri. 2009

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We investigate the charge-density wave (CDW) instability in single and double layers, as well as in the bulk $2{\text{H-NbSe}}_{2}$. We demonstrate that the density functional theory correctly describes the metallic CDW state in the bulk $2{\text{H-NbSe}}_{2}$. We predict that both monolayer and bilayer ${\text{NbSe}}_{2}$ undergo a CDW instability. However, while in the bulk the instability occurs at a momentum ${\mathbf{q}}_{\text{CDW}}\ensuremath{\approx}\frac{2}{3}\ensuremath{\Gamma}M$, in freestanding layers it occurs at ${\mathbf{q}}_{\text{CDW}}\ensuremath{\approx}\frac{1}{2}\ensuremath{\Gamma}M$. Furthermore, while in the bulk the CDW leads to a metallic state, in a monolayer the ground state becomes semimetallic, in agreement with recent experimental data. We elucidate the key role that an enhancement of the electron-phonon matrix element at $\mathbf{q}\ensuremath{\approx}{\mathbf{q}}_{\text{CDW}}$ plays in forming the CDW ground state.

Subfields: Atomic and Molecular Physics, and Optics, Electronic, Optical and Magnetic Materials, Materials Chemistry

View Paper: https://www.semanticscholar.org/paper/f80efaa340cff63a3f6ddf37f42ce3769c7cbaef