Nearly-free-electron system of monolayer Na on the surface of single-crystal HfSe$_2$

TL;DR

This study investigates a monolayer of sodium on HfSe$_2$, revealing nearly free-electron behavior with an effective mass close to free electrons, due to large lattice spacing and weak hybridization, making it a model for 2D materials.

Contribution

It demonstrates that Na monolayer on HfSe$_2$ exhibits nearly free-electron behavior, highlighting the role of lattice constant and hybridization in electronic properties.

Findings

Na monolayer shows effective mass ~1m_e

Weak hybridization due to large lattice constant

System serves as a model for 2D materials

Abstract

The electronic structure of a single Na monolayer on the surface of single-crystal HfSe$_2$ is investigated using angle-resolved photoemission spectroscopy. We find that this system exhibits an almost perfect "nearly-free-electron" behavior with an extracted effective mass of $\sim\!1m_e$, in contrast to heavier masses found previously for alkali metal monolayers on other substrates. Our density-functional-theory calculations indicate that this is due to the large lattice constant, causing both exchange and correlation interactions to be suppressed, and to the weak hybridization between the overlayer and the substrate. This is therefore an ideal model system for understanding the properties of two-dimensional materials.