  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.
