  If a material with an odd number of electrons per unit cell turns out to be
insulating, Mott localisation may be invoked as an explanation. This is widely
accepted for the layered compound 1T-TaS2, which has a low-temperature
insulating phase comprising charge order clusters with 13 unpaired orbitals
each. But if the stacking of layers doubles up the unit cell to include an even
number of orbitals, the nature of the insulating state is ambiguous. Here,
scanning tunnelling microscopy (STM) reveals two distinct terminations of the
charge order in 1T-TaS2, the sign of such a double-layer stacking pattern.
However, spectroscopy at both terminations allows us to disentangle unit-cell
doubling effects and determine that Mott localisation alone is enough to drive
gap formation. We also observe the collapse of Mottness at an extrinsically
restacked termination, demonstrating that the microscopic mechanism of
insulator-metal transitions lies in degrees of freedom of interlayer stacking.

Strongly Correlated Electrons;Materials Science