Momentum resolved fingerprint of Mottness in Nb₃Br₈

The insulating behavior of crystalline solids can be described within the band theoretical framework or based on strong local interactions. For an even number of electrons per unit cell, either band- or Mott-insulators can theoretically occur. However, unambiguously identifying an unconventional Mott-insulator with an even number of electrons experimentally has remained a longstanding challenge due to the lack of a momentum-resolved fingerprint. This challenge has recently become pressing for the layer dimerized van der Waals compound Nb₃Br₈, which exhibits a puzzling magnetic field-free diode effect when used as a weak link in Josephson junctions, but has previously been considered to be a band-insulator. In this work, we present a unique momentum-resolved signature of a Mott-insulating phase via angle-resolved photoemission spectroscopy (ARPES): the periodicity of the highest occupied band along the out-of-plane dimerization direction k_z has a momentum space separation of ∆k_z = 2π/d, whereas the valence band maximum of a band insulator would be separated by less than ∆k_z = π/d, where d is the average spacing between the layers. Moreover, the momentum resolved signature employed here could be used to detect quantum phase transition between bandand Mott-insulating phases in van der Waals heterostructures, where interlayer interactions and correlations can be easily tuned to drive such transition.