Correlated flat bands and quantum spin liquid state in a cluster Mott insulator

Flat bands are rare in pristine solids and are unstable against electronic correlations or other types of long-range order. Unlike atomic-scale Hubbard systems or Moiré materials, where electronic correlations are either localized or long-ranged, pristine flat band systems with short-range interactions that do not break symmetry spontaneously are less known and intriguing. These systems could bridge the gap between atomic Mott insulators and Moiré correlated insulators, offering a unique platform to explore their mysterious relation. Using an analytical analysis, further verified by numerical calculations, we show that monolayer Nb 3 Cl 8 is a unique flat band system with short-range interactions. We present clear evidence that it is a cluster Mott insulator, which nicely explains the electronic structure observed in angle-resolved photoemission spectroscopy. We further propose that monolayer Nb 3 Cl 8 may constitute a rare example of molecular quantum spin liquid with flat bands.