revealed by scanning tunneling spectroscopy

A key to unraveling the high-temperature superconductivity mechanism in cuprates is to understand the Metal-insulator transition in Mott insulators that can be induced by either doping charge or raising temperature. While the doping dependence of the Mott gap in cuprates has been studied extensively, it is difficult to explore the gap's temperature dependence in these systems due to its large size (typically 1~2 eV) and the resulting high temperature scale required to melt the Mott-state. Here we use scanning tunneling microscopy/spectroscopy to investigate the temperature dependence of the electronic structure of the single-layer Mott insulator 1T-TaSe₂ which has a much more modest Mott gap of ~ 0.1 eV. We observe that the gap closes rapidly at a temperature scale that is significantly smaller than the gap size. This is accompanied by changes in the exotic orbital texture of the upper Hubbard band that are consistent with reduced screening at elevated temperatures. This phenomenon is unique among strongly correlated systems and is compatible with recent theoretical predictions where spin fluctuations are responsible for the rapid gap closing.