Evidence for the Renormalization of the Mott Gap by Spin Correlations in Van der Waals Mott insulator RuBr₃

The interplay between spin and charge degrees of freedom in Mott insulators remains a central topic in strongly correlated electron systems. Using variable-temperature scanning tunneling microscopy/spectroscopy (STM/STS), we investigate the Van der Waals magnet RuBr₃, a candidate Kitaev quantum spin liquid material. At low temperatures, RuBr₃ exhibits a large correlated Mott gap exceeding 3 eV. As temperature increases, the gap softens significantly, decreasing to ~0.7 eV by 200 K. This evolution coincides with magnetic phase transitions: a sharp reduction occurs near the Néel temperature (T_N ≈ 34 K) upon loss of long-range antiferromagnetic order, followed by a gradual decline through the Kitaev paramagnetic regime. Comparison with α-RuCl₃ reveals that the dominant spin correlations—whether long-range order or short-range Kitaev interactions—govern the Mott gap renormalization. Our results highlight the essential role of spin-charge coupling in Mott physics and provide key insights into the electronic behavior of Kitaev materials across different magnetic phases.