image: Fundamental properties of Nb3Cl8
Credit: ©Science China Press
In condensed matter physics, electron–electron interactions often give rise to exotic quantum states that defy conventional band theory. Among them, Mott insulators are especially intriguing: although band theory predicts metallic conductivity, strong Coulomb repulsion localizes the electrons, turning the material into an insulator. These correlation-driven states underpin many emergent quantum phases such as high-temperature superconductivity and quantum spin liquids. However, experimental access to such correlated states has long been limited to extreme conditions—low temperatures, high pressures, or irreversible tuning.
A research team led by Prof. Xiaobo Lu at the International Center for Quantum Materials, Peking University, together with collaborators from the Chinese Academy of Sciences and Beijing Institute of Technology, has now reported the first clear evidence of a Mott insulating state persisting up to room temperature in the kagome compound Nb₃Cl₈. Their study demonstrates that Nb₃Cl₈, a van der Waals layered material with a breathing kagome lattice, provides a highly tunable platform for exploring strong electron correlations under practical conditions.
The findings have been published in National Science Review (2025), under the title “Evidence of Mott insulator with thermally induced melting behavior in kagome compound Nb₃Cl₈.”
The researchers fabricated atomically thin Nb₃Cl₈ flakes through mechanical exfoliation and encapsulated them between hexagonal boron nitride (hBN) layers in a dual-gate device geometry. Electrical transport measurements revealed ambipolar semiconducting behavior with a Fermi level close to the conduction band (as shown in Fig. 1). Even at high carrier densities of around 1.3 × 10¹³ cm⁻², Nb₃Cl₈ remained strongly insulating. The extracted interaction parameter, rₛ ≈ 60, confirms the system lies deep in the strongly correlated regime.
To probe the nature of the energy gap, the team employed an ingenious detection scheme using a monolayer graphene (MLG) sheet as a sensitive charge sensor separated from Nb₃Cl₈ by an ultrathin hBN dielectric. By tracking the shift of graphene’s charge neutrality point (CNP) under varying gate voltages, they directly mapped the chemical potential of Nb₃Cl₈ and extracted its temperature-dependent energy gap (as shown in Fig. 2).
Two striking results emerged. First, the gap size was found to be about 1.10 eV at 100 K, decreasing to 0.86 eV at 200 K, consistent with theoretical and optical measurements. Second—and most remarkably—the gap exhibited a strong thermal melting behavior, rapidly shrinking to 0.63 eV at 300 K. Such pronounced temperature dependence is a hallmark of a Mott insulator, in sharp contrast to conventional semiconductors like bilayer WSe₂, whose bandgap remains nearly constant over the same temperature range (as shown in Fig. 3). This “melting” of the Mott gap arises from the weakening of electron correlations by thermal fluctuations at elevated temperatures
Together with density functional theory plus Hubbard U calculations showing the splitting of half-filled flat bands into upper and lower Hubbard bands, the experimental results provide compelling evidence that Nb₃Cl₈ hosts a Mott insulating state stable up to room temperature. This discovery not only establishes Nb₃Cl₈ as a new member of the kagome correlated materials family but also opens a promising route toward room-temperature exploration of flat-band correlation physics, moiré engineering, and quantum device applications.