Direct observation of massless excitons and linear exciton dispersion

Excitons in two-dimensional (2D) materials provide a unique platform for exploring many-body physics. In this work, we report the first direct observation of linearly dispersive, massless excitons in freestanding monolayer hexagonal boron nitride (hBN) using momentum-resolved electron energy-loss spectroscopy (Q-EELS). The measured exciton group velocity of 8×10^5 m/s shows excellent agreement with ab initio GW-BSE theory, and the lowest dipole-allowed transition is identified at 6.6 eV, resolving a long-standing debate on the band gap of monolayer hBN. These results provide unambiguous evidence of massless excitons in a 2D material and establish momentum-resolved EELS as a powerful probe of exciton band structure, opening new opportunities for studying excitonic transport and collective quantum phases.