P-d Correlation-Determined Charge Order Stiffness and Corresponding Quantum Melting in Monolayer 1T-TiSe2

1T-TiSe2, a promising candidate of the sought-after excitonic insulator, possesses an enigmatic charge density wave (CDW) order of which the microscopic origin is formidable to settle owing to the chicken-and-egg entanglement between the electron and lattice degrees of freedom. Its CDW experiences an intriguing but elusive quantum melting and eventually enters the superconducting phase under metal intercalation, suggesting the possible role of melted-order fluctuation in gluing the electron paring. Employing the spectroscopic imaging scanning tunneling microscope (STM), we access the pure electronic behavior by visualizing the CDW melting process of monolayer 1T-TiSe2 in both the space and energy-band dimensions. In real space, the native lattice imperfections disturb the local order parameter and stimulate the melting of CDW. In energy-band space, different states exhibit varying stiffness against the melting stimuli, yielding distinctive melted textures. The evolution of CDW topological defects and the structure factor in the quantum melting process provide a straightforward avenue to evaluate the CDW coherency, which shows that the CDW stiffness scales with the strength of the p-d Coulomb correlation. Our study reveals the quantum melting of CDW with an altering band-orbital-correlation character and puts compelling emphasis on the indispensable role of excitonic interaction in stabilizing the charge order of monolayer TiSe2.