P‑d Correlation-Determined\nCharge Order Stiffness\nand Corresponding Quantum Melting in Monolayer 1T-TiSe<sub>2</sub>

1T-TiSe<sub>2</sub>, a promising candidate of the sought-after\nexcitonic insulator, possesses an enigmatic charge density wave (CDW)\norder of which the microscopic origin is formidable to settle owing\nto the chicken-and-egg entanglement between the electron and lattice\ndegrees of freedom. Its CDW experiences an intriguing but elusive\nquantum melting and eventually enters the superconducting phase under\nmetal intercalation, suggesting the possible role of melted-order\nfluctuation in gluing the electron paring. Employing the spectroscopic\nimaging scanning tunneling microscope (STM), we access the pure electronic\nbehavior by visualizing the CDW melting process of monolayer 1T-TiSe<sub>2</sub> in both the space and energy-band dimensions. In real space,\nthe native lattice imperfections disturb the local order parameter\nand stimulate the melting of CDW. In energy-band space, different\nstates exhibit varying stiffness against the melting stimuli, yielding\ndistinctive melted textures. The evolution of CDW topological defects\nand the structure factor in the quantum melting process provide a\nstraightforward avenue to evaluate the CDW coherency, which shows\nthat the CDW stiffness scales with the strength of the p-d Coulomb\ncorrelation. Our study reveals the quantum melting of CDW with an\naltering band-orbital-correlation character and puts compelling emphasis\non the indispensable role of excitonic interaction in stabilizing\nthe charge order of monolayer TiSe<sub>2</sub>.