Termination-dependent charge transfer enhances the Kondo coupling strength in CeCoIn₅

The reduction of beam spot sizes at modern synchrotron facilities has transformed angle-resolved photoemission spectroscopy (ARPES) into a powerful spectro-microscopy probe. In the layered heavy-fermion compound CeCoIn₅, this enables us to resolve and compare distinct surface terminations hosting distinctive electronic structures. Using spatially resolved ARPES, we identify two terminations exhibiting markedly different degrees of c-f hybridization. We show that these differences originate from variations in surface charge transfer, with the more electron-rich termination exhibiting stronger hybridization. Controlled in situ alkali dosing further enhances the surface electron density and concomitantly increases the Kondo coupling strength, providing direct spectroscopic evidence for tunable hybridization in a heavy fermion compound. These results establish a new route for visualizing and manipulating the Kondo interaction in quasi-two-dimensional heavy-fermion systems.