  The integration of scanning tunneling microscopy (STM) and electron spin
resonance (ESR) spectroscopy has emerged as a powerful and innovative tool for
discerning spin excitations and spin-spin interactions within atoms and
molecules adsorbed on surfaces. However, the origin of the STM-ESR signal and
the underlying mechanisms that govern the essential features of the measured
spectra have remained elusive, thereby significantly impeding the future
development of the STM-ESR approach. Here, we construct a model to carry out
precise numerical simulations of STM-ESR spectra for a single hydrogenated Ti
adatom and a hydrogenated Ti dimer, achieving excellent agreement with
experimental observations. We further develop an analytic theory that
elucidates the fundamental origin of the signal as well as the essential
features in the measured spectra. These new theoretical developments establish
a solid foundation for the on-demand detection and manipulation of atomic-scale
spin states, with promising implications for cutting-edge applications in spin
sensing, quantum information, and quantum computing.
