  Hyperfine interactions between electron and nuclear spins have been widely
used in material science, organic chemistry, and structural biology as a
sensitive probe to the local chemical environment through spatial
identification of nuclear spins. With the nuclear spins identified, the
isotropic and anisotropic components of the hyperfine interactions in turn
offer unique insight into the electronic ground-state properties of the
paramagnetic centers. However, traditional ensemble measurements of hyperfine
interactions average over a macroscopic number of spins with different
geometrical locations and nuclear isotopes. Here, we use a scanning tunneling
microscope (STM) combined with electron spin resonance (ESR) to measure
hyperfine spectra of hydrogenated-titanium (Ti) atoms on MgO/Ag(100) and
thereby determine the isotropic and anisotropic hyperfine interactions at the
single-atom level. By combining vector-field ESR spectroscopy with STM-based
atom manipulation, we characterize the full hyperfine tensor of individual
Ti-47 and Ti-49 atoms and identify significant spatial anisotropy of hyperfine
interaction for both isotopes when they are adsorbed at low-symmetry binding
sites. Density functional theory calculations reveal that the large hyperfine
anisotropy arises from a highly anisotropic distribution of the ground-state
electron spin density. Our work highlights the power of ESR-STM-enabled
single-atom hyperfine spectroscopy as a powerful tool in revealing ground-state
electronic structures and atomic-scale chemical environments with
nano-electronvolt resolution.
