Anisotropic hyperfine interaction of surface-adsorbed single atoms

TL;DR

This study employs ESR-STM to measure hyperfine interactions at the single-atom level, revealing significant anisotropy and providing detailed insights into the electronic ground state and local chemical environment of surface-adsorbed atoms.

Contribution

It introduces a novel method combining ESR and STM to characterize the full hyperfine tensor of individual atoms, uncovering anisotropic hyperfine interactions at low-symmetry sites.

Findings

Significant hyperfine anisotropy observed in single Ti atoms.

Density functional theory explains the anisotropy via electron spin density distribution.

Single-atom hyperfine spectroscopy reveals detailed electronic and chemical environment information.

Abstract

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.