In this paper several electron spin resonance–scanning tunneling microscopy (ESR-STM) experiments are described. In this technique, the tip of a STM scans a surface that contains isolated paramagnetic spin centers. The individual spin centers apply a time-dependent perturbation on the tunneling electrons, giving rise to a time-dependent component of the tunneling current—at the Larmor frequency [J(${\mathrm{\omega }}_{\mathit{L}}$)]. This occurs only when the tunneling region is close to the spin center. Therefore, J(${\mathrm{\omega }}_{\mathit{L}}$) is spatially localized. Reproducibility of the spatial localization together with a reproducible tip-dependent frequency shift were observed. In higher magnetic fields the average linewidths, together with the fluctuations in the frequency of the signal (probably due to the electric fields near the spin center), are larger. The proposed mechanism is that electric-dipole-moment oscillations, which are modulated by the Larmor frequency (the Zeeman effect), will give a significant J(${\mathrm{\omega }}_{\mathit{L}}$) component close to the tunneling region by modulating both the barrier height and width.

• Received 16 March 1993

DOI:https://doi.org/10.1103/PhysRevB.48.4887