Tuning single-atom electron spin resonance in a vector-magnetic field*

Scanning tunneling microscopy (STM) has been one of highly versatile surface science tools to investigate electronic and magnetic properties of individual atoms and molecules on surfaces. Recently, successful combination of electron spin resonance (ESR) technique and STM has demonstrated electron spin of individual atoms on ultrathin insulating MgO films can be coherently driven. Utilizing a 2D vector magnetic field and the local stray field from the magnetic STM tip, we optimize the ESR signal and characterize the role of the tip field. We demonstrate single atom ESR using the tip-field only, under zero external magnetic field, which promises to make this technique available in many existing STM systems.