Free coherent evolution of a coupled atomic spin system initialized by electron scattering

Sander Otte's group at TU Delft reported a novel pump-probe ESR-STM method using the Nanonis Mimea in combination with a radio frequency RF source and the Unisoku USM-1300 ultra-low temperature STM. Once again, the powerful and unbeatable combination of Nanonis Mimea and Unisoku STM demonstrated that only by working with the best possible equipment can one push the limits of what is possible.

The new method allowed the observation of the free coherent flip-flop evolution of two coupled spin 1/2 atoms (hydrogenated titanium atom dimers) on a magnesium oxide surface. The fast spin flips of the entangled atoms were measured by combining the energy resolution of ESR-STM and the time resolution of DC pump-probe spectroscopy. Previous ESR-STM measurements were performed in combination with microwave AC pulsing schemes and allowed the observation of the coherent time evolution of a single atomic spin. However, in the case of a pair of entangled spins, a faster initial excitation is required than can be achieved with microwave pulses. The initial excitation must be fast due to competition with the intrinsic dynamics determined by the coupling strength. The initialisation rate itself is limited by the RF frequency of the power available at the STM tip. Initialisation was performed by coherence-maintaining pulses in the tunneling current. Spins were flipped instantaneously and it was shown that only the spin immediately below the tip is affected by the spin excitation.

Method allows for looking into insights of the physics of electron spin scattering which are not accessible to the ESR-STM pulsed methods. Demonstrated approach of a very local and nearly instantaneous coherent spin flip offers routes to coherent manipulation of extended atomic spin arrays as essential building blocks for advanced spintronic devices as well as studies into the propagation of spin waves.