Science 329, 1628–1630 (2010)

The scanning tunnelling microscope (STM) can image surfaces with atomic resolution but some processes involving single atoms happen too fast to be recorded by this instrument. Now scientists at IBM's Almaden Research Center in the United States and the EPFL in Switzerland have improved the temporal resolution of the STM by a factor of one million while retaining its ability to image individual atoms and molecules.

The STM works by measuring the electric currents that tunnel from the ultrasharp tip of the microscope to the surface being imaged as the tip is scanned over it. The current through the tunnel junction formed by the tip and the surface depends on the distance between them. The magnetic properties of surfaces can be probed with a magnetic tip and the IBM team uses a variation on this approach to achieve high temporal and spatial resolution at the same time.

In the latest experiments a strong voltage pulse (the pump) is applied to the tunnel junction to create excited spin states on the surface, and a weaker voltage pulse (the probe) is applied at a later time to monitor these states. By repeating this measurement many times, with different delays between the pump and probe pulses, the IBM team finds that the spin-relaxation times of individual iron–copper dimers on a surface can vary between 50 and 250 ns.