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Transport properties

Scanning tunnelling microscope runs faster

31 Oct 2007 Belle Dumé

Ordinary scanning tunnelling microscopes (STMs) could soon be collecting images 100-times faster than ever before thanks to a simple but clever modification by physicists in the US. The team added a specially-designed radio-frequency circuit to an existing STM, which allows it to operate at 10 MHz. Such RF-STMs could help researchers gain a better understanding of how atoms and electrons move in solids (Nature 450 85).

RF-STM image

STMs have been around for 25 years and are routinely used to obtain atomic-scale images. STMs work by measuring the currents produced by electrons as they tunnel from a conducting sample to the sharp probe tip of the microscope. However, one major drawback of STMs is that they are limited in their temporal resolution, or bandwidth, to frequencies of about 10-100 kHz because of stray capacitances in the tunnel current readout circuits. These “parasitic” elements, which are introduced when amplifying the tiny current signals so they can be read, degrade the bandwidth by creating electrical short-circuits at high frequencies.

Now, Kamil Ekinci and colleagues at Boston University and Cornell University have now succeeded in increasing this bandwidth to 10 MHz — a 100-fold improvement. The researchers did this by incorporating a resonant inductor-capacitor circuit into the STM that essentially nulls out the parasitic elements allowing changes in the tunnelling current to be measured over much shorter timescales than before.

“The main advantage is that we have now have access to the ‘high-frequency signal components’ of the tunnel current,” Ekinci told physicsworld.com. “This means that if the current changes on fast time scales, we can detect it. Large bandwidth basically allows us to see these faster signals.”

The new technique permits two types of novel application: fast temperature and motion/position measurements at the nanometre scale. “Researchers may want to modify their existing STM set-ups using our technique, something that is quite straightforward to do,” suggests Ekinci. “The tool may be useful for people studying exotic materials like superconductors and they may be able to perform some new measurements not possible before. The technique might also come in handy for research on electron spin currents and may allow quantum-limited position measurements. The list goes on.”

Ekinci’s team is already building the next generation of RF STMs where they can better control the temperature and vacuum conditions in the microscope. “Once this is done, it will be a powerful tool that we can use for experiments,” he adds.

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