STM and beyond

In scanning tunneling microscopy (STM) one brings a sharp metallic tip into atomic-scale proximity with a sample, and then measures the net electric current across the voltage-biased tunnel junction.  The tunneling current is typically proportional to the energy-integrated local density of states (LDOS) of the sample surface, therefore via scanning tunneling spectroscopy one can acquire spatially resolved LDOS by differential conductance imaging. After a Fourier transform, one obtain quasiparticle interference patterns, where the topology of the band structure is encoded.

The tunneling current manifests the time-averaged charge transfer, and is usually measured at DC. However, the current fluctuates by itself due to the intrinsic randomness of tunnel events. This fluctuation, called shot noise, provides information on the tunneling charge carriers. The recent technical breakthrough allows for local noise spectroscopy with superb spatial resolution inherited from STM. With local noise spectroscopy, one can directly visualize paired and unpaired electrons.

Beyond the standard STM, various measurements are possible with simple instrumental development. For example, when the tip and sample become superconducting, a Josephson tunnel junction naturally forms, enabling a direct probe of electron pair density by measuring the Josephson critical current. By introducing radio-frequency components to STM, one can access dynamics at high frequencies up to GHz.