Charge accumulation imaging (CAI) is a low-temperature method that allows us to probe the quantum structure of conducting layers covered by insulator. In this way it extends the reach of scanning probe microscopy to electron systems buried below the sample's surface. To understand how CAI works, think of the tip as a tiny nanoscale capacitor plate. As electrons enter the sample, image charges are induced on the tip, just like for a standard parallel plate capacitor. A circuit constructed from cryogenic transitors amplifies the tip's image charge with an incredible sensitivity of 0.01 electrons per root Hertz.

The animation to the right shows the CAI method in action. In this example, charges enter a two-dimensional electron layer burried within a GaAs-AlGaAs sample from below -- by crossing a tunneling barrier. Both positive and negative charges are shown in red, whereas the blue lines indicate the electric field emanating from the sample. The Q-meter represents the low-temperature sensor circuit.

The figure to the left shows an example from the introductory paper. In this case, a two-dimensional electron system was prepared to have a steep density minimum near the center of the scanned area. The series of 2.75 µm x 2.75 µm charge images shows remarkable behavior in response to a perpendicular magnetic field. The dark spot, which first emerges at 1.5 T, marks a distinct region of low charge accumulation. The edges of the spot follow a profile of constant electron density, sufficient to fill exactly two quantum levels (Landau levels). These observations are intimately connected to the quantum Hall effect.

a movie

	Scanning tunneling micrscopy (STM) is a powerful technique based on the quantum tunneling of electrons from a conducting sample to a sharp tip. In addition to topographical mapping of the surface on the atomic scale, low-temperature operation allows the density of states to be locally resolved. We have an ongoing interest in applying STM to study nanoscale electronic structure in semiconducting and superconducting systems.

Our system features a processing chamber directly connected to the cryostat with STM. Processing includes evaporation, sputtering and ion milling in UHV conditions. Our STM design is based on the Beetle style, shown schematically above. It is thermally compensated and highly mechanically stable, while allowing for three-dimensional sample coarse positioning.