Recent science magazine covers have taken to displaying dazzling images of the surfaces of metals and semiconductors. These come from scanning tunneling microscopes, which let scientists look at images of individual atoms and even smaller features.
The operating principle of the scanning tunneling microscope is radically different from other microscopes. It makes use of the fact that solids are covered with a microscopic “atmosphere” of electrons. The instrument lowers a tiny metal needle to within billionth of an inch of the surface, and the “atmospheres” of the solid and needle overlap.
When this happens, electrons tunnel between the needle and the surface, producing a minute electric current. By moving the needle across the surface while simultaneously raising and lowering it to keep the current constant, the tip accurately traces contours as small as a fraction of an atomic diameter.
Because the scanning tunneling microscope depends on a tunneling current, it...