Tunneling

Tunneling is the weird phenomenon that results in particles having less energy than their potential energy - ie they have negative kinetic energy. That's strange I hear you say, and indeed it is. As with all these things classical mechanics says it's impossible, so you need to look to the murky world of quantum theory to understand it.

 Tunnelling is a consequence of the continuity of wavefunctions at barriers. A wavefunction that is not zero at the inside edge of the barrier decays towards zero inside the barrier. If the amplitude has not reached zero at the outer edge of the barrier it will stop decaying and resume the oscillation it had on the other side of the barrier (but with a smaller amplitude)
 Once the concept of tunneling was understood, people (IBM to be precise) used this phenomenon to develop a microscope that can be used to picture or even manipulate individual atoms. The Scanning Tunneling Microscope uses the fact the when fine point is very close to a surface, electrons can 'tunnel' accross the gap. This flow of electrons causes a measurable current which can then be used to determin the distance of the tip from the surface. The following extract is from the IBM website and it explains how the fantastic images (also stolen from the IBM website!) were created.

• A tip is scanned over a surface at a distance of a few atomic diameters in a point-by-point and line-by-line fashion. At each point the tunneling current between the tip and the surface is measured. The tunneling current decreases exponentially with increasing distance and thus, through the use of a feedback loop, the vertical position of the tip can be adjusted to a constant distance from the surface.

• The amount of these adjustments is recorded and defines a grid of values which can be displayed as a grayscale image.

• Instead of assigning the values to a color we can also use them to deform the grid in the direction perpendicular to the surface.

• Now we can bring back the grayscale and paint each square according to an average of the four defining grid points.

• graph taken from: Physical Chemistry, Peter Atkins, 6th Ed, OUP (1998)