Atomic world > Scanning Tunneling Microscope(STM) > Tunneling effect

 Fig. 2 Classical mechanics governs the motions we see in daily life.

If you throw a plastic ball towards a concrete wall, it will bounce back because it cannot penetrate through the wall (Fig. 2). The motions we see in daily life are governed by the principles of mechanics which were discovered by Isaac Newton hundreds of years ago. These principles are collectively known as classical mechanics.

In the early 20th century, however, physicists found that classical mechanics does not apply to the atomic world. When the size of an object is very small, its motion is governed by another set of principles called quantum mechanics. In the quantum world, things are completely different. Matter shows both particle-like properties and wave-like properties. This is known as wave-particle duality.

For example, we usually think of electron as a particle. But when a large number of electrons hit a thin barrier, quantum mechanics predicts that the electrons will behave more like a wave. Part of the electrons still bounces back, but strangely enough, a certain number of electrons do pass through the barrier (Fig. 3)! This is analogy to water waves tunneling through a dike (Fig. 4). This strange effect is called electron tunneling.

 Fig. 3 When a large number of electrons hit a thin barrier, some are able to pass through even though this is not allowed in classical mechanics. Fig. 4 Analogy of electron tunneling: water tunneling through a dike.

The motion of electrons produces an electric current. Therefore, when electrons tunnel through a barrier, we are able to detect a current behind the barrier. This current is called the tunneling current. The tunneling current I decreases exponentially with the thickness d of the barrier:

I = a e-2 Kd

 Fig. 5 An example of the dependence of tunneling current on the thickness of barrier. The shaded region indicates that the current decreases by more than 7 times within 1 nm.

where a and K are constants that depend on the properties of the barrier, the energy of the electron beam, etc. Fig. 5 shows an example of a = 2.6 nA (nanoamperes, 1 nA = 10-9 A) and K = 1.02 × 109 m-1. We see that the current is very sensitive to the thickness of wall. A slight increase in thickness reduces the current significantly.