Aharonov-Bohm effect

Aharonov-Bohm effect [‚ä·hä′rō‚nȯf ′bām i‚fekt]

(quantum mechanics)

An effect manifested when a beam of electrons is split into two beams that travel in opposite directions around a region containing magnetic flux and are then recombined, whereby the intensity of the resulting beam oscillates periodically as the enclosed magnetic field is changed.

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Aharonov-Bohm effect

The predicted effect of an electromagnetic vector or scalar potential in electronic interference phenomena, in the absence of electric or magnetic fields on the electrons.

The fundamental equations of motion for a charged object are usually expressed in terms of the magnetic field and the electric field. The force on a charged particle can be conveniently written as in the equations below, where q is the

particle's charge, is its velocity, and the symbol × represents the vector product. Associated with is a scalar potential V defined at any point as the work W necessary to move a charge from minus infinity to that point, V = W/q. Generally, only the difference in potentials between two points matters in classical physics, and this potential difference can be used in computing the electric field. Similarly, associated with is a vector potential, a convenient mathematical aid for calculating the magnetic field. See Electric field, Potentials

In quantum mechanics, however, the basic equations that describe the motion of all objects contain and V directly, and they cannot be simply eliminated. Nonetheless, it was initially believed that these potentials had no independent significance. In 1959, Y. Aharonov and D. Bohm discovered that both the scalar and vector potentials should play a major role in quantum mechanics. They proposed two electron interference experiments in which some of the electron properties would be sensitive to changes of or V, even when there were no electric or magnetic fields present on the charged particles. The absence of and means that classically there are no forces acting on the particles, but quantum-mechanically it is still possible to change the properties of the electron. These counterintuitive predictions are known as the Aharonov-Bohm effect.

Surprisingly, the Aharonov-Bohm effect plays an important role in understanding the properties of electrical circuits whose wires or transistors are smaller than a few micrometers. The electrical resistance in a wire loop oscillates periodically as the magnetic flux threading the loop is increased, with a period of h/e (where h is Planck's constant and e is the charge of the electron), the normal-metal flux quantum. In single wires, the electrical resistance fluctuates randomly as a function of magnetic flux. Both these observations, which were made possible by advances in the technology for fabricating small samples, reflect an Aharonov-Bohm effect. They have opened up a new field of condensed-matter physics because they are a signature that the electrical properties are dominated by quantum-mechanical behavior of the electrons, and that the rules of the classical physics are no longer operative. See Quantum mechanics