Noise measurements show quantum effects in conduction at room temperature
For over 90 years, physicists have known that there are consequences to the fact that electrical charge is not continuous, but instead comes in quanta called electrons, each with a unit of charge. When charge is flowing in an electrical circuit, the average current alone does not convey all available information about the conduction process. Instead, the fluctuations around the average, called the current noise, can tell much more. For example, if electrons came through a circuit precisely at regular time intervals, there would be no fluctuations – no noise. In contrast, if electrons came through in big bunches, there would be large fluctuations – enhanced noise, compared to the case of randomly arriving electrons. This noise, due to the discrete charge of the electron, is often called shot noise.At the nanometer scale, the quantum mechanical character of electrons becomes important. If two pieces of metal are touching at the level of only a few atoms, then there are only a few “channels” available to the wave-like electrons to get from one side of the junction to the other. One prediction based on this idea is that the electrical conductance (current divided by voltage) should preferentially take on certain values in simple metal junctions, integer multiples of the “quantum of conductance”, 2e2/h, where e is the electronic charge and h is Planck’s constant. A second prediction is that the current noise should be suppressed at those conductance values. In the past this has been observed by others in metal junctions and semiconductor point contacts at temperatures near absolute zero.In a recent paper from the Natelson lab, graduate student Patrick Wheeler and collaborators were able to use radio-frequency measurement techniques to observe both conductance quantization and suppression of the current noise at room temperature in atomic-scale gold junctions. The junctions were made using an apparatus originally built by undergraduate researchers. The figure at left shows a diagram of the apparatus, where the two ends of a notched wire may be brought in and out of contact with atomic precision. At right, the blue curve is a histogram of the conductance values that are observed as the junction is made and broken thousands of times. The histogram shows prominent peaks near integer multiples of the quantum of conductance. The red curve shows the simultaneously measured noise, with suppressions apparent near the special conductance values.These measurements demonstrate clearly that quantum effects can be seen in electrical properties when matter is examined at the nanometer scale, even at room temperature. Careful study of the noise and its dependence on the current itself will lead to new insights into how heat is generated and dissipated at the nanoscale, a topic of critical interest as electronic devices continue to be reduced in size toward the atomic limit.
Reference: P. J. Wheeler, J. N. Russom, K. Evans, N. S. King, and D. Natelson, “Shot noise suppression at room temperature in atomic-scale Au junctions”, Nano Letters 10, in press (2010).http://pubs.acs.org/doi/abs/10.1021/nl904052r