The twang of a nano-size guitar string shows the graininess of nature, says S.Ananthanarayanan.

The Greeks thought matter must be made of atoms because they had to set a limit to how small things could get. The idea of infinite smallness presented a philosophical problem. If a distance could be divided without end, its length would consist of an infinite number of rapidly shrinking portions to be covered. It was then a wonder how we ever got from one place to another. As algebra and the nature of converging series, which give us the answer, had not been developed, the Greeks said there was a limit to how far things could be divided. They must, then, reduce to something indivisible, the atoms. What we have now discovered as atoms do have a structure and consists of smaller parts. But it turns out the Greeks were right, it is of the so called elementary particles that atoms, and hence all matter, does consist.

The maverick Nobel laureate, Richard Feynman, had an instructive angle to a thought experiment about very small particles. We now know that gases consist of molecules, sub-microscopic particles, in constant motion. It is because the particles are so small, and so many, that the pressure that their motion exerts of the walls of a container is not staccato, but constant. And this is why the pressure is the same all over the volume of gas in a container.

The thought experiment Feynman spoke of is called Maxwell’s demon, named after James Clerk Maxwell, its creator. The demon is imagined as a creature, about the size of the molecules of gas, controlling a trapdoor in the wall of a gas container. Now, as the demon can see the molecules, he can watch for each of them, as they approach the container wall, and open the trap door just when a molecules is headed straight for it. In this way, the demon can allow the fastest molecules to escape from the container, and the pressure and temperature within the container would steadily fall, although there was no refrigerator or exhauster at work!

Feynman shot down the idea of such a demon (which Maxwell did too) by considering what the effect of bombardment by high speed projectiles would be on the demon and the components of the trap door. Feynman quickly calculates that we cannot imagine that the dexterity of an agent operating such a mechanism in normal conditions would be the same in the very small world.

Yutian Wen, N. Ares, F. J. Schupp, T. Pei, G. A. D. Briggs?and E. A. Laird, at the Universities of Oxford and Lancaster write in the journal, Nature Physics, of an experiment that brings into view the identity of individual electrons that make up an electric current. A nano-tube of carbon, an electrical conductor of atom-sized dimensions, is set humming like a guitar string when a current is passed, an effect that has been shown to arise from the energy imparted to the carbon tube by staccato passage of electrons from the electrical connections of the nano-tube.

The paper in Nature Physics says that evidence of individual electrons has been observed in mechanical arrangements of nanometer dimensions, but the electrons disturb the delicate components and make it difficult to carry out measurements. In the current work, the authors used a carbon nano-tube, 800 nanometers, or less than a micron in length, and they show that the passage of single electrons through the filament causes a sustained mechanical vibration, like a guitar string that has been plucked, And the vibration has the features of a laser, but result is mechanical vibration rather than emission of light.

A nano-tube of carbon is suitable for such work because of the properties of the carbon atom. It is the tendency for atoms to seek a stable state with eight electrons in their outer orbit. They hence combine with other atoms and share electrons to make up the number. The carbon atom has six electrons, and four of them are in the outer orbit. As this is half-way to eight electrons, carbon is able to form a variety of chemical bonds. And in a network of carbon atoms, each one can hand-hold with three or four other carbon atoms, to form structures of great mechanical strength. And one such structure is that of the nano-tube, a microscopic connector, channel, with optical, electrical and mechanical properties that can be adapted for use in electronic microchips

The group in Oxford and Lancaster suspended the carbon nano-tube with electrical connectors at either end, so that a current could pass through. While the entry of single electrons affects the nano-tube physically, physical changes in the nano-tube would affect the current through the junctions at the ends. The result is that if the electrical and mechanical effects could find a match, one could feed the other, for sustained oscillation, the paper says.

Suitable electrical voltages, including a high-frequency, oscillating signal, were hence applied to the apparatus, at the supports called the ‘source’ and the ‘drain’. As thermal disturbances would obscure the effects being looked for, the apparatus was cooled practically down to – 273°C, or absolute zero. Mechanical movements of the nano-tube were detected using the effect such motion had on the electrical currents flowing. And it was found that when the oscillating signal matched mechanical frequency, there was a peak in the level of vibration.

The remarkable feature noticed was that even there was no oscillating signal applied, but only a steady voltage, the nano-tube was in vibration. And this vibration was the strongest near about the frequency of resonance when the oscillating signal was present. This vibration was apparently the result of the disturbance caused by single electrons crossing into the nano-tube, and in the absence of the oscillating signal, these disturbances were not regular, but staccato.

In the case of a laser, the medium collects packets of incoming energy and further packets of energy can induce or stimulate emission of more packets, in a coherent fashion. In the case of the carbon nano-tube, there was no ‘stimulated emission’, but the vibration was ‘self-excited’ and a result of the irregular arrival of electrons, modulated by the effect of the nano-tube deformations at the junctions.

The frequency at which the nano-tube vibrates was found to be 231 million cycles per second. The frequency standard for tuning instruments in Western music is the ‘middle A’, which is 440 cycles per second. The table shows the frequencies of the overtones, or sounds with doubles frequencies. We can see that the nano-tube vibrates at the 20th overtone of middle A. This is above the limit of our hearing, which is between the 5th and 6th overtones. Or even of animals and insects, which can hear till a little over the 8th overtone.

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