Traffic jam in a mountain pass

Electrons flowing through nanowires of some one hundred atoms wide do not behave as predictably as scientists supposed. They can cause a traffic jam in the wire. With this discovery, to be published in the journal Nature on 5 September and online on 29 August, University of Groningen physicists have solved a 20-year-old mystery.

It all started in 1988. The Dutch physicist Bart van Wees, who has since moved to the University of Groningen, where he is a professor of applied physics at the Zernike Institute of Advanced Materials, discovered a strange phenomenon in the conductivity of nanowires. When he made these wires wider, the conductivity didn’t increase proportionally, but in steps. Van Wees discovered that quantum processes governed the conductivity and he could describe this in an elegant formula.

But in the first stage, with the thinnest wires, a tiny deviation was found in the step-by-step increase, a small peak that shouldn’t be there. At first, it was brushed aside as some sort of error in the measurements, but the error turned out to be persistent and repeatable. In 1995, physicists came to the inevitable conclusion that it wasn’t an error – there was some as yet unexplained physics going on in these nanowires. The phenomenon was even given a name: Zero Bias Anomaly (ZBA).

This kept experimental and theoretical scientists at work. Hundreds of scientific papers have been published but no one could come up with a full explanation. Several years ago, a PhD student in the lab of University of Groningen professor Caspar van der Wal also made some nanowires. His measurements showed the enigmatic ZBA. ‘But we also saw some other interesting trends in our data’, Van der Wal recounts. So he started a separate PhD project to investigate the mystery that had kept many of his colleagues occupied for so long.

A PhD student, Javaid Iqbal from Pakistan, made a great many nanowires from very pure materials and did measurements near absolute zero (at 50 milliKelvin). This way, he obtained very accurate results.

The nanowires, by the way, are not really wires with a conductive core in an insulating layer. The ‘wire’ takes shape between two electrodes that create what is called saddle potential, a field that is sensed by passing electrodes and makes them ‘feel’ as if they are going through a mountain pass. On both sides, the pass is closed off by a cliff face and the electrodes have to pass through this narrow channel.

Iqbal’s work produced the enigmatic rogue peak, but when he increased the voltage in the wire, a second peak occurred. ‘Others have seen a double peak, but they thought this meant their nanowire device was malfunctioning’, Van der Wal explains.

But the very accurate measurements in his lab proved that this wasn’t the case. The double peak was real. In addition, Iqbal could make the wires longer (by putting a series of electrodes in line so that he could elongate his ‘mountain pass’ at will) and the number of peaks he observed increased with the length of the wire.

To explain these results, Van der Wal called in the help of some theoretical physicists who had done extensive work on the ZBA, particularly a group from Israel who had previously predicted the occurrence of double peaks at higher voltages. ‘But their theory didn’t explain why the number of peaks would increase with the length of the nanowire.’ Together with colleagues from Spain and Germany, an explanation for all the data was worked out. It boils down to this: an electron (or more than one) gets stuck in the mountain pass.

Why does it get stuck? The electrons moving through the nanowire behave like waves. These waves bounce off the steep ascent of the pass, or against the side walls. But they also sense each other’s presence. This way, a complicated combined effect takes place inside the pass, where different physical phenomena affect the electrons. ‘We call this “many body physics”, a complicated process that cannot be described by one simple formula.’

The net result of the combined effect is that an electron gets stuck, partially blocking the pass. And this blockade is what causes the peaks in the experiments. In longer wires, two or more electrons may get stuck in the (longer) pass, causing two or more peaks.

So after twenty years, there finally seems to be an explanation for the enigmatic peaks. But how important is it? Very, explains Van der Wal. ‘The behaviour of electrons in this type of very narrow wire is much more complex than we anticipated.’ The traffic jams in the pass can actually change the characteristics of passing electrons, such as the spin (a gyroscope-like rotation). And these nanowires are being used on a large scale in research. They form part of what’s called a Quantum Dot , a device that is used as a ‘bit’ in the building of quantum computers. In all applications using nanowires, physicists have to take into account the effect the wires can have on the electrons.

The discovery was published on the Nature website on 28 August and will appear in print on 5 September. Along with the paper by the Van der Wal group, another paper describing the same peaks is being published, with largely the same conclusions.

The fact that Van der Wal has solved a mystery which started with a discovery by his colleague Bart van Wees is a coincidence. ‘In fact, my work is concerned with totally different systems. But it was Van Wees who took me on here in Groningen.’ Van der Wal works mainly on optical nanodevices. ‘But I may do some further work on the nanowires.’ For although the mystery is now solved, there are still some details that need sorting out. ‘And that will lead to some very interesting discussions’, says Van der Wal with enthusiasm.

See also a News & Views commentary on this study in Nature Physics.

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Reference

Odd and even Kondo effects from emergent localization in quantum point contacts, Nature, online 29 Augustus 2013, in print on 5 September.

M. J. Iqbal¹, Roi Levy², E. J. Koop¹, J. B. Dekker¹, J. P. de Jong¹, J. H. M. van der Velde¹, D. Reuter³, A. D. Wieck³, R. Aguado⁴, Yigal Meir^2,5 and C. H. van derWal^1,

DOI:10.1038/nature12491

1 Zernike Institute for Advanced Materials, University of Groningen, NL-9747AG Groningen, The Netherlands.

2 Department of Physics, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel.

3 Angewandte Festkörperphysik, Ruhr-Universität Bochum, D-44780 Bochum, Germany.

4 Instituto de Ciencia de Materiales de Madrid (ICMM), Consejo Superior de Investigaciones Científicas (CSIC), Sor Juana Ines de la Cruz 3, 28049 Madrid, Spain.

5 Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel.