Nature publication: how one electron beats a billion electron pairs

The team of scientists from TU Delft, Philips and TASC has succeeded to bridge two superconducting contacts with a nanoscale wire of semiconductor material. A tiny section of the wire (about 70 nm long) functions as a box which confines a small and controllable number of electrons, called a quantum dot. Van Dam and co-workers demonstrate that a supercurrent – an electrical current without resistance – can flow through such a semiconductor quantum dot. They also demonstrate that the direction of the supercurrent can be reversed by adding just a single electron to the quantum dot.

In superconductors, electrons form strongly bound pairs due to the existence of an attractive interaction. These so-called Cooper pairs can flow through the superconductor without resistance, giving rise to a supercurrent. In the sixties it was shown that when two superconductors are linked together – e.g. by a thin layer of non-superconducting metal or insulating oxide – Cooper  pairs can pass from one superconductor to the other enabling a zero-resistance supercurrent through the non-superconducting link.

In the experiment reported by Van Dam and collaborators, the observed supercurrent through the semiconductor quantum dot is different from supercurrents through other kinds of linking materials. Instead of being transported as Cooper pairs, the two electrons of each pair split up and flow one-by-one through the quantum dot (see Fig. 1). If this sequential transport is sufficiently fast the broken pair of electrons can form a pair again on the other side of the quantum dot, resulting in a supercurrent. This one-by-one transport is a direct consequence of the strong electrostatic repulsion between electrons, which have the same (negative) charge. To minimize the energy cost due to their electrostatic repulsion, the electrons of a Cooper pair find it more convenient to pass through the small quantum dot one-by-one rather than simultaneously.

The team of scientists has demonstrated that the supercurrent flow – a billion Cooper pairs per second – can be reversed by adding only one electron to the quantum dot. This striking effect is due to the quantum mechanical nature of the quantum dot. Similar to electrons in atoms, the electrons of a quantum dot move along closed orbits of well-defined energy – for this reason quantum dots are often referred to as ‘artificial atoms’. These orbits provide the path for electrons going from one side of the quantum dot to the other. Interestingly, the quantum mechanical nature of the orbits used by the electrons to cross the dot determines the direction of the supercurrent. Adding an extra electron to the quantum dot changes the nature of the orbits involved in the electron transport leading to supercurrent reversal.

Until now, supercurrents through semiconductor quantum dots had not been observed. This was mainly due to the lack of a controllable connection between quantum dots and superconducting contacts. The team of scientists solved this problem by using indium arsenide semiconductor nanowires (diameter ~ 60 nm) combined with two gate electrodes, namely two thin metal lines deposited on top of the nanowires and electrically connected to external batteries with tunable voltages (see Fig. 2). The two gate electrodes confine electrons in a small nanowire section, resulting in the formation of the quantum dot. Changing the voltage on these gates enables control of the number of confined electrons and of the coupling between the quantum dot and the superconducting contacts.

This research is part of the Dutch FOM concentration group ''Solid state quantum information processing'' at the Kavli Institute of Nanoscience Delft. Funding was also obtained from the European Union and the Japanese International Cooperative Research Project (ICORP).

Figure 1:

Schematic representation of Cooper pair transport through a semiconductor quantum dot. The quantum dot is defined in a semiconductor nanowire (blue) where Cooper pairs are formed due to the attached superconducting contacts (light gray). Owing to the repulsive electrostatic interaction, Cooper pairs break and electrons are transported one-by-one through the quantum dot. After passing the dot, a Cooper pair is formed again in the nanowire section on the right hand side of the quantum dot.

Figure 2:

Colored scanning electron microscope image of the superconducting device.

The nanowire (blue, diameter ~ 60 nanometer) is connected to wide superconducting contacts (light gray). The quantum dot is defined in the nanowire by applying negative voltages to the two closely-spaced narrow gates (gray).

Contact:

Dr. Jorden van Dam: +31 15 2786085, j.a.vandam@tudelft.nl

Science information officer Ineke Boneschansker, telephone number: 015 278 8499 or 06 140 151 19, e-mail: i.boneschansker@tudelft.nl

See also the press release of June 12^th 2006.