"Mating behavior" of electrons revealed

Triplet superconductivity first detected experimentally

The picture shows the top view of a finished tunnel element and below the schematic structure of the layer sequences inside the component. The superconducting layers are marked in gray and consist of the metal niobium. Between the superconducting layers are normally conducting and ferromagnetic barriers, through which the Cooper pairs must tunnel through. Crucial is the ferromagnetic alloy layer (Heusler), which only lets through the triplet-Cooper pairs. © RUB
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A breakthrough in quantum physics has now been achieved by an international research team: they have investigated the "mating behavior" of electrons and were able to prove for the first time that there are electron pairs, so-called Cooper pairs, which have a parallel orientation of their angular momentum - spins.

The experimental findings of the researchers led by Professor Kurt Westerholt and Professor Hartmut Zabel of the Ruhr-Universität Bochum (RUB) as well as the University of Kiel and the University of Santa Barbara in the USA could contribute in the future to produce new and energy-efficient components. The researchers report on their findings in the journal "The Physical Review" of the American Physical Society.

Electron pairs in the singlet state

Without electrical resistance, we could save a lot on the utility bill and make a vital contribution to the energy problem, if there were not a few more problems. Many metals and also oxides show a superconducting state, but only at low temperatures. The superconducting effect is created by Cooper pairs, which move through the metal together and "without resistance".

In each Cooper pair, the electrons are arranged so that their total angular momentum becomes zero. Every electron has an angular momentum, the so-called spin, with the value 1/2. If one electron rotates to the left (-1/2) and the other to the right (+1/2), then the sum of both angular momenta is zero. This effect, which is found only in superconductors, is called singlet state.

Superconducting Cooper pairs

If you bring superconductors into contact with ferromagnets, then the Cooper pairs are broken up on the shortest distance and the superconductor becomes the normal conductor, Cooper pairs in a singlet state can not survive in a ferromagnet. However, researchers from the RUB, among others, have predicted theoretically a new type of Cooper pair that has a better chance of survival in ferromagnets. In these Cooper pairs, the spins are aligned in parallel and thus have a finite angular momentum of value 1. Since this angular momentum can have three orientations in space, it is also called a triplet state. display

"Obviously, there is always a certain small percentage of Cooper pairs that are in the triplet state but quickly switch back to the singlet state, " explains Westerholt. "The challenge was to experimentally prove these triplet-Cooper pairs."

Tunnel current from Cooper pairs

Superconductors can be used to produce highly sensitive magnetic field detectors that can even detect magnetic fields originating from brain waves. These detectors are called SQUID - superconducting quantum interference device -, which are devices that use superconducting quantum properties. The central part in these components are so-called tunnel barriers, consisting of a sequence of superconducting layer, insulator and again superconducting layer.

A Cooper pair can then "tunnel" through a very thin insulator layer. Tunnel many Cooper pairs, then they form a tunnel current. Of course you must not make the barrier too thick, otherwise the tunnel current will dry up. One to two nanometers in thickness are ideal, according to Professor Hermann Kohlstedt of the University of Kiel.

Double success in Bochum and Kiel

If you replace a part of the tunnel barrier with a ferromagnetic layer, the Cooper pairs are broken up in the barrier and do not reach the superconductor on the other side. The tunnel current decreases drastically. However, triplet Cooper pairs tunnel much better through the ferromagnetic barrier, according to Dirk Sprungmann, who was also involved in this study. If one succeeds in converting part of the singlet Cooper pairs into triplet Cooper pairs, then the tunneling current should be much stronger and pass through a thicker ferromagnetic layer.

That's exactly what the physicists tested in Bochum and Kiel. They ran the Cooper pairs through ferromagnetic barriers that were up to ten nanometers thick. With this the physicists have achieved a double success. On the one hand, they were able to experimentally prove that triplet-Cooper pairs exist. On the other hand, they were able to show that the tunneling current is larger than for singlet-Cooper pairs in conventional tunnel junctions.

New ferromagnetic tunnel barriers

"These new ferromagnetic tunneling barriers may possibly be used for novel devices, " says Martin Weides of the University of Santa Barbara. Among other things, the scientists' research confirmed the theoretical work of a Norwegian research team published just a few weeks earlier.

(Ruhr-University Bochum, 02.12.2010 - DLO)