The results achieved by this research team headed by Prof. Kurt Westerholt and Prof. Hartmut Zabel (Department of Physics and Astronomy at RUB) could contribute to new, power saving components in the future. The researchers reported on their findings in the American Physical Society's noted journal "The PhysicalReview".
Electron pairs in singlet state
If it were possible to eliminate electrical resistance we could reduce our electric bill significantly andmake a significant contribution to solving the energy problem, if it were not for a few otherproblems. Many metals as well as oxides demonstrate a superconductive state, however only at lowtemperatures. The superconductive effect results from Cooper pairs that migrate through the metaltogether "without resistance". The electrons in each Cooper pair are arranged so that theircomposite angular momentum is zero. Each electron has an angular momentum, the so-called spin,with a value of 1/2. When one electron spins counterclockwise (-1/2) and the other clockwise (+1/2),the total of the two spin values is zero. This effect, found only in superconductors, is called thesinglet state.
Superconductive Cooper pairs
If a superconductor is brought into contact with a ferromagnetic material, the Cooper pairs arebroken up along the shortest path and the superconductor becomes a normal conductor. Cooperpairs cannot continue to exist in a singlet state in a ferromagnetic material. Researches at RUB (Prof.Konstantin Efetov, Solid State Physics) among others have, however, theoretically predicted a newtype of Cooper pair, which has a better chance of survival in ferromagnetic materials. In such Cooperpairs the electrons spin in parallel with one another so that they have a finite spin with a value of 1.Since this angular momentum can have three orientations in space, it is also known as the tripletstate. "Obviously there can also be only one certain, small fraction of Cooper pairs in a triplet state,which then quickly revert to the singlet state" explained Prof. Kurt Westerholt. "The challenge was toverify these triplet Cooper pairs experimentally".
Tunnel current from Cooper pairs
Superconductors allow us to produce highly sensitive detectors for magnetic fields, which even allowdetection of magnetic fields resulting from brain waves. These detectors are called SQUID's(superconducting quantum interference devices) – components which use the superconductivequantum properties. The central feature in these components consists of so-called tunnel barrierswith a series of layers made up of a superconductor, insulator and another superconductor.Quantum mechanics allows a Cooper pair to be "tunneled" through a very thin insulating layer.Tunneling of a large number of Cooper pairs results in a tunnel current. "Naturally the barrier cannotbe too thick, otherwise the tunnel current subsides. A thickness of one to two nanometers is ideal",according to Prof. Hermann Kohlstedt (CAU).
Double success in Bochum und Kiel
If part of the tunnel barrier is replaced by a ferromagnetic layer, the Cooper pairs are broken upwhile they are still in the barrier and do not reach the superconductor on the other side. The tunnelcurrent decreases drastically. "Triplet Cooper pairs can, however, be tunneled much better throughsuch a ferromagnetic barrier", says Dirk Sprungmann, who was involved as Ph.D. student. If we aresuccessful in converting a portion of the singlet Cooper pairs to triplet Cooper pairs, the tunnelcurrent should be significantly stronger and be able to pass through a thicker ferromagnetic layer.This is precisely what the physicists in Bochum and Kiel tested. They allowed the Cooper pairs to passthrough ferromagnetic barriers with thicknesses of up to 10 nanometers. With this attempt thephysicists achieved a double success. On the one hand they were able to experimentally verify theexistence of triplet Cooper pairs, and, on the other, they demonstrated that the tunnel current isgreater than for singlet Cooper pairs in conventional tunnel contacts. "These new ferromagnetictunnel barriers may possibly be used for new types of components", states Dr. Martin Weides (SantaBarbara). With their research findings the scientists confirmed, among other things, the theoreticalwork of a Norwegian research team published only a few weeks before.
Source: Ruhr-University Bochum