Electron ‘pairing’: Triplet superconductivity proven experientially for first time

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 Physical

Electron pairs in singlet state

If it were possible to eliminate electrical resistance we could reduce our electric bill significantly and
make a significant contribution to solving the energy problem, if it were not for a few other
problems. Many metals as well as oxides demonstrate a superconductive state, however only at low
temperatures. The superconductive effect results from Cooper pairs that migrate through the metal
together “without resistance”. The electrons in each Cooper pair are arranged so that their
composite 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 the
singlet state.

Superconductive Cooper pairs

If a superconductor is brought into contact with a ferromagnetic material, the Cooper pairs are
broken up along the shortest path and the superconductor becomes a normal conductor. Cooper
pairs 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 new
type of Cooper pair, which has a better chance of survival in ferromagnetic materials. In such Cooper
pairs 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 triplet
state. “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 to
verify these triplet Cooper pairs experimentally”.

Tunnel current from Cooper pairs

Superconductors allow us to produce highly sensitive detectors for magnetic fields, which even allow
detection of magnetic fields resulting from brain waves. These detectors are called SQUID’s
(superconducting quantum interference devices) — components which use the superconductive
quantum properties. The central feature in these components consists of so-called tunnel barriers
with 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 cannot
be 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 up
while they are still in the barrier and do not reach the superconductor on the other side. The tunnel
current decreases drastically. “Triplet Cooper pairs can, however, be tunneled much better through
such a ferromagnetic barrier”, says Dirk Sprungmann, who was involved as Ph.D. student. If we are
successful in converting a portion of the singlet Cooper pairs to triplet Cooper pairs, the tunnel
current 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 pass
through ferromagnetic barriers with thicknesses of up to 10 nanometers. With this attempt the
physicists achieved a double success. On the one hand they were able to experimentally verify the
existence of triplet Cooper pairs, and, on the other, they demonstrated that the tunnel current is
greater than for singlet Cooper pairs in conventional tunnel contacts. “These new ferromagnetic
tunnel barriers may possibly be used for new types of components”, states Dr. Martin Weides (Santa
Barbara). With their research findings the scientists confirmed, among other things, the theoretical
work of a Norwegian research team published only a few weeks before.

Title picture

D. Sprungmann, K. Westerholt, H. Zabel, M. Weides, H. Kohlstedt: Evidence of triplet
superconductivity in Josephson junctions with barriers of the ferromagnetic Heusler alloy Cu2MnAl.
Physical Review B 82 (2010), DOI: 10.1103/PhysRevB.82.060505

Further information

Prof. Hartmut Zabel, Prof. Kurt Westerholt, Experimental Physics IV — Solid State Physics, Department
of Physics and Astronomy at RUB, Tel. +49 (0)234/32-23650, -23621, Email: [email protected].
[email protected]

Prof. Dr. Hermann Kohlstedt, Nanoelektronik, Technische Fakultät Kiel, Christian-Albrechts-
Universität Kiel, [email protected], +49 (0)431/880-6075

Editor: Jens Wylkop

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