Electron 'pairing': Triplet superconductivity proven experientially for first time

December 01, 2010

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 Review".

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: hartmut.zabel@rub.de. Kurt.westerholt@rub.de

Prof. Dr. Hermann Kohlstedt, Nanoelektronik, Technische Fakultät Kiel, Christian-Albrechts- Universität Kiel, hko@tf.uni-kiel.de, +49 (0)431/880-6075

Editor: Jens Wylkop

Ruhr-University Bochum

Related Magnetic Fields Articles from Brightsurf:

Physicists circumvent centuries-old theory to cancel magnetic fields
A team of scientists including two physicists at the University of Sussex has found a way to circumvent a 178-year old theory which means they can effectively cancel magnetic fields at a distance.

Magnetic fields on the moon are the remnant of an ancient core dynamo
An international simulation study by scientists from the US, Australia, and Germany, shows that alternative explanatory models such as asteroid impacts do not generate sufficiently large magnetic fields.

Modelling extreme magnetic fields and temperature variation on distant stars
New research is helping to explain one of the big questions that has perplexed astrophysicists for the past 30 years - what causes the changing brightness of distant stars called magnetars.

Could megatesla magnetic fields be realized on Earth?
A team of researchers led by Osaka University discovered a novel mechanism called a ''microtube implosion,'' demonstrating the generation of megatesla-order magnetic fields, which is three orders of magnitude higher than those ever experimentally achieved.

Superconductors are super resilient to magnetic fields
A Professor at the University of Tsukuba provides a new theoretical mechanism that explains the ability of superconductive materials to bounce back from being exposed to a magnetic field.

A tiny instrument to measure the faintest magnetic fields
Physicists at the University of Basel have developed a minuscule instrument able to detect extremely faint magnetic fields.

Graphene sensors find subtleties in magnetic fields
Cornell researchers used an ultrathin graphene ''sandwich'' to create a tiny magnetic field sensor that can operate over a greater temperature range than previous sensors, while also detecting miniscule changes in magnetic fields that might otherwise get lost within a larger magnetic background.

Twisting magnetic fields for extreme plasma compression
A new spin on the magnetic compression of plasmas could improve materials science, nuclear fusion research, X-ray generation and laboratory astrophysics, research led by the University of Michigan suggests.

How magnetic fields and 3D printers will create the pills of tomorrow
Doctors could soon be administering an entire course of treatment for life-threatening conditions with a 3D printed capsule controlled by magnetic fields thanks to advances made by University of Sussex researchers.

Researchers develop ultra-sensitive device for detecting magnetic fields
The new magnetic sensor is inexpensive to make, works on minimal power and is 20 times more sensitive than many traditional sensors.

Read More: Magnetic Fields News and Magnetic Fields Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.