Triplet Superconductivity Proven For First Time

I suspect that anything that combines superconductivity and ferromagneticscould be important.  In the meantime wehave confirmed the existence of triplet superconductivity

There is no indication of how all this might be convincingly applied asyet, but it certainly looks promising. We will have to wait on this.

The triplet state is formed when the spinning pairs line up in parallelrather than in line making it easier to ‘tunnel’ across the ferromagneticbarrier.

Triplet Superconductivity Proven ForFirst Time

by StaffWriters

Bochum, Germany (SPX) Dec 02, 2010

The electrons in each Cooper pair are arranged so thattheir composite angular momentum is zero. Each electron has an angularmomentum, the so-called spin, with a value of 1/2.

The results achieved by this research team headed by Prof. Kurt Westerholt andProf. Hartmut Zabel (Department of Physics and Astronomy at RUB) could contribute to new, powersaving components in the future. The researchers reported on their findings inthe American Physical Society's noted journal "The Physical Review".

If it were possible toeliminate electrical resistance we could reduce our electric bill significantlyand make a significant contribution to solving the energy problem, if it werenot for a few other problems. Many metals as well as oxides demonstrate asuperconductive state, however only at low temperatures.

The superconductiveeffect results from Cooper pairs that migrate through the metal together"without resistance". The electrons in each Cooper pair are arrangedso that their composite angular momentum is zero. Each electron has an angularmomentum, the so-called spin, with a value of 1/2.

When one electronspins counterclockwise (-1/2) and the other clockwise (+1/2), the total of thetwo spin values is zero. This effect, found only in superconductors, is calledthe singlet state.

Superconductive Cooper pairs

If a superconductor is brought into contactwith a ferromagnetic material, the Cooper pairs are broken up along theshortest path and the superconductor becomes a normal conductor.

Cooper pairs cannotcontinue to exist in a singlet state in a ferromagnetic material. Researches atRUB (Prof. Konstantin Efetov, Solid State Physics) amongothers have, however, theoretically predicted a new type of Cooper pair, whichhas a better chance of survival in ferromagnetic materials.
In such Cooper pairsthe electrons spin in parallel with one another so that they have a finite spinwith a value of 1. Since this angular momentum can have three orientations inspace, it is also known as the triplet state.

"Obviously therecan also be only one certain, small fraction of Cooper pairs in a tripletstate, which then quickly revert to the singlet state" explained Prof.Kurt Westerholt. "The challenge was to verify these triplet Cooper pairsexperimentally".

Tunnel current from Cooper pairs

Superconductors allow us to produce highlysensitive detectors for magnetic fields, which even allow detection of magneticfields resulting from brain waves. These detectors are called SQUID's (superconductingquantum interference devices) - components which use the superconductivequantum properties.

The central feature inthese components consists of so-called tunnel barriers with a series of layersmade up of a superconductor, insulator and another superconductor.

Quantum mechanicsallows a Cooper pair to be "tunneled" through a very thin insulatinglayer. Tunneling of a large number of Cooper pairs results in a tunnel current."Naturally the barrier cannot be too thick, otherwise the tunnel currentsubsides. A thickness of one to two nanometers is ideal", according toProf. Hermann Kohlstedt (CAU).

Double success in Bochum und Kiel

If part of the tunnel barrier is replaced by aferromagnetic layer, the Cooper pairs are broken up while they are still in thebarrier and do not reach the superconductor on the other side. The tunnelcurrent decreases drastically.

"Triplet Cooperpairs can, however, be tunneled much better through such a ferromagneticbarrier", says Dirk Sprungmann, who was involved as Ph.D. student. If weare successful in converting a portion of the singlet Cooper pairs to tripletCooper pairs, the tunnel current should be significantly stronger and be ableto pass through a thicker ferromagnetic layer.

This is precisely whatthe physicists in Bochum and Kiel tested. They allowed the Cooper pairs topass through ferromagnetic barriers with thicknesses of up to 10 nanometers.

With this attempt thephysicists achieved a double success. On the one hand they were able toexperimentally verify the existence of triplet Cooper pairs, and, on the other,they demonstrated that the tunnel current is greater than for singlet Cooperpairs in conventional tunnel contacts.

"These newferromagnetic tunnel barriers may possibly be used for new types ofcomponents", states Dr. Martin Weides (Santa Barbara). With their research findingsthe scientists confirmed, among other things, the theoretical work of aNorwegian research team published only a few weeks before.

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

No comments:

Post a Comment