Magnetism is involved in the mechanism behind high temperature superconductivity

Paul Boughton

Scientists from Queen Mary, University of London and the University of Fribourg (Switzerland) have found evidence that magnetism is involved in the mechanism behind high temperature superconductivity.

Writing in the journal Nature Materials, Dr Alan Drew from Queen Mary's Department of Physics and his colleagues at the University of Fribourg report on the investigation of a new high temperature superconductor, the so-called oxypnictides. They found that these exhibit some striking similarities with the previously known copper-oxide high temperature superconductors - in both cases superconductivity emerges from a magnetic state. Their results go some way to explaining the mechanisms behind high temperature superconductors.

Engineer Live spoke with Alan Drew about his research.

1. What are oxypnictides and what are their properties?

Oxypnictides are a class of materials that include oxygen, a pnictogen (from group-V, especially phosphorus and arsenic) and one or more other elements.

The oxypnictide in the paper (SmFeAsO[1-x]F[x]) is a member of a new type of high temperature superconducting family, the iron based superconductors. They are all based on layers of Iron and Arsenic separated by layers of something else - either metal-oxides (such as Lanthanum or Samarium Oxide) or other metals (such as Barium and Potassium, Calcium and Sodium or Strontium and Samarium).

The main property that has renewed the interest in these materials is that they superconduct at high temperatures - presently up to 56K (-217 Celcius). Whilst this still seems very cold to the casual observer, their cousins (the copper oxide based high temperature superconductors) have a maximum superconducting critical temperature of 138K (-135 Celcius) which is well above the boiling point of Nitrogen. This is very important for applications, as liquifying nitrogen is relatively easy when compared to other gases such as Helium and since it is abundant in the air we breath there is a ready supply.

Even though 56K seems much lower than the present record of 138K, this is the highest temperature that has been observed in a layered copper- free superconductor. The important point here is that the highest temperature superconductor that should exist according to the conventional theory of superconductivity is 39K (and even this requires a special modification to the theory). There is no accepted theory of high temperature superconductivity - just a number of competing proposals that all have problems of one sort or another. In essence, no one really understands why high temperature superconductors superconduct, so having an entirely new family to explore will hopefully increase our understanding of the high temperature superconductors and allow a theory to be established.

2. How do they compare with copper-oxide high temperature superconductors?

As in the copper oxides, the superconducting critical temperature in the pnictides is much higher than conventional theory predicts and the materials have a layered structure. Also, the superconductivity appears upon doping away from a parent material that exhibits antiferromagnetism. These materials are therefore prime candidates for being "unconventional" superconductors, where the mechanism is most likely based on electronic or magnetic correlations rather than on the conventional electron-phonon interaction (a phonon is a vibration of the crystal structure and as far as current understanding goes, can only be responsible for superconductors up to 39K).

Nevertheless, these pnictides also provide several unique aspects which clearly distinguish them from the copper oxides (cuprates). For example the undoped parent compounds are metals (even though fairly poor ones) as opposed to the cuprates which are so-called "Mott- Hubbard" insulators. Furthermore, band structure calculations (where the conducting electrons reside) suggest that several (probably all five) of the subbands are crossing participate in the metallic response. This is in contrast to the cuprates, where due to a strong crystallographic distortion of the copper-oxide octahedra, only a single band is relevant. there also appears to be a sizeable direct electronic orbital overlap, that opens up various possibility for competing kinds of interactions (leading to magnetic frustration and low-energy excitations which may be relevant for the superconductivity).

It is therefore of great importance to obtain further insight into the differences and similarities of the pnictide and cuprates. One of the prominent issues concerns the question of how magnetism and superconductivity evolve upon charge carrier doping in these pnictides and whether they coexist.

3. What did you learn about them in you experiment?

We learned that static (i.e is very slow to change) magnetism and superconductivity coexist over a fairly wide range of charge carrier doping and that dynamic (i.e is fluctuating as a function of time) magnetism coexists over the full range of charge carrier doping.

Since the highest superconducting critical temperature of the new pnictides appears to correspond to the one with the most prominent magnetism, this suggests that the two could be linked.

Understanding the underlying mechanism behind high temperature superconductors will hopefully allow us to design new superconductors that superconduct at much higher temperatures, possibly bringing them into mainstream consumer technologies.

4. Why has it been so difficult to get to the root of the nature of superconductivity?

There are many competing interactions that we can measure experimentally, not all of which are directly responsible for superconductivity, but can mask those that are. Some of this is related to the quality of the materials grown - it is a large materials science challenge to grow pure and defect free materials. These defects often affect or even dominate the experimental results. In other words, there are plenty of red-herrings!

It is also an extremely difficult problem to solve with many many effects going on all at once and it is like no other we have solved before. There has been a huge effort to improve the theoretical language used to describe systems where electrons interact strongly with one another and there is still some way to go, in my opinion.

5. What potential applications does the new superconductor pose?

There are many current and future applications of superconductors, although presently we are not in a position to exploit the new pnictide high temperature superconductors, as the material science of them still has a long way to go. It is early days yet! Therefore I have chosen to answer more generally what superconductors can do for us.

Power utilities have also begun to use superconductor-based transformers and "fault current limiters" - the first superconducting transformer was installed in a utility power network in March of 1997. ABB also recently announced the development of a 6.4MVA (mega-volt- ampere) fault current limiter - the most powerful in the world. This new generation of high temperature superconducting fault limiters is being called upon due to their ability to respond in just thousandths of a second to limit tens of thousands of amperes of current.

Another development is the use of high temperature superconducting wires to transmit electricity. Both the US and Japan have plans to replace underground copper power cables with superconducting BSCCO cable-in-conduit cooled with liquid nitrogen. By doing this, more current can be routed through existing cable tunnels. In one instance 250 pounds of superconducting wire replaced 18,000 pounds of vintage copper wire, making it over 7000% more space-efficient. In May of 2001 residents of Copenhagen, began receiving their electricity through high-temperature superconducting material.

An electrical current flowing round a loop of superconducting wire can also continue indefinitely, producing some of the most powerful electromagnets known to man. These magnets are used in CAT/MRI scanners and there is a huge impact of superconductors on the medicine because of this. Pretty much every CAT/MRI scanner uses a superconducting magnet that generates a magnetic field up to 60,000 times the earth's magnetic field strength. Superconducting magnets are also used to ‘float’ the MagLev train, and to steer the proton beam of the Large Hadron Collider (LHC) at CERN.

As well as creating magnetic fields, superconductors can be used to sense tiny magnetic fields, such as those that are present in the tiny metallic springs in land mines (most of a landmine is not metallic, therefore is not very magnetic and difficult to sense by conventional magnetic techniques). Indeed, one is able to build a ground penetrating, hand-held, extremely sensitive land-mine detector from a SQUID (Superconducting QUantum Interference Device) magnetometer.

Envisaged future applications of superconductors exist also in ultrafast electronic devices and in quantum computing. The latter has the potential to revolutionise data communication and encryption, offering levels of protection (encryption) many orders of magnitude better than what we currently have.

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