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Turn up the temperature

A more elaborate picture is developing of what makes some materials superconduct at relatively high temperatures. With it come hints for how to design materials with still higher transition temperatures.

Discovered a hundred years ago, superconductivity describes the flow of electric current without resistance in metals. Most metals do not become superconducting until cooled to within about ten degrees of absolute zero. But the discovery, in 1987, of materials that become superconducting at much higher temperatures rekindled an old dream that one day room-temperature superconductivity might be achieved.

One of the mysteries of the high-temperature superconductors is the link between their layered crystal structure and their transition temperatures (at which they become superconducting). These ceramic materials contain multiple layers of superconducting copper oxide, separated by insulating material — a kind of superconducting baklava. As physicists learned to synthesize more complex versions of these materials, with first one, then two, then three superconducting layers, the superconducting transition temperature shot up, from about 40 K in the single-layer materials to above 130 K in the three-layer materials1,2 (Fig. 1). But there it has stood for almost a decade — adding more layers unfortunately drives the transition temperature back down again. So what went wrong?

Figure 1: Layering effect.

a, A single layer of lanthanum copper oxide becomes superconducting at a temperature of 40 K, as the electrons in the layer order into pairs. b, If the number of copper-oxide layers is increased to three (with layers of insulating material between them), the temperature at which the material becomes superconducting rises to 130 K, because electron pairs are now able to ‘tunnel’ between the layers. c, But if the number of layers is increased beyond three, the transition temperature does not continue to rise — in fact, it falls. Chakravarty et al.3 propose that the transfer of charge from the inner to the outer layers nucleates a second order parameter, in addition to the ordered pairing of electrons, and that this drives down the transition temperature.

On page 53 of this issue, Chakravarty, Kee and Völker3 propose an explanation. Their theory brings together several insights into the physics of high-temperature superconductivity, and links three important effects — tunnelling, interlayer charge distribution and hidden order.

The first element of Chakravarty and colleagues' analysis3 is electron tunnelling between the superconducting layers. Inside a superconductor, electrons are bound into pairs. When two superconducting layers are brought close together, these pairs can ‘tunnel’ or jump from one to the other. (Here ‘tunnelling’ refers to the quantum-mechanical motion of a wave/particle through a region of space that would be forbidden in classical mechanics.) This tunnelling effect couples the two superconductors, and is called a ‘Josephson coupling’ after its discoverer.

About ten years ago (in a now disproven theory), Chakravarty, Sudbø, Anderson and Strong proposed that interlayer tunnelling is actually the origin of high-temperature superconductivity4. Chakravarty et al.3 have revived the idea of interlayer coupling, but now they argue that, although it is not the main engine of superconductivity in the layers, such coupling is responsible for the rise in transition temperature when going from one-layer to three-layer materials.

The authors3 also take account of how charge is distributed between the layers of the superconductor. A remarkable aspect of high-temperature superconductors is that, in many ways, these materials are closer to insulators than to metals. The mother compounds of high-temperature superconductors are insulating, and develop superconductivity only when extra charge carriers (electrons, or their positively charged counterparts, holes) are introduced into the material by chemical doping. Some years ago, Japanese physicists Ohta, Tohyama and Maekawa5 predicted that in multilayer high-temperature materials the charge would redistribute, with the result that doping in the interior layers would in fact be less than in the outer layers. This idea has been confirmed by NMR measurements6,7 of the doping profile across the multilayer.

But central to the concepts underlying the new theory is the so-called order parameter. The Russian physicist Lev Landau was the first to realize that when matter develops new forms of order, the collective motion of its constituent particles can be described by variables that he called ‘order parameters’. The advantage of Landau's order-parameter approach is that it means that the collective large-scale properties of a system can be easily separated from the gothic details of its microscopic motions. Using this concept, Landau and Vitaly Ginzburg8 were able to develop an order-parameter theory for the macroscopic properties of superconductors more than a decade before the pairing mechanism for superconductivity was devised by John Bardeen, Leon Cooper and Robert Schrieffer.

High-temperature superconductors are far more complex than their low-temperature counterparts, and there are many indications that their unique properties result from the competition between more than one type of order parameter. The electron correlations that are responsible for high-temperature superconductivity are still a mystery. But Chakravarty et al.3 bypass this unsolved problem by using the order-parameter approach to analyse the interaction between the superconductivity in each layer of the material.

The authors have melded these ideas of tunnelling, charge transfer and order into a simple Landau–Ginzburg expression for the total energy of the system. Looking beyond the order connected with correlated electrons, the basic idea of their theory is that, for more than three layers of copper oxide, a second, competing order parameter nucleates in the interior under-doped layers and lowers the superconducting transition temperature. By fitting the properties of a single-layer material and using the measured doping profile, they are then able to compute how the superconducting transition temperature evolves with the number of layers, obtaining good qualitative agreement with experimental data.

A key prediction of Chakravarty and colleagues' theory3 is that the ‘pseudogap’ should be larger in the inner layers of a multilayer superconductor. In normal superconductors, when electrons pair up, a gap develops in the electron energy spectrum; the higher the transition temperature of the metal, the larger the gap. But high-temperature superconductors break this rule: under-doped materials with lower transition temperatures actually develop a larger gap than optimally doped materials, even at high temperatures where they have not yet become superconducting. The nature of this so-called pseudogap is a matter of deep controversy9. But, according to one school of thought, it is produced by a second order parameter that competes with superconductivity, just as Chakravarty et al.3 envisage. There are various experiments under way10 that will shortly be able to test this idea and perhaps probe the nature of the hidden order inside the layers.

An attractive feature of this new theory3 is that it brings together several prevalent ideas about high-temperature superconductivity and provides a simple account of why the superconducting transition temperature first rises then falls as the number of superconducting layers increases. That we might now understand why the transition temperatures in these curious layered materials have been limited is exciting indeed — and from this understanding, the prospect of designing materials with still higher transition temparatures looks more realistic.


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