Introduction
Graphite is commonly used to write or draw things that can be kept indefinitely or erased completely — a property that encourages creativity. Creativity of a different kind has led Weller and co-workers1 to the discovery of superconducting graphite with unusual properties, as reported on page 39 of this issue. They manipulated the material to become a superconductor with a critical temperature Tc as high as 11.5 K by simply adding the common element calcium to it (or the less-common element ytterbium with a corresponding Tc of 6.5 K). In previous work2, 3 with potassium and sodium as dopants the critical temperature did not exceed 1 K, except with high-pressure techniques that yielded an encouraging value of 5 K. The new results1 have been reproduced and expanded quickly4 and theoretical activity has initiated a trail of explanations (see, for example, page 42; ref. 5).
A new material is always a fascinating playground for physicists. Physics tries to explain the world in which we live. But that world is not a static entity. It expands because we create ideas and materials that pose new intellectual challenges. Most people would look for signs of enrichment in architecture or on the internet. However, condensed-matter physics has a symbiotic relationship with advances in materials and materials control, which are sometimes part of the everyday world, and sometimes more hidden.
The silicon technology, which has brought us integrated circuits, has also brought us the quantum Hall effect. It appears that matter has become a very helpful instrument for experimental physics to create unprecedented boundary conditions. Where would physicists have gone for systems of two-dimensional electrons had it not been for semiconductor technology? Some researchers go one step further and construct materials by changing the boundary conditions of the system in a judicious manner and hoping for new discoveries in such untrodden territory. They are guided by intuitive ideas about possible causes of a particular phenomenon, and even when the intuition is wrong, as demonstrated by the case of polarons and high-temperature superconductivity, it may lead, serendipitously, to spectacular discoveries.
An example of this materials-driven experimental physics approach is published in this issue by Weller et al.1 and theoretically addressed by Csányi et al.5. Graphite pencils write because graphene sheets are loosely bound and can easily be transferred to paper, which might end up representing a poem, a musical composition or a mathematical equation. Weller et al. found that on inserting electron-donating atoms such as ytterbium and calcium between the graphene sheets, the material superconducts up to temperatures as high as 6.5 K and 11.5 K. Later work4 confirms these results. The magnetization curve4 shows a sharp transition with a width of only 0.5 K, leading to a full Meissner state of magnetic flux expulsion. This result is due to improved materials preparation techniques.
Figure 1 shows the crystal structure of C6Ca. The calcium atoms donate charge to the graphene planes and they also push the planes further away from each other. In previous work the aim was either to maximize the distance or the metal concentration. Surprisingly, in this case the mutual separation is less than in KC8, but the increase in Tc is two orders of magnitude. This is quite different from the case of C60 in which an increase in distance is accompanied by an increase in Tc. The same can be said about the amount of charge transfer. The measured critical fields along the different directions also add to the story. In the ab plane, the superconducting coherence length is 35 nm, whereas in the c direction it is 13 nm. Obviously there is some anisotropy but much less than expected naively. The consequence is that despite their increased separation the coherent coupling between the different graphene planes is rather strong, and will have to be taken into account by theoreticians. Indeed, the coherent coupling perpendicular to the planes plays an important role in the work by Csányi et al.5 as well as in another analysis that has appeared recently6.
Figure 1: Crystal structure of C6Ca.
The unit cell is rhombohedral with the calcium atoms in green and graphene sheet in red.
Full size image (28 KB)The experimental progress in graphite might stimulate increased attention toward a controversial result concerning superconductivity in ropes of multiwalled nanotubes7. By inserting dopant atoms between the nanotubes it might be possible to mimic the situation found now for graphite.
For those working on superconducting devices these new materials are not likely to be the most appealing. The fabrication route is time-consuming and not readily compatible with thin-film technology. Certainly, point-contact spectroscopy or optical spectroscopy will be carried out soon to determine the energy gap of these materials. The bottom line is that nature continues to pose questions to physicists — questions without easy answers but which open new routes to the creative process. Physics will not be ruled by carbon copies of the past.