Perspectives: Superconductivity The Race to Beat the Cuprates

Superconductors are materials that lose all electrical resistance below a specific temperature, known as the critical temperature (Tc). Large-scale applications, for example, in superconducting cables, require materials with high (ideally room temperature) Tc’s, but most superconductors have very low Tc’s, typically a few kelvin or less. The discovery of a layered copper oxide (cuprate) with a Tc of 38 K (see panel A in the first figure) in 1986 [1] raised hopes that high temperature superconductivity might be within reach. By 1993, cuprate Tc’s of 133 K at ambient pressure had been achieved [2,3], but efforts to further increase cuprate Tc’s have not been fruitful. Two reports by Schön et al. [4,5] in the current issue of science –applying a similar technique to two very different materials– drastically alter the perception that planar cuprates are the only route to high temperature superconductivity. Schön et al. use a field-effect device introduced in previous investigations to transform insulating compounds into metals [6]. On page 2430, they show that copper oxide materials with a ladder structure (panel B in the first figure) can be superconducting [4], even without the high pressure applied in previous studies of related compounds. Even more spectacularly, they report on page 2432 that the Tc of a noncuprate molecular materials, C60 (panel C in the first figure), known before to superconduct at 52 K upon hole doping [7], can be raised by hole doping with intercalated CHBr3 to 117 K [5], not far from the cuprate record. Simple extrapolations suggest that the Tc could be increased even further, effectively ending the dominance of cuprates in the high-Tc arena. The idea behind the studies is conceptually simple. Field-effect doping exploits the fact that under a strong, static electric field, charge (electrons or holes) will accumulate at the surface of the material, effectively modifying the electronic density in that region. This is necessary to stabilize superconductors away from nominally insulating compositions. The dielectric portion of the field-effect device must be able to sustain electric fields large enough to induce a sufficient number of holes per atom or molecule for the material under study to become superconducting. In addition, the interface with the studied material must be as perfect as possible. Doping through a field-effect device [4,5] avoids imperfections that cause the system to deviate locally from its average properties. Such imperfections are inevitably induced by chemical doping. Disorder has not been seriously considered by most cuprate high-Tc theorists, but its important role is slowly emerging. Some phase diagrams of cuprates may have to be redrawn when doping is introduced through a field-effect device [8].