Superconductivity occurs when a material is cooled to a specific low temperature (Tc), which eliminates all electrical resistance allowing the electrons to flow freely. This remarkable phenomenon was discovered in 1911 but is still not fully understood. Accurately predicting superconducting transition temperatures is one of the most difficult theoretical problems in the field of condensed-matter physics. In this respect, study of the simplest elemental metals is of great fundamental interest.
Lithium is considered to be "the simplest" metal in the sense that it has a highly symmetrical structure (bcc), and its electronic properties are well described within a nearly free electron model. Under ambient pressure no sign of superconductivity in lithium has been detected. Lithium attracted a lot of interest after Neaton and Ashcroft (Nature 400, 141 (2000)) predicted that under pressure it may undergo several structural transitions, possibly leading to a "paired-atom" phase with low symmetry and near-insulating properties resembling molecular hydrogen. This work initiated a wave of experimental and theoretical activity. It should be noted that lithium is highly reactive and difficult to experiment with at high pressures. Hanfland et al (Nature 408, 174 (2000)) succeeded in finding a new phase in X-ray diffraction experiments at high pressures. These transformations indicate strong electron-lattice coupling and superconductivity in lithium with high Tc of 60 to 80 K might be expected according to calculations (Physical Review Letters 86, 1861 (2001)).
After two years efforts, two groups succeeded in establishing superconductivity in lithium. Nature 419, 597 (2002) published an article by Shimizu et al from Osaka University, Japan in which they found a drop in electrical resistance at pressures greater than 30 GPa which was suppressed by strong magnetic fields indicating the presence of superconductivity. They found that Tc increases to 20 K at 48 GPa. These results are in basic agreement with the work of the Geophysical Laboratory of the Carnegie Institution of Washington, DC, USA in collaboration with the High Pressure Group at the Max Planck Institute for Chemistry in Mainz (Science, 17 October). This work used a new diamond-anvil cell technique, combined with resistivity magnetic susceptibility for the reliable determination of superconductivity. It was found that Tc of lithium ranges from 9 K at 23 GPa (230.000 atmospheres) to 16 K at 80 GPa. The observed values for Tc are in apparent contradiction with theory which predict much higher Tc. More sophisticated theoretical treatments, similar to those proposed for metallic hydrogen, may be required.
The same research team (MPI Mainz/GL) recently discovered superconductivity in boron, also a light element (Science 293 (2001)). A relatively high Tc =11 K was found and Tc also increases with pressure. The recent findings will undoubtedly motivate both theoretical and experimental activity in searching for high-temperature superconductivity in light element compounds at ambient pressures. The last experiments are also important steps in tackling the lightest of elements - hydrogen. Theory predicts that at pressures of 300-400 GPa molecular hydrogen will become a metal with room-temperature superconductivity.
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