Compressed Sulfur as a Superconductor
Superconductors are materials that offer no resistance to electric current. For pure sulfur it was predicted on the basis of theoretical models that the transition to the superconductor should take place at a pressure of more than 550 GPad. However, scientists have now observed that sulfur already becomes superconductive at 93 GPa. With this, the old ideas are disproved and an experimental system for new models is available. A group of scientists from the Carnegie Institution and the Russian Academy of Sciences report in Nature (November 27 issue) the surprising observation that sulfur becomes a superconductor at 93 GPa (0.93 million atmospheres). At this pressure, the critical temperature (Tc) was 10 K or -263oC. With increasing pressure, the temperature for superconductivity increased, increasing by 0.06 K per GPa (up to 14K). At a pressure of 160 GPa (the highest measured pressure in the present experiments), the Tc increased again, this time to 17 K. In a similar study, published in Physical Review Letters of November 24, 1997, the same authors report measurements Niobium with up to 132 GPa pressure.
A material becomes a superconductor when it loses its electrical resistance. The phenomenon has been known since 1911. It is among the observations most counterintuitive to physical intuition. Within the last decade, superconducting materials have been discovered at temperatures high enough to allow applications, particularly in the computer and electrical energy fields. Most studies have focused on oxide ceramics.
The mechanisms of superconductivity in materials are of great theoretical interest; in many cases, however, they are controversial. Investigations of simple substances such as the pure elements that might be superconductive, including a review of the effects of pressure on the Tc, are essential to an understanding of the underlying physics. Such studies are crucial for the development of new, technologically viable superconductors.
The authors of both publications are Viktor Struzhkin, Russell Hemley and Ho-kwang Mao from the Carnegie Geophysical Laboratory and NSF Center for High Pressure Research and Yuri Timofeev from the Institute of High-Pressure Physics of the Russian Academy of Sciences. The group used the Megabar high-pressure diamond anvil cell in conjunction with a magnetic susceptibility technique that they have perfected over the past several years. The technique allowed them to determine the superconducting transition temperature without having to place electrical lead strips on the sample. This allowed them to perform their measurements on very small samples (down to 0.04 mm in diameter with a thickness of a few thousandths of a millimeter). Tests of the method in the megabar pressure range (above 100 GPa) were performed on niobium, which has a critical temperature of 9.5 K at atmospheric pressure. But this dropped to 4.5 K (instead of increasing) at 132 GPa.
The conversion of sulfur from an insulator to a superconductor at 93 GPa was unexpected. A few years ago scientists had observed changes in the optical properties of sulfur which indicated that the material transformed into a metal at about 90 GPa (at room temperature) with a corresponding change in the crystal structure, and that at about 160 GPa changes its structure. The latest theoretical calculations had predicted that sulfur would only become a superconductor at a much higher pressure (over 550 GPa). The new results show that at the first transition (at 90 GPa) the material changes directly from a non-conductor to a superconductor. The results provide an important example of large-scale changes in physical properties under pressure.
According to the authors, their results are particularly remarkable because the metallic phases of sulfur have the highest transition temperatures of all elementary solids measured to date. Sulfur as a superconductor is now in the company of the heavier elements in the fifth main group of the periodic table. This fact should now enable a critical examination of the theories of superconductivity. At the end of their article the authors write: "Given the relative ease with which the electronic structure of elemental sulfur can be calculated and the knowledge of the structure of the high-pressure crystals, this element should be suitable for important tests of new possible mechanisms.”
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