Can diamond now be a superconductor?
Apr 4, 2003
A physicist in South Africa claims to have created a new superconducting state of matter at room temperature. Johan Prins of the University of Pretoria observed the superconducting state in experiments with diamonds that had been doped with oxygen (Semiconductor Science and Technology 18 S131).
Diamond is a semiconductor and Prins has long been interested in using n-type diamond as a “cold” cathode to replace the “hot” cathodes found in television tubes and many other devices. Moreover, he believes that the results of his experiments on n-type diamond surfaces – made by exposing the diamond to energetic oxygen ions – can only be explained by a new type of superconducting state. “If it is not superconductivity then it must be violating the second law of thermodynamics,” he says.
In his experiments, Prins measures the current that flows between the diamond and a gold-plated probe as the distance between them is varied. When a voltage of +1000 V is applied, the current always settles down at a value of about 0.5 mA for separations up to about 16 µm, after which it falls to zero. A current also flows in the opposite direction when a voltage of –1000 V is applied, but it decreases more rapidly with distance. The experiments are performed at room temperature in a vacuum of 10–6 mbar.
Prins argues that a thin “electron-charge” layer is formed in the vacuum just above the surface of the diamond, and that a depletion layer of positive charges forms in the diamond. This is similar, he says, to the Schottky diode that is generated between an n-type semiconductor and a metal. Prins then applies the equations that describe electron transport through a Schottky diode to his system. He finds that as more and more electrons are extracted from the diamond, the density of electrons in this layer reaches a critical value at which a Bose–Einstein-type condensate of electron pairs forms. Current continues to flow from the diamond cathode through this layer to the anode, even though there is no voltage across the layer – a sign of superconductivity.
However, the rest of the diamond community remains to be convinced. Richard Jackman of University College London, who edited the special issue of the journal in which Prins’ papers appear, describes them as “largely theoretical papers, thought provoking and very controversial – the end conclusions remain open to debate”.
Prins admits that he must show that the state can expel magnetic fields to conclusively prove that the state is superconducting. However, he has recently retired and does not have the facilities to perform such an experiment. He has offered to fly his samples to another lab but has not yet found any volunteers. Prins and two colleagues are also trying to secure patents on the ideas.
In addition, Prins is half-way through writing six theoretical papers that will, he claims, fully explain the results and shed new light on the mechanisms underlying high-temperature superconductivity.
About the author
Peter Rodgers is Editor of Physics World