For nearly thirty years, the search for superconductors operating at room temperature has been conducted in the field of exotic materials — cuprates — that can carry currents without energy loss in the form of heat at temperatures down to -109 degrees Celsius. However, scientists say they repeated this record with the use of hydrogen sulfide molecules. When they subjected a tiny sample of this material to pressure similar to that found in the core of the Earth, it became superconducting at a temperature of -83 degrees Celsius.
“If the result is reproduced, it will be a historic discovery,” says Robert Cava, a solid state chemist at Princeton University.
In accordance with the established theory of superconductivity, called the BCS theory in honor of the creators, John Bardeen, Leon Cooper and Robert Schrieffer, vibrations in crystal atoms can cause electrons to form "Cooper pairs" that can flow through a crystal without resistance. The BCS theory was developed in the 1950s, but many physicists believe that it cannot explain superconductivity in cuprates, which were discovered in 1986, or in iron pnictides, discovered in 2006.
Scientists believe that the BCS theory can lead to the discovery of other types of superconductors operating at high temperatures, especially in materials containing light elements like hydrogen. They create more frequent vibrations that form stronger bonds between the electron pairs.
The latter study draws on the work of Neal Ashcroft, a physicist at Cornell University in Ithaca, New York, who studied the superconducting potential of hydrogen compounds. More specifically, he investigated a recent theoretical prediction by a group of Chinese physicists who suggested that hydrogen sulphide should be superconducting at temperatures of the order of -100 degrees Celsius and under a pressure of 1.6 million atmospheres. Such pressure compresses electrons into Cooper pairs and makes them more resistant to thermal fluctuations.
Mikhail Yeremets and his colleagues from the Max Planck Institute for Chemistry in Mainz, Germany, placed a sample of hydrogen sulfide in one hundredth of a millimeter across between two diamond anvils, and then measured with electrodes, how the electrical resistance of the material changes when the system cools down to almost absolute zero. Scientists have discovered that at a pressure of 1.8 million atmospheres and a temperature of -109 degrees Celsius, the resistance suddenly dropped, which means a transition to superconductivity occurred.
Scientists attribute this unexpected indicator of the critical temperature to the fact that hydrogen sulfide molecules contain a relatively large number of hydrogen atoms. These molecules, scientists say, should remain superconducting at this temperature.
Also, scientists cite some evidence that they saw a lower transition temperature (of the order of -183 degrees Celsius), when they replaced the heavy sulfur deuteride with sulfur hydride. The heavier the atoms, the slower the crystal vibrations and the worse the superconductivity. The work of scientists was published in arXiv.org in early December.
If the result of the work is confirmed by other groups, there will be a significant increase in the critical temperature necessary to achieve superconductivity. The current record (magnesium diboride) is set at -234 degrees Celsius. Physicists who have familiarized themselves with the work write that the results of Yeremets and his colleagues literally created a stir in the scientific community, and there are no complaints about the paper conclusions yet.
Alexander Gurevich, a theorist at the University of the Old Dominion in Norfolk, agrees that the results will mean "a significant breakthrough in the study of superconductivity", but he is very cautious in his statements. He says that the authors still need to demonstrate the so-called Meissner effect, during which the material “pushes” the magnetic field lines as it cools to the superconducting state. The physicist also expresses the hope that other scientists will quickly reproduce the experiments of researchers.
The discovery of Yeremets and his colleagues suggests that it may be possible to find high critical temperatures in other hydrogen-containing substances such as fullerenes or aromatic hydrocarbons. They, the scientist says, could be turned into superconductors by mixing with other elements in small proportions, without the use of high pressure. Kava, however, warns that it is too early to talk about possible applications of the discovery - things can go wrong with other hydrides.
The article is based on materials .
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