Researchers at the Max Planck Institute for Chemistry in Mainz and Johannes Gutenberg University Mainz observed that hydrogen sulfide becomes superconductive at minus 70 degree Celsius – when the substance is placed under a pressure of 1.5 million bar. This corresponds to half of the pressure of the earth's core. With their high-pressure experiments the researchers in Mainz have thus not only set a new record for superconductivity, their findings have also highlighted a potential new way to transport current at room temperature with no loss.

For many solid-state physicists, superconductors that are suitable for use at room temperature are still a dream. Up to now, the only materials known to conduct current with no electrical resistance and thus no loss did so only at very low temperatures. Accordingly, special copper oxide ceramics, so-called cuprates, took the leading positions in terms of transition temperature, i.e., the temperature at which the material loses its resistance. The record for a ceramic of this type is roughly minus 140 degrees Celsius at normal air pressure and minus 109 degrees Celsius at high pressure. In the ceramics, a special, unconventional form of superconductivity occurs. For conventional superconductivity, temperatures of at least minus 234 degrees Celsius have so far been necessary.

A team led by Dr. Mikhael Eremets, head of the working group "High pressure chemistry and physics" at the Max Planck Institute for Chemistry, working in collaboration with Dr. Vadim Ksenofontov und Sergii Shylin of the Institute of Inorganic Chemistry and Analytical Chemistry at Johannes Gutenberg University Mainz has now observed conventional superconductivity at minus 70 degrees Celsius in hydrogen sulfide (H2S). To convert the substance, which is a gas under normal conditions, into a superconducting metal the scientists did however have to subject it to a pressure of 1.5 megabar or 1.5 million bar.

His team has also been the first to prove in an experiment that there are conventional superconductors with a high transition temperature. Theoretical calculations had already predicted this for certain substances including hydrogen sulfide.

The researchers generated the extremely high pressure required to make hydrogen sulfide superconductive at comparatively moderate negative temperatures in a special pressure chamber smaller than one cubic centimeter in size. The two diamond tips on the side, which act as anvils, are able to constantly increase the pressure that the sample is subjected to. The cell is equipped with contacts to measure the electrical resistance of the sample. In another high-pressure cell, the researchers were able to investigate the magnetic properties of a material that also change at the transition temperature. After the researchers had filled the pressure chamber with liquid hydrogen sulfide, they increased the pressure acting on the sample gradually up to roughly two megabar and changed the temperature for each pressure level. They took measurements of both resistance and magnetization to determine the material's transition temperature. The magnetization measurements provide very useful information, because a superconductor possesses ideal diamagnetic properties.

The researchers believe that it is mainly hydrogen atoms that are responsible for hydrogen sulfide losing its electrical resistance under high pressure at relatively high temperatures: Hydrogen atoms oscillate in the lattice with the highest frequency of all elements, because hydrogen is the lightest. As the oscillations of the lattice determine the conventional superconductivity – and do this more effectively the faster the atoms oscillate – materials with high hydrogen content exhibit a relatively high transition temperature. In addition, strong bonds between the atoms increase the temperature at which a material becomes superconducting. These conditions are met in H3S, and it is precisely this compound that develops from H2S at high pressure.

This story is reprinted from material from Johannes Gutenberg University with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.