Cool lab: A diamond anvil cell (blue box) was used to produce the high pressure of 2.6 million atmospheres. | Photo Credit: J. Adam Fenster
Starting from hydrogen sulphide, infamous for its rotten-egg smell, a group of researchers at University of Rochester, Intel corporation and University of Nevada in the U.S. have created a material that is superconducting at 15 degrees Celsius. That is, it shows zero resistance to the flow of electricity through it. Such a material would have hitherto unheard of applications from power supplies to quantum computers. The only caveat is that it needs ultrahigh pressure of about 2 million atmospheres to achieve this transition, putting off any thoughts of application to the future. The research, published in Nature, has sparked off animated discussions in the world of physics.
“Ever since Wigner and Huntington (1935) predicted that solid hydrogen can be metallised by application of a pressure of about 25 Gigapascal (GPa), the high-pressure physics community has been after metallisation of hydrogen,” says G. Baskaran, Distinguished Professor of Physics at IIT Madras and Distinguished Visiting Research Chair at Perimeter Institute, Waterloo, Canada, who is an expert in the field of superconductivity. “Ashcroft (1968) gave a new impetus to this search and suggested that within conventional BCS mechanism of superconductivity, in view of light mass of H atom, superconducting Tc of metallic hydrogen will reach room temperature scales,” explains Prof. Baskaran
In 2015, a breakthrough happened when a group led by M.I. Eremets managed to apply pressure on hydrogen sulphide and get a superconductor at 200 kelvin (minus 73 degrees Celsius). Prof. Baskaran adds, “It was followed by superconductivity in few more hydride superconductors. Until recently, the record holder was Lanthanum superhydride (LaH10). Its Tc is about 250 K, which is room temperatures in polar regions!”.
The researchers made three tests to verify that this phase was indeed a superconductor: First, they measured resistance as a function of temperature and found that it did fall to a vanishingly small value below the critical temperature, Tc. A true superconductor would, if placed in a magnetic field, try to push out the field from its interior. This is called perfect diamagnetism, and the group ascertained that the magnetic susceptibility was that of a diamagnet.
Thirdly, it is known that sufficiently high magnetic fields can destroy superconductivity in a material. The researchers applied high magnetic fields up to 9 tesla and showed the suppression of the transition temperature to lower and lower values as the magnetic field increased. If you extrapolate this relationship, they show that at around 62 tesla the magnetic field would destroy the superconductivity completely.
Commenting on the high pressure that was needed to create this material, Ranga P. Dias, from the University of Rochester and the corresponding author of the paper, says in an email to The Hindu that the pressure they needed was 267 Gigapascals, or 2.6 million atmospheres. “The pressure at the centre of the Earth is 360 GPa, so it is 75% of the pressure at centre of the Earth,” he adds.
They achieved this by squeezing a tiny volume of the substance between the jaws of a diamond anvil. “We have broken many diamonds. It’s very challenging to keep this material to very high pressure because of the hydrogen diffusion,” he reveals.
While they know that the substance is a metal in the normal state, they do not know much about its structure. This is because it is needed to study it using x-ray diffraction and that does not work in this system. “The next challenge will be to produce these materials that are stable (or metastable) at ambient pressure via ‘compositional tuning’ so they will be even more economical to mass produce,” says Dr. Dias.
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