SCIENCE

Super-discovery

60thamizhavel Bismuth brotherhood: (From left) Thamizhavel, Ramakrishnan, Om Prakash and Anil at TIFR | Amey Mansabdar

Proving bismuth to be a superconductor has set wheels turning among scientists

The room is a maze of cables and pipelines running seemingly haphazardly between intimidating cylinders. S. Ramakrishnan, professor of low temperature physics, points to a complicated apparatus sticking out of the floor of his office saying, “You are in luck. We’ve just started cooling the laboratory again.” We look around for some door that leads to what is touted as the third coldest ultra-low temperature laboratory in the world, only to realise that the “lab” is this equipment and much of what juts below it, in the basement.

This humble contraption at the Tata Institute of Fundamental Research (TIFR) had performed an experiment which stunned the scientific community, and could spark new theories. Ramakrishnan and his team of three proved that bismuth is a superconductor at extremely low temperatures, almost close to the absolute zero (-273 degrees Celsius/ 0 Kelvin).

Superconductors are materials which transmit electricity with high efficiency, leading to no transmission loss. All known superconductors exhibit this trait only at very low temperatures. Scientists are trying to find a material which can superconduct at ambient temperatures.

So then, why is the TIFR discovery making waves? After all, one needed to reach ridiculously low temperatures—which can have no practical use—to show bismuth’s superconducting nature. It’s bismuth, and not the temperature, that is the newsmaker. As per the conventional theory of superconductivity, put forward by the Nobel-winning Bardeen-Cooper-Schrieffer trio, bismuth should not superconduct, however low the temperature. One requirement for superconductivity is the presence of abundant free electrons—bismuth has far fewer than most conventional conductors (1017 per cubic cm in bismuth, as against 1022 per cubic cm in metallic conductors). Also, while in superconductors the electron velocity is much higher than the ionic velocity, in bismuth, the two are comparable.

What this means is that since the existing theory cannot explain the phenomenon, theoreticians will have to develop a new theory on superconductivity. This throws open the possibility of finding new superconductors among materials which weren’t considered and, perhaps, one at ambient temperatures.

61coldwork

The paper, published in the journal Science, has four authors: Ramakrishnan, his postdoctoral student Om Prakash Shukla, A. Thamizhavel (assistant professor, condensed matter physics) and Anil Kumar (electronics support). The editors were surprised that the discovery had just four authors, all from the same institute. It took the TIFR team about a year to satisfy the referees, answering every doubt, before the article was published.

It look much less time for the scientific community to take note of the discovery. A global expert on bismuth, Kamran Behnia from CRNS, France, hailed it as an important discovery; theoretician Marvin Cohen from the US said it could pave the way for a better theory to explain the mysterious world of superconductors; and G. Bhaskaran of Institute of Mathematical Sciences, Chennai, called the paper the “tip of the iceberg.”

Ramakrishnan has been fascinated with bismuth since his postdoctoral days. It’s a quiet element that sits unassumingly on the periodic table, surrounded by natural and radioactive poisons (arsenic, lead, mercury polonium and radon). But, bismuth is so harmless that it is even used in children’s toys and as a gastric medication. It is easily available and inexpensive. In the last ten years, material scientists have favoured bismuth for research, given that many physical properties like dimagnetism, thermoelectric effect and Nernst heat effect were first observed in the metal.

“In 1990, I asked my postdoctoral guide Frank Pobell to try and see if bismuth would superconduct. He wasn’t convinced.” The lowest temperature laboratory in the world, at Helsinki, had already tested bismuth till a temperature as low as 10 milli-Kelvin and concluded that it wouldn’t superconduct. “But the thought stayed with me even after I joined TIFR, 16 years ago.” Obtaining grants, however, took time, and it was only in 2007 that he got cracking.

First, he had to build a laboratory that could reach those ultra-frigid temperatures. Traditionally, such laboratories are housed in a separate building, insulated from vibrations, magnetic waves and disturbances that can impede cooling. Ramakrishnan had only his office space, so they decided to compress the laboratory into a series of cylinders, propped up on pillars kept in a bed of sand. Nobel-winning scientist on superfluids R.C. Richardson saw the initial attempts on a visit, and said, “Good luck Rama, if you get to one milli-Kelvin.” He wasn’t belittling the effort, but was just sceptical. In 2011, Ramakrishnan informed him that he had reached 40 micro-Kelvin, leaving Richardson amazed. The laboratory had cost only Rs 1 crore, much of which was the cost of liquid helium, the coolant.

With the jugaad laboratory, Ramakrishnan got the right student in Shukla, who set to work earnestly, using ultra pure bismuth crystals that Thamizhavel prepared in his furnace. “I made Shukla work on two papers, because in case this didn’t work out, he would have wasted precious years.” In the end, that proved unnecessary. The eureka moment happened in August 2015, when Shukla read the instruments and realised bismuth was superconducting. The bush-shirt-and-capri clad researcher didn’t do a jig, pop a bottle of champagne or go out screaming. “I just got busier, calibrating each reading, leaving no question unanswered,” says Shukla.

What next? “We’ve done the initial discovery. This has to be taken forward by others, as I am close to retirement,” says Ramakrishnan. He hopes this discovery will pave way for finding superconductors at higher temperatures. For instance, MRI machines work on superconductors, but need temperatures of at least -269 degrees Celsius. For this, they need liquid helium that costs Rs 1,500 a litre. A scan thus costs around Rs 4,000. If there were superconductors at the temperature of liquid nitrogen (-196 degrees C), the cost of the scan would only be a fraction, as a litre of nitrogen comes at Rs 5.

Ramakrishnan’s laboratory, meanwhile, is getting a lot of work. They have received samples from the University of Cambridge, which require testing to be done at ultra-low temperatures. And the manufacturer, who made the laboratory on Ramakrishnan’s instructions, has already sold two other models of the same.

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