Scientists Observe Superconductivity in Single Crystals of Pure Bismuth

According to a team of researchers at the Tata Institute of Fundamental Research (TIFR) in Mumbai, India, pure bismuth — a semimetal with a rhombohedral structure — is superconducting at ultra-low temperatures.

Artificially grown bismuth crystal illustrating the stairstep structure, with a 1 cm3 cube of bismuth metal. Image credit: Heinrich Pniok / CC BY-NC-ND 3.0.

At low temperatures, many elemental metals, such as lead or aluminum, transition to being superconducting – a state in which electric current passing through these materials can persist indefinitely with no power source.

In bismuth, a single electron is shared by 100,000 atoms, making this semimetal’s carrier density quite low, which has made it seem unlikely it could be a superconductor.

In fact, bismuth has been shown to resist superconductivity all the way down to super-cold temperatures of 10 mK (millikelvin).

“The search for superconductivity in bulk bismuth began more than half a century ago,” the researchers said.

“Although, the superconductivity was observed at high pressures, in amorphous form, thin films, metal hetero-structures, granular nanowires and nanoparticles of bismuth, bulk bismuth under ambient conditions remained in the normal state down to 10 mK.”

“The superconductivity in bulk bismuth is thought to be unlikely because of the extremely low carrier density.”

The TIFR team, led by Professor S. Ramakrishnan, observed bulk superconductivity in single crystals of pure bismuth below 0.53 mK under ambient pressure with an estimated critical magnetic field of 0.000005 Tesla (nearly 1/8 of Earth’s magnetic field).

The observation makes bismuth one of the two lowest carrier density superconductors to date.

“The discovery was made by observing a diamagnetic signal using a home made ultra sensitive magnetometer which is housed in a state of the art TIFR copper nuclear refrigerator built in 2011,” Prof. Ramakrishnan and co-authors said.

Because bismuth doesn’t fit neatly into the standard picture of superconductivity, further theoretical work is necessary to fully explain the findings.

The research was published online Dec. 1 in the journal Science.

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Om Prakash et al. Evidence for bulk superconductivity in pure bismuth single crystals at ambient pressure. Science, published online December 1, 2016; doi: 10.1126/science.aaf8227