Superconductivity of Bismuth Observed at .5 mK

Scientists from India report that pure Bismuth (Bi) is superconducting at ultralow temperatures, a discovery that cannot be explained by standard models of superconductivity.

For more than a century, researchers have studied Bi, the 83rd element of the periodic table, and it continues to draw enormous scientific interest due to its unusual electronic properties. Bulk Bi at ambient pressure is a semi-metal and it remains in a normal state down to 0.010 K.

Unlike metals where there is roughly one mobile electron per atom, in a semi-metal like Bi the concentration of mobile electrons is extremely low. In Bi, 100,000 atoms share a single mobile electron. Due to this extremely low carrier density and earlier studies showing Bi resistant to superconductivity at 10 mK, scientists thought its superconductivity in bulk was unlikely.

Such thinking has changed, however, in light of the findings published in Science by a research team at the Tata Institute of Fundamental Research. Led by Professor S. Ramakrishnan, the TIFR researchers discovered superconductivity of a high-quality single crystal of Bi (99.998 percent pure) at 0.53 mK with a critical field of 0.000005 Tesla (nearly 1/8 of earth’s magnetic field). The team used a homemade ultra sensitive magnetometer housed in a copper nuclear refrigerator to observe the experiment’s diamagnetic signal.

The finding makes Bismuth one of the two lowest carrier density superconductors to date and suggests, according to the TIFR team, that a new standard model in superconductivity is needed since its assumption that electronic (Fermi) energy is much larger than lattice (vibration) energy fails in Bismuth.