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Superconductivity of pure Bismuth crystal at 0.00053 K

Bulk rhombohedral Bismuth (Bi) at ambient pressure is a semi-metal and it remains in the 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 (100,000 atoms share a single mobile electron).

Hence, the superconductivity (SC) in bulk is thought to be very unlikely due to this extremely low carrier density.

Now, a group of TIFR scientists led by Professor S. Ramakrishnan have discovered superconductivity of a high quality single crystal of Bi (99.998% pure) at 0.00053 K with a critical field of 0.000005 Tesla (nearly 1/8 of earth's magnetic field). 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.

This discovery cannot be explained by standard models of superconductivity. A new theory is necessary since the assumption that the electronic (Fermi) energy is much larger than the lattice (vibration) energy used in standard models fails in Bismuth.



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