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First evidence of natural superconductivity

¹ Museo di Storia Naturale & Dipartimento di Scienze della Terra, Università di Firenze, Via G. La Pira, 4, 50121 Firenze, Italy
² Dipartimento di Chimica & INSTM (U.d.R. Firenze), Università di Firenze, Via della Lastruccia, 3, 50019 Sesto Fiorentino, Italy

^* Corresponding author, e-mail: dibenefr{at}geo.unifi.it

The singular conductive properties of natural covellite were investigated through magnetic AC susceptibility measurements. Two natural hexagonal covellite lamellae coming from Calabona (Italy) and Butte (USA), respectively, were investigated from 30 to 1.5 K. At 1.63(5) K, an abrupt increase of the diamagnetism (Meissner effect) was observed, thus evidencing a transition from a poor conducting to a superconducting state. The transition is independent on small applied DC fields (< 50 Oe), whereas larger values prevent the superconducting state to be attained. These features confirm covellite as a Class I superconductor.

This peculiar property of covellite, never observed before on natural materials, can be mainly related to its structural features. The layered framework of covellite, where CuS₃ planes are isolated by covalently bonded S₂ planes, provides an electron excess, which can be easily involved in the bulk conduction.

Key-words: superconductivity, covellite, AC susceptibility, dissipative currents.