Superconductivity at carrier density 1017 cm-3 in quasi-one-dimensional Li0.9Mo6O17

Quasi-one-dimensional systems, having tendencies toward density-wave order in competition with superconductive pairing in their ground states, may give rise to unconventional superconductivity, a central theme in condensed matter physics. Partial density-wave gapping of electronic bands at the Fermi surface in such systems can yield superconductivity at very low carrier density that challenges Bardeen-Cooper-Schrieffer (BCS) theory since the pairing energy scale may approach or exceed the Fermi energy and render screening of the Coulomb interaction ineffective. Here we present low-T magnetotransport measurements on the quasi-one-dimensional conductor Li0.9Mo6O17 showing the metallic state from which superconductivity emerges (T-c similar or equal to 2 K) to possess among the lowest known carrier densities, similar to 10(17) cm(-3), and a ratio of T-c to Fermi temperature within the BCS-Bose-Einstein-condensation crossover regime. A semimetallic state caused by a density-wave-induced Fermi surface reconstruction with highly anisotropic electron and hole pockets is implied. The degree of interpocket nesting appears to determine whether the extreme low-density ground state is superconducting or undergoes additional Fermi surface gapping.