Upper Critical Magnetic Field far above the Paramagnetic Pair-Breaking Limit of Superconducting One-Dimensional Li0.9Mo6O17 Single Crystals

The upper critical field H-c2 of purple bronze Li0.9Mo6O17 is found to exhibit a large anisotropy, in quantitative agreement with that expected from the observed electrical resistivity anisotropy. With the field aligned along the most conducting axis, H-c2 increases monotonically with decreasing temperature to a value 5 times larger than the estimated paramagnetic pair-breaking field. Theories for the enhancement of H-c2 invoking spin-orbit scattering or strong-coupling superconductivity are shown to be inadequate in explaining the observed behavior, suggesting that the pairing state in Li0.9Mo6O17 is unconventional and possibly spin triplet.