Anisotropic fully gapped superconductivity in quasi-one-dimensional Li_0.9⁢Mo₆⁢O₁₇

Superconductivity in quasi-one-dimensional Li_0.9⁢ Mo₆⁢ O₁₇ emerges from an exotic, nonmetallic normal state that exhibits signatures of Tomonaga-Luttinger liquid behavior, emergent symmetry, and excitonic order. The high upper critical field, 𝐻𝑐⁢², in Li_0.9⁢ Mo₆⁢ O₁₇ suggests that that the favored pairing state is spin-triplet in nature. Here, we report measurements of the magnetic penetration depth down to 0.08K (𝑇/𝑇_𝑐≲0.04) and the specific heat down to 0.4K (𝑇/𝑇_𝑐≲0.2), and show that they are consistent with a moderately coupled, fully gapped superconducting state with marked gap anisotropy and a minimum (Δ_min≃0.4𝑘⁢_B𝑇_𝑐) occurring over a very narrow region in 𝑘 space. Combined with knowledge of 𝐻_𝑐⁢2, these measurements support the presence of a nodeless and possibly odd-parity spin-triplet superconducting order parameter in Li_0.9⁢ Mo₆⁢ O_17.