A powerful approach to analysing quantum systems with dimensionality d>1 involves adding a weak coupling to an array of one-dimensional (1D) chains. The resultant quasi-1D (q1D) systems can exhibit long-range order at low temperature, but are heavily influenced by interactions and disorder due to their large anisotropies. Real q1D materials are therefore ideal candidates not only to provoke, test and refine theories of strongly correlated matter, but also to search for unusual emergent electronic phases. Here we report the unprecedented enhancement of a superconducting instability by disorder in single crystals of Na2−δMo6Se6, a q1D superconductor comprising MoSe chains weakly coupled by Na atoms. We argue that disorder-enhanced Coulomb pair-breaking (which usually destroys superconductivity) may be averted due to a screened long-range Coulomb repulsion intrinsic to disordered q1D materials. Our results illustrate the capability of disorder to tune and induce new correlated electron physics in low-dimensional materials.

We present electronic-structure calculations, electrical resistivity data, and the first specific-heat measurements in the normal and superconducting states of quasi-one-dimensional M 2 Mo 6 Se 6 ͑M =Tl,In,Rb͒. Rb 2 Mo 6 Se 6 undergoes a metal-insulator transition at ϳ170 K: electronic-structure calculations indicate that this is likely to be driven by the formation of a dynamical charge-density wave. However, Tl 2 Mo 6 Se 6 and In 2 Mo 6 Se 6 remain metallic down to low temperature, with superconducting transitions at T c = 4.2 K and 2.85 K, respectively. The absence of any metal-insulator transition in these materials is due to a larger in-plane bandwidth, leading to increased interchain hopping which suppresses the density wave instability. Electronic heat-capacity data for the superconducting compounds reveal an exceptionally low density of states D E F = 0.055 states eV −1 atom −1 , with BCS fits showing 2⌬ / k B T c Ն 5 for Tl 2 Mo 6 Se 6 and 3.5 for In 2 Mo 6 Se 6 . Modeling the lattice specific heat with a set of Einstein modes, we obtain the approximate phonon density of states F͑͒. Deconvolving the resistivity for the two superconductors then yields their electron-phonon transport coupling function ␣ tr 2 F͑͒. In Tl 2 Mo 6 Se 6 and In 2 Mo 6 Se 6 , F͑͒ is dominated by an optical "guest ion" mode at ϳ5 meV and a set of acoustic modes from ϳ10 to 30 meV. Rb 2 Mo 6 Se 6 exhibits a similar spectrum; however, the optical phonon has a lower intensity and is shifted to ϳ8 meV. Electrons in Tl 2 Mo 6 Se 6 couple strongly to both sets of modes, whereas In 2 Mo 6 Se 6 only displays significant coupling in the 10-18 meV range. Although pairing is clearly not mediated by the guest ion phonon, we believe it has a beneficial effect on superconductivity in Tl 2 Mo 6 Se 6 , given its extraordinarily large coupling strength and higher T c compared to In 2 Mo 6 Se 6 .