Phonon Mode Spectroscopy, Electron-Phonon Coupling, And The Metal-Insulator Transition In Quasi-One-Dimensional M2mo6se6

We present electronic-structure calculations, electrical resistivity data, and the first specific-heat measurements in the normal and superconducting states of quasi-one-dimensional M2Mo6Se6 (M=Tl, In, Rb). Rb2Mo6Se6 undergoes a metal-insulator transition at similar to 170 K: electronic-structure calculations indicate that this is likely to be driven by the formation of a dynamical charge-density wave. However, Tl2Mo6Se6 and In2Mo6Se6 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-EF =0.055 states eV(-1) atom(-1), with BCS fits showing 2 Delta/k(B)T(c)>5 for Tl2Mo6Se6 and 3.5 for In2Mo6Se6. Modeling the lattice specific heat with a set of Einstein modes, we obtain the approximate phonon density of states F(omega). Deconvolving the resistivity for the two superconductors then yields their electron-phonon transport coupling function alpha F-2(tr)(omega). In Tl2Mo6Se6 and In2Mo6Se6, F(omega) is dominated by an optical "guest ion" mode at similar to 5 meV and a set of acoustic modes from similar to 10 to 30 meV. Rb2Mo6Se6 exhibits a similar spectrum; however, the optical phonon has a lower intensity and is shifted to similar to 8 meV. Electrons in Tl2Mo6Se6 couple strongly to both sets of modes, whereas In2Mo6Se6 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 Tl2Mo6Se6, given its extraordinarily large coupling strength and higher T-c compared to In2Mo6Se6.