Phonon mode spectroscopy, electron-phonon coupling, and the metal-insulator transition in quasi-one-dimensionalM2Mo6Se6

Abstract

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 .

Strong-coupling electron-phonon superconductivity in noncentrosymmetric quasi-one-dimensionalK2Cr3As3

“…The PDOS also shows a peak at this frequency. Similar low-energy peaks has also been observed in M 2 Mo 6 X 6 compounds, and they have been attributed to the Einstein-like vibrations of M atoms [23,24]. However, we can see from Fig.…”

Synthesis, crystal structure, and electrical and magnetic properties of BaMo 6 Te 6 : A novel reduced molybdenum telluride containing infinite chains of trans-face shared Mo 6 octahedra

“…Recent local density approximation calculations show that in all members of the M 2 Mo 6 Se 6 series, a single 1D helix band crosses the Fermi level. Its in-plane dispersion is reduced by a factor of ten in Rb 2 Mo 6 Se 6 compared with Tl 2 Mo 6 Se 6 and In 2 Mo 6 Se 6 while the out-of-plane dispersion is practically unchanged [22]. This reduction is sufficient to explain the trend from metallic conductivity followed by strongly-coupled superconductivity in [24].…”

A disorder-enhanced quasi-one-dimensional superconductor

“…Upturns or divergence in ρ ( T ) have been widely reported in q1D materials and variously attributed to localization1819202122, multiband TLL physics23, DW formation2425, incipient density fluctuations16 and proximity to Mott instabilities8. Differentiating between these mechanisms has proved challenging, in part due to the microscopic similarity between localized electrons and randomly-pinned DWs in 1D.…”