We elucidate the origin of the phonon-mediated superconductivity in 2H-NbS2 using the ab initio anisotropic Migdal-Eliashberg theory including Coulomb interactions. We demonstrate that superconductivity is associated with Fermi surface hot spots exhibiting an unusually strong electronphonon interaction. The electron-lattice coupling is dominated by low-energy anharmonic phonons, which place the system on the verge of a charge density wave instability. We also provide denitive evidence for two-gap superconductivity in 2H-NbS2, and show that the low-and high-energy peaks observed in tunneling spectra correspond to the Γ-and K-centered Fermi surface pockets, respectively. The present ndings call for further eorts to determine whether our proposed mechanism underpins superconductivity in the whole family of metallic transition metal dichalcogenides. PACS numbers: 74.70.Xa, 63.20.kd, 74.20.Fg, 74.25.Jb Transition metal dichalcogenides (TMDs) have generated considerable interest in recent years, since they provide an ideal playground for studying semiconductors, metals, and superconductors in two dimensions using the same structural template [13]. In the case of superconducting TMDs, one remarkable feature is that Cooper pair condensation usually coexists with a charge density wave (CDW) [4], raising the question on whether superconductivity and CDW co-operate or compete in these compounds [512].Within the family of superconducting TMDs, 2H-NbS 2 stands out as the only system for which a CDW phase has not been observed [13, 14]. This suggests that a comparative analysis of NbS 2 and other superconducting TMDs may help to clarify the interplay between the superconductive and the CDW instabilities in the entire family. 2H-NbS 2 is a phonon-mediated superconductor with a critical temperature T c = 5.7 K. Scanning tunneling spectroscopy (STS) measurements on this compound revealed two pronounced features in the density of states (DOS) at 0.53 meV and 0.97 meV below the critical temperature, providing strong indications of two-gap superconductivity [14]. However, so far microscopic calculations have considered only a single-gap scenario [15, 16].In this work we investigate the nature of the superconducting gap and the pairing mechanism in 2H-NbS 2 using the fully anisotropic ab initio Migdal-Eliashberg theory, and describe both electron-phonon and electronelectron interactions without any adjustable parameters. Our key nding is that a very signicant contribution to the superconducting pairing comes from the lowenergy anharmonic phonons with wavevectors near the line connecting the M and L points. These are the same phonons responsible for the CDW instability in other TMDs [8, 11, 1719], indicating that superconductivity in NbS 2 is intimately connected with a latent CDW. In agreement with the STS experiments of Ref. 14, we nd two distinct and anisotropic superconducting gaps.All calculations reported in this work were performed using density functional theory (DFT) in the local density approximation [20,21]. We employed...

Using the ab initio anisotropic Eliashberg theory including Coulomb interactions, we investigate the electron-phonon interaction and the pairing mechanism in the recently-reported superconducting Ca-intercalated bilayer graphene. We find that C6CaC6 can support phonon-mediated superconductivity with a critical temperature Tc = 6.8–8.1 K, in good agreement with experimental data. Our calculations indicate that the low-energy Caxy vibrations are critical to the pairing, and that it should be possible to resolve two distinct superconducting gaps on the electron and hole Fermi surface pockets.

Using state-of-the-art many-body calculations based on the "GW plus cumulant" approach, we show that electron-plasmon interactions lead to the emergence of plasmonic polaron bands in the band structures of common semiconductors. Using silicon and group IV transition-metal dichalcogenide monolayers (AX_{2} with A=Mo,W and X=S, Se) as prototypical examples, we demonstrate that these new bands are a general feature of systems characterized by well-defined plasmon resonances. We find that the energy versus momentum dispersion relations of these plasmonic structures closely follow the standard valence bands, although they appear broadened and blueshifted by the plasmon energy. Based on our results, we identify general criteria for observing plasmonic polaron bands in the angle-resolved photoelectron spectra of solids.

Articles you may be interested inReduced dimensionality spin-orbit dynamics of CH3 + HCl CH4 + Cl on ab initio surfaces We report a new experimental determination of the branching ratio for formation of ground state Cl( 2 P 3/2 ) and spin-orbit excited Cl( 2 P 1/2 ) fragments subsequent to excitation of the HCl molecule to the repulsive A 1 ⌸ electronic state. Our work extends previous experimental measurements well into the long-wavelength tail of the A 1 ⌸←X 1 ⌺ ϩ absorption profile. The branching into the spin-orbit excited Cl( 2 P 1/2 ) fragment is found to reach a maximum near 220 nm, before falling to zero at threshold. Our new measured branching fractions are reproduced nearly quantitatively by new calculations which supplement those reported previously ͓M. H. Alexander, B. Pouilly, and T. Duhoo, J. Chem. Phys. 99, 1752 ͑1993͔͒.

We present an implementation of time-dependent density functional perturbation theory for spin fluctuations, based on planewaves and pseudopotentials. We compute the dynamic spin susceptibility self-consistently by solving the time-dependent Sternheimer equation, within the adiabatic local density approximation to the exchange and correlation kernel. We demonstrate our implementation by calculating the spin susceptibility of representative elemental transition metals, namely bcc Fe, fcc Ni and bcc Cr. The calculated magnon dispersion relations of Fe and Ni are in agreement with previous work. The calculated spin susceptibility of Cr exhibits a soft-paramagnon instability, indicating the tendency of the Cr spins to condense in a incommensurate spin density wave phase, in agreement with experiment. arXiv:1707.05219v1 [cond-mat.mtrl-sci]