The EPW (Electron-Phonon coupling using Wannier functions) software is a Fortran90 code that uses densityfunctional perturbation theory and maximally localized Wannier functions for computing electron-phonon couplings and related properties in solids accurately and efficiently. The EPW v4 program can be used to compute electron and phonon self-energies, linewidths, electron-phonon scattering rates, electron-phonon coupling strengths, transport spectral functions, electronic velocities, resistivity, anisotropic superconducting gaps and spectral functions within the Migdal-Eliashberg theory. The code now supports spin-orbit coupling, time-reversal symmetry in noncentrosymmetric crystals, polar materials, and k and q-point parallelization. Considerable effort was dedicated to optimization and parallelization, achieving almost a ten times speedup with respect to previous releases. A computer test farm was implemented to ensure stability and portability of the code on the most popular compilers and architectures. Since April 2016, version 4 of the EPW code is fully integrated in and distributed with the Quantum ESPRESSO package, and can be downloaded through QE-forge at http://qe-forge.org/gf/project/q-e.

The movement of dislocations in a crystal is the key mechanism for plastic deformation in all materials. Studies of dislocations have focused on three-dimensional materials, and there is little experimental evidence regarding the dynamics of dislocations and their impact at the atomic level on the lattice structure of graphene. We studied the dynamics of dislocation pairs in graphene, recorded with single-atom sensitivity. We examined stepwise dislocation movement along the zig-zag lattice direction mediated either by a single bond rotation or through the loss of two carbon atoms. The strain fields were determined, showing how dislocations deform graphene by elongation and compression of C-C bonds, shear, and lattice rotations.

We probe the accuracy limit of ab initio calculations of carrier mobilities in semiconductors, within the framework of the Boltzmann transport equation. By focusing on the paradigmatic case of silicon, we show that fully predictive calculations of electron and hole mobilities require many-body quasiparticle corrections to band structures and electron-phonon matrix elements, the inclusion of spin-orbit coupling, and an extremely fine sampling of inelastic scattering processes in momentum space. By considering all these factors we obtain excellent agreement with experiment, and we identify the band effective masses as the most critical parameters to achieve predictive accuracy. Our findings set a blueprint for future calculations of carrier mobilities, and pave the way to engineering transport properties in semiconductors by design. arXiv:1803.05462v1 [cond-mat.mtrl-sci]

We combine the fully anisotropic Migdal-Eliashberg theory with electron-phonon interpolation based on maximally-localized Wannier functions, in order to perform reliable and highly accurate calculations of the anisotropic temperature-dependent superconducting gap and critical temperature of conventional superconductors. Compared with the widely used McMillan approximation, our methodology yields a more comprehensive and detailed description of superconducting properties, and is especially relevant for the study of layered or low-dimensional systems as well as systems with complex Fermi surfaces. In order to validate our method we perform calculations on two prototypical superconductors, Pb and MgB2, and obtain good agreement with previous studies.

Deposition of semiconductors and metals from chemical precursors onto planar substrates is a well-developed science and technology for microelectronics. Optical fibers are an established platform for both communications technology and fundamental research in photonics. Here, we describe a hybrid technology that integrates key aspects of both engineering disciplines, demonstrating the fabrication of tubes, solid nanowires, coaxial heterojunctions, and longitudinally patterned structures composed of metals, single-crystal semiconductors, and polycrystalline elemental or compound semiconductors within microstructured silica optical fibers. Because the optical fibers are constructed and the functional materials are chemically deposited in distinct and independent steps, the full design flexibilities of both platforms can now be exploited simultaneously for fiber-integrated optoelectronic materials and devices.

New candidate ground states at 1:4, 1:2, and 1:1 compositions are identified in the well-known Fe-B system via a combination of ab initio high-throughput and evolutionary searches. We show that the proposed oP12-FeB2 stabilizes by a break up of 2D boron layers into 1D chains while oP10-FeB4 stabilizes by a distortion of a 3D boron network. The uniqueness of these configurations gives rise to a set of remarkable properties: oP12-FeB2 is expected to be the first semiconducting metal diboride and oP10-FeB4 is shown to have the potential for phonon-mediated superconductivity with a T(c) of 15-20 K.

A double-walled carbon nanotube is used to study the radial charge distribution on the positive inner electrode of a cylindrical molecular capacitor. The outer electrode is a shell of bromine anions. Resonant Raman scattering from phonons on each carbon shell reveals the radial charge distribution. A self-consistent tight-binding model confirms the observed molecular Faraday cage effect, i.e., most of the charge resides on the outer wall, even when this wall was originally semiconducting and the inner wall was metallic.

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...