Theory of electron-plasmon coupling in semiconductors

The ability to manipulate plasmons is driving new developments in electronics, optics, sensing, energy, and medicine. Despite the massive momentum of experimental research in this direction, a predictive quantum-mechanical framework for describing electron-plasmon interactions in real materials is still missing. Here, starting from a many-body Green's function approach, we develop an ab initio approach for investigating electron-plasmon coupling in solids. As a first demonstration of this methodology, we show that electron-plasmon scattering is the primary mechanism for the cooling of hot carriers in doped silicon, it is key to explain measured electron mobilities at high doping, and it leads to a quantum zero-point renormalization of the band gap in agreement with experiment

Two-gap superconductivity in heavily n-doped graphene: ab initio Migdal-Eliashberg theory

Graphene is the only member of the carbon family from zero- to three-dimensional materials for which superconductivity has not been observed yet. At this time, it is not clear whether the quest for superconducting graphene is hindered by technical challenges, or else by the fluctuation of the order parameter in two dimensions. In this area, ab initio calculations are useful to guide experimental efforts by narrowing down the search space. In this spirit, we investigate from first principles the possibility of inducing superconductivity in doped graphene using the fully anisotropic Migdal-Eliashberg theory powered by Wannier-Fourier interpolation. To address a best-case scenario, we consider both electron and hole doping at high carrier densities, so as to align the Fermi level to a van Hove singularity. In these conditions, we find superconducting gaps of $s$-wave symmetry, with a slight anisotropy induced by the trigonal warping, and, in the case of $n$-doped graphene, an unexpected two-gap structure reminiscent of MgB$_2$. Our Migdal-Eliashberg calculations suggest that the observation of superconductivity at low temperature should be possible for $n$-doped graphene at carrier densities exceeding $10^{15}$ cm$^{-2}$

Electron-phonon interactions from first principles

This article reviews the theory of electron-phonon interactions in solids from the point of view of ab-initio calculations. While the electron-phonon interaction has been studied for almost a century, predictive non-empirical calculations have become feasible only during the past two decades. Today it is possible to calculate from first principles many materials properties related to the electron-phonon interaction, including the critical temperature of conventional superconductors, the carrier mobility in semiconductors, the temperature dependence of optical spectra in direct and indirect-gap semiconductors, the relaxation rates of photoexcited carriers, the electron mass renormalization in angle-resolved photoelectron spectra, and the non-adiabatic corrections to phonon dispersion relations. Here we review the theoretical and computational framework underlying modern electron-phonon calculations from first principles, as well as landmark investigations of the electron-phonon interaction in real materials. In the first part of the article we summarize the elementary theory of electron-phonon interactions and their calculations based on density-functional theory. In the second part we discuss a general field-theoretic formulation of the electron-phonon problem, and establish the connection with practical first-principles calculations. In the third part we review a number of recent investigations of electron-phonon interactions in the areas of vibrational spectroscopy, photoelectron spectroscopy, optical spectroscopy, transport, and superconductivity.Comment: 68 pages, 18 PNG figures. Expanded following suggestions from reviewers and colleagues. Added approx. 50 references. Updated arXiv references. Fixed typos (thanks everyone for your feedback). To appear in Reviews of Modern Physic

Quasiparticle GWGW band structures and Fermi surfaces of bulk and monolayer NbS2_2

In this work we employ the $GW$ approximation in the framework of the SternheimerGW method to investigate the effects of many-body corrections to the band structures and Fermi surfaces of bulk and monolayer NbS$_2$. For the bulk system, we find that the inclusion of these many-body effects leads to important changes in the band structure, especially in the low-energy regime around the Fermi level, and that our calculations are in good agreement with recent ARPES measurements. In the case of a free-standing monolayer NbS$_2$, we observe a strong increase of the screened Coulomb interaction and the quasiparticle corrections as compared to bulk. In this case we also perform calculations to include the effect of screening by a substrate. We report in detail the results of our convergence tests and computational parameters, to serve as a solid basis for future studies.Comment: 15 pages, 18 figure

Towards predictive many-body calculations of phonon-limited carrier mobilities in semiconductors

We probe the accuracy limit of {\it 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.Comment: 11 pages and 8 figure