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Excitonic-insulator instability and Peierls distortion in one-dimensional semimetals Barborini, Matteo ; ; et al in Physical Review. B (2022), 105(7), 075122 The charge density wave instability in one-dimensional semimetals is usually explained through a Peierls-like mechanism, where the coupling of electrons and phonons induces a periodic lattice distortion ... [more ▼] The charge density wave instability in one-dimensional semimetals is usually explained through a Peierls-like mechanism, where the coupling of electrons and phonons induces a periodic lattice distortion along certain modes of vibration, leading to a gap opening in the electronic band structure and to a lowering of the symmetry of the lattice. In this work, we study two prototypical Peierls systems: the one-dimensional carbon chain and the monatomic hydrogen chain with accurate ab initio calculations based on quantum Monte Carlo and hybrid density functional theory. We demonstrate that in one-dimensional semimetals at T=0, a purely electronic instability can exist independently of a lattice distortion. It is induced by spontaneous formation of low energy electron-hole pairs resulting in the electronic band gap opening, i.e., the destabilization of the semimetallic phase is due to an excitonic mechanism. [less ▲] Detailed reference viewed: 112 (14 UL)Collective electronic excitations in charge density wave systems: The case of CuTe Cudazzo, Pier Luigi ; Wirtz, Ludger in Physical Review. B (2021), 104(12), 125101 The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW ... [more ▼] The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW) transition in solids. In particular it allows us to disentangle purely phononic mechanisms from the excitonic insulator scenario which is associated to a purely electronic mechanism. As a matter of fact, while the the loss function of the excitonic insulators should display negative dispersive features associated to the softening of neutral electronic excitations at the CDW wave vector above the critical temperature, no softening is expected when the driving force is purely phononic. Here we perform a microscopic analysis of the dynamical charge response of CuTe, a material that displays a low-temperature Peierls-like CDW instability. By means of first-principles time-dependent density functional calculations of the loss function, we characterize the plasmon dispersion along the different directions, highlighting the role of the intrinsic structural anisotropy and the effects of the crystal local fields that are responsible for the periodic reappearance of the spectra of the first Brillouin zone as well as the formation of an acousticlike plasmon. Finally, we demonstrate that also in this system, in analogy with other materials displaying excitonic insulator instability, the low energy region of the loss function presents negative dispersive structures at momentum transfer close to the CDW wave vector. This is a feature common to both excitonic insulator transition and Peierls distortion that further highlights how the difference between the two mechanisms is at most quantitative. [less ▲] Detailed reference viewed: 47 (8 UL)Exciton band structure of molybdenum disulfide: from monolayer to bulk ; Cudazzo, Pier Luigi ; et al in Electronic Structure (2021), 3 Exciton band structures analysis provides a powerful tool to identify the exciton character of materials, from bulk to isolated systems, and goes beyond the mere analysis of the optical spectra. In this ... [more ▼] Exciton band structures analysis provides a powerful tool to identify the exciton character of materials, from bulk to isolated systems, and goes beyond the mere analysis of the optical spectra. In this work, we focus on the exciton properties of molybdenum sisulfide (MoS 2 ) by solving the ab initio many-body Bethe–Salpeter equation, as a function of momentum, to obtain the excitation spectra of both monolayer and bulk MoS 2 . We analyse the spectrum and the exciton dispersion on the basis of a model excitonic Hamiltonian capable of providing an efficient description of the excitations in the bulk crystal, starting from the knowledge of the excitons of a single layer. In this way, we obtain a general characterization of both bright and darks excitons in terms of the interplay between the electronic band dispersion (i.e. interlayer hopping) and the electron–hole exchange interaction. We identify for both the 2D and the 3D limiting cases the character of the lowest-energy excitons in MoS 2 , we explain the effects and relative weights of both band dispersion and electron–hole exchange interaction and finally we interpret the differences observed when changing the dimensionality of the system. [less ▲] Detailed reference viewed: 64 (1 UL)First-principles description of the exciton-phonon interaction: A cumulant approach Cudazzo, Pier Luigi in Physical Review. B (2020), 102 Electron-phonon coupling leads to intriguing effects in the spectra of materials. Current approximations to calculate spectra most often describe this coupling insufficiently. Starting from basic ... [more ▼] Electron-phonon coupling leads to intriguing effects in the spectra of materials. Current approximations to calculate spectra most often describe this coupling insufficiently. Starting from basic equations of many-body perturbation theory, we derived a cumulant formulation for neutral excitation spectra that contains excitonic effects and the coupling between excitons and phonons. The cumulant approach allows us to include dynamical effects arising from the electron-phonon coupling in a simple and intuitive way. It can be implemented as a postprocessing of state-of-the-art GW-plus-Bethe-Salpeter calculation of excitonic states and a density functional perturbation theory calculation of phonons and electron-phonon coupling. We demonstrate that, in order to obtain a consistent treatment of exciton-phonon coupling, diagrams have to be taken into account that can be neglected when the effect of lattice vibrations is treated in a static or quasistatic approximation. From the application of this approach to a model system, we analyzed the main features of the exciton-phonon interaction and provided a general picture of their link with the properties of materials such as exciton mass and exciton Bohr radius. [less ▲] Detailed reference viewed: 157 (8 UL)Correlation satellites in optical and loss spectra Cudazzo, Pier Luigi ; in Physical Review Research (2020), 2 Coupling of excitations leads to intriguing effects on the spectra of materials. We propose a cumulant formulation for neutral electronic excitations which opens the way to describe effects such as double ... [more ▼] Coupling of excitations leads to intriguing effects on the spectra of materials. We propose a cumulant formulation for neutral electronic excitations which opens the way to describe effects such as double plasmon satellites or exciton-exciton coupling. Our approach starts from the GW+Bethe-Salpeter approximation to many-body perturbation theory which is based on a quasiparticle picture, and it adds coupling of excitations through a consistent inclusion of dynamically screened interactions. This requires one to consider scattering contributions that are usually neglected. The result is formulated in a way that highlights essential physics, that can be implemented as a postprocessing tool in first-principles codes, and that suggests which kind of materials and measurements should exhibit strong effects. This is illustrated using a model. [less ▲] Detailed reference viewed: 127 (8 UL) |
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