Wehling, Tim Oliver

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Wehling, Tim Oliver
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Wehling, Tim Oliver
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Wehling, T. O.
Wehling, Tim
Wehling, T.
Wehling, Tim O.
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    Many-body instabilities in two-dimensional materials
    (2020-09-07)
    Berges, Jan
    ;
    Wirtz, Ludger
    ;
    Wehling, Tim Oliver
    ;
    Wehling, Tim Oliver
    Two-dimensional materials constitute a prominent showplace of competing many-body instabilities such as superconductivity and charge-density waves (CDWs). Even though these phenomena populate rich phase diagrams, the underlying physics can be reduced to well-defined interactions between basic particles. Here, we focus on CDWs in the metallic transition-metal dichalcogenides (TMDCs), whose origin has been controversial for decades. We approach the problem by “downfolding” the full system onto models that are complex enough to capture the relevant processes but at the same time simple enough to be dealt with efficiently. They describe the low-energy electrons, phonons—the vibrations of the lattice—, and their interaction. All model parameters are obtained from first principles, using methods such as density-functional theory and constrained density-functional perturbation theory (cDFPT). In this context, we developed a computational scheme that allows us to calculate how the electrons renormalize the phonons and thus effect said CDWs. Regarding the octahedral TMDC NbSe₂, we find that the presence of a CDW lays the ground on which electronic correlations drive the system into an insulating phase. Independently, we show that in the trigonal–prismatic TMDCs, the electrons and phonons qualitatively behave very similar but that quantitative energy differences finally determine the material specifics. Importantly, our cDFPT results reveal that the CDWs in these materials stem from interactions with the low-energy electrons alone. On this basis, we calculate the CDW phase diagram of TaS₂ and thus explain contradictory experimental findings. Furthermore, introducing the concept of fluctuation diagnostics to electron–phonon physics, we pinpoint the microscopic origin of these CDWs. We can rule out purely electronic mechanisms and confirm the importance of the momentum dependence of the electron–phonon coupling, which we trace back to massive Dirac fermions.
    doctoral thesis
      843  1215
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    Coulomb interaction and phonons in doped semiconducting and metallic two-dimensional materials
    (2017-05-17)
    Schönhoff, Gunnar
    ;
    Czycholl, Gerd
    ;
    Wehling, Tim Oliver
    ;
    Wehling, Tim Oliver
    Two-dimensional (2D) materials present a rapidly developing field of research with sometimes highly unusual and uniquely two-dimensional physics. Starting with graphene, many recent studies have investigated 2D materials, with results for properties encompassing such different topics as Dirac electrons, charge ordering, and superconductivity. To get closer to a predictive theory of the phases of 2D materials, this thesis systematically tackles the previously unclear problem of the Coulomb interaction and its influence on the electronic and many-body properties, with the focus on transition metal dichalcogenides (TMDCs). While this influence can be quite strong due to the low dimensionality and the corresponding reduced screening, there is so far no comprehensive understanding of the Coulomb interaction in 2D, its effects, and the possibilities for engineering it. Furthermore, the interplay between electron-electron interaction, electron-phonon interaction, and screening is far from being fully understood. The goal of this thesis is to improve on this by providing a description of the Coulomb interaction for the example of the TMDCs as well as to develop a material-systematic database on the basis of ab-initio calculations for electrons and phonons. We use Density Functional Theory to describe the electronic structure, Density Functional Perturbation Theory for the phonons and the electron-phonon interaction, and the Random Phase Approximation to obtain Coulomb matrix elements. In addition to both semiconducting and metallic TMDCs, we look at functionalized graphene C8H2. The first step is a quantification of the Coulomb interaction and the screening in the TMDCs along with calculations for the plasmonic spectra, which turn out to be highly susceptible to environment and doping. Secondly, we discuss the influence of the interaction on different many-body instabilities and find a small suppression of superconducting order in semiconducting TMDCs, depending again on doping and the dielectric environment, while the magnetic order in metallic TMDCs is enhanced. If we further include the phonons, we see that superconductivity is predicted to be a global phenomenon in the doped semiconducting TMDCs and C8H2, and that charge density waves at different wave vectors are supposedly occurring in all TMDCs.
    doctoral thesis
      362  238