Benedikt Scharf

Effects of optical and surface polar phonons on the optical conductivity of doped graphene

Using the Kubo linear response formalism, we study the effects of intrinsic graphene optical and surface polar phonons (SPPs) on the optical conductivity of doped graphene. We find that inelastic electron-phonon scattering contributes significantly to the phonon-assisted absorption in the optical gap. At room temperature, this midgap absorption can be as large as 20-25% of the universal ac conductivity for graphene on polar substrates due to strong electron-SPP coupling. The midgap absorption, moreover, strongly depends on the substrates and doping levels used. With increasing temperature, the midgap absorption increases, while the Drude peak, on the other hand, becomes broader as inelastic electron-phonon scattering becomes more probable. Consequently, the Drude weight decreases with increasing temperature.

Magneto-optical conductivity of graphene on polar substrates

We theoretically study the effect of polar substrates on the magneto-optical conductivity of doped monolayer graphene, where we particularly focus on the role played by surface polar phonons (SPPs). Our calculations suggest that polaronic shifts of the intra- and interband absorption peaks can be significantly larger for substrates with strong electron-SPP coupling than those in graphene on non-polar substrates, where only intrinsic graphene optical phonons with much higher energies contribute. Electron-phonon scattering and phonon-assisted transitions are, moreover, found to result in a loss of spectral weight at the absorption peaks. The strength of these processes is strongly temperature-dependent and with increasing temperatures the magneto-optical conductivity becomes increasingly affected by polar substrates, most noticeably in polar substrates with small SPP energies such as HfO

Coulomb drag between massless and massive fermions

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Marrying Excitons and Plasmons in Monolayer Transition-Metal Dichalcogenides

. The inclusion of a Landau level-dependent scattering rate to account for Coulomb impurity scattering does not alter this qualitative picture, but can play an important role in determining the lineshape of the absorption peaks, especially at low temperatures, where impurity scattering dominates.

Magnetic properties of HgTe quantum wells

We theoretically investigate the frictional drag induced by the Coulomb interaction between spa- tially separated massless and massive fermions at low temperatures. As a model system, we use a double-layer structure composed of a two-dimensional electron gas (2DEG) and a n-doped graphene layer. We analyze this system numerically and also present analytical formulae for the drag re- sistivity in the limit of large and small interlayer separation. Both, the temperature and density dependence are investigated and compared to 2DEG-2DEG and graphene-graphene double-layer structures. Whereas the density dependence of the transresistivity for small interlayer separation differs already in the leading order for each of those three structures, we find the leading order con- tribution of density dependence in the large interlayer separation limit to exhibit the same density dependence in each case. In order to distinguish between the different systems in the large interlayer separation limit, we also investigate the subleading contribution to the transresistivity. Furthermore, we study the Coulomb drag in a double-layer structure consisting of n-doped bilayer and monolayer graphene, which we find to possess the same qualitative behavior as the 2DEG-graphene system.

Magnetic Proximity Effects in Transition-Metal Dichalcogenides: Converting Excitons

Just as photons are the quanta of light, plasmons are the quanta of orchestrated charge-density oscillations in conducting media. Plasmon phenomena in normal metals, superconductors and doped semiconductors are often driven by long-wavelength Coulomb interactions. However, in crystals whose Fermi surface is comprised of disconnected pockets in the Brillouin zone, collective electron excitations can also attain a shortwave component when electrons transition between these pockets. Here, we show that the band structure of monolayer transition-metal dichalcogenides gives rise to an intriguing mechanism through which shortwave plasmons are paired up with excitons. The coupling elucidates the origin for the optical side band that is observed repeatedly in monolayers of WSe

Theory of thermal spin-charge coupling in electronic systems.

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Probing Majorana-like states in quantum dots and quantum rings

and WS

Excitonic Stark effect in MoS 2 monolayers

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Proximitized Materials.

but not understood. The theory makes it clear why exciton-plasmon coupling has the right conditions to manifest itself distinctly only in the optical spectra of electron-doped tungsten-based monolayers.