Quasiparticles in graphene and other 2d materialsmodulation by a surface acoustic wave and contribution to coulomb drag

Zusammenfassung

Two-dimensional (2D) materials, such as graphene, h-BN, black phosphorus (BP) and transition metal dichalcogenides (TMDCs), have recently shown a lot of potential in the area of electronics and photonics. Assembling them into vertical van der Waals heterostructures is currently one of the hottest research fields with a wide range of newly observed phenomena. In this thesis, we have investigated various quasiparticles in graphene and other 2D materials, with special attention to their coupling and related phenomena as well as their interaction with light, the latter being modulated or mediated by a surface acoustic wave (SAW). In particular, we have addressed plasmons in graphene and its heterostructures, their hybridization with phonons, and their coupling with light by means of SAW, Coulomb drag effect in graphene, exciton-plasmon coupling by SAW in TMDCs, BP and metal and the effect of strain produced by a SAW on the phonons of graphene. We have first theoretically studied the plasmons in single (SLG) and double layer graphene (DLG) systems and their coupling with surface and hyperbolic phonons of a h-BN interlayer and the substrate leading to hybridized surface plasmon-phonon polaritons (SPPPs) and hyperbolic plasmon-phonon polaritons (HPPPs). We have demonstrated that a SAW can be used to launch propagating SPPPs in SLG and DLG system with h-BN interlayers on a piezoelectric substrate like AlN and ZnO, where the SAW-induced surface modulation acts as a dynamic diffraction grating providing the extra necessary momentum for the generation of the polaritons. The h-BN interlayer between the graphene and the piezoelectric substrate and between the two graphene layers significantly changes the dispersion of the SPPPs and increases their lifetime. We have also theoretically studied the electron-electron interactions in Coulomb drag in the case of a DLG system separated by a thin h-BN interlayer. We have demonstrated that these interactions can be influenced by various quasiparticles like plasmons and phonons, and also by exchange and correlation (XC) effects. We have numerically evaluated the effect of these quasiparticles and XC on the drag resistivity, and have shown how all these contributions altogether can modify the drag resistivity allowing us to replicate the experimental data reported in the literature by our theoretical model with high accuracy. We have continued our theoretical studies of 2D materials with TMDCs (MoS_2 and WS_2) and BP. We have proposed the coupling between excitons in these 2D materials and plasmons in a thin silver film by means of a SAW, in a similar way as in the case of graphene, demonstrating that the strong coupling regime is achieved. Finally, we have put forward the experimental study of the modulation of the Raman-active optical phonon modes of SLG by the dynamic strain created by a SAW resonator on a LiNbO_3 substrate. Here we have showed that the SAW strain field induces a variation in the Raman scattering intensity as large as 15 % and a shift in the phonon modes of up to 10 cm^(-1) for a maximum strain of 0.24 % at a SAW frequency of 400 MHz. Thus, SAWs are proven to be powerful tools to modulate the vibrational properties of graphene by means of the high-frequency localized deformations tailored by the acoustic transducers, which can also be extended to other 2D materials and van der Waals heterostructures.