Limits on graphene's charge carrier mobility and what we can do to overcome them --- interactions and disorder in epitaxial graphene on silicon carbide

SiC（0001）上的外延石墨烯是用于具有精确厚度控制的晶片尺寸生长的石墨烯的最佳实现。由于与衬底的相互作用很小，锥形能带结构被很好地保留。虽然该材料系统原则上适用于电子器件中的应用-背栅和掺杂已经成功实施-相对于悬浮的石墨烯片降低的载流子迁移率似乎是一个不可逾越的障碍。高速电子石墨烯器件中的输运本质上受到来自热光学声子的电荷载流子的散射的限制。此外，电荷载流子迁移率受到诸如衬底、缺陷或杂质的存在的外部效应的损害。尽管以前的努力，更好地了解内在和外在的影响是非常可取的。我们将使用时间和角度分辨光电子能谱（ARPES）来访问石墨烯中热载流子的衰变通道，并将其与高分辨率静态ARPES以及简单的模型计算相结合，以研究不同衬底，缺陷和杂质对石墨烯电子结构的影响。石墨烯/SiC（0001）系统为这种研究提供了理想的场所，因为它允许在石墨烯-SiC界面之间插入各种原子（例如Au、Ge和H）以及通过原子或分子吸附进行化学掺杂。此外，可以通过用氩离子轰击故意破坏石墨烯晶格来获得受控的缺陷密度。在第二步中，我们将建立在通过这些研究获得的知识的基础上，设计具有优化的石墨烯-SiC界面和掺杂水平的石墨烯/SiC（0001）系统。为此，我们将利用不同准粒子相互作用与衬底屏蔽，化学掺杂以及故意创建的缺陷浓度的相互作用，并找到一种组合，从而为石墨烯的电荷载流子提供最佳条件。最后，使用选择性相干声子激发与飞秒光脉冲在中红外波长，我们将试图提示石墨烯中不同的准粒子相互作用之间的平衡，并诱导新的电子性质缺乏平衡。红外激活的A_2u面外声子的共振激发将导致在原本不耦合的π带和σ带之间的平均非零耦合。这种耦合可能会触发相变，进入预测的量子自旋霍尔相，甚至进入瞬态超导相。静态和时间分辨ARPES的结合对于更完整地理解相互作用和无序是必不可少的，因为只有良好的时间和能量分辨率才能最终揭示石墨烯中特殊电子结构的复杂性。

Epitaxial graphene on SiC(0001) is the favorite realization of graphene for wafer-sized growth with precise thickness control. Due to the small interaction with the substrate, the conical band structure is well-preserved. While the material system is in principle suitable for the application in electronic devices --- back gating and doping have been successfully implemented --- the reduced carrier mobility with respect to suspended graphene sheets seems to be an insuperable obstacle. Transport in high-speed electronic graphene devices is intrinsically limited by the scattering of the charge carriers from hot optical phonons. In addition, the charge carrier mobility is impaired by extrinsic effects, such as the presence of a substrate, defects or impurities. Despite previous efforts, a better understanding of both intrinsic and extrinsic effects is highly desirable. We will use time- and angle- resolved photoemission spectroscopy (ARPES) to access the decay channels of hot carriers in graphene and combine it with high-resolution static ARPES as well as simple model calculations to investigate the effects of different substrates, defects and impurities on the electronic structure of graphene. The graphene/SiC(0001) system offers an ideal playground for such investigations as it allows for the intercalation of various atoms (e.g. Au, Ge and H) in between the graphene-SiC interface as well as chemical doping by atom or molecule adsorption. Furthermore, controlled defect densities can be obtained by deliberately destroying the graphene lattice with Argon ion bombardment. In a second step, we will build on the knowledge gained through these studies to engineer a graphene/SiC(0001) system with an optimized graphene-SiC interface and doping level. For this purpose, we will exploit the interplay of different quasiparticle interactions with substrate screening, chemical doping, as well as deliberately created defect concentrations, and find a combination that results in the best possible conditions for graphene's charge carriers. Finally, using selective coherent phonon excitation with femtosecond optical pulses at mid-infrared wavelengths, we will attempt to tip the equilibrium balance between different quasiparticle interactions in graphene and induce novel electronic properties absent in equilibrium. Resonant excitation of the IR-active A_2u out-of-plane phonon will result in an average non-zero coupling between the otherwise uncoupled pi- and sigma-bands. This coupling might trigger phase transitions into the predicted quantum spin Hall phase or even into a transient superconducting phase. The combination of static and time-resolved ARPES is essential for a more complete understanding of interactions and disorder as only both a good time and energy resolution will ultimately reveal the intricacies of the peculiar electronic structure in graphene.