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

在碳化硅(0001)上外延石墨烯是最受欢迎的实现石墨烯的晶片尺寸生长和精确厚度控制的方法。由于与衬底的相互作用很小，锥形能带结构被很好地保存下来。虽然该材料系统原则上适用于电子器件的应用-背门和掺杂已经成功实施-相对于悬浮的石墨烯薄膜，载流子迁移率的降低似乎是一个不可逾越的障碍。高速电子石墨烯器件的输运本质上受到热光学声子载流子散射的限制。此外，载流子迁移率受到外在效应的影响，例如衬底、缺陷或杂质的存在。尽管之前做出了努力，但更好地理解内在和外在影响是非常可取的。我们将使用时间和角度分辨光电子能谱(ARPES)来获取石墨烯中热载流子的衰变通道，并将其与高分辨率静态光电子能谱以及简单的模型计算相结合，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/碳化硅(0001)体系为此类研究提供了一个理想的平台，因为它允许各种原子(如Au、Ge和H)插入石墨烯-碳化硅界面之间，以及通过原子或分子吸附进行化学掺杂。此外，可以通过用Ar离子轰击故意破坏石墨烯晶格来控制缺陷密度。在第二步中，我们将在这些研究中获得的知识的基础上，设计出具有优化的石墨烯-碳化硅界面和掺杂水平的石墨烯/碳化硅(0001)系统。为此，我们将利用不同准粒子相互作用与衬底筛选、化学掺杂以及故意创造缺陷浓度的相互作用，并找到一种组合，为石墨烯的电荷载流子带来可能的最佳条件。最后，利用中红外波段飞秒光脉冲的选择性相干声子激发，我们将尝试倾斜石墨烯中不同准粒子相互作用之间的平衡，并诱导出平衡中不存在的新的电子性质。红外活性的A_2U离面声子的共振激发将导致未耦合的pi带和sigma带之间的平均非零耦合。这种耦合可能会触发相变到预测的量子自旋霍尔相，甚至触发到瞬时超导相。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，因为只有同时具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.