Surface Modification of Dirac Materials

固体中的电子不一定完全像自由空间中的电子一样。特别是，它们可能有一个不同于实际电子质量的“有效质量”，它描述了固体中电子对力的反应方式。最近，已经发现了电子质量为零的材料。在这种材料中，电子以恒定的速度运动，与能量无关，类似于光的粒子以恒定的速度运动。这些材料被称为“狄拉克材料”，因为描述电子运动的狄拉克方程与描述电子接近光速运动的狄拉克方程是一样的。狄拉克材料包括石墨烯（单原子厚度的碳原子平面）和硒化铋的金属表面。该项目将研究如何利用石墨烯和硒化铋的缺陷来改变材料的电子特性，如诱导磁性或从导电行为过渡到绝缘行为。本项目将研究双层石墨烯和强拓扑绝缘体Bi2Se3两种狄拉克电子材料系统的表面改性及其对电子输运性质的影响。采用机械剥离法制备双层石墨烯（BLG），并用超高压He+离子照射诱导空位。原子去除截面的差异可能会导致缺陷浓度的上下对称性和A-B对称性被打破。进一步的对称性破坏可以通过门控来实现。缺陷的亚晶格对称性的破坏有望通过RKKY效应导致BLG中的铁磁性，本工作将在各向异性磁电阻和自旋注入实验中寻找缺陷BLG中铁磁性的证据。采用分子束外延的方法在绝缘衬底上制备Bi2Se3薄膜，并在不破真空的情况下在薄膜上原位进行电子输运测量。将研究的中心问题是制备薄膜中的界面和体掺杂，退火或暴露于环境中的掺杂，以及磁性和非磁性杂质的散射。

Electrons in solids need not behave exactly like electrons in free space. In particular, they may have an 'effective mass' which is different from the real electron mass, and describes the way the electron in the solid responds to a force. Recently, materials have been discovered in which the electron mass is exactly zero. In such materials the electron moves at a constant speed regardless of its energy, akin to particles of light which travel at constant speed. These materials are called 'Dirac materials' because the electron motion is described by the same Dirac equation which describes electrons moving close to the speed of light. Dirac materials include graphene (a single atom thick plane of carbon atoms) and the metallic surface of bismuth selenide. This project will study how defects in graphene and bismuth selenide can be used to change the electronic properties of the materials, such as inducing magnetism or a transition from conducting to insulating behavior. Technical AbstractThis project will study the surface modification and resulting modification of electronic transport properties of two Dirac electronic materials systems, bilayer graphene and strong topological insulator Bi2Se3. Bilayer graphene (BLG) will be prepared by mechanical exfoliation, and irradiated with He+ ions in UHV to induce vacancies. Differences in cross section for atom removal are expected to result in breaking the top-bottom and A-B symmetries in defect concentration. Further symmetry-breaking could be provided by gating. Broken sublattice symmetry of defects is expected to lead to ferromagnetism in BLG via the RKKY effect, and this effort will search for evidence of ferromagnetism in defective BLG in anisotropic magnetoresistance as well as in spin-injection experiments. Thin films of Bi2Se3 will be prepared by molecular beam epitaxy on insulating substrates, and electronic transport measurements will be performed in situ on the films without breaking vacuum. The central issues that will be studies are interface and bulk doping in as-prepared films, doping upon annealing or exposure to ambient, and scattering by magnetic and non-magnetic impurities.