Spatially-Resolved Electronic and Magnetic Structure of 2D Van der Waals Materials and Heterostructures

非技术描述：在这个项目中，一类被称为“二维(2D)范德华材料”的材料正在研究中。这些材料由单独的原子片(面)组成，这些原子片彼此之间只有非常弱的键(范德瓦尔斯键)。在这种材料中，电子主要在薄片内运动，产生电子轨道的特殊性质(即电子的“量子态”)。从基础物理的角度研究电子的这些轨道是非常有趣的，这些轨道在小功率、高速晶体管等电子器件中具有潜在的应用前景。一种名为低温扫描隧道显微镜(LT-STM)的仪器正在用于这项研究，该仪器最近安装在卡内基梅隆大学，由NSF主要研究仪器计划提供资金。有了这台仪器，材料顶层原子平面上的单个原子都能成像，电子轨道也能成像。这项研究还包括：(1)使用LT-STM仪器中的磁性“探头”研究材料的磁性；(2)将成堆的原子片组装在一个片到另一个片中，这些原子片上包含不同类型的原子。形成这种“异质结构”导致了电子的新型轨道，同样具有在各种电子设备中的潜在应用。项目人员参与了对这种独特的LT-STM仪器操作的研究生和本科生的培训，以及协助外部用户，并向来访的中学生演示纳米科学。技术描述：初步研究重点是过渡金属二卤化物(TMD)材料，如MoS2和WSe2。通常，在由两种不同材料的原子层组成的异质双层中，两层中的晶格是旋转排列的，但由于两层中不同的晶格常数(原子之间的分离)而形成了干涉图案。这种模式对异质双分子膜中的电子态有很大的影响。特别是，价带边和导带边附近的态被发现限制在干涉图案的单位晶胞内，即类似于“量子点”的限制态。相对较大的晶胞尺寸(约10 nm)导致对最低能态的强烈限制，导致非常平坦(宽度小于1 meV)的电子带从通常的价带和导带边缘分离出来。这些材料在低温下可能会出现新的物理现象，这是由于平带的大密度态引起的不稳定性；可以想象，异质双分子膜的限制态可以应用于例如量子计算。这项研究工作利用低温自旋分辨扫描隧道显微镜和光谱来研究包括异质结构在内的各种二维材料的能带结构，这些材料表现出磁性、超导和电荷密度波现象。自旋分辨扫描隧道显微镜为参与的学生提供了独特的机会，并通过向外部用户开放而使其他研究项目受益。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical Description: In this project, a class of materials known as "two-dimensional (2D) Van der Waals materials" are being studied. These materials consist of individual sheets (planes) of atoms, with the sheets bonded to each other with only very weak bonds (Van der Waals type bonds). In such materials, the electrons move mainly within a sheet, producing special properties for the electron orbitals (that is, the "quantum states" of the electrons). It is very interesting from a fundamental physics point of view to study these orbitals of the electrons, and the orbitals have potential application in electronic devices such as low-power, high-speed transistors. An instrument known as a low-temperature scanning tunneling microscope (LT-STM), recently installed at Carnegie Mellon University and funded through the NSF Major Research Instrumentation program, is being used for the research. With this instrument, the individual atoms on the top atomic plane of the material are imaged, and the electron orbitals are also imaged. The research also includes: (i) study of magnetic properties of the materials, using a magnetic "probe" in the LT-STM instrument; and (ii) assembling of stacks of atomic sheets, containing different types of atoms in one sheet to the next. Forming such "heterostructures" leads to new type of orbitals for the electrons, again with potential application in variety of electronic devices. The project personnel are involved in graduate and undergraduate student training on this unique LT-STM instrument operation, as well as in assisting external users, and demonstration on nanoscience to visiting middle school children.Technical Description: Initial studies are focusing on transition metal dichalcogenide (TMD) materials, such as MoS2 and WSe2. Typically in heterobilayers composed of atomic layers of two different materials, the lattices in the two layers are rotationally aligned, but nevertheless an interference pattern forms due to the different lattice constants (separation between atoms) in the two layers. This pattern is found to have a large influence on the electronic states in the heterobilayer. In particular, states near the valence and conduction band edges are found to be confined within the unit cell of the interference patterns, i.e. analogous to confined states of "quantum dots". The relatively large size of the unit cell (about 10 nm) leads to strong confinement of the lowest energy states, leading to a very flat (width of less than 1 meV) electronic bands that are split off from the usual valence and conduction band edge. New types of physics phenomena may be manifest in these materials at low temperature, due to instabilities arising from the large density of states of the flat bands; conceivably, the confined states of the heterobilayers could find application, e.g., in quantum computing. The research effort utilizes low-temperature spin-resolved scanning tunneling microscopy and spectroscopy to investigate the band structures of a variety of two-dimensional materials, including heterostructures, that exhibit magnetic, superconducting and charge density wave phenomena. The spin-resolved scanning tunneling microscope offers unique opportunities for the students involved and benefits other research projects by being available to external users.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.