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，正在研究一类被称为“二维（2D）范德华材料”的材料。这些材料由单独的原子片（平面）组成，这些原子片彼此之间只有非常弱的键（范德华式键）。在这种材料中，电子主要在薄片内运动，从而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，并且这些轨道在电子器件如低功耗、高速晶体管中有潜在的应用。一种被称为低温扫描隧道显微镜（LT-STM）的仪器，最近安装在卡内基梅隆大学，由美国国家科学基金会主要研究仪器项目资助，正在用于这项研究。利用这台仪器，可以对材料上原子平面上的单个原子进行成像，也可以对电子轨道进行成像。研究还包括：(i)研究材料的磁性，在LT-STM仪器中使用磁性“探针”；（二）原子片堆的组装，一张到另一张包含不同类型的原子。形成这样的“异质结构”导致了电子的新型轨道，在各种电子设备中也有潜在的应用。项目人员参与了这种独特的LT-STM仪器操作的研究生和本科生培训，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：最初的研究集中在过渡金属二硫化物（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层中，两层中的晶格是旋转排列的，但由于两层中晶格常数（原子之间的分离）不同，形成了干涉图案。发现这种模式对异质层中的电子态有很大的影响。特别是，价带和导带边缘附近的状态被发现被限制在干涉图案的单位胞内，即类似于“量子点”的限制状态。相对较大的单元电池尺寸（约10纳米）导致最低能态的强烈限制，导致从通常的价带和导带边缘分离出非常平坦的电子带（宽度小于1 meV）。由于平带的大密度态引起的不稳定性，这些材料在低温下可能会出现新的物理现象；可以想象，异质层的受限状态可以在量子计算等领域得到应用。该研究利用低温自旋分辨扫描隧道显微镜和光谱学来研究各种二维材料的能带结构，包括表现出磁性、超导和电荷密度波现象的异质结构。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，并为外部用户提供了其他研究项目。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.