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

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

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.