Route to Quantum Anomalous Hall Effect Using Magnetic Van der Waals Heterostructures
Abstract
Advances in electronics and information technology are ultimately driven by fundamental advances in materials science. Electrical transport measurements of silicon and germanium paved the way for the development of the transistor, for example, while studies of giant magnetoresistance in magnetic multilayers has led to new memory devices such as MRAM. Atomically thin, two-dimensional (2D) materials derived from van der Waals crystals are an emerging class of materials with high potential for new disruptive technologies. Their simple layered structure further allows different species to be vertically stacked, forming more complex heterostructures that cannot be produced using traditional growth methods. Graphene and 2D semiconductors have already been combined to create tunable photodetectors with extremely large efficiencies, for instance. Recently, the library of 2D materials has grown to include compounds hosting exotic quantum behavior, such as superconductors, ferromagnets, as well as systems with nontrivial band topology. Their incorporation in unique device geometries may allow for the development of novel quantum electronics with enhanced functionalities compared to traditional semiconductor-based devices. In particular, topological insulators (TIs), where the surface states are protected by time reversal symmetry, can be potentially harnessed for ultralow dissipation circuitry. By introducing ferromagnetism to the surface states, it is further possible to realize a Chern insulator with edge channels. Experimentally, this system will show a quantized Hall conductance even at zero magnetic field, also known as the quantum anomalous Hall effect (QAHE). In comparison, the ordinary quantum Hall effect generally requires very high magnetic fields, and so may be less relevant for technology applications. Remarkably, the QAHE has been observed in TI thin films doped with magnetic ions. Yet, the Hall quantization is robust only at temperatures below 100mK. The cause has been attributed to the bulk conduction of dopant electrons, which requires ultralow temperatures to localize. Since the dopants are the magnetic ions themselves, there is a direct competition between ferromagnetism and insulating behavior of the bulk, both of which are required to observe the QAHE. Here, a different approach to realize the QAHE is proposed that avoids this competition, and so may potentially allow for its observation at elevated temperatures. By exploiting the structure and versatility of 2D materials, one can instead magnetize the surface states of the TI by placing it in proximity to a ferromagnetic insulator. Since the magnet contributes no itinerant carriers, the bulk conduction is determined by the intrinsic properties of the TI itself, which can be optimized separately during growth. Recently a class of layered magnetic insulators (CrX3, X = I, Br, Cl) has been found that show robust ferromagnetism down to single layers with transition temperatures above 10K, while high-mobility surface states have been previously demonstrated in bulk insulating BiSbTeSe2 (BSTS), whose structure is also layered. The PIÕs group shall interface these two materials using the established techniques of van der Waals heterostructures. Graphene shall be used both to contact the BSTS and electrostatically gate the surface states in order to bring the Dirac point within the exchange gap induced by the CrX3. The samples will be prepared entirely in inert atmosphere to ensure atomically clean interfaces between the different materials. Magnetotransport measurements shall be performed at temperatures down to 25mK with the goal of observing the QAHE above 1K. In order to further increase the quantization temperature, subsequent measurements involving new magnetic insulators with higher transition temperatures shall be performed when the materials become available, while the application of high pressures can enhance the exchange coupling across the interface.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Apr 29, 2019
- Source ID
- W911NF1910267
Entities
People
- Adam Tsen
Organizations
- Army Contracting Command
- United States Army
- University of Waterloo