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Summary
The concept of topological invariants in topological quantum materials represent a new way to classify materials. The emergent field of topological quantum matter has its origins in the quantum Hall effect (QHE) discovered in 1980, which forms the basis for the resistance standard in the International System of Units (SI). Since this time, the field of topological materials has expanded into many materials systems known as topological insulators (TI). They behave as an insulator in their bulk interior while possessing metallic surfaces or edges, thereby permitting charges to move only on their surfaces/edges. In the bulk, TIs have an energy gap separating the conduction and valance bands, similar to an insulator. On the surface/edge there are special states that lie within the bulk energy band gap and permit metallic surface conduction. The goal of this project is to characterize and understand the origins of topological quantum materials with an emphasis on metrology applications, such as in new SI quantum resistance standards that do not require an applied magnetic field and fault-tolerant quantum computing applications.
Description
Dichalcogenide-Based Topological Insulators
A family of TI materials can by synthesized by combining binary compounds of Bismuth (Bi) or Antimony (Sb) with Selenium (Se) and Tellurium (Te) to form Bi2Se3, Bi2Te3, and Sb2Te3 compounds. In these material compounds, the spin of the electron has a strong interaction with the motion of the electron, giving rise to what is called a large spin-orbit coupling. This results in a shift of the electron energy levels with the consequence that these materials become topological insulators. Our efforts to synthesize these materials include molecular-beam-epitaxy (MBE) growth of thin films, and furnace growth of 3-dimensional crystals. In the MBE method, researchers heat up Knudsen cells containing the various elements. The elements then evaporate and combine as they condense on a substrate. Reflection-high-energy-electron-diffraction (RHEED) measurements are used to monitor the quality of the growing film (Fig. 1(a)). Each oscillation of RHEED intensity in Fig. 1(b) corresponds to the growth of one "quintal" layer consisting of five repeating atomic layers of the material. Figure 1(c) shows an atomic force microscopy image of an MBE grown Sb2Te3 thin film.
Currently, TI material with magnetic dopants is being developed to realize the quantum anomalous Hall effect (QAHE) for a new quantum resistance standard without the need for a large superconductor magnet. This would result in a smaller resistance standard instrument that could be deployable to NIST customers. The current focus for magnetic TI alloys includes Cr-(Bi0.33Sb0.67)2Te3 grown on STO substrates, as well as other alloys involving Mn, and V. Correlation between macroscopic resistance measurements and microscopic measurements is made by pre-patterning contacts on the STO substrates before growth (Fig. 2).
Topological Superconductivity
Topological superconductivity can be engineered by the proximity effect of combining superconducting materials with TIs. Under the right conditions, topological superconductivity is predicted to host a type of quasiparticle called a Majorana fermion. These particles follow non-Abelian statistics, which allows for quantum information storage that is robust against error and noise, making them ideal qubits. Current efforts in this area include combining various TI materials with superconducting substrates. Additionally, topological superconductivity can be found in certain flat band graphene systems, such as rhombohedral graphene and moiré systems, which are currently under investigation.