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Charge and Spin Transport in Topologically Non-trivial Solid States


Interfacial coupling between the top and the bottom surface states in thin-film 3-dimensional (3D) topological insulators (TIs) destroys the momentum-spin (k-s) locking chirality, and thus prohibits the good transport properties of TI surface states. Our theoretical investigations in 3D TI thin films show that this effect only occurs near the band gaps due to the spin mismatch between the opposing surfaces. By tuning the position of the Fermi level, interfacial tunneling and in-plane semi-classical transport present unique signatures due to this mismatch. These results indicate the possibility to restore the bulk transport properties of 3D TIs in the case of thin films.

As a real-space non-trivial topological order in solid states, the skyrmion phase in B20 compounds or heavy metal interfaces is considered as a strong candidate to implement next generation storage units or spintronic devices. Using coherent transport modeling, we demonstrate that a `topological spin Hall effect' (TSHE) can be achieved in some circumstances due to individual magnetic skyrmions. In order to evaluate the device application possibilities, a topological charge analysis is carried out to identify and quantify the topological protection in each skyrmion. Based on this analysis, an on-wafer solution to individually create magnetic skyrmions is proposed. The feasibility and stability of the proposal is numerically evaluated by solving Landau-Lifshitz-Gilbert (LLG) equation.

Besides the physics that governs the operations in electronic or spintronic devices, the requirement of signal integrity gives rise to another fundamental limit to enhance the performance of next-generation devices. System designers have proposed many solutions to overcome this limit, such as differential signaling or multi-port passive networks. Characterization of these designs calls for reliable algorithms to remove the effects of access structures: the de-embedding techniques. Taking impedance mismatch as a perturbation quantity, we establish a simplified model of uniform multi-conductor transmission lines. A novel multi-port de-embedding technique is proposed theoretically and tested experimentally.

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