Electrically tunable surface-to-bulk coherent coupling in topological insulator thin films (1104.1404v3)

Abstract: We study coherent transport in density tunable micro-devices patterned from thin films of the topological insulator (TI) Bi2Se3. The devices exhibit pronounced electric field effect, including ambipolar modulation of the resistance with an on/off ratio of 500%. We show that the weak antilocalization (WAL) correction to conductance is sensitive to the number of coherently coupled channels, which in a TI includes the top and bottom surface and the bulk carriers. These are separated into coherently independent channels by the application of gate voltage and at elevated temperatures. Our results are consistent with a model where channel separation is determined by a competition between the coherence time and surface-bulk scattering time.

• The paper demonstrates that gate voltage modulates coherent coupling between surface and bulk states in Bi2Se3, achieving a 500% on-off resistance ratio.
• It employs weak antilocalization analysis with the HLN formula to quantify phase coherence and the impact of electric fields and temperature on channel decoupling.
• The findings pave the way for designing spintronic devices and exploring fault-tolerant quantum computing through controlled electronic transport.

Electrically Tunable Surface-to-Bulk Coherent Coupling in Topological Insulator Thin Films

The paper of topological insulators (TIs) represents a significant advance in condensed matter physics, drawing attention due to their unique electronic properties characterized by conducting surface states and insulating bulk states. This paper investigates the electrically tunable surface-to-bulk coherent coupling in thin films of the TI Bi$_2$Se$_3$, focusing on the mechanism of channel separation and its influence on electronic transport.

Key Contributions

The research utilizes micro-devices crafted from Bi$_2$Se$_3$ thin films with tunable charge density, emphasizing the electric field effect and its capability to modulate resistance, exhibiting an on-off ratio of 500%. Weak antilocalization (WAL) is employed as a key technique to discern phase coherence effects, highlighting the role of both surface and bulk states. The paper establishes that the WAL correction is adjustable based on the gate voltage and temperature, thus affecting the number of coherently coupled channels.

Experimental Insights

The researchers deployed micro-devices demonstrating distinct WAL features at low magnetic fields with parameters that are influenced by the electric field through gating. The WAL effect, sensitive to phase coherence time, was analyzed using the Hikami-Larkin-Nagaoka (HLN) formula, revealing that the prefactor $\alpha$ corresponding to WAL was tunable, indicating modulation of coherent channel coupling. Notably, the mechanism for channel decoupling involves a competition between phase coherence time and surface-to-bulk scattering time, where the application of a gate voltage induces a spatial separation by forming a depletion layer that suppresses coupling between bulk and surface states.

Implications and Speculations on Future Developments

The findings propose that controlling the surface-to-bulk coupling in TIs can significantly influence electronic properties, thereby offering insights into designing devices with tailored transport characteristics. From a theoretical perspective, these results contribute to a deeper understanding of coherence effects in TIs, particularly in relation to charge carrier dynamics within coupled channels.

The practical implications are substantial for the development of spintronic devices that capitalize on TI surface states for enhanced performance. Furthermore, the ability to manipulate these channels could aid in the realization of exotic states like Majorana fermions, which are promising for fault-tolerant quantum computing. Future research could extend these principles to other TI materials with different crystal structures or paper the impact of additional external perturbations, such as strain or magnetic fields, offering broader implications for device applications and novel physics in topological matter.

This paper, by elucidating the controllable nature of coherence in TIs, lays the groundwork for potential advancements in both fundamental physics and applied technologies related to topological states.