Gate-Voltage Control of Chemical Potential and Weak Anti-localization in Bismuth Selenide

Abstract: We report that Bi$2$Se$_3$ thin films can be epitaxially grown on SrTiO${3}$ substrates, which allow for very large tunablity in carrier density with a back-gate. The observed low field magnetoconductivity due to weak anti-localization (WAL) has a very weak gate-voltage dependence unless the electron density is reduced to very low values. Such a transition in WAL is correlated with unusual changes in longitudinal and Hall resistivities. Our results suggest much suppressed bulk conductivity at large negative gate-voltages and a possible role of surface states in the WAL phenomena. This work may pave a way for realizing three-dimensional topological insulators at ambient conditions.

• The paper demonstrates effective gate-voltage control of the chemical potential in Bi₂Se₃ films, marking a key transition from bulk- to surface-dominated transport.
• It employs molecular beam epitaxy to grow high-quality films and achieves carrier density tuning with changes up to ±2.5×10¹³ cm⁻².
• The paper reveals that weak anti-localization behavior, modulated by electron density, underscores strong spin-orbit coupling in topological surface states.

Overview of "Gate-Voltage Control of Chemical Potential and Weak Anti-localization in Bi$_2$Se$_3$"

This paper examines the electronic properties of Bi$_2$Se$_3$ thin films, a promising material for realizing three-dimensional topological insulators. Topological insulators are characterized by an insulating bulk and conducting surface states that possess novel electronic properties, which are protected by time-reversal symmetry and can be manipulated for various quantum computing and spintronic applications. The authors of this study focus on the ability to control the chemical potential in these films via a gate voltage, and they investigate the implications of this control on weak anti-localization (WAL) behaviors.

Key Findings

1. Epitaxial Growth and Tunability: The authors report successful growth of Bi$_2$Se$_3$ thin films on SrTiO$_{3}$ substrates using molecular beam epitaxy. This setup allows for significant tunability in carrier density when a back-gate voltage is applied.
2. Weak Anti-localization: The study reveals that while WAL is typically gate-voltage independent, it shows sensitivity to electron density reductions at very low values. The presence of WAL indicates strong spin-orbit coupling, which is a signature of topological surface states.
3. Chemical Potential Modulation: Through the use of gate voltage, researchers demonstrate a depletion of bulk conductivity accompanied by changes in longitudinal and Hall resistivities. This suggests that surface states play a critical role in WAL phenomena.
4. Transport Regimes: Three distinct transport regimes are identified at varying gate voltage levels, corresponding to different degrees of carrier depletion. Particularly, a transition between bulk-dominated and surface-dominated conductivities is highlighted.

Numerical Insights

• The study measures carrier densities reaching changes up to ±2.5×10$^{13}$ cm$^{-2}$, a clear demonstration of the effectiveness of gate control.
• A striking increase in $B_\phi$, indicative of reduced phase coherence length as the carrier density is reduced, is observed in the regime where bulk conduction is suppressed. This behavior suggests enhanced interaction effects in the surface states.

Implications and Future Directions

The results provide significant insights into the manipulation of both bulk and surface states in topological insulators via an electric field, paving the way for practical applications in device fabrication. The work underscores the potential to utilize Bi$_2$Se$_3$ at ambient conditions, overcoming previous challenges that demanded vacuum or low-temperature environments.

Further research could focus on exploring different substrate materials to enhance the quality of the films and extend the method to other topological insulator materials. There is also an opportunity to explore more complex heterostructures, potentially leading to devices with highly controlled electronic properties. The study brings forward the prospect of integrating topological insulators in electronic components, which could significantly impact future developments in quantum computing and spintronics.

In summary, the research articulates how structural manipulation of Bi$_2$Se$_3$ and gate voltage application can critically affect the electronic landscape of topological insulators, offering a feasible route for tuning their properties for advanced technological applications.