High-precision realization of robust quantum anomalous Hall state in a hard ferromagnetic topological insulator

Abstract: The discovery of the quantum Hall (QH) effect led to the realization of a topological electronic state with dissipationless currents circulating in one direction along the edge of a two dimensional electron layer under a strong magnetic field. The quantum anomalous Hall (QAH) effect shares a similar physical phenomenon as the QH effect, whereas its physical origin relies on the intrinsic spin-orbit coupling and ferromagnetism.Here we report the experimental observation of the QAH state in V-doped (Bi,Sb)2Te3 films with the zero-field longitudinal resistance down to 0.00013+-0.00007h/e2 (~3.35+-1.76 ohm), Hall conductance reaching 0.9998+-0.0006e2/h and the Hall angle becoming as high as 89.993+-0.004degree at T=25mK. Further advantage of this system comes from the fact that it is a hard ferromagnet with a large coercive field (Hc>1.0T) and a relative high Curie temperature. This realization of robust QAH state in hard FMTIs is a major step towards dissipationless electronic applications without external fields.

• The paper achieves a high-precision QAH state in V-doped (Bi,Sb)₂Te₃, reporting a Hall conductance of 0.9998±0.0006e²/h and nearly zero longitudinal resistance.
• It employs intrinsic ferromagnetism to realize the QAH effect without external magnetic fields by leveraging self-magnetization in V-doped films.
• The research highlights enhanced Curie temperature and reduced carrier density in V-doped systems, paving the way for energy-efficient quantum devices.

High-Precision Confirmation of the Quantum Anomalous Hall State in Hard Ferromagnetic Topological Insulators

This paper presents an empirical study on the realization of the Quantum Anomalous Hall (QAH) effect in vanadium (V)-doped (Bi,Sb)₂Te₃ thin films, supporting the robustness of the QAH state in such materials. The researchers achieve this by employing V-doped topological insulators (TIs) as opposed to the more traditionally used chromium (Cr) doping, thereby making significant strides in the precision and stability of the QAH effect.

The experimental setup utilizes V-doped (Bi,Sb)₂Te₃ wherein ferromagnetism is introduced intrinsically, avoiding the need for an external magnetic field. The results demonstrate zero-field longitudinal resistance as low as 0.00013±0.00007h/e² (~3.35±1.76Ω) and a Hall conductance approaching 0.9998±0.0006e²/h. Hall angle measurements reach 89.993±0.004º at a low temperature of 25mK, underscoring the high-precision realization of the QAH state in these hard ferromagnetic TIs (FMTIs).

Key Findings

The study delivers several key findings:

1. Precision of QAH State: The work reports highly accurate measurements of Hall resistance and conductance in V-doped TIs, equivalent to the quantum Hall (QH) state but without the need of an external magnetic field.
2. Ferromagnetic Properties: V-doped Sb₂Te₃ exhibits a more robust ferromagnetic state than Cr-doped systems, with a coercive field (H_c) exceeding 1.0T, significant for spontaneous QAH effect development.
3. High Curie Temperature: The V-doped thin films show a higher Curie temperature (T_C) compared to their Cr counterparts, enhancing the feasibility of the QAH effect at relatively higher temperatures.
4. Spontaneous Self-Magnetization: The study highlights that V-doped films can achieve the QAH state without any magnetic field training, indicating the strong ferromagnetic order and self-driven QAH state development.
5. Decreased Carrier Density: The dual doping in V-doped systems results in a lower carrier density, beneficial for tuning the chemical potential towards a charge-neutral state with reduced effort.

Implications

The successful demonstration of the QAH effect in V-doped TIs promises significant advancements in metrological and spintronic applications. The ability to achieve dissipationless electronic pathways without external fields aligns well with the ongoing drive towards energy-efficient quantum devices. Furthermore, this study establishes a threshold for precision in QAH systems using V-doped TIs, proposing an augmented path towards higher temperature operation of quantum anomalous Hall devices via enhanced Curie temperatures.

Future Research Directions

This research opens several avenues for further exploration:

• Temperature Optimization: Future work could focus on optimizing these materials for higher temperature operations of the QAH state, leveraging insights into magnetic anisotropy and energy barriers.
• Material Engineering: Experimenting with different doping concentrations, film thicknesses, and alternative substrates could yield additional enhancements in the stability and tunability of the QAH effect.
• Theoretical Insights: Complementary theoretical studies examining the impact of impurity band localization and the role of exchange interactions could further elaborate on the underlying physics of the robust QAH state in V-doped films.

This study marks a significant step in the development of filling the gap between the theoretically predicted and experimentally realized QAH states. By demonstrating the advantages of V-doped systems over Cr-doped TIs, a path is paved for more practical and reliable implementation of quantum Hall states into everyday electronic applications without reliance on external magnetic fields.