Topological Insulators: Materials of the Future

Introduction

Prior to the 1980s, it was widely believed that materials could only be conducting or insulating, or somewhere in between. However, research beginning in that decade began hinting at the possibility of materials exhibiting both conducting and insulating qualities at once. This groundbreaking concept was experimentally confirmed in 2007 with the discovery of topological insulators (TIs), materials that conduct electricity on their surface but remain insulators in their bulk, or interior. This duality, which arises from the quantum properties of their structures, makes them highly efficient and stable conductors. As a result, TIs are ideal candidates for use in electronic and quantum computing devices, where minimizing energy loss is paramount. With their unique conductivity, quantum properties, and potential for engineering modifications, TIs have the ability to revolutionize future technologies. Current research aims to optimize their properties while improving preparation methods for scalable production and exploring their applications in existing devices. Although some challenges remain, TIs are steadily progressing toward real-world implementation.

Insights on Conductivity

The defining characteristic of TIs is their strong surface conductivity. Unlike conventional materials, where resistance due to energy loss slows electron movement, TIs have protected surface states, or surfaces that are resistant to disruption. Several papers have examined the conductivity of TIs under various conditions, testing whether TIs can remain highly conductive under real-world influences. One of these studies, conducted by Huang, et al. (2022), found that charged Coulomb impurities within materials increase TI conductivity. The authors observed that, in the presence of impurities, electron puddles form and facilitate electron movement throughout the material (Huang, et al., 2022). In TIs, naturally occurring charged impurities actually enhance functionality rather than being a drawback. In contrast to materials that suffer from reduced electron flow due to impurities, TIs maintain—and even improve—their conductivity. Thus, TIs can be integrated into a wide range of devices and applications without the need for excessive external treatments to maintain their conductive nature, ultimately making them more scalable for widespread use in various industries.

Several other factors besides Coulomb impurities have been shown to impact TI conduction. While charge impurities are internal and found within materials, there are a variety of external factors that can be applied to materials as well. Studying how materials react to external influences aids in the creation of conduction-variable devices, which can adjust conductivity levels to meet the needs of specific scientific or technological applications. For example, the application of external electric fields and on-site energy differences have been found to increase TI conductivity, while exchange fields decrease it (Nguyen, et al., 2020). TIs have the potential to be manipulated in order to fit the conductivity needs for any electronic system. If, for example, a specific TI in a computer needed to have increased conductive power, one way of doing that would be by applying an electric field around it. By modifying external conditions, TIs could serve as adaptable components in the next generation of electronics.

On top of the effects of certain external fields on TIs, Nguyen, et al. (2020) also found that TIs can maintain a stable conductivity, even at zero temperature. In typical conductive materials, a decrease in temperature results in slower electron flow, as thermal energy is no longer available to facilitate the movement of electrons. However, the quantum properties of TIs ensure that their surface states remain conductive, even in the absence of thermal energy. This characteristic is particularly significant for quantum computing and space exploration, where devices often operate at extremely low temperatures. So, on top of being used inside of everyday devices, TIs even have the potential to be used in satellites in outer space. Their conductive powers and adaptability are why they will, without a doubt, play a major role in the future of technology.

Quantum Hall Effect and Quantum Applications

TIs are uniquely suited for quantum applications due to their ability to support stable quantum states, a crucial feature for advancing quantum technologies. One notable property of TIs is the fact that they exhibit the quantum Hall effect (QHE), a phenomenon where electrons form quantized, or “fixed”, energy and conduction levels under an applied magnetic field. This results in highly stable edge states where electrons move along specific pathways and conductivity remains stable. In fact, it was the discovery of the QHE in 1980 that led to the creation of the first theoretical models of topological insulators. Because QHE can only be observed in 2D electron systems, scientists theorized that some materials could demonstrate QHE on their exterior surfaces (which are two-dimensional) but remain insulating on the inside. Since then, the effect has been observed in several TIs, including the TI BiSbTeSe₂, as demonstrated in a 2014 study by Xu, et al. This research highlighted the remarkable stability of the edge states exhibited by BiSbTeSe₂, a trait that is essential for encoding data into quantum devices. The presence of strong QHE in TIs like BiSbTeSe₂ is why TIs have extensively researched in the field of quantum physics and quantum device development.

Modern quantum devices, such as fault-tolerant quantum computers and ultra-precise sensors, require highly stable and precise parts, as even slight instability could lead to errors. Although TIs may not currently be ready to be integrated into quantum systems, they could be the key to developing advanced quantum devices of the future. In particular, one aspect that should be studied further is enhancing the longevity of stable quantum states in TIs and devising methods to maximize this stability. A crucial area for further research is enhancing the longevity of stable quantum states within TIs by developing innovative methods to maximize this stability. That way, the efficient electron flow induced by the QHE can be optimized for real-world usage.

Modified TIs and Practical Applications

The theoretical and experimental results from the studies discussed above show great promise for TIs. However, to bridge the gap between the theoretical insights and practical applications, TIs will likely need to be engineered or modified to optimize them for use. Certain practical requirements, such as being able to operate at room temperature or integrating into existing technologies seamlessly, may not be met by all existing TIs, which necessitates enhancements. Because this field is relatively new and still being studied, there haven’t been many efforts made to scientifically engineer improved TIs. One experiment that did, however, was detailed by Zhao, et al. (2024), where the TI Bi₁.₁Sb₀.₉STe₂ was doped with tin (Sn), resulting in a compound named Sn-doped Bi₁.₁Sb₀.₉STe₂, or BSST. This modified compound demonstrated surface conductivity at room temperature (Zhao, et al., 2024), eliminating the need for cooling devices that are typically needed for quantum devices. While other materials may require expensive, high-energy cooling systems to function, BSST proved to be highly temperature resilient. By modifying an existing TI, researchers were able to bypass a major limitation for many quantum materials and prove BSST to be both cost and energy efficient. Likewise, other TIs could be modified in a similar manner to improve their performance in practical applications. In fact, in a paper written by Yue, et al. (2018), it was discovered that synthetic TIs, due to their larger surface-to-volume ratio, significantly enhance the conduction properties of the surface states. In this way, creating synthetic TIs may actually be a necessity in the future for systems that require unusually high conduction.

There are various ways to create synthetic TIs, and part of integrating TIs into the real world is determining which method is most effective. Yue, et al. (2018) explored the development of synthetic TIs, showcasing how material engineering can enhance the practical usability of TIs. The research highlighted several synthesis methods, such as chemical vapor deposition (CVD), solvothermal processes, and molecular beam epitaxy (MBE), and described how each one can be tailored to enhance the surface-to-volume ratio, optimize the properties of the topological surface states, and ensure the material’s performance in real-world devices. Moreover, Yue, et al. (2018) demonstrated how adjusting these methods could improve the scalability of TIs, which is crucial for their widespread integration into technologies. Ultimately, the goal of TI research is to find methods that not only engineer TIs with the desired properties, but to also make the process of producing TIs cost and energy effective, while ensuring that TIs can be produced at a large scale.

Future Directions for Research

Despite their immense promise, TIs currently face several challenges that must first be addressed before their full potential can be reached. Tian et al. (2017) outlined key areas for improvement, including the synthesis of high-quality crystals, precise control over doping, and enhanced scalability for industrial production. High-quality crystal synthesis is needed to ensure that TIs exhibit their characteristic surface states without being worn down. Additionally, precise doping control is necessary so that the introduced material properly binds to the TI and functions as expected. Without proper doping control, unintended effects like a disruption of the crystal lattice could occur.

In today’s day and age, there is a growing emphasis on the idea of sustainability, which is a concern that TIs can address. Because TIs don’t require a lot of energy to conduct electricity with little to no resistance, they can aid in the creation of low-energy-consumption devices that could revolutionize industries by reducing global energy demands. With TIs ability to reduce global energy consumption while enabling sustainable technological innovations, the need for TIs in our future is evident. Research today should focus on determining the ability to mass-produce TIs with consistent performance in a cost-effective manner, as this is the next step for their practical implementation.

Conclusion

TIs are a major step forward in materials science, with their properties offering opportunities for innovation. Their unique combination of surface conductivity, quantum properties, and modifiability puts them at the forefront of modern innovation. Over the past 40 years, countless research studies have proven their incredible capacity to transform electronic and quantum devices. In the present day, advancements in material engineering bring us a step closer to bridging the gap between laboratory discoveries and industrial applications. By overcoming challenges related to scalability and environmental adaptability, TIs have the potential to revolutionize technology and address global challenges like energy efficiency and sustainable development. While the research continues to expand, one thing is clear: TIs are redefining what is possible in science and engineering.

References

Huang, Y., Skinner, B., & Shklovskii, B. I. (2022). Conductivity of Two-Dimensional Small Gap Semiconductors and Topological Insulators in Strong Coulomb Disorder. Journal of Experimental and Theoretical Physics, 135(4), 409–425. https://doi.org/10.1134/s1063776122100065

Nguyen, V. C., Hoi, B. D., & Yarmohammadi, M. (2020). Electrical conductivity of statically perturbed topological crystalline insulators. Journal of Physics D: Applied Physics, 53(42), 425301. https://doi.org/10.1088/1361-6463/ab9d9c

‌ Xu, Y., Miotkowski, I., Liu, C., Tian, J., Nam, H., Alidoust, N., Hu, J., Shih, C.-K., Hasan, M. Z., & Chen, Y. P. (2014). Observation of topological surface state quantum Hall effect in an intrinsic three-dimensional topological insulator. Nature Physics, 10(12), 956–963. https://doi.org/10.1038/nphys3140

Zhao, W., Xing, K., Chen, L., Vu, T., Akhgar, G., He, Y., Bake, A., Wang, X., & Karel, J. (2024). Quantum interference effects in a 3D topological insulator with high-temperature bulk-insulating behavior. Applied Physics Review, 11(1), 011419. https://doi-org.ccny-proxy1.libr.ccny.cuny.edu/10.1063/5.0168129

Yue, C., Jiang, S., Zhu, H., Chen, L., Sun, Q., & Zhang, D. (2018). Device Applications of Synthetic Topological Insulator Nanostructures. Electronics, 7(10), 225. https://doi.org/10.3390/electronics7100225

Tian, W., Yu, W., Shi, J., & Wang, Y. (2017). The Property, Preparation and Application of Topological Insulators: A Review. Materials, 10(7), 814. https://doi.org/10.3390/ma10070814