(Nanowerk Spotlight) In the ever-evolving landscape of technology, researchers are constantly seeking ways to create faster, more efficient, and versatile devices. Over the years, scientists have explored various degrees of freedom in the electronic properties of materials to develop novel technologies. From conventional electronics, which utilize the charge of an electron, to spintronics, which exploit the spin degree of freedom, and now to valleytronics, which focuses on the valleys in the electronic band structure, researchers continue to push the boundaries of what is possible.
The concept of valleytronics emerged in the early 2000s when scientists began exploring the potential of two-dimensional (2D) materials like graphene and transition metal dichalcogenides (TMDs) for novel electronic applications. Researchers discovered that these materials exhibited unique electronic properties, including multiple local energy extrema or valleys, which could be manipulated for various applications.
One of the pivotal works in the field of valleytronics is the 2007 theoretical paper by Rycerz, Tworzydło, and Beenakker titled "Valley filter and valley valve in graphene" published in the journal Nature Physics. This paper demonstrated that valley polarization in graphene could be achieved and controlled, paving the way for further research in valleytronics.
Another influential paper in the field of valleytronics is the experimental study titled "Valley polarization in MoS2 monolayers by optical pumping" by Kin Fai Mak, Keliang He, Jie Shan, and Tony F. Heinz, published in Nature Nanotechnology in 2012. This paper demonstrated that valley polarization could be achieved in monolayer MoS2 using circularly polarized light, marking an important milestone for valleytronics.
Since the publication of these influential papers, researchers have been working to unlock the potential of valleytronics, exploring new materials, and developing innovative techniques for valley manipulation and detection. One significant recent research finding is the work led by Professor JaeDong Lee's team, which resolved the stability issue of valley spin by identifying the formation of valley domains in molybdenum disulfide, a 2D monolayer semiconductor material. This discovery has the potential to pave the way for more applications in low-power, high-speed information storage platforms (read more: Valleytronics core theory for future high-efficiency semiconductor technology).
Electronics, Spintronics, and Valleytronics - A Comparative Overview
Before diving into the intricacies of valleytronics, it is essential to understand its predecessors, electronics and spintronics, and the differences between these three paradigms.
Conventional electronics rely on the manipulation of the charge of an electron to transmit and process information. Electronic devices, such as transistors, diodes, and capacitors, exploit the movement of electrons and holes in semiconductors to perform various tasks. The limitations of electronics, including the downscaling of transistor dimensions and associated energy dissipation issues, have prompted the exploration of alternative technologies, such as spintronics and valleytronics.
Spintronics, or spin-based electronics, exploit the spin degree of freedom of electrons, in addition to their charge, for information processing and storage. Spintronic devices, such as magnetic tunnel junctions and spin valves, make use of the electron's intrinsic angular momentum (spin) to achieve enhanced functionality, higher storage densities, and reduced power consumption compared to conventional electronics. However, spintronic devices are not without their limitations, including the need for specialized materials and the difficulty of manipulating spins in a controlled manner.
Valleytronics builds upon the foundation laid by electronics and spintronics by exploiting a new degree of freedom in the electronic band structure of materials - the valleys. Valleys represent distinct energy extrema in the electronic band structure, and their manipulation can enable information processing and storage with potentially higher speeds, increased storage capacity, and reduced energy consumption. Valleytronics is still an emerging field, with researchers exploring novel materials and methods to fully harness the potential of valley degrees of freedom.
At the core of valleytronics lie the unique electronic properties of two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs). These materials often exhibit multiple local energy extrema, or valleys, in their electronic band structures, which can be controlled and manipulated for various applications. The ability to manipulate these valleys forms the basis of valleytronics.
Two-dimensional materials play a crucial role in valleytronics due to their strong spin-orbit coupling, nontrivial Berry curvature, and valley-dependent selection rules. These features facilitate a strong coupling between the electron's spin and valley degrees of freedom, allowing for the control and manipulation of valleys using external fields, such as electric, magnetic, and optical fields. The reduced dimensionality of 2D materials leads to enhanced confinement of charge carriers, which can result in strong valley-dependent transport phenomena, such as the valley Hall effect and valley-dependent thermoelectric response, central to the development of valleytronic devices. Furthermore, the wide range of 2D materials offers a versatile platform for exploring different valleytronic properties and designing novel devices tailored to specific applications.
A key aspect of valleytronics is the presence of valley-dependent selection rules in many 2D materials, such as TMDs. These selection rules enable the direct manipulation of valley polarization using circularly polarized light, which is essential for creating and controlling valley polarization—an essential aspect of valleytronics. Additionally, the rich variety of 2D materials provides researchers with ample opportunities to tailor materials and their properties to specific valleytronic applications, further cementing their importance in the field.
In summary, the fundamentals of valleytronics revolve around the unique electronic properties of 2D materials and their ability to provide an ideal platform for controlling and manipulating valley degrees of freedom. This strong foundation sets the stage for further exploration and development of valleytronic devices with a wide range of potential applications.
Valleytronics is based on a set of key principles that govern the behavior of valleys and their potential use in devices. The valley index is a quantum number that labels the different valleys in the electronic band structure, akin to the spin index in spintronics. Valley polarization refers to the selective population of one valley over the other, which can be achieved using external fields like circularly polarized light or strain.
Valley-selective transport phenomena occur due to the preferential scattering of carriers in one valley over the other, resulting in effects such as the valley Hall effect, valley-dependent thermoelectric response, and valley-selective tunneling. Valley coherence, the quantum mechanical superposition of valley states, can enable quantum control of valley degrees of freedom, thereby paving the way for quantum valleytronic devices.
Valleytronics offers several benefits that could revolutionize the electronics industry. Firstly, it provides an additional degree of freedom for information processing and storage, potentially leading to devices with greater functionality and versatility. Secondly, valleytronic devices may consume less energy than their electronic and spintronic counterparts, as they rely on the manipulation of valleys instead of charge or spin. Lastly, valleytronics could help address some of the limitations of conventional semiconductor technologies, such as the downscaling of transistor dimensions and associated energy dissipation issues.
Valleytronics has a wide range of applications that span across various domains. Valleytronic logic gates, which perform Boolean operations using valley degrees of freedom, can be developed by exploiting valley polarization and valley-selective transport. Nonvolatile valley-based memory devices can be created by utilizing valley polarization to store information, in a manner similar to spintronic memory devices that use spin polarization.
The valley-selective optical response of 2D materials can be harnessed for the development of valley-based optoelectronics, including light-emitting diodes (LEDs), photodetectors, and solar cells with improved efficiency and functionality. Furthermore, valley degrees of freedom can be exploited for quantum computing and communication applications. For example, valley qubits could be created by harnessing the quantum coherence of valley states in 2D materials, enabling the development of quantum valleytronic devices.
Recent Research Developments in Valleytronics
A study published in Nature Communications has brought attention to the promising potential of tin(II) sulfide (SnS) as a candidate valleytronic material. This research, led by Jie Yao of the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and Shuren Lin of UC Berkeley's Department of Materials Science and Engineering, demonstrated that SnS could provide substantial improvements in computing power and data storage density (read more: "Valleytronics discovery could extend limits of Moore's Law").
Unlike previously investigated candidate valleytronics materials, SnS can absorb different polarizations of light and selectively reemit light of different colors at different polarizations at room temperature without additional biases. This property allows for concurrent access to both the electronic and valleytronic degrees of freedom, significantly increasing the computing power of circuits made with this material.
Furthermore, SnS has an advantage over other materials due to its ease of processing. The material's unique properties make it a strong contender for the development of operational valleytronic devices, which could one day be integrated into electronic circuits. Additionally, the distinctive coupling between light and valleys in SnS may pave the way for the creation of hybrid electronic/photonic chips in the future.
This research is part of the broader "Beyond Moore's Law" initiative at Berkeley Lab, which aims to evaluate and develop next-generation electronic materials and computing technologies. By partnering with industry professionals, the initiative seeks to accelerate the commercialization and scale-up of these technologies, ensuring that the pace of progress in computing power and data storage density continues to advance.
The exploration of SnS as a potential valleytronic material exemplifies the ongoing efforts to bridge the gap between theoretical concepts and practical applications in valleytronics. As researchers continue to investigate and develop materials like SnS, the field of valleytronics moves closer to realizing its potential to revolutionize the electronics industry and shape the future of computing technology.
Graphene and 2D Materials
Quantum computers, which harness the power of quantum phenomena to achieve significantly faster computation speeds, have gained considerable attention recently. However, the necessity of operating these computers at ultralow temperatures has hindered their practicality. Valleytronics may offer a solution.
A recent breakthrough by scientists from IIT Bombay and Max-Born Institut demonstrates that pristine graphene can be used in valleytronics (Optica, "Light-induced valleytronics in pristine graphene"). Previously thought to be impossible due to graphene's inherent symmetry, the researchers used tailored light polarization to break the symmetry and induce valley polarization. This achievement enables valley operations in graphene at room temperature, which could pave the way for accessible, general-purpose quantum computers.
Scientists find a way to use , a promising technology for encoding and processing quantum information. (Image: IIT Bombay)
This approach is applicable to both gapped and gapless materials, such as monolayer graphene and molybdenum disulfide. The protocol is also robust against variations in key parameters, like dephasing times, wavelengths, and laser pulse time delays.
This work advances the field of valleytronics, opening up the possibility of ultra-fast valley switching at petahertz rates.
Challenges and Future Prospects
Despite the tremendous potential of valleytronics, there are several challenges that researchers must overcome. These include material and fabrication issues such as defects, disorder, and scalability, as well as the development of a more comprehensive theoretical understanding and experimental techniques for valley manipulation and detection.
As researchers continue to make progress in addressing these challenges, the future prospects for valleytronics remain promising. The field is likely to see significant advances in the coming years, both in terms of fundamental understanding and the realization of practical devices. The development of new materials and innovative techniques for valley manipulation will play a crucial role in unlocking the full potential of valleytronics, ultimately leading to a new generation of electronic and quantum devices with unprecedented capabilities.
Valleytronics is an exciting new frontier in the world of condensed matter physics and materials science, offering the potential to revolutionize the electronics industry and quantum computing. Building upon the foundations laid by electronics and spintronics, valleytronics explores a new degree of freedom in the electronic properties of materials, with the promise of transforming our electronic landscape and shaping the future of computing. As researchers continue to unlock the full potential of valley degrees of freedom in 2D materials, we can look forward to a new era of technological innovation marked by enhanced functionality, energy efficiency, and the ability to overcome the limitations of conventional semiconductor technologies.
By providing a comparative overview of electronics, spintronics, and valleytronics, it becomes evident that each successive paradigm seeks to build upon the successes and learn from the limitations of its predecessors. As the field of valleytronics continues to mature, it is likely to witness significant breakthroughs that will pave the way for a new generation of electronic and quantum devices with unprecedented capabilities.
The interdisciplinary nature of valleytronics research, encompassing condensed matter physics, materials science, and engineering, will drive the development of new materials and innovative techniques for valley manipulation. As researchers and industry professionals collaborate to overcome the challenges inherent in the field, valleytronics holds the potential to not only complement but also surpass the achievements of electronics and spintronics in shaping our future technological landscape. With important recent discoveries, such as the one by Professor JaeDong Lee's team, valleytronics research is poised to become applicable in more varied fields, accelerating the advancement of low-power, high-speed information storage platforms.