Experimental studies of elementary particles and atoms as well as the application of quantum mechanics to describe their behavior have revolutionized our understanding of the structure of matter and led to unprecedented rates of technical progress in the 20th century.
Even the smallest of differences between how various types of atoms and molecules interact can lead to vastly different properties in bulk materials. For example, one material might conduct electrical current while another acts as an insulator or even superconductor — all related to how electrons are allowed to move and interact within them.
Recently, scientists have begun to study more exotic materials, those whose surfaces behave differently from the properties of the overall bulk material. One of the most interesting of these are known as topological insulators.
“Topological insulators are novel materials that allow [electricity conduction without heat loss] along the edges but remain insulating in the bulk,” explained Xiaoling Wang, professor at the University of Wollongong, Australia, in an email. “Such edge state conduction is associated with quantum mechanical properties unique to the bulk state of topological insulators, and differentiates topological insulators from ordinary insulators, which are insulating in the bulk as well as along the edges.”
These materials are very promising for applications in future electronics. However, their usefulness could be enhanced if the electrons running along their surfaces share the same polarization — this refers to the direction of their internal rotation, also referred to as their “spin”.
This would lead to the application of topological insulators in spintronics, a field of electronics whose operation depends on the spins of current carriers with the potential to bring about more efficient computer memory, hard drives, quantum computers, and much more.
Searching for the right material
To find such materials, Wang and his colleague Muhammad Nadeem at the University of Wollongong studied the interaction of electrons in two-dimensional (2D) (that is, one atom thick) ribbons of different materials.
“The significance of 2D materials is multifold,” Nadeem explained, outlining of the rationale behind their research direction. “On the one hand, 2D materials host unique physical properties [that are] tunable via extrinsic stimuli [such as electric field, strain, or illumination]. On the other hand, they promise energy-efficient and miniaturized devices for information and communication technologies due to exotic quantum mechanical functionalities and reduced dimensionalities.”
In their study published in Advanced Physics Research, the physicists analyzed antiferromagnets — materials in which atomic spins align in a regular pattern with neighboring spins having opposite directions. Topological insulators based on these materials are of particular interest since antiferromagnets are minimally influenced by an external magnetic field, safeguarding electronics built using these materials against outside perturbations.
To study these materials, the team used a well-established model that describes the interaction of electrons in solids. They found that 2D antiferromagnetic ribbons turn into topological insulators with polarized electrons at their boundary edge — an interface or perimeter where the material transitions from one state to another, often exhibiting distinct properties or behaviors — but only under very specific conditions.
“This work predicts the existence and practical realization of a new class of topological quantum material called topological Dirac spin-gapless materials. These materials host unique chiral edge states,” Wang said. “In other words, all the edge states carry the same spin-polarization, a key feature for devising spintronic technologies.”
The physical behavior they predict is not specific to one particular material, which opens up many possibilities for future practical applications in industry and science.
“The proposed mechanisms are material-independent and can be employed for emergent spintronic devices, such as graphene spintronics,” explained Nadeem.
Although this theoretical result is exciting, there is still a lot of work to be done to put it into practice and build real spintronic devices based on the materials they predicted.
“This work needs further attention on two fronts,” concluded Nadeem. “First, modeling and simulation for energy efficiency and performance of quantum devices based [topological insulators with polarized electrons at the boundary edge], and second, finding suitable materials which host robust spin-gapless chiral edge states and are integrable with current spintronic technologies.”
Reference: Muhammad Nadeem and Xiaolin Wang, Topological Dirac Spin-Gapless Materials — a New Horizon for Topological Spintronics Without Spin-Orbit Interaction, Advanced Physics Research (2023). DOI: 10.1002/apxr.202300028
Feature image credit: Pawel Czerwinski on Unsplash