A material with high electron mobility is analogous to a highway without traffic. Electrons flowing through the material experience smooth sailing, encountering minimal obstacles or congestion to impede their path. The higher a material’s electron mobility, the more efficient its electrical conductivity becomes, resulting in less energy loss. Advanced materials exhibiting high electron mobility are crucial for developing more efficient and sustainable electronic devices capable of accomplishing more with less power.
In a recent breakthrough, physicists at MIT, the Army Research Lab, and other institutions have achieved an unprecedented level of electron mobility in a thin film of ternary tetradymite. This class of mineral is naturally found in deep hydrothermal deposits of gold and quartz.
The researchers grew pure, ultrathin films of the material, minimizing defects in its crystalline structure. This nearly perfect film, significantly thinner than a human hair, exhibits the highest electron mobility observed in its class.
The team estimated the material’s electron mobility by detecting quantum oscillations during the passage of electric current. These oscillations signify the quantum mechanical behavior of electrons within a material. The researchers identified a specific rhythm of oscillations characteristic of high electron mobility, surpassing any previously recorded levels in ternary thin films of this class.
Jagadeesh Moodera, a senior research scientist in MIT’s Department of Physics, explains, “Before, what people had achieved in terms of electron mobility in these systems was like traffic on a road under construction — you’re backed up, you can’t drive, it’s dusty, and it’s a mess. In this newly optimized material, it’s like driving on the Mass Pike with no traffic.”
Published in the journal Materials Today Physics, the team’s results highlight the potential of ternary tetradymite thin films for future electronics. Applications include wearable thermoelectric devices that efficiently convert waste heat into electricity and spintronic devices, which process information using an electron’s spin, consuming significantly less power than conventional silicon-based devices.
Hang Chi, a former research scientist at MIT and now at the University of Ottawa, describes the use of quantum oscillations as an effective tool for measuring a material’s electronic performance: “We are using this oscillation as a rapid test kit. By studying this delicate quantum dance of electrons, scientists can start to understand and identify new materials for the next generation of technologies that will power our world.”
The study involved collaboration between Chi and Moodera, along with Patrick Taylor, formerly of MIT Lincoln Laboratory, Owen Vail and Harry Hier of the Army Research Lab, and Brandi Wooten and Joseph Heremans of Ohio State University.
The name “tetradymite” originates from the Greek words “tetra” (four) and “dymite” (twin), describing the mineral’s crystal structure. This structure consists of rhombohedral crystals “twinned” in groups of four, sharing identical crystal structures and a side. Tetradymites are composed of bismuth, antimony tellurium, sulfur, and selenium. In the 1950s, scientists discovered the semiconducting properties of tetradymites, making them ideal for thermoelectric applications. The mineral, in its bulk crystal form, could passively convert heat into electricity.
Later, in the 1990s, the late Institute Professor Mildred Dresselhaus proposed that the mineral’s thermoelectric properties could be significantly enhanced by focusing on its microscopic, nanometer-scale surface, where electron interactions are more pronounced.
Chi explains, “It became clear that when you look at this material long enough and close enough, new things will happen. This material was identified as a topological insulator, where scientists could see very interesting phenomena on their surface. But to keep uncovering new things, we have to master the material growth.”
To grow thin films of pure crystal, the researchers employed molecular beam epitaxy.
This method involves firing a beam of molecules at a substrate, typically in a vacuum and under precisely controlled temperatures. As the molecules deposit on the substrate, they condense and gradually build up, one atomic layer at a time. By controlling the timing and type of molecules deposited, scientists can grow ultrathin crystal films in precise configurations, minimizing defects.
Co-author Taylor elaborates on the growth process: “Normally, bismuth and tellurium can interchange their position, which creates defects in the crystal. The system we used to grow these films came down with me from MIT Lincoln Laboratory, where we use high-purity materials to minimize impurities to undetectable limits. It is the perfect tool to explore this research.”
The team grew thin films of ternary tetradymite, each approximately 100 nanometers thin. They then tested the film’s electronic properties by searching for Shubnikov-de Haas quantum oscillations. This phenomenon, discovered by physicists Lev Shubnikov and Wander de Haas, demonstrates that a material’s electrical conductivity can oscillate when exposed to a strong magnetic field at low temperatures. This occurs because the material’s electrons occupy specific energy levels that shift as the magnetic field changes.
These quantum oscillations can reveal a material’s electronic structure and the behavior and interactions of its electrons. For the MIT team, the oscillations could determine a material’s electron mobility. The presence of oscillations indicates that the material’s electrical resistance can change, implying that electrons are mobile and can flow easily.
The team searched for signs of quantum oscillations in their new films by exposing them to ultracold temperatures and a strong magnetic field. They then ran an electric current through the film and measured the voltage along its path while adjusting the magnetic field.
Chi describes their findings: “It turns out, to our great joy and excitement, that the material’s electrical resistance oscillates. Immediately, that tells you that this has very high electron mobility.”
The team estimates that the ternary tetradymite thin film exhibits an electron mobility of 10,000 cm2/V-s, the highest recorded for any ternary tetradymite film. They believe that the film’s record mobility stems from its low defect and impurity levels, achieved through their precise growth strategies. Fewer defects in a material translate to fewer obstacles for electrons, allowing them to flow more freely.
Moodera emphasizes the significance of their findings: “This is showing it’s possible to go a giant step further when properly controlling these complex systems. This tells us we’re in the right direction, and we have the right system to proceed further, to keep perfecting this material down to even much thinner films and proximity coupling for use in future spintronics and wearable thermoelectric devices.”
This research received support from the Army Research Office, National Science Foundation, Office of Naval Research, Canada Research Chairs Program, and the Natural Sciences and Engineering Research Council of Canada.
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