The exotic behavior of electrons in quantum materials has perplexed scientists for decades. Experimental investigations are essential for understanding the physical properties of quantum materials, which have the potential to dramatically improve technology for communication, computing and sensing. Today, several complementary experimental techniques are being used to illustrate the behavior of electrons in quantum materials, and help unlock their full potential.
Recent findings by Moore Foundation grantees at Princeton University have provided insight into one of these unusual properties: the topological insulator. Topological insulators are materials that allow electrical current to flow with minimal resistance on their surface, but not through their bulk.
Princeton’s Robert Cava (center) has played critical roles in the discovery and materials development of topological insulators, and Nai Phuan Ong (right) is a pioneer in experiments designed to successfully detect the current carried by surface electrons in topological insulators. Both are investigators in the foundation’s Emergent Phenomena in Quantum Systems (EPiQS) Initiative. EPiQS investigator and Princeton physics professor Ali Yazdani is also pictured (left).
A possible path to faster, more efficient electronics
In a study published in Science Advances, a research team led by Ong and Cava created a semiconductor that transitions to a Weyl metal under pressure. Weyl metals contain emergent massless particles, called Weyl fermions, that travel through the material with minimum resistance, a property that may lead to faster and more efficient electronics in the future.
These researchers investigated the role of pressure in lead tin telluride, a semiconductor that is also a topological insulator. In particular, the team focused on a property in this semiconductor called the Berry curvature, which mimics a magnetic field acting on electrons. The Berry curvature arises intrinsically from the electronic band of a semiconductor and acts like an effective “magnetic field” that can exert a huge force on electrons.
The team found that, while under high pressure, lead tin telluride transitions from a semiconductor to a Weyl metal that displays a large Berry curvature. Pressure can change the fundamental behavior of a material—for example: turning a lump of graphite into diamond.
In this case, the maximum pressure the team applied to the lead tin telluride semiconductor was 2.8 gigapascals. For comparison, the pressure needed to turn graphite into diamond is 10 gigapascals. The team says their findings “confirm lead tin telluride as the first known example of a material that becomes a Weyl metal under pressure.”
Earlier this year, Zahid Hasan, also an EPiQS investigator at Princeton, showed how synthetic routes (rather than pressure) can be used to alter topological properties. His team discovered how the flow of electricity can be radically changed by adjusting the pattern of layers in an engineered material, consisting of alternating layers of a topological insulator and an ordinary insulator. In 2015, Hasan’s remarkable discovery of the elusive Weyl fermion in another material, tantalum arsenide, was named a Top Ten Breakthrough in physics by Physics World magazine.
Hasan’s team expanded on this work in a recent study in Nature Physics, showing how topological objects known as Weyl nodes can annihilate one another under high magnetic fields. A recent Physics World article discussed Weyl fermions in detail. Hasan was also featured in a Scientific American article titled The Strange Topology That Is Reshaping Physics.
Pushing the frontiers of condensed matter physics
Cava, Ong and Hasan are part of a larger group of leading physicists at Princeton who are pursuing discovery-driven research with the potential to transform our understanding of how complex quantum matter organizes itself. These topics are of interest to all EPiQS investigators.
“The work coming out of Princeton is very exciting to see and we believe the kind of environment they have set up enables researchers to collaborate and push the frontiers of condensed matter physics,” said Dusan Pejakovic, Program Director of the EPiQS Initiative. “These findings serve as an example of what is possible at an institution where there is an ongoing investment in people and facilities. Princeton’s tradition of encouraging researchers to investigate risky or challenging scientific problems that may not have immediate application, yet advance our understanding of the field.”
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