Researchers develop new class of quantum critical metal that could advance electronic devices

“The insights gained from this discovery could lead to the development of electronic devices with extreme sensitivity, driven by the unique properties of quantum critical systems,” said Si, a professor of physics and astronomy at the Harry C. and Olga K. Wiess Institute and director of Rice’s Extreme Quantum Materials Alliance.

Quantum phase transitions

At the heart of this research is the concept of quantum phase transitions. Just as water changes between solid, liquid, and gaseous states, electrons in quantum materials can shift between different phases as their environment changes. But unlike water, these electrons obey the rules of quantum mechanics, leading to much more complex behavior.

Quantum mechanics introduces two important effects: quantum fluctuations and electronic topology. Even at absolute zero, where thermal fluctuations disappear, quantum fluctuations can still cause changes in the organization of electrons, leading to quantum phase transitions. These transitions often result in extreme physical properties known as quantum criticality.

Furthermore, quantum mechanics gives electrons a unique property that is tied to topology. This is a mathematical concept that, when applied to electronic states, can yield unusual and potentially useful behaviors.

The study was conducted by Si’s group in a long-term collaboration with Silke Paschen, co-author of the study and professor of physics at the Vienna University of Technology, and her research team. Together, they developed a theoretical model to investigate these quantum effects.

The theoretical model

The researchers considered two types of electrons: some moving slowly, like cars stuck in traffic, and others moving quickly in a hard shoulder. Although the slow-moving electrons appear stationary, their spins can point in any direction.

“Normally, these spins would form an orderly pattern, but the lattice in which they appear in our model does not allow for such ordering, leading to geometric frustration,” Si said.

Instead, the spins form a more fluid arrangement known as a quantum spin liquid, which is chiral and chooses a direction in time. When this spin liquid couples with the fast-moving electrons, it has a topological effect.

The research team found that this coupling also causes a transition to a Kondo phase, where the spins of the slow electrons lock onto the fast ones. The study reveals the complex interplay between electronic topology and quantum phase transitions.

Normal electric transport

When electrons move through these transitions, their behavior changes dramatically, particularly in the way they conduct electricity.

One of the key findings concerns the Hall effect, which describes how an electric current bends under the influence of an external magnetic field, Paschen said.

“The Hall effect has a component that is turned on by the electronic topology,” she said. “We show that this effect experiences a sudden jump across the quantum critical point.”

Implications for future technology

This discovery advances our understanding of quantum materials and opens up new possibilities for future technology. A key part of the research team’s finding is that the Hall effect responds dramatically to the quantum phase transition, Si said.

“Thanks to the topology, this reaction takes place in a small magnetic field,” he said.

These special properties can lead to the development of new types of electronic devices, such as sensors with extreme sensitivity. These sensors can revolutionize sectors such as medical diagnostics or environmental monitoring.

Co-authors of the study include Wenxin Ding of Anhui University in China, a former postdoctoral researcher in Si’s group at Rice, and Rice alumna Sarah Grefe ’17 of California State University.

The research was supported by the U.S. National Science Foundation, the Air Force Office of Scientific Research, the Robert A. Welch Foundation, and a Vannevar Bush Faculty Fellowship.