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

A new study led by Qimiao Si of Rice University has unveiled a new class of quantum-critical metals, shedding light on the complex interactions of electrons within quantum materials. Published in Physical Exam Letters On September 6, the research explores the effects of Kondo coupling and chiral spin liquids within specific lattice structures.

“The insights gained from this discovery could lead to the development of extremely sensitive electronic devices that leverage the unique properties of critical quantum systems,” said Si, the Harry C. and Olga K. Wiess Professor of Physics and Astronomy 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 from solid to liquid to gas, electrons in quantum materials can change from one phase to another depending on changes in their environment. But unlike water, these electrons follow the rules of quantum mechanics, leading to much more complex behaviors.

Quantum mechanics introduces two key 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 called quantum criticality.

Additionally, quantum mechanics gives electrons a unique property related to topology, a mathematical concept that, when applied to electronic states, can produce unusual and potentially useful behaviors.

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

The theoretical model

The researchers studied two types of electrons: Some move slowly, like cars stuck in traffic, and others move quickly on a highway. Although slow electrons appear stationary, their spin can point in any direction.

“Normally, these spins would form an ordered pattern, but the lattice they inhabit in our model does not allow for such neatness, 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 over time. When this spin liquid couples to the fast-moving electrons, it has a topological effect.

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

Usual electric transport

As electrons go through these transitions, their behavior changes dramatically, including how they conduct electricity.

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

“The Hall effect contains a component that is activated by electronic topology,” she explained. “We show that this effect undergoes a sudden jump beyond the quantum critical point.”

Implications for future technologies

This discovery advances our understanding of quantum materials and opens new possibilities for future technologies. An important part of the research team’s findings is that the Hall effect responds drastically to the quantum phase transition, Si said.

“Thanks to topology, this response occurs in a tiny magnetic field,” he said.

These unusual properties could lead to the development of new types of electronic devices, such as extremely sensitive sensors that could revolutionize fields like 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.