The Fascinating World of Quantum Materials: Unveiling a New Class of Quantum Critical Metals

A groundbreaking study led by Rice University’s Qimiao Si has uncovered a new class of quantum critical metal, providing unprecedented insights into the complex interactions of electrons within quantum materials. Recently published in Physical Review Letters on Sept. 6, this research delves into the effects of Kondo coupling and chiral spin liquids within specific lattice structures.

“The revelations from this breakthrough could pave the way for the creation of electronic devices with unparalleled sensitivity, driven by the distinctive properties of quantum-critical systems,” explained Si, the Harry C. and Olga K. Wiess Professor of Physics and Astronomy and director of Rice’s Extreme Quantum Materials Alliance.

Exploring Quantum Phase Transitions

Central to this study is the concept of quantum phase transitions. In a manner analogous to water transitioning between solid, liquid, and gas states, electrons in quantum materials can undergo shifts between different phases based on changes in their environment. However, unlike water molecules, these electrons adhere to the principles of quantum mechanics, resulting in significantly more intricate behaviors.

Quantum mechanics introduce two crucial effects: quantum fluctuations and electronic topology. Even at absolute zero where thermal fluctuations cease to exist, quantum fluctuations can induce alterations in the organization of electrons, leading to quantum phase transitions. These transitions often manifest in extraordinary physical characteristics known as quantum criticality.

Furthermore, quantum mechanics confer electrons with a distinctive property linked to topology, a mathematical concept that, when applied to electronic configurations, can engender unusual and potentially advantageous behaviors.

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

The Intricacies of the Theoretical Model

The researchers examined two categories of electrons: some moving sluggishly akin to vehicles stuck in traffic, and others moving swiftly in a fast lane. Despite the appearance of being stationary, the spins of the slow-moving electrons can orient in any direction.

“Under normal circumstances, these spins would arrange themselves in an orderly fashion, but the lattice they inhabit in our model does not allow for such regularity, leading to geometric frustration,” elucidated Si.

Instead, the spins adopt a more fluid configuration termed a quantum spin liquid, which is chiral and selects a temporal direction. When this spin liquid interacts with the fast-moving electrons, it yields a topological influence.

The research team unearthed that this interaction also triggers a transition into a Kondo phase, where the spins of the sluggish electrons align with those of the faster ones. The study illuminates the intricate interplay between electronic topology and quantum phase transitions.

Novel Insights into Electrical Transport

As electrons navigate through these transitions, their conduct undergoes a significant transformation, particularly in terms of their electrical conduction.

One of the most notable findings pertains to the Hall effect, which elucidates how an electrical current deflects under the influence of an external magnetic field, as highlighted by Paschen.

“The Hall effect encompasses a component facilitated by electronic topology,” she remarked. “We demonstrate that this effect exhibits a sharp leap across the quantum critical point.”

Implications for Future Technological Advancements

This groundbreaking discovery elevates our comprehension of quantum materials and unlocks a realm of potential for future technologies. A pivotal aspect of the research team’s revelation is that the Hall effect demonstrates a profound response to the quantum phase transition, as noted by Si.

“Thanks to the topological aspect, this response occurs in a minuscule magnetic field,” he added.

The extraordinary properties unveiled could herald the development of innovative electronic devices such as highly sensitive sensors that could revolutionize fields like medical diagnostics and environmental monitoring.

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

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