A recent study has introduced a new category of quantum critical metal, providing insight into the complex behavior of electrons in quantum materials. The research examines the implications of Kondo coupling and chiral spin liquids within certain lattice structures.
A recent study led by Qimiao Si at Rice University has revealed a revolutionary class of quantum critical metal, providing insights into the complex interactions of electrons in quantum materials. This research was published in Physical Review Letters on September 6 and focuses on the impacts of Kondo coupling and chiral spin liquids within selected lattice frameworks.
“The findings from this research could pave the way for highly sensitive electronic devices that leverage the distinct properties of quantum-critical systems,” remarked 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
This research centers around the idea of quantum phase transitions. Much like water changes forms between solid, liquid, and gas, electrons in quantum materials can transition between various phases based on their surroundings. However, unlike water, these electrons obey the principles of quantum mechanics, resulting in more complex behaviors.
Quantum mechanics introduces two primary effects: quantum fluctuations and electronic topology. Even at absolute zero, where thermal fluctuations cease to exist, quantum fluctuations can still influence the arrangement of electrons, resulting in quantum phase transitions. These transitions often produce extreme physical attributes known as quantum criticality.
Additionally, quantum mechanics grants electrons a distinctive characteristic related to topology—a mathematical theory that can lead to unique and potentially useful electron behaviors when applied to electronic states.
The study was conducted by Si’s team in long-standing collaboration with Silke Paschen, co-author and a physics professor at the Vienna University of Technology, along with her research group. Together, they created a theoretical model to examine these quantum characteristics.
The theoretical model
The researchers analyzed two categories of electrons: some that move slowly, akin to cars in traffic, and others that zip along in a fast lane. Although the slower electrons seem motionless, their spins can orient in any direction.
“Typically, these spins would arrange themselves in an organized manner, but the lattice in our model disrupts this order, resulting in geometric frustration,” explained Si.
Consequently, the spins arrange into a more fluid structure called a quantum spin liquid, which exhibits chirality and selects a temporal direction. When this spin liquid interacts with the fast-moving electrons, it produces a topological effect.
The research team found that this interaction instigates a transition into a Kondo phase, wherein the spins of the slow electrons synchronize with those of the faster ones. This study highlights the intricate relationship between electronic topology and quantum phase transitions.
Typical electrical transport
As electrons navigate these transitions, their behavior alters significantly, especially regarding their conductivity.
An important discovery regarding the Hall effect, which describes how electrical currents bend due to an external magnetic field, was made by Paschen.
“The Hall effect contains a component that is influenced by electronic topology,” she noted. “We have demonstrated that this effect undergoes a sudden transition at the quantum critical point.”
Implications for future technology
This discovery enhances our understanding of quantum materials and opens doors to new technological advancements. A key finding from the research team is that the Hall effect reacts dramatically during the quantum phase transition, Si mentioned.
<p”Thanks to the topology, this reaction occurs in a tiny magnetic field,” he added.
These extraordinary properties could potentially lead to the creation of innovative electronic devices, such as highly sensitive sensors that could transform sectors like medical diagnostics or environmental monitoring.
Co-authors of the study include Wenxin Ding from Anhui University in China, a former postdoctoral researcher in Si’s group at Rice, and Rice alumna Sarah Grefe ’17 from 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.
