A recent study by Rice University physicist Qimiao Si sheds light on the mysterious behaviors of quantum critical metals, materials that challenge conventional physics at low temperatures.
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The research focuses on quantum critical points (QCPs), where materials hover on the boundary between two distinct phases, such as magnetism and nonmagnetism. The study's insights enhance our understanding of these peculiar metals and their connection to high-temperature superconductors, which can conduct electricity without resistance at comparatively high temperatures.
Central to this research is the concept of quantum criticality—a state in which materials become highly sensitive to quantum fluctuations, the microscopic disturbances that influence electron behavior. Unlike ordinary metals, which adhere to well-established principles, quantum critical metals behave in unexpected ways, showcasing unusual and collective properties that have puzzled scientists for decades. These systems, often referred to as “strange metals,” offer a window into the complex world of quantum materials.
Our work dives into how quasiparticles lose their identity in strange metals at these quantum critical points, which leads to unique properties that defy traditional theories.
Qimiao Si, Harry C. and Olga K. Wiess Professor and Director, Rice University
Quasiparticles, which represent the collective behavior of electrons acting as if they were individual particles, are fundamental to energy and information transfer in materials. However, at QCPs, these quasiparticles disappear in a phenomenon known as Kondo destruction. In this process, the magnetic moments in the material stop interacting with the electrons as they normally would, leading to a profound transformation of the metal's electronic structure.
This transformation is particularly evident in the Fermi surface, which maps the possible electron states within the material. As the system crosses the QCP, the Fermi surface undergoes an abrupt shift, dramatically altering the material's properties and its overall behavior.
Universal Behavior Across Materials
The study expands its scope beyond heavy fermion metals—materials characterized by exceptionally heavy electrons—to include copper oxides and specific organic compounds. These "strange metals" challenge the traditional Fermi liquid theory, which typically describes electron motion in most metals. Instead, their behavior aligns with universal constants like Planck's constant, which defines the quantum relationship between energy and frequency.
The researchers uncovered a phenomenon they term dynamical Planckian scaling, where the temperature dependence of electronic properties mirrors universal patterns seen in phenomena like cosmic microwave background radiation and black-body radiation, which models the behavior of stars. This finding reveals a common organizational framework across different quantum critical materials, providing valuable insights into the development of advanced superconductors.
Broader Implications
The implications of this research extend to other quantum materials, including iron-based superconductors and compounds with intricate lattice structures. A notable example is CePdAl, a material where the competition between two forces—the Kondo effect and RKKY interactions—dictates its electronic behavior.
By investigating these transitions, scientists aim to uncover similar phenomena in other correlated materials, where complex interelectronic interactions dominate.
Studying how these forces influence materials at QCPs could shed light on transitions in other systems with correlated or intricate electronic relationships, broadening our understanding of quantum materials.
This research was co-authored by Haoyu Hu and Lei Chen from Rice University’s Department of Physics and Astronomy, Extreme Quantum Materials Alliance, and Smalley-Curl Institute. The work received support from the National Science Foundation, Air Force Office of Scientific Research, Robert A. Welch Foundation, Vannevar Bush Faculty Fellowship, and European Research Council.
Journal Reference:
Hu, H., et al. (2024) Quantum critical metals and loss of quasiparticles. Nature Physics. doi.org/10.1038/s41567-024-02679-7.