Quasiparticle loss in extreme quantum materials

A new study by Rice University physicist Qimiao Si unravels the enigmatic behaviors of quantum critical metals — materials that defy conventional physics at low temperatures. Published in Nature Physics Dec. 9 , the research examines quantum critical points (QCPs), where materials teeter on the edge between two distinct phases such as magnetism and nonmagnetism. The findings illuminate the peculiarities of these metals and provide a deeper understanding of high-temperature superconductors, which conduct electricity without resistance at relatively high temperatures.

Key to this study is quantum criticality, a delicate state where the material becomes ultrasensitive to quantum fluctuations — microscopic disturbances that alter electron behavior. While ordinary metals obey well-established principles, quantum critical metals defy these norms, exhibiting strange and collective properties that have long puzzled scientists. Physicists call such systems “strange metals.”

“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,” said Si, the Harry C. and Olga K. Wiess Professor of Physics and Astronomy and director of Rice’s Extreme Quantum Materials Alliance.

Quasiparticles, representing the collective behavior of electrons acting like individual particles, play a crucial role in energy and information transfer in materials. However, at QCPs, these quasiparticles vanish in a phenomenon known as Kondo destruction. Here magnetic moments in the material cease their usual interaction with electrons, dramatically transforming the metal’s electronic structure.

This change is evident in the Fermi surface, a map of possible electron states within the material. As the system crosses the QCP, the Fermi surface abruptly shifts, significantly altering the material’s properties.

Universal behavior across materials

The study extends beyond heavy fermion metals — materials with unusually heavy electrons — to include copper oxides and certain organic compounds. All of these strange metals exhibit behaviors that defy traditional Fermi liquid theory, a framework used to describe electron motion in most metals. Instead, their properties align with fundamental constants such as Planck’s constant, governing the quantum relationship between energy and frequency.

The researchers identified a condition called dynamical Planckian scaling, where the temperature dependence of electronic properties mirrors universal phenomena like cosmic microwave background radiation and the radiation of the “black body” that approximates the behavior of stars. This discovery underscores a shared organizational pattern across various quantum critical materials, offering insights into creating advanced superconductors.

Broader implications

The research implications extend to other quantum materials, including iron-based superconductors and those with intricate lattice structures. One example is CePdAl, a compound where the interplay of two competing forces — the Kondo effect and RKKY interactions — determines its electronic behavior. By studying these transitions, scientists hope to decode similar phenomena in other correlated materials, where complex interelectronic relationships dominate.

Observing how these forces shape the material at QCPs could help scientists better understand transitions in other correlated materials or those with complex interelectronic relationships.

This research, co-authored by Haoyu Hu and Lei Chen from Rice’s Department of Physics and Astronomy, Extreme Quantum Materials Alliance and Smalley-Curl Institute, was supported by the National Science Foundation, Air Force Office of Scientific Research, Robert A. Welch Foundation, Vannevar Bush Faculty Fellowship and European Research Council.

Journal: Nature Physics
Article Title: Quantum critical metals and loss of quasiparticles
Article Publication Date: 9-Dec-2024

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