For years, physicists have been fascinated by a quantum phenomenon observed in “strange metals” – a class of superconducting materials. These materials exhibit a unique scattering behavior of electrons, influenced by temperature. Understanding this phenomenon in unconventional metals could hold the key to solving various quantum material puzzles, such as high-temperature superconductivity. In two groundbreaking papers, a collaboration of international researchers, including physicists from Cornell University, unravels the microscopic explanation for this intriguing “Planckian” scattering in the compound PdCrO2. Remarkably, its nearly identical sister compound, PdCoO2, does not exhibit the same behavior. This research opens up new possibilities for understanding quantum materials and their potential applications in efficient energy transfer.
Planckian scattering refers to the rate at which electrons collide with material imperfections and with each other, which increases linearly with temperature. In many strange metals, the time between electron collisions is determined by Planck’s constant and temperature. Interestingly, a majority of the known high-temperature superconductors display this characteristic when heated above their superconducting temperature. It has long been believed that deciphering the origin of high-temperature superconductivity lies in understanding the common thread across these materials, which leads to the universal Planckian time scale.
The researchers aimed to provide a quantitatively accurate description of the mysterious Planckian scattering rate in strongly interacting metals by comparing PdCrO2 and PdCoO2. These compounds are well-known for their clean crystal structures and documented properties. Their study, published in the Proceedings of the National Academy of Sciences (PNAS), titled “T-linear Resistivity From Magneto-Elastic Scattering: Application to PdCrO2,” establishes a comprehensive microscopic theory for the origin of Planckian scattering times.
PdCrO2, a magnetic chromium oxide mineral called a “delafossite,” serves as the material of choice for this study. It represents a paradigmatic example of an “interesting correlated material” with two species of electrons – one set of mobile electrons that conduct electricity freely and another set of immobile electrons that exhibit magnetism. The presence of electron magnetism in PdCrO2 plays a crucial role in the observed Planckian electrical transport behavior. In contrast, its sister compound, PdCoO2, lacks any signs of magnetism and does not display Planckian behavior.
While magnetism is a key factor, it alone does not explain the origin of Planckian timescales. The researchers discovered that an unexpected cooperative process involving the interaction of electrons with the crystal’s vibrations and localized spins, which are fundamental building blocks of magnetism, holds the missing piece of the puzzle. Juan Felipe Mendez Valderrama, a doctoral student in physics and co-lead author, explains that by considering this previously overlooked interaction, scientists can identify new materials where this cooperative process dominates, leading to entirely new phenomena.
This research collaboration involved scientists from various institutions, including Cornell University, the Weizmann Institute of Science, the Max Planck Institute, and the University of St. Andrews. The collaboration between Debanjan Chowdhury, assistant professor of physics at Cornell, and Erez Berg from the Weizmann Institute of Science, began in the summer of 2022 when their shared ideas converged during a physics workshop at the Aspen Center. Their joint efforts have resulted in a theory that aligns remarkably well with experimental observations.
Understanding the mysteries of Planckian scattering and its role in quantum materials opens up new avenues for discovering materials with unique properties. For physicists, this breakthrough has significant implications for advancing the understanding of high-temperature superconductivity and finding more efficient ways of energy transfer. While superconducting materials remain challenging to comprehend and model theoretically, the study of simpler, well-characterized compounds like PdCrO2 provides a foundation for building theories and unlocking the potential of these extraordinary materials.
The research conducted by an international collaboration of physicists sheds light on the enigmatic phenomenon of Planckian scattering in strange metals. Their breakthrough representation of the microscopic origin of this scattering in PdCrO2, compared to its sister compound PdCoO2, provides valuable insights into the complexities of quantum materials. By considering the cooperative process between electrons, crystal vibrations, and localized spins, scientists can uncover novel materials and phenomena. As researchers continue to explore the mysteries of quantum materials, the hope is to gain fundamental insights that will pave the way for advancements in energy transfer and other quantum applications.