For years, scientists have been baffled by a quantum phenomenon that occurs in “strange metals” – a class of superconducting materials where electrons scatter at high rates influenced by temperature. Understanding why this scattering happens in certain unconventional metals could provide valuable insights into various quantum material puzzles, including the elusive high-temperature superconductivity. In a breakthrough discovery, a team of international researchers, including physicists from Cornell University, has shed light on this phenomenon by comparing two closely related compounds known as PdCrO2 and PdCoO2.

The rate at which electrons collide with imperfections in a material and with each other, known as Planckian scattering, increases in a linear fashion as the temperature rises. By studying the crystals of PdCrO2 and PdCoO2, which are highly pure and well-documented, the researchers have for the first time provided a precise quantitative explanation for the origin of the mysterious Planckian scattering rate in strongly interacting metals.

The study, titled “T-linear Resistivity From Magneto-Elastic Scattering: Application to PdCrO2,” was published in the Proceedings of the National Academy of Sciences (PNAS) on August 28.

The characteristic time between electron collisions in numerous strange metals, including many high-temperature superconductors, is determined by the Planck’s constant and the temperature. This has led scientists to believe that the key to understanding high-temperature superconductivity lies in uncovering the common thread connecting these materials and their universal Planckian time scale.

Debanjan Chowdhury, assistant professor of physics at Cornell University and a co-author of the paper, explains that the motivation behind this joint theory and experiment collaboration was to find a material example where all the properties relevant for electrical transport are accurately known. By doing so, they aimed to develop a microscopic theory for the origin of Planckian scattering times. PdCrO2 became the ideal candidate, as its properties have been extensively studied and are in excellent agreement with the experiment.

Although understanding these superconducting materials is challenging, given their complexity, Chowdhury emphasizes the importance of unraveling their mysteries for more efficient energy use. The researchers focused on PdCrO2 as a simpler, well-characterized material to build a theory around this phenomenon.

PdCrO2 is a magnetic “delafossite” – a type of chromium oxide mineral – that exhibits characteristics of an “interesting correlated material.” It consists of two species of electrons: one group of mobile electrons that conduct electricity freely and another group of immobile electrons that demonstrate magnetism. Interestingly, PdCoO2, its sister compound, shares similar properties but lacks any sign of magnetism. While electrical transport in PdCrO2 abides by the Planckian timescale, it does not in PdCoO2.

However, magnetism alone does not explain the origin of Planckian timescales. The researchers discovered that the missing piece of the puzzle lies in a cooperative process, where electrons interact with crystal vibrations and localized spins – the basic building blocks of magnetism. Juan Felipe Mendez Valderrama, a doctoral student in physics and co-lead author from the Weizmann Institute of Science, explains that this interaction, previously overlooked, plays a crucial role in understanding the origin of Planckian timescales. By identifying candidate materials where this interaction is dominant and manipulating its ingredients, scientists can explore entirely new phenomena.

The collaboration involved researchers from the Weizmann Institute of Science, the Max Planck Institute, and the University of St. Andrews. The decision to join forces came about in summer 2022 when Debanjan Chowdhury and Erez Berg, long-time collaborators, realized they had the same ideas for resolving the experimental puzzle while attending a summer workshop at the Aspen Center for Physics.

The experimental study that inspired this theory, called “Investigation of Planckian Behavior in a High-Conductivity Oxide: PdCrO2,” was also published in PNAS on August 28, with all the co-authors mentioned above.

While this study focused on a specific material, PdCrO2, the insights gained from it are expected to apply to a broader range of materials where electrical transport exhibits the mysterious Planckian timescale. Scientists hope that this newfound understanding will lead to fundamental breakthroughs in the field and open doors for future advancements in high-temperature superconductivity.

The study received support from the National Science Foundation and the Max Planck Society.

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Frequently Asked Questions (FAQ)

1. What is Planckian scattering?
   Planckian scattering refers to the rate at which electrons collide with material imperfections and each other. This scattering rate increases linearly with temperature and plays a crucial role in many quantum phenomena.
2. What are strange metals?
   Strange metals are a class of superconducting materials where electrons scatter at high rates influenced by temperature. They exhibit unconventional behavior compared to other metallic systems.
3. What is high-temperature superconductivity?
   High-temperature superconductivity refers to the phenomenon of electric current flowing through a material with zero resistance at temperatures higher than those achievable with conventional superconductors. It has the potential to revolutionize electrical energy transfer for more efficient use.
4. Why is understanding high-temperature superconductivity important?
   Understanding high-temperature superconductivity is crucial for developing more efficient methods of electrical energy transfer. It has significant implications for various industries and technologies, including power transmission and energy storage.
5. What is the significance of the compound comparison study?
   By comparing the compounds PdCrO2 and PdCoO2, researchers gained insights into the origin of Planckian scattering timescales. This study provides a quantitative explanation for this phenomenon and opens doors for further investigations into a larger class of materials.