This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A team of scientists from Caltech and Yale University has shown for the first time how to accurately quantify an important quantum phenomenon in metals, called the Kondo effect, for specific real materials. Unlike previous approaches, which for decades have relied on simplified models to qualitatively describe the effect, the new work uses the actual atomic and electronic structures of materials to solve the problem directly.
The work represents a step toward simulations of important quantum materials such as high-temperature superconductors, in which the motions of individual electrons depend so sensitively on what other electrons are doing at any moment that they cannot be averaged together. The team describes the new technique and results in a paper published in Science. The lead authors are Linqing Peng (Ph.D.) and Tianyu Zhu of Yale University.
Both Peng and Zhu started working on the project in the lab of Garnet Chan, Bren Professor of Chemistry and director of the Rudolph A. Marcus Center for Theoretical Chemistry at Caltech. "It is now possible to predict the properties of some complicated materials purely through computation without referring to experiment," says Chan, who is the senior author of the paper and a Simons Investigator in Physics.
"These first materials that we have studied are like a baby step, or a prototype problem, along the way to more complex phenomena such as high-temperature superconductors and quantum magnets." For many of the materials used in computing, including semiconductors such as silicon, interactions between electrons are so weak that they are insignificant to understanding overall behavior. But to learn more about so-called strongly correlated materials needed for quantum applications, pinning down how electrons interact and bounce off each other is crucial. The Kondo effect comes into play in the simplest example of such a strongly correlated material: the case where a single magnetic atom, such as iron or manganese, is embedded as an impurity in a metal, such as copper.
When that bulk material is cooled below a certain temperature (known as the Kondo temperature), something odd happens. When a normal metal is cooled, its electrical resistance decreases steadily, making the material a better conductor at lower and lower temperatures. When a metal with a magnetic impurity reaches its Kondo temperature, its resistance reaches a minimum and then increases.
"That is the signature of the Kondo effect, and it's a property of the electrons in the impurity interacting with the electrons traveling through the bulk metal," Chan explains. Physicists in the 1970s (including, importantly, alumnus Kenneth Wilson, Ph.D. '61) established a theoretical understanding of the general physics around the Kondo effect. It is a prototypical example of what physicists call a many-body problem because it involves trying to describe the behavior of a system with many, many particles—in this case, electrons in the metal—that interact in complex ways, making it impossible to solve by looking at the behavior of each particle independently.
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