Testing Quantum Theory With Precision

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Comparing experimental results and theoretical calculations can be difficult for quantum materials. These are materials that have special properties, such as superconductivity, that can only be understood using the rules of quantum mechanics. One way that scientists compare experiments and computations is to use sample materials that isolate and emphasize an atomic line with one dimensional (1D) properties. In this study, scientists grew thin films of layered copper-oxygen (cuprate) materials to isolate 1D copper chains. This allowed them to test theories of how electrons interact in quantum materials. They grew the films under conditions that allowed them to carefully modify the films’ chemistry and electronic structure. They then measured the electronic structure. The research was possible in part because of a specialized synchrotron X-ray beam line designed and built for this purpose.

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Describing how the properties of quantum materials interact and testing related theories are mathematically very complex and time consuming. This work enabled a direct comparison of computational results against experimental measurements. The study indicates that standard theory is not sufficient and requires a new term to fit the experimental data. The work will help scientists refine theories that are essential to describing and engineering new quantum materials and effects. This could eventually lead to new quantum electronic devices.

It is currently impossible to computationally solve the electronic structure of multi-dimensional quantum materials. 1D theory is computationally possible but difficult to test because most materials have 3D structures. The structure of inherently layered 2D cuprate materials can be rearranged, when synthesized in the ultra-thin limit, resulting in 1D copper-oxygen chains that run parallel to the material surface. However, to fully test theories of electron interactions and transport, researchers also need well characterized “doping” defects in the cuprate oxygen stoichiometry.

In this research, scientists figured out a synthesis method, using ozone during molecular beam epitaxial growth, to add extra oxygen atoms that grab electrons from the copper atoms and create holes in the electronic structure. This was done at a thin film deposition station connected to a synchrotron X-ray beamline that was designed with a sensitive X-ray photoemission spectroscopy capability that can map out the resulting electronic structure. By comparing experimental results with theory, the researchers showed that the standard theory of electron interactions and transport could not predict the 1D doping effects without a modification used to show an unusually strong attraction between certain electrons at longer separations. This attraction is mediated by atomic vibrations. Understanding the coupling between chemistry, defects, vibrations, and the spin direction of electrons are a necessary part of engineering quantum materials for future devices. This work provides a needed direct connection between theory and experiment at the level of correlated electron theory.

Reference: Chen Z, Wang Y, Rebec SN, et al. Anomalously strong near-neighbor attraction in doped 1D cuprate chains. Science. 2021;373(6560):1235-1239. doi: 10.1126/science.abf5174

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