May 26, 2018

Cracking the Code of Superconductivity and Magnetism

Scientists validated a decades-old prediction of a crossover from random (left) to coherent (right) electronic excitations upon cooling a single crystal of a cerium palladium compound. Image on the left shows the theoretical intensity at higher temperature where the electron excitations are random (dark blue). The image on the right with highs and lows in intensity (yellow to blue) corresponds to low temperature. The theoretical (left half) and inelastic neutron scattering data (right half) are overlaid for comparison. The strong variation in intensity at low temperature is due to the emergence of a coherent wave-like behavior of electronic excitations. @ Argonne National Laboratory

The Science

Warm a material and the electrons move randomly. Cool this material and the electrons follow patterns. Those patterns matter if you’re designing super-efficient transmission power lines or ultra-powerful magnets. A crossover was predicted at the point where random motion, called incoherence in electronic excitations, becomes coherent waves. Exotic properties such as superconductivity are observed at these lower temperatures. For the first time, scientists validated this prediction with inelastic neutron scattering. Their experiments close the gap between what’s seen and what’s expected in terms of electron behavior.

The Impact

Advances in neutron scattering and theoretical tools enabled detailed insights into strongly interacting electrons. Scientists need these insights. With them, they can predict useful behaviors and create new materials. Such materials could lead to novel memory, more efficient computing, and advanced sensors.

Summary

Strongly correlated electron systems have been studied for over five decades. Scientists predicted that at high temperatures, electrons in materials fluctuate in a random fashion. Upon cooling, the incoherent electronic fluctuations become coherent wave-like excitations. A quantitative understanding of the crossover from random to coherent electronic excitations is believed to be important to many phenomena in strongly correlated electron systems, including high-temperature superconductivity. Theory has long assumed a gradual loss of coherence with increasing temperature. Only recently has it become possible to perform realistic calculations to more completely include both strong local correlations and electronic band structures. These calculations combine density functional theory with dynamical mean field theory. Previous experiments could not verify the predicted loss because of the inherent limitations of the tools. Now, scientists have overcome these limitations with advancements in measuring inelastic neutron scattering at high flux sources. Neutron scattering data were collected from a cerium palladium compound over large volumes of momentum and energy transfer. The measured intensities showed strong momentum dependence due to the formation of coherent electron bands at low temperatures. The quantitative agreement between experiment and theory shows a robust understanding of the temperature dependence of this electron coherence. The research demonstrated how the latest advances in neutron scattering instrumentation enable quantitative comparisons with computational methods for calculating the behavior of strongly correlated electrons. This is vital for predicting emergent phenomena such as high-temperature superconductivity and colossal magnetoresistance with tremendous potential for applications and design of new materials.

Coherent band excitations in CePd3: A comparison of neutron scattering and ab initio theory E.A. Goremychkin, H. Park, R. Osborn, S. Rosenkranz, J.P. Castellan, V.R. Fanelli, A.D. Christianson, M.B. Stone, E.D. Bauer, K.J. McClellan, D.D. Byler, and J.M. Lawrence Science 359, 186 (2018). DOI: 10.1126/science.aan0593