A team of researchers, led by the U.S. Department of Energy's (DOE) Argonne National Laboratory, has used recent advances in neutron spectroscopy at ISIS and the Spallation Neutron Source (SNS) to show that inelastic neutron scattering measurements of the dynamic magnetic susceptibility of CePd3 provide a benchmark for ab initio calculations based on dynamical mean field theory (DMFT). Their work, recently published in Science, is a step towards predicting the properties and functionality of strongly correlated materials, allowing us to explore their potential to be used in novel ways.
Investigating strongly correlated electron systems
For more than five decades, scientists have been studying strongly correlated electron systems, in which electron-electron interactions are too strong to be ignored, and which give rise to useful properties such as superconductivity or magnetism. The experimental techniques used, such as photoemission spectroscopy, have limitations, but until recently it was not thought to be feasible to measure the electronic structure of materials with correlated electrons using neutrons.
With the development of a new generation of inelastic neutron scattering (INS) spectrometers with large position-sensitive detectors, efficient measurements in 4D have become possible in single crystals, rotating the sample during the data collection. A team of researchers has made use of these new capabilities to delve into the behaviour of correlated electron systems, and demonstrated that a neutron probe overcomes the limitations of other techniques.
According to Dr Ray Osborn, a senior scientist at Argonne, "Neutrons are absolutely essential for this research. Neutron scattering is the only technique that is sensitive to the whole spectrum of electronic fluctuations in four dimensions of momentum and energy, and the only technique that can be reliably compared to realistic theoretical calculations on an absolute intensity scale."
Merlin, ARCS and MAPS
For their recently published research, the team carried out measurements on time-of-flight spectrometer Merlin at ISIS and the wide Angular-Range Chopper Spectrometer (ARCS) at SNS. Previous INS measurements used the time-on-flight (TOF) chopper spectrometer MAPS. MAPS has been in operation since 2000, and was the first chopper spectrometer to employ a large array of position sensitive detectors, and the first to be designed solely for the purpose of measuring excitations in single crystals - changing the way that the neutron community thought about INS.
Helen Walker, Merlin instrument scientist, says, “Merlin was designed to be a high intensity, medium energy resolution spectrometer, to measure excitations in 4D with multiple incident energies in correlated materials, to complement MAPS, and has been in operation since 2008. The experimental results from MERLIN on high temperature Fe-based superconductors have provided pivotal information to understand the mechanism of superconductivity in Fe-based materials." [ref.1]
Their studies of the strongly correlated electron system (CePd3, a cerium-palladium compound) revealed how the electronic excitations change from incoherent electronic bands at high temperature to coherent hybridized bands at low temperatures. As their experimental data can be placed on an absolute scale by normalizing the intensity to a vanadium standard, the team were able to use it to produce a direct comparison of experiment and theory for the first time. The results show that it is now possible to accurately determine the electronic structure of strongly correlated materials by incorporating local correlations into band structures through the combination of density functional theory (DFT) and dynamical mean field theory (DMFT). As such, it is now possible to measure and theoretically account for the dynamic magnetic susceptibility arising from correlated electron bands that will allow us to understand fully many anomalous properties of this class of materials, for example heavy fermion superconductivity and its relation to high temperature superconductivity.
It should be possible to extend this work to materials with even stronger electronic correlations, such as heavy fermions, where electrons are 1000 times heavier than the expected from the free-electron theory, topological materials and the insights gained into the properties and functionality of these materials will open up their potential to be used in novel ways.
Goremychkin EA et al. Coherent band excitations in CePd3: A comparison of neutron scattering and ab initio theory. Science. 359(6372), 186-191 (2018).
Fanelli VR et al. Q -dependence of the spin fluctuations in the intermediate valence compound. , J. Phys: Condens. Matter 26, 225602 (2014)
[ref.1] Christianson, A. D. et al., Unconventional superconductivity in Ba0.6K0.4Fe2As2 from inelastic neutron scattering. Nature 456, 930_932 (2008).
Lumsden M.D. et al., Evolution of spin excitations into the superconducting state in FeTe1-xSex, Nature Phys. 6, 182 (2010)