The Mystery of the Lightweight Electrons

When electrons propagate in an ordered solid-state material, they are scattered by defects, lattice vibrations, and other electrons. This has the effect of making the electrons seem as though they are heavier than non-interacting electrons  (i.e., electrons described by density functional theory). Now, researchers have discovered a system in which this common tenet of many-body theory is violated. The compound is LiCu₂O₂, a mixed-valence copper oxide in which planes of Cu(I) ions are coupled to chains of Cu(II) via common oxygen ligands. Copper oxides are important for superconductivity applications but are difficult to understand due to their complex charge, spin, and orbital interactions.

At ALS Beamline 7.0.2, the researchers studied LiCu₂O₂ using angle-resolved photoemission spectroscopy (ARPES), a technique that allowed them to measure the band structure of the material and to deduce the electron mass. They found that the valence electrons on the Cu(II) chains behave as expected, with strong Coulomb interactions between them driving the system into a charge-transfer insulating state. A big surprise, however, came upon inspection of the Cu(I)-derived valence electrons. These turned out to be effectively 2.5 times lighter than what density functional theory predicts, a consequence of their coupling to Cu(II).

The study points out that state-of-the-art electronic-structure calculation techniques may be intrinsically inappropriate for describing certain electron–orbital interactions (in this case, ligand-to-d hybridizations) in late transition-metal oxides. Such descriptions are crucial for understanding the electronic structure of other copper oxides, some of which have wide-ranging applications due to their electronic, magnetic, and catalytic properties.