Variation of electron-electron interaction in pyrochlore structures

We conduct a comprehensive ab initio investigation of electron-electron interactions within the pyrochlore structures of R2Ru2O7, R2Ir2O7, Ca2Ru2O7, and Cd2Ru2O7, where R denotes a rare-earth element. Utilizing a multiorbital Hubbard model, we systematically explore the effects of various rare-earth elements and applied high pressure on the correlation strength in these compounds. Our calculations on the Coulomb interaction parameter U and the bandwidth W reveal that the chemical pressure for R2Ru2O7 and R2Ir2O7 leads to an unusual increase in U/W ratio, hence, an increase in correlation strength. Contrary to conventional understanding of bandwidth control, our study identifies that the Hubbard U is more influential than the bandwidth W behind the metal-insulator landscape of R2Ru2O7 and R2Ir2O7, leading to an interaction-controlled metal-insulator transition. We also find unexpected behavior in physical pressure. Whereas physical pressure leads to a decrease in the correlation strength U/W as usual in R2Ru2O7, the effect is notably small in Ca2Ru2O7 and Cd2Ru2O7, which provides an important clue to understanding the unusual pressure-induced metal-insulator transition observed experimentally.