%0 Journal Article %T The (eg £¿ eu) £¿ Eg product Jahn¨CTeller effect in the neutral group-IV vacancy quantum bits in diamond %J - %D 2019 %R https://doi.org/10.1038/s41524-019-0158-3 %X The product Jahn¨CTeller effect may occur for such coupled electron¨Cphonon systems in solids where single electrons occupy double degenerate orbitals. We propose that the excited state of the neutral XV split-vacancy complex in diamond, where X and V labels a group-IV impurity atom of X£¿=£¿Si, Ge, Sn, Pb and the vacancy, respectively, is such a system with eg and eu double degenerate orbitals and Eg quasi-localized phonons. We develop and apply ab initio theory to quantify the strength of electron¨Cphonon coupling for neutral XV complexes in diamond, and find a significant impact on the corresponding optical properties of these centers. Our results show good agreement with recent experimental data on the prospective SiV(0) quantum bit, and reveals the complex nature of the excited states of neutral XV color centers in diamond %U https://www.nature.com/articles/s41524-019-0158-3