Pu 239 nuclear magnetic resonance in the candidate topological insulator PuB4

We present a detailed nuclear magnetic resonance (NMR) study of Pu239 in bulk and powdered single-crystal plutonium tetraboride (PuB4), which has recently been investigated as a potential correlated topological insulator. This study constitutes the second-ever observation of the Pu239 NMR signal, and provides unique on-site sensitivity to the rich f-electron physics and insight into the bulk gaplike behavior in PuB4. The Pu239 NMR spectra are consistent with axial symmetry of the shift tensor showing for the first time that Pu239 NMR can be observed in an anisotropic environment and up to room temperature. The temperature dependence of the Pu239 shift, combined with a relatively long spin-lattice relaxation time (T1), indicate that PuB4 adopts a nonmagnetic state with gaplike behavior consistent with our density functional theory calculations. The temperature dependencies of the NMR Knight shift and T1-1-microscopic quantities sensitive only to bulk states-imply bulk gaplike behavior confirming that PuB4 is a good candidate topological insulator. The large contrast between the Pu239 orbital shifts in the ionic insulator PuO2 (∼+24.7%) and PuB4 (∼-0.5%) provides a new tool to investigate the nature of chemical bonding in plutonium materials.