# ${}^{239}$Pu nuclear magnetic resonance in the candidate topological insulator PuB$_4$

## TL;DR

This study reports the first detailed ${}^{239}$Pu NMR analysis of PuB$_4$, revealing its non-magnetic, gap-like bulk behavior consistent with topological insulator properties, and demonstrating NMR's sensitivity to anisotropic plutonium environments.

## Contribution

It provides the second-ever observation of ${}^{239}$Pu NMR, showing its applicability in anisotropic environments and offering insights into the bulk electronic structure of PuB$_4$ as a candidate topological insulator.

## Key findings

- ${}^{239}$Pu NMR spectra show axial symmetry and room temperature observability.
- PuB$_4$ exhibits non-magnetic, gap-like behavior consistent with topological insulator characteristics.
- Contrast in orbital shifts between PuO$_2$ and PuB$_4$ offers new insights into chemical bonding in plutonium materials.

## Abstract

We present a detailed nuclear magnetic resonance (NMR) study of ${}^{239}$Pu in bulk and powdered single-crystal plutonium tetraboride (PuB$_4$), which has recently been investigated as a potential correlated topological insulator. This study constitutes the second-ever observation of the ${}^{239}$Pu NMR signal, and provides unique on-site sensitivity to the rich $f$-electron physics and insight into the bulk gap-like behavior in PuB$_4$. The ${}^{239}$Pu NMR spectra are consistent with axial symmetry of the shift tensor showing for the first time that ${}^{239}$Pu NMR can be observed in an anisotropic environment and up to room temperature. The temperature dependence of the ${}^{239}$Pu shift, combined with a relatively long spin-lattice relaxation time ($T_1$), indicate that PuB$_4$ adopts a non-magnetic state with gap-like behavior consistent with our density functional theory (DFT) calculations. The temperature dependencies of the NMR Knight shift and $T_1^{-1}$--microscopic quantities sensitive only to bulk states--imply bulk gap-like behavior confirming that PuB$_4$ is a good candidate topological insulator. The large contrast between the ${}^{239}$Pu orbital shifts in the ionic insulator PuO$_2$ ($\sim$~+24.7~\%) and PuB$_4$ ($\sim$~-0.5~\%) provides a new tool to investigate the nature of chemical bonding in plutonium materials.