# The Curious Case of NiRh$_2$O$_4$: A Spin-Orbit Entangled Diamond Lattice Paramagnet

## TL;DR

This paper investigates NiRh$_2$O$_4$, revealing it as a spin-orbit entangled diamond lattice paramagnet with frustrated exchange interactions, using ab initio and model Hamiltonian approaches to explain its magnetic properties and excitations.

## Contribution

It introduces a comprehensive theoretical framework combining ab initio calculations and model Hamiltonians to explain the magnetic behavior of NiRh$_2$O$_4$, emphasizing spin-orbit coupling and exchange frustration.

## Key findings

- Identification of a spin-orbit entangled non-magnetic ground state.
- Prediction of dispersive gapped magnetic modes and dark states.
- Demonstration that exchange frustration suppresses exciton condensation.

## Abstract

Motivated by the interest in topological quantum paramagnets in candidate spin-$1$ magnets, we investigate the diamond lattice compound NiRh$_2$O$_4$ using {\it ab initio} theory and model Hamiltonian approaches. Our density functional study, taking into account the unquenched orbital degrees of freedom, shows stabilization of $S=1$ and $L=1$ state. We highlight the importance of spin-orbit coupling, in addition to Coulomb correlations, in driving the insulating gap, and uncover frustrating large second-neighbor exchange mediated by Ni-Rh covalency. A single-site model Hamiltonian incorporating the large tetragonal distortion is shown to give rise to a spin-orbit entangled non-magnetic ground state, largely accounting for the entropy, magnetic susceptibility, and inelastic neutron scattering results. Incorporating inter-site exchange within a slave-boson theory, we show that exchange frustration can suppress exciton condensation. We capture the dispersive gapped magnetic modes, uncover `dark states' invisible to neutrons, and make predictions for future experiments.