Motivated by the interest in topological quantum paramagnets in candidate spin-1 magnets, we investigate the diamond lattice compound NiRh2O4 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.
@article{arxiv.1905.11403,
title = {The Curious Case of NiRh$_2$O$_4$: A Spin-Orbit Entangled Diamond Lattice Paramagnet},
author = {S. Das and D. Nafday and Tanusri Saha-Dasgupta and Arun Paramekanti},
journal= {arXiv preprint arXiv:1905.11403},
year = {2019}
}