Interchain interactions, multimagnon condensation, and strain effect in the chain compound NaVOPO4

Employing first-principles modeling and many-body methods, the magnetic properties of the spin-1/2 chain compound ${\mathrm{NaVOPO}}_{4}$ are explored. The extensive first-principles calculations establish an intricate three-dimensionally-coupled model that consists of weakly alternating $J\ensuremath{-}{J}^{\ensuremath{'}}$ antiferromagnetic chains running along cris-cross directions between two consecutive $ab$ planes, connected via two subleading couplings: a ferromagnetic exchange along the $c$ direction (${J}_{c}$) and a weaker antiferromagnetic exchange (${J}_{a}$) along the body-diagonal direction. The exact diagonalization and density matrix renormalized group study has been carried out on a two-dimensional spin model with $J\ensuremath{-}{J}^{\ensuremath{'}}\ensuremath{-}{J}_{c}$ and effective ${J}_{d}$ couplings, constructed based on the full model, for numerical ease. The ${J}_{c}\ensuremath{-}{J}_{d}$ phase diagram is found to host a disorder phase with a finite spin gap for comparable values of ${J}_{c}$ and ${J}_{d}$, arising out of the competing nature of these two interactions, other than two ordered phases. The calculated thermodynamic properties of this model provide a fair description of experimentally measured data. The predominant manifestation of ${J}_{c}$ and ${J}_{d}$ in the disorder phase happens in the stabilization of a multimagnon condensed phase upon gap closing by application of an external magnetic field. We further explore the effect of tensile uniaxial strain, which is found to drive the system from a gapful to a gapless ground state.