DFT+DMFT study of spin-charge-lattice coupling in covalent LaCoO$_3$

We study energetics and the nature of both homogeneous and mixed spin (MS) states in LaCoO$_3$ incorporating structural changes of the crystal volume expansion and the Co-O bond disproportionation (BD) during the spin-state transition using the density functional theory plus dynamical mean field theory (DFT+DMFT) method. DFT+DMFT predicts that energetics of both excited spin states are almost the same while DFT+U calculations of the same structures energetically favor the MS states and produce various metastable solutions whose energetics depend sensitively on final spin states. Within DFT+DMFT, the homogeneous spin state in the expanded crystal volume shows the multiconfigurational nature with non-negligible occupancy probabilities of both high spin (HS) and low spin (LS) states along with $d^6$ and $d^7$ charge configurations indicating the dynamically fluctuating nature of spin and charge states due to the Co-O covalency. The nature of the MS state under the BD structure reveals that Co sites with the long Co-O bonds develop a Mott insulating state and favor HS with a $d^6$ configuration, while more covalent Co sites with the short Co-O bonds occupy more LS states with a $d^7$ configuration and behave as a band insulator, as a result, charge ordering is induced in the BD structure from the spin-state ordering. We also find that both energetics and electronic structure sensitively depend on the Co-O covalency effect, which can be tuned by changing the double counting potential and the resulting $d-$occupancy ($N_d$), and $N_d$ close to 6.7 is consistent with the nature of the spin-state transition. Our results show that structural changes during the spin-state transition can play an important role in understanding energetics and electronic structure of LaCoO$_3$.

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