Tunable site- and orbital-selective Mott transition and quantum confinement effects in La0.5Ca0.5MnO3 nanoclusters

We present a dynamical mean-field theory study of the charge and orbital correlations in finite-size La0.5Ca0.5MnO3 (LCMO) nanoclusters. Upon nanostructuring LCMO to clusters of 3 nm diameter, the size reduction induces an insulator-to-metal transition in the high-temperature paramagnetic phase. This is ascribed to the reduction in charge disproportionation between Mn sites with different nominal valence [H. Das, Phys. Rev. Lett. 107, 197202 (2011)PRLTAO0031-900710.1103/PhysRevLett.107.197202]. Here we show that upon further reducing the system size to nanoclusters of a few atoms, quantum confinement effects come into play. These lead to the opposite effect: the nanocluster turns insulating again and the charge disproportionation between Mn sites and the orbital polarization are enhanced. Electron doping by means of external gate voltage on few-atom nanoclusters is found to trigger a site- and orbital-selective Mott transition. Our results suggest that LCMO nanoclusters could be employed for the realization of technological devices, exploiting the proximity to the Mott transition and its control by size and gate voltage.