Spin reorientation transition driven by polaronic states in Nd2CuO4

Abstract

We show that the spin reorientation transition in Nd₂CuO₄ is due to the electronic interactions between the bulk and the surface. Electron-like doping due to the small oxygen deficiency in Nd₂CuO₄ gives rise to the localized Nd²⁺ states on the surface while the bulk has dominating Nd³⁺ states. Nd²⁺ states act like bound polaronic in-gap states on the surface, which are hybridized with the Cu–O charge transfer band. Polaron–polaron interactions lead to ferromagnetic correlations and also decrease the energy gap across the spin reorientation transition. Drastic variation in the density of states along with valence band narrowing at low temperature indicates the bulk–surface interaction between the Nd³⁺ and Nd²⁺ states through the Cu–O spin cloud. Short range electron–lattice interaction due to the bound polaronic states with large negative Seebeck coefficient (−535 μV K⁻¹ at 300 K) suggests electron doped Nd₂CuO₄ as a potential thermoelectric material.