Multiple electronic-valence elements in A-site perovskite manganites: a route to high metallicity and an orbital order coexistence

With respect to the double-exchange hopping mechanism, the role of chemical element sitting at perovskite A-site (generally hosting an alkali-earth or a rare-earth atom) has always been considered as “silent,” being the corresponding conduction band of the A-site element too far from Fermi’s energy level. In order to make such an atomic site active within the transport mechanism, a possible strategy calls for a partial insertion of multiple-valence ions which also show the requested electronic properties (i.e. conduction band crossing EF). In manganites, the ideal candidate isindeed Mn-ions themselves. Here we show that a partial substitution of Mn ions at perovskite A-site (therefore named as A-site perovskite manganites) is indeed possible in both La-deficient LaxMnO3 and off-stoichiometric LaxSryMnO3 manganite thin films. By combining polarization-dependent x-ray absorption spectroscopy and resonant inelastic x-ray spectroscopy, the relevant Mn2+ content is demonstrated, and it is unambiguously assigned its crystallographic site (namely, the perovskite A-site). Similarly to traditional manganites, Mn2+ substitution induces the required Mn3+/Mn4+ mixed population. However, differently from the latter, the Mn2+ ions at perovskite A-site are electronically involved in the transport mechanisms, having their electronic bands crossing the Fermi energy. Such an energetic configuration favours the hopping of electrical charge through that site (usually silent), in addition to the traditional Mn3+/Mn4+ hopping path (named Multiple double-exchange mechanism), thus contributing to the ferromagnetic and metallic state. Furthermore, to an highly metallic and ferromagnetic state, it surprisingly corresponds also a strong Mn orbital order. Indeed, the tendency of the orbitals to order locally usually strongly compete with the kinetic energy of the free charge carriers, which however tends to destroy long range orbital order. Multiple double-exchange mechanism is here demonstrated that destroys such a dichotomy by sustaining the co-existence of highly metallic states with orbital ordered phase. This will open unexplored perspectives in both theoretical and experimental possibilities based on such a coexistence of spin/orbital order (generally in competition with each other), and more in general in fundamental studies on transport mechanism in strongly correlated electrons systems.