Electronic reconstruction in (SrMnO3)n/(LaMnO3)m digital superlattices probed by O K edge x-ray linear dichroism

The (SrMnO3)n(LaMnO3)2n (SMO/LMO, n=8,5,1) superlattices can be seen as a way to introduce Sr doping in LaMnO3 (LMO) (La:Sr=2:1, i.e. the optimal doping for the high Curie temperature CMR compound La1-xSrxMnO3) without introducing substitutional disorder. The metallicity and magnetism of the samples is tuned by the thickness of the constituent blocks, that determine the density and the coupling between the interfaces. X-ray natural and magnetic linear dichroism (XNLD, XMLD) and magnetic circular dichroism (XMCD) at the Mn L23 edge have been successfully employed to characterize the orbital occupation and the magnetic moment orientation, both in antiferromagnetic (AF) and ferromagnetic (F) phases. L edge spectra are dominated by the Mn3+ contribution, so little information is obtained about the strain and charge reconstruction effect on the energy levels of the SMO (where Mn mainly assumes 4+ valence), and about new states created at the interfaces[1]. In this respect, because of the hybridization between the oxygen 2p and the empty metal states, the O K edge XAS can give indication on both components of the SL, and on the interfacial charge transfer. It was demonstrated that in oxides, the oxygen 1s2p transition absorption spectrum (O K edge) maps the unoccupied density of states (DOS) of predominant metal character, as in the O2- ion the 2p orbital is nominally full [2]. We can then take advantage of O K edge absorption spectra collected with linear polarized light to study the competition between bulk and interface effects in different period SLs. Linear dichroism analysis show that the presence of the interfaces not only acts on the constituent blocks of the superlattices by doping, but also modifies the band structure in a non-trivial way. The interface contribution strongly emerges in the n=1 sample, also providing an explanation for the observed XNLD and XMLD signals at the L edge. Furthermore O K edge analysis can provide direct evidence of the validity of several band structure calculations reported in literature[3].