Low energy excitations in cuprate superconducting superlattices and in BaCuO_2+x and CaCuO₂ constituent blocks: disentangling the role of conduction planes and charge reservoirs

Abstract

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This is a copy of the slides presented at the meeting but not formally written up for the volume.

Abstract

We propose a novel approach to correlate electronic properties with superconductivity in high critical temperature cuprates. We have disassembled the superconductor in the constituent blocks and investigated separately the charge reservoir (CR) and the conduction planes or infinite layer (IL) blocks, which consist of BaCuO2+x (BCO) and CaCuO2 (CCO) respectively. Artificial BCO/CCO superconducting superlattices [1], obtained by stacking in a sequence the two blocks, have been also measured to study the effect of charge transfer (from the CR to the IL block). On such samples, in form of c-axis oriented thin films, we have performed polarization dependent X-ray Absorption Spectroscopy (XAS) and Resonant Inelastic X-ray Scattering (RIXS) measurements at the Cu L3 edge, to investigate the hole doping effect and the low energy excitations. XAS results clearly show a significant number of Cu d(z2) holes, both in the pure BCO samples and the BCO/CCO superlattices linked to the presence of apical oxygen. The high energy shoulder related to the existence of itinerant Cu 3d holes hybridised with O 2p (doping holes) is evident in the XAS spectra of the pure BCO and the BCO/CCO superconducting superlattices. The relative shoulder intensity decreases by increasing the CCO block thickness, in agreement with the expected decrease of the doping level. RIXS spectra show similar dd-excitations in the ab-planes for BCO, CCO and 2X2 superlattice, as a consequence of similar in-plane crystal field effects [2]. On the contrary, the charge transfer excitations [3] in ab-plane look similar only for the BCO film and the 2X2 superlattice. Namely, fluorescence activates at the absorption edge because of the metal character of both the CR block and the superconducting superlattice. On the contrary in CCO (insulator) the onset of the fluorescence signal is moved at higher photon energies, in agreement with the presence of a charge transfer band gap. In addition, we have detected an excitation at 0.5 eV in BCO and BCO/CCO superlattices, but not in CCO. This excitation could be interpreted as the stabilization energy of the Zhang-Rice (ZR) singlet [4] in doped compounds, or as the x2-y2 to z2 dd excitation. All the results are in agreement with the scenario of charge transfer from the BCO charge reservoir block to the CCO infinite layer block. [1] G. Balestrino et al., Phys. Rev. B 62, 9835 (2000) [2] P. Kuiper et al., Phys. Rev. Lett. 80, 5204 (1998) [3] G. Ghiringhelli et al. Phys. Rev. Lett. 92, 117406 (2004) [6] N.B.Brookes et al., Phys. Rev. Lett. 87, 237003 (2001).