Raman spectroscopy study of the interface structure in (CaCuO₂)_n/(SrTiO₃)_m superlattices

Abstract

Raman spectra of CaCuO 2 /SrTiO 3 superlattices show clear spectroscopic marker of two structures formed in CaCuO 2 at the interface with SrTiO 3 . For non-superconducting superlattices, grown in low oxidizing atmosphere, the 425 cm -1 frequency of oxygen vibration in CuO 2 planes is the same as for CCO films with infinite layer structure (planar Cu-O coordination). For superconducting superlattices grown in highly oxidizing atmosphere, a 60 cm -1 frequency shift to lower energy occurs. This is ascribed to a change from planar to pyramidal Cu-O coordination because of oxygen incorporation at the interface. Raman spectroscopy proves to be a powerful tool for interface structure investigation. Superconductivity indeed does appear only if the superlattices are grown under strongly oxidizing conditions, due to extra oxygen ions entering at the interface. 4,5,6 The larger the number of these oxygen ions is, the higher the hole doping of the CCO block and the T c are (a maximum T c 40 K has been obtained). 4 Excess oxygen ions occupy the apical position for the Cu ions of the CuO 2 planes close to the interface. 4,5,6 Therefore, different structures are present in the CCO layers close to the boundaries with SrTiO 3 and in those ones far from them. Indeed, the latter should retain their IL structure, with purely planar Cu-O coordination, whereas pyramidal Cu-O coordination can occur in the former. Recently, the occurrence of similar structural conditions has been also foreseen in films of IL system ACuO 2 (A = Ca, Sr, Ba) in a theoretical paper by Zhong et al. 7 and experimentally proven in the case of SrCuO 2 /SrTiO 3 superlattices. 8 Zhong et al. 7 found that, in order to reduce the electrostatic instability, naturally present in these highly polar systems, the structure changes, below a critical ACuO 2 thickness, from the CuO 2 planar (IL) to a chain structure by a tilting of the 3 CuO 2 planes. This transition is obtained by moving oxygen atoms from the CuO 2 planes to the Ca planes, thus leading to the presence of apical oxygen.Raman spectroscopy demonstrated to be a powerful tool to investigate layered structures 9,10 and to provide a careful characterization of film samples. 11 In particular, in cuprates with IL structure, the effects of the hole doping of CuO 2 planes, obtained by either introduction of excess oxygen atoms 12 or excitation through the charge transfer gap, 13 has

Tcup to 50 K in superlattices of insulating oxides

“…We have thus grown some superlattices on LaAlO 3 (LAO) substrate. As previously found in the samples used for a Raman study [15], the SLs grown on LAO show a much lower T c than those grown on NGO. The R(T ) for a representative sample is shown in figure 4 together with the R(T ) curve of the optimal SL grown on NGO.…”