Research projects

The research project aims the search for high critical temperature (T_c) superconductors. Oxides with Cu in mixed valence states 2+/3+ (d⁸/d⁹) have been prepared and characterised. Synthesis methods include the unusual high pressure and high temperature technique. Chemical and structural details have been studied using synchrotron X-ray and neutron diffraction techniques, as well as muon spectroscopy. Throughout the project, the study of M_1-xCu_xSr₂RCu₂O_7+x (R=rare earth and M=Fe and Mo) superconductors has been addressed. The partial substitution of Cu by other cations in high oxidation states, such as Fe^₃₊/Fe^₄₊ or Mo^₅₊, gives rise to a high concentration of holes in the CuO₂ planes. This study has led to the completion of three Doctoral Theses. Among the main results, Mo_0.3Cu_0.3Sr₂TmCu₂O_7+d and FeSr₂YCu₂O_7+d superconducting phases with high Tc (83K and 70K, respectively) are notable. In addition, the coexistence of superconductivity and magnetism in FeSr2YCu2O7.85 has been proven. In the final stage of the project, the search for superconductivity has focused on different transition metal cations with mixed valence. 

The recent discovery of superconductivity in Nd_0.8Sr_0.2NiO₂ thin films has motivated the study of the family of nickelates (R_1-xSr_x)_n+1Ni_nO2_n+1 (R= rare earth) as potential high Tc  superconductors. These oxides, are obtained via topotactic reduction of the Ruddlesden-Popper phases, (R_1-xSr_x)_n+1Ni_nO_3n+1 with CaH₂. The prepared materials, with composition R_0.8Sr_1.2NiO₃, contain Ni in square-plane coordination forming infinite chains (spin-ladder) along the c axis and mixed valence Ni⁺(d⁹)/ Ni²⁺(d⁸), with similar features to cuprates, thus suggesting their potential behavior as high T_c superconductors.