Towards a new era of superconductors

Irene Vega

Superconductors are materials that support the flow of electric current with zero resistance, that is, without loss of energy.

The work of Araceli Gutiérrez Llorente, professor at the URJC in the department of Applied Mathematics, Materials Science and Engineering and Electronic Technology, has focused on the study of nickel oxides (nickelates), investigated for several decades as possible analogues of superconducting copper oxides (cuprates), discovered in 1986. In cuprates (unconventional superconductors) the nature of the interaction that leads to the formation of the superconducting state is still unknown and is different from the mechanism that operates in so-called superconductors. conventional, described by the BCS (Bardeen-Cooper-Schrieffer) theory.

“In 2019, researchers at Stanford University (USA) observed superconductivity in a nickelate. This discovery aroused great interest because it made it possible to experimentally investigate superconductivity in another family of oxides. However, the synthesis process is complex, which means that the number of experimental works is still very small, although these are crucial to elucidate the nature of the interaction that gives rise to the condensation of electrons in a non-superconducting state. conventional,” says Araceli Gutiérrez Llorente. The results of her research carried out with these superconductors are collected in a scientific article, published in the journal Advanced Science, of which she is the main author.

In this study, the results of experiments carried out with superconducting nickels are presented, the synthesis of which requires two steps. First, a non-superconducting phase is prepared, which must have a high structural quality. In the second phase, one-third of the oxygen atoms are removed from that structure so that the remaining oxygen atoms and nickel atoms form planes. “Once this structure is achieved, to observe superconductivity, the material must be cooled below its critical temperature, in this case, below 260ºC below zero. Furthermore, the structural study of the superconducting phase requires high-resolution electron microscopy,” explains Professor Gutiérrez Llorente.

The experiments carried out show essential parameters of the initial phase so that it can generate the superconducting phase and properties of the metallic state (non-superconducting) of the reduced phase.

This work has been carried out in collaboration with researchers from the carbon oxides group. Laboratoire Albert Fert (CNRS - Université Paris Saclay, France) and with scientists from the electron microscopy STEM groups of Laboratoire de Physique de Solides (Paris Saclay) and Thales (French multinational company). The published study is part of Araceli Gutiérrez Llorente's two-year stay at the Laboratoire Albert Fert, financed by the European Union.

Left: Superconducting transition. Resistivity as a function of temperature (normalized to its value at room temperature) of the nickelate after successive reduction processes.

Center: Schematic of the crystalline structure of the superconducting phase.

Right: High-resolution electron microscopy image of the superconducting phase obtained, where the absence of apical oxygens and the formation of NiO2 planes can be observed.