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The defence will be in French.
Abstract:
In August 2019, the American team of H.Y. Hwang at Stanford discovered for the first time a superconducting (SC) state in a thin film of a two-dimensional nickelate Nd0.8Sr0.2NiO2, with a critical temperature Tc = 15 K. This 2D structure, known as “infinite-layer”, is analogous to high-Tc SC cuprates, and thus generating a real interest within the community. Subsequently, SC was achieved in iso-structural Ln1-xAxNiO2 systems (Ln = La, Pr, Nd, or Eu, and A = Sr, Ca) with a Tc forming a dome as a function of doping for 0.1 ≤ x(A) ≤ 0.3. Questions remain regarding the nature and origin of superconductivity in these “infinite-layer” nickelates, which will require numerous experimental investigations.
Further work has been conducted on the same Ln1-xAxNiO2 phases, but synthesized in the form of bulk samples, such as powders and single crystals. All of which turned out to be non-SC. This thesis is set within this context and aims to address the question: is it possible to obtain “infinite layer” nickelates in the form of bulk SC materials ?
Our research focused on the study of the synthesis and structural and physical characterizations of the polycrystalline solid solution Nd1-xCaxNiO2 for 0 ≤ x(Ca) ≤ 1. The goal is to synthesize the entire series and study its structural and physical characteristics. Structural analyses help identify potential causes for the presence or absence of SC in the phases Nd1-xCaxNiO2.
The synthesis of the “infinite-layer” phase was successful for 0 ≤ x(Ca) ≤ 0.50. Several defects have been highlighted in our samples : a secondary quasi-amorphous and unknown phase, which we have named NdNiOw, has been identified but its structure could not be determined. The crystallites of the Nd1-xCaxNiO2 phases contain intercalation defects, distorted NiO2 planes, nickel-site vacancies, and the presence of residual oxygen at the apical sites of the nickel.
Physical measurements of our Nd1-xCaxNiO2 samples at low temperature show no signs of SC transition. One of the main causes is likely the presence of these numerous structural defects in the crystallites.
Our powders are electrically insulating, and under a weak magnetic applied field at low temperature, the secondary quasi-amorphous NdNiOw phase biases the magnetization measurements of the samples. However, under strong magnetic fields, its magnetic contribution is less problematic. It is then possible to distinguish the behavior of the Nd1-xCaxNiO2 phases (0.05 ≤ x(Ca) ≤ 0.50), which are paramagnetic down to 20 K, with mainly antiferromagnetic interactions without long-range ordering.
