STRUCTURAL AND ELECTRONIC PROPERTIES OF Nb3Sn IN THE HIGH PRESSURE AND LOW TEMPERATURE REGION

Abstract

Currently, the A15 type compound Nb3Sn represents the most ap propriate superconductor for applications requiring magnetic fields well above 10 T, such as the future particle accelerators and fusion reactors. The stress-strain dependence of Nb3Sn properties is a research topic long since. It is known in particular that an applied uniaxial and/or transverse load remarkably affects Nb3Sn superconducting properties by degrading both its critical current density and critical temperature: consequently such strain sensitivity is a key factor to be considered when dealing with the above mentioned high-field applications. The response of Nb3Sn to mechanical stresses is then a topic of strong interest. In order to shed light onto the degradation of the superconducting properties as function of stress and strain, a first approach concerns the investigation of the structural and superconducting properties when a high hydrostatic pres sure is applied. Recently, X-ray diffraction (XRD) experiments on the two A15 com pounds - Nb3Al and Nb3Ga - as a function of an applied hydrostatic pressure, revealed a pressure-induced isostructural phase transition which has been subsequently suggested to be an Electronic Topological Transi tion (ETT) by means of first-principle calculations. In this thesis work the structural and superconducting properties of Nb3Sn as a function of the hydrostatic pressure have been studied by means of XRD, XAFS (X-ray absorption spectroscopy), Density Func tional Theory (DFT) and Density Functional Perturbation Theory (DFPT) calculations, revealing that also Nb3Sn could exhibit an anomaly as a function of the hydrostatic pressure below 10 GPa. XRD at high pressure suggested that Nb3Sn has an anomaly in the compressibility at around 5- 6 GPa. XAFS measurements confirm the presence of a structural anomaly, that can be related to dimerization effect of the Nb chains that charac terizes the Nb3Sn structure. Based on first-principle calculations, the phonon and electronic contributions to this anomaly have been investi gated: in particular, from the simulations it can be inferred that the ob served anomaly presumably originates from phonons and thus from lattice contributions. This work, with the goal to shed light on the nature of the recently observed pressure-induced structural anomaly on A15 compounds, repre sents an effort toward the general understanding of the behaviour of A15 Nb-based superconductors as a function of pressure.