Preface Special Issue on novel superconducting and magnetic materials

Superconductivity and magnetism -- and their entanglement in a single material -- are among the most studied phenomena in condensed matter physics and continue to pose new challenges for fundamental research and exciting opportunities for technological applications. The last decade has witnessed ground-breaking discoveries in both fields: high-temperature superconductivity in compressed hydrides, unconventional superconductivity in iron-based materials and new types of magnetic states in spin-orbit coupled materials with topological and nematic characteristics. The prediction of material-specific properties and the interpretation of superconducting and magnetic phase transitions have been crucially aided by advances in ab-initio electronic structure methods within the density functional theory and its extensions. This special issue gathers together selected theoretical and experimental contributions on novel aspects of superconductivity and magnetism, %that have been collected in memory of Prof. Sandro Massidda. The collection aims to provide an updated view on timing issues and challenges in this active research field that have been at the hearth of Sandro's scientific interests. As commemorated in the obituary by Continenza and Colombo, Sandro has dedicated his scientific work to the development and application of \textit{ab-initio} computational and theoretical methods, yet never losing focus to the ultimate goal of theoretical and computational physics, that is to support, complement and understand the experimental observations

Doping-induced insulator-metal transition in the Lifshitz magnetic insulator NaOsO3

By means of first principles schemes based on magnetically constrained density functional theory and on the band unfolding technique we study the effect of doping on the conducting behaviour of the Lifshitz magnetic insulator NaOsO3. Electron doping is treated within a supercell approach by replacing sodium with magnesium at different concentrations (MgxNa1-xOsO3, x = 0.125, 0.25, 0.375, 0.5). Undoped NaOsO3 is subjected to a temperaturedriven Lifshitz transition involving a continuous closing of the gap due to longitudinal and rotational spin fluctuations (Kim et al 2016 Phys. Rev. B 94 241113). Here we find that Mg doping suppresses the insulating state, gradually drives the system to a metallic state (via an intermediate bad metal phase) and the transition is accompanied by a progressive lowering of the Os magnetic moment. We inspected the role of longitudinal spin fluctuations by constraining the amplitude of the local Os moments and found that a robust metal state can be achieved below a critical moment. In analogy with the undoped case we conjecture that the decrease of the local moment can be controlled by temperature effects, in accordance with the theory of itinerant electron magnetism