Dottorato in FISICA

Titolo della tesi: Ab initio materials design of superhydrides: a quest to high-Tc superconductivity at room pressure

This thesis presents a collection of my main results on the study of high-pressure superhydrides, using first-principles methods for crystal structure prediction and superconductivity. Superhydrides, i.e. compounds that under high pressure incorporate a large amount of hydrogen in their crystal structure, are extremely exciting. Among them, superconductors with critical temperatures (Tc’s) close to, or even above room temperature were found at extremely high pressures (higher than one million atmospheres). In less than one decade, the study of superhydrides has achieved groundbreaking results, and has given rise to a very active research community. This rapid progress was largely driven by an extremely successful synergy between first-principles calculations based on Density Functional Theory and high-pressure experiments. During the course of my thesis, the focus shifted from finding materials with higher and higher superconducting Tc’s, to the search for ambient-pressure high-Tc superconductors, which may find their way into technological applications. In this regard, the attention is shifting from binary to ternary hydrides, containing two elements other than hydrogen. This thesis, based on the results contained in four papers published in 2018-2021, presents my research efforts in this direction. The main research goal was to deepen the understanding of the essential features that lead to the formation of high-Tc hydride superconductors, with the overarching goal of reaching high-Tc superconductivity at ambient pressure. After a general introduction to the history and open questions in the field, I discuss in some detail the theoretical foundations of methods for crystal structure prediction and superconductivity employed in the thesis. These methods are applied in the second part of the thesis to the study of three different systems: sodalite-like yttrium hydrides, calcium boron hydrides, and lanthanum ternary hydrides. The hope of the author is that some of the ideas outlined in this thesis will stimulate future research in ternary hydrides.