The main aim of the project is the accurate characterization of the high-pressure superconducting phase diagram of a subset of complex boro-hydrides, with chemical formula ABH4, where A is an alkali metal or alkaline earth, which our preliminary results identify as a viable candidate for high-Tc superconductivity.
In particular, we will evaluate the possibility of stabilizing hydrogen-deficient phases (ABH3,ABH2), metallizing them by external pressure or doping, and estimate the relative critical temperature. In order to have accurate results, the vibrational properties will be evaluated within the stochastic self-consistent harmonic approximation (SSCHA). This will allow us to correctly take into account quantum lattice effects, which can have dramatic consequences on the superconducting phase diagram of hydrogen-rich solids.
The project combines two recently-established groups in theoretical condensed-matter physics, with complementary expertises in ab-initio crystal structure prediction and quantum lattice effects, which have been independently active in the study of superconductivity in high-pressure hydrides, since the ground-breaking report of high-Tc conventional superconductivity in SH3 in 2015.
The potential impact of the project is very high, in terms of purely scientific impact and international visibility. Indeed, the proposed topic is currently at the forefront of condensed matter research, because an improved understanding of possible mechanisms to achieve superconductivity above liquid nitrogen temperature in commercially-available materials has the potential to open a plethora of large-scale applications for superconductors, ranging from electrical grid components to magnetic levitation trains. In addition, the two groups have an extended network of international collaborators, which ensures a high visibility for the results of the collaboration.
The proposed project is highly INNOVATIVE, for two main reasons:
1) The study of high-pressure hydrides is a new field, with an extremely rapid progress in the last three years: The discovery papers have been cited more than 1000 times in less than 3 years, and Mikhail Eremets, author of the original H3S discovery, has been named one of the "ten people who mattered this year" by Nature in 2015.
2) The methods we employ are state-of-the-art, and at the forefront of what is currently employed for conventional superconductivity: (i) the possibility of predicting Xtal structure is rapidly changing the scope of the field of ab-initio calculations from the description of the properties of existing materials to material design; (ii) the investigation of quantum lattice effects on the vibrational spectrum and electron-phonon interaction is an emerging trend in the field of ab-initio calculations. These effects are particularly crucial in hydrides, where they have been shown to influence the energetics, structural stability and superconductivity in a highly non-trivial and unpredictable way.
The combination of the two approaches (i) and (ii) within a single project is quite unusual, as the main groups who work on this topic worldwide usually belong to different communities, with little overlap and collaborations.
The potential IMPACT of the project is very high, in terms of both applied and fundamental research.
In fact, if the most ambitious aim of the project, i.e. the identification of the synthesis route of a new high-temperature superconductor close to ambient pressure, is reached, this could enable a whole range of large-scale applications, which are hindered by the high refrigeration and manufacturing costs of currently-known superconductors. On the contrary, the materials we plan to study in the present project are commercially available and inexpensive, since they are routinely used for hydrogen storage research and applications.
Although the project does not include an experimental partner, we are in contact, through national and international collaborations, with the main international actors of the field, as evidenced by our publication list, and this would allow us to test our theoretical predictions in a timely manner.
Even if we do not reach this most ambitious goal within the time-frame of the present project, we are quite confident that we will obtain a full characterization of the high-pressure phase diagram of two-three complex hydrides, which per se represents a major achievement. Indeed, the study of matter at extreme pressures is an emerging trend worldwide; in the last years, high-pressure experiments have widely expanded our understanding of materials properties, challenging the conventional intuition of physics and chemistry. Well-known examples are the formation of insulating, transparent phases of alkali metals, binary compounds of noble gases, chemically-forbidden phases such as superhydrides, etc. Apart from superconductivity, the high-pressure phase diagram of complex hydrides is likely to also give rise to a plethora of compounds, of relevance to several different applications.
In addition to the purely scientific aspects, the present project has a high potential in terms of international visibility.
In fact, the Physics Department of Sapienza has been at the forefront of theoretical research on high-pressure hydrides since the very beginning, hosting also two workshops on the subject(Superhydrides 2016, RomeSC2018) in the last two years. The present project combines two parallel lines of research independently established by two different groups at La Sapienza in the last three years, and contributed significantly to the microscopical understanding of these system. Both groups have a tradition for excellence, with several recognized contributions to the field of density functional theory, superconductivity and quantum lattice effects.