In december 2014, superconductivity with a record critical temperature of 203 K was reported in a sulfur hydride (SH3) at a pressure of 200 GPa. SH3 is not only the current record-holder for high-temperature superconductivity, but also the first example of a new superconductor which was entirely predicted from first principles.This discovery has stimulated an intense activity to search for new and better superconductors, which can operate at higher temperature and lower pressures.
In this project, we propose to study the high-pressure behaviour of ternary hydrides (gallates and alienates of alkali metals and alkaline earths); these are commercial materials which are routinely used in hydrogen storage applications. Compared to binary hydrides, like SH3, ternary hydrides permit to tune independently bonding characteristics and charge doping, and we believe this could be effectively exploited to reduce the pressure needed to achieve metallisation and superconductivity. In addition to exploring the high-pressure phase diagram with complementary experiments and ab-initio calculations, we will use machine learning techniques to identify the main features that lead to metallization, and thus superconductivity, in high-pressure hydrides.
Our team comprises experts in ab-initio (DFT) calculations and crystal structure prediction, theory of superconductivity, machine learning and high-pressure spectroscopy and thus possesses all necessary expertise to carry out the project.
Our final vision, i.e. finding a high-Tc superconductor that could be synthesized (close to) ambient pressure, has a truly ground-breaking potential. The technologies enabled by this discovery would in fact have a transformative impact on many different aspects of the society: energy storage and distribution, transportation, sensoring and imaging, supercomputing etc.
Reaching this goal in one year is unrealistic, but this timeframe is sufficient to make fundamental advancements in several directions.
1) Innovative methods: many of the techniques we plan to use (high pressure, Xtal structure prediction, machine learning) have been developed or have reached their full potential only in the last 5-10 years. Used in combination, they are rapidly shifting the focus of condensed matter research from the description of materials properties to material design.
The importance of this new approach is testified by the increasing number of publications, conferences, and journals dedicated to material design, and by the emergence of new big consortia, such as the Materials Genome Project in the US, NOMAD in Europe, MARVEL in Switzerland, etc. To fully exploit the potential of these methods, it is essential to build teams with complementary expertise, to understand mutual advantages and limitations. Collaborations like that we propose in this project are a big step forward in this direction.
2) Finding new superconductors: the report of a Tc of 203 K in SH3 was probably the biggest breakthrough of the last 30 years in the field of SC. The original papers of the experimental report and the theoretical prediction have received more than 500 citations in the last two years, testifying their high importance. The full potential of the discovery, however, will be realized only with the experimental discovery of other high-Tc high-pressure hydrides. Obtaining SC in commercial solid hydrides would allow to use simpler procedures and obtain a higher reproducibility than in the SH3 case, for which samples are synthesized directly in the diamond anvil cell from SH2 and H2 using complex cryogenic loading procedures.
3) Understanding the physics and chemistry of hydrides at high pressure: Following the excitement of the SH3 discovery, there has been an "explosion" of computational predictions of the high-pressure phase diagrams of binary hydrides, which offered only partial glimpses on a fascinating and puzzling physical and chemical behavior. Setting up a database of ternary hydrides and analyzing it with machine learning techniques is an important step towards a systematic understanding.
The field of material design, and in particular that of SCs at high pressure, is evolving very rapidly and the international competition is quite strong. What makes our collaboration unique is the in-house combination of state-of-the-art experimental and theoretical techniques, which gives us an important competitive advantage over other groups.
Note that all project members have a collaborated in the past on topics related to the present proposal (superconductivity, electronic structure and high-pressure), as testified by joint publications,[1] and have an excellent international visibility in their fields (superconductivity, electronic structure, high-pressure, data science); the main international collaborations are described in the last sub-section of this proposal.
In addition, both the PI (Lilia Boeri) and the theory group of Luciano Pietronero have entered the field of high-pressure SC in hydrides since the very beginning, with several publications and invited talks on the topic;[2]-[3] in 2016, Luciano Pietronero was also one of the main organizers of the first international workshop on superhydrides (http://www.superhydrides.net/), which gathered in Rome more than 80 researchers active on the topic. This guarantees that the results
[1] See, for example: L. Boeri, E. Cappelluti, G.B. Bachelet, L. Pietronero, PRB 2002; L. Boeri, M. Giantomassi, G.B. Bachelet, O.K. Andersen, PRB 2007; C. Marini, L. Boeri, P. Postorino et al., PRB 2008; F. Capitani, L. Boeri, P. Postorino et al., JPCM 2016.
[2] C. Heil and L. Boeri, PRB 2015; J.A. Flores-Livas, L.B. et al., PRB 2016 and PRM 2017; C. Kokail, C.Heil and L. B., PRB 2016; C. Kokail, L. B., W. von der Linden, cond-mat/1705.06977.
[3] L. Ortenzi, E. Cappelluti, L. Pietronero, PRB 2016; A.P. Durajski, R. Szczsniak, L.P. AdP (2016).