We have worked on four work packages (WPs): WP1, where we have focused on new methodological developments to incorporate efficiently ionic quantum effects on the ab initio calculation of structural, vibrational, and superconducting properties of materials; WP2, where we have characterized the properties of superconducting hydrogen-based compounds in order to understand clearly the reason why high-Tc is possible; WP3, where we have investigated the possibility of high-temperature superconductivity in few systems at ambient pressure; and WP4, where we have performed novel predictions of compounds.
In WP1 first we have finalized the implementation of the Stochastic Self-Consistent Harmonic Approximation (SSCHA) method, which can efficiently calculate the structural and vibrational properties of materials including ionic quantum effects and the consequent anharmonicity in a non-perturbative way. Secondly, we have developed a completely new method to calculate the electron-phonon interaction including non-linear effects. The novel theoretical framework is now well-defined and the computational implementation of it is finalized in its first prototype.
The characterization part performed in WP2 has provided very interesting results, pushing the state-of-the-art in the field. With the study of structural and electronic properties of hundreds of compounds, we have unveiled that creating an electronic network of delocalized electronic states is the key to enhance the critical temperature (Tc) in hydrogen-based superconductors. In fact, we have defined a new descriptor, only based on electronic properties, that can predict the critical temperature within 60 K. We have also worked on characterizing the role of quantum effects and anharmonicity in these superconductors and determined that, remarkably, these effects can stabilize superconductors at much lower pressures than expected classically, which opens hopes for discovering high-Tc compounds even at ambient pressure. Also we have understood that the symmetry of the chemical bonding is the key to understand the impact of ionic quantum anharmonicity on a superconductor.
The work performed on WP3 has shown us that, despite many metastable states can exist in ambient pressure metal hydrides like PdH, these are not expected to increase the superconducting critical temperature. Also that, despite the a priori interesting perspective, the recently synthesized hydrogen boride monolayer is not a superconductor either. These results help us focus the search on particular systems in WP4 and discard others.
The ultimate quest performed in WP4 has turned out to be very successful and we have been able to predict several new hydrogen-based superconductors that are expected to superconduct above the boiling temperature of liquid nitrogen and be metastable at ambient pressure. First, we have demonstrated that Mg2IrH6 is a compound that is metastable at ambient pressure and has a superconducting critical temperature of around 80 K. Secondly, we have predicted that a ternary compound with a perovskite like structure, RbPH3, is thermodynamically stable at moderate pressures (around 25 GPa) and remains metastable down to ambient pressure with a Tc of around 100 K. This work is particularly important because it verifies one of the main hypotheses of this proposal: lattice anharmonic effects stabilize high-Tc materials and one needs to include these effects to be able to discover high-Tc hydrides at low and ambient pressures.
