Periodic Reporting for period 4 - SuperH (Discovery and Characterization of Hydrogen-Based High-Temperature Superconductors)
Reporting period: 2023-08-01 to 2025-01-31
Superconductors are one of the most fascinating materials existing today due to their capacity to conduct electricity with no loss. Their technological applications are however limited by the low temperatures at which materials lose their superconducting behavior. New hopes flourished in the last few years after the discovery of superconductivity above 200 K in hydrides, which clearly shows that hydrogen-based superconductors can be high-temperature superconductors, at least at high pressures. At the moment, due to their recent discovery and the big room still open for discoveries, hydrogen-based superconductors offer the most realistic path towards ambient temperature and pressure superconductivity, a dream of physics that would bring a strong technological revolution.
As some of the experimental discoveries had been somewhat anticipated by first-principles theoretical calculations, the potential of this type of calculations to guide the experimental work in the right track is clearly acknowledged by the scientific community. However, hydrogen ions are the lightest in the periodic table and, consequently, are subject to strong quantum fluctuations, which can strongly affect the structural and superconducting properties of hydrogen-based superconductors. Incorporating these ionic quantum fluctuations into first-principles calculations is not a simple task, especially because it usually means that high-order terms in the potential that determines the dynamics of the ions need to be included in the calculations in a non-perturbative way.
The overall objective of the project is to contribute with first-principles calculations to the discovery of new hydrogen-based superconductors at low pressures, even at ambient pressure. In order to overcome the difficulties imposed by the crucial role of ionic fluctuations, we will develop new and efficient computational tools to calculate structural, vibrational, and superconducting properties of compounds fully incorporating these effects in a non-perturbative way. Making use of these developed methods, we will work on predicting new high-temperature superconducting materials at low pressures.
As some of the experimental discoveries had been somewhat anticipated by first-principles theoretical calculations, the potential of this type of calculations to guide the experimental work in the right track is clearly acknowledged by the scientific community. However, hydrogen ions are the lightest in the periodic table and, consequently, are subject to strong quantum fluctuations, which can strongly affect the structural and superconducting properties of hydrogen-based superconductors. Incorporating these ionic quantum fluctuations into first-principles calculations is not a simple task, especially because it usually means that high-order terms in the potential that determines the dynamics of the ions need to be included in the calculations in a non-perturbative way.
The overall objective of the project is to contribute with first-principles calculations to the discovery of new hydrogen-based superconductors at low pressures, even at ambient pressure. In order to overcome the difficulties imposed by the crucial role of ionic fluctuations, we will develop new and efficient computational tools to calculate structural, vibrational, and superconducting properties of compounds fully incorporating these effects in a non-perturbative way. Making use of these developed methods, we will work on predicting new high-temperature superconducting materials at low pressures.
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.
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.
The methodological work performed on WP1 has finalized the implementation of the SSCHA method, a state-of-the-art method to calculate the impact of ionic quantum anharmonic effects on the thermodynamic, transport, and superconducting properties of materials, and has developed a completely new method to calculate the electron-phonon interaction at a non-linear level from first-principles, clearly advancing the state pf the art.
Our results obtained related to the characterization of hydrogen-based materials have also provided results that advance the state of the art. In fact, we have discovered a correlation between the networking value and Tc. This has provided for the first time the knowledge of when and why hydrogen-based materials are high-Tc superconductors. These has allowed us to estimate with a reasonable accuracy with very cheap calculations the critical temperature of hydrogen-based superconductors, allowing a fast screening of candidate materials.
Finally, we have shown that metastable high-Tc hydrides with a critical temperature of around 100 K are possible and can be synthesized at moderate pressures and quench to ambient pressure. We have provided a couple of materials like Mg2IrH6 and RbPH3 of this kind. In the future new similar predictions will come, specially given that our work has shown that anharmonic effects are crucial to stabilize high-Tc hydrides at ambient pressure and our methodology (SSCHA) to estimate them is open source and can be used by all the community. Experimentalists have already plenty of materials to try to synthesize, and will have more in the near future, thanks, in part, to the work of this project.
Our results obtained related to the characterization of hydrogen-based materials have also provided results that advance the state of the art. In fact, we have discovered a correlation between the networking value and Tc. This has provided for the first time the knowledge of when and why hydrogen-based materials are high-Tc superconductors. These has allowed us to estimate with a reasonable accuracy with very cheap calculations the critical temperature of hydrogen-based superconductors, allowing a fast screening of candidate materials.
Finally, we have shown that metastable high-Tc hydrides with a critical temperature of around 100 K are possible and can be synthesized at moderate pressures and quench to ambient pressure. We have provided a couple of materials like Mg2IrH6 and RbPH3 of this kind. In the future new similar predictions will come, specially given that our work has shown that anharmonic effects are crucial to stabilize high-Tc hydrides at ambient pressure and our methodology (SSCHA) to estimate them is open source and can be used by all the community. Experimentalists have already plenty of materials to try to synthesize, and will have more in the near future, thanks, in part, to the work of this project.