Periodic Reporting for period 3 - SuperH (Discovery and Characterization of Hydrogen-Based High-Temperature Superconductors)
Okres sprawozdawczy: 2022-02-01 do 2023-07-31
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 three 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 already predicted superconducting hydrogen-based compounds; and WP3, where we have investigated the possibility of high-temperature superconductivity in systems at ambient pressure.
In WP1 first we have finalized the implementation of the Stochastic Self-Consistent Harmonic Approximation 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 advanced a lot and is close to be finished. 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.
In WP3 we are currently studying the possibility of high-temperature superconductivity in metastable states of PdH; the superconducting properties of the promising hydrogen boride monolayer recently synthesized; and the electronic ground state of polyacene chains, which are the building blocks of organic superconductors. Even if we have identified many metastable states in PdH, none of them seems to have an outstanding Tc despite the unconfirmed experimental evidence. The hydrogen boride monolayer also does not seem to be a good superconductor according to our results. Finally, we are identifying an important role of spin fluctuations in polyacenes.
In WP1 first we have finalized the implementation of the Stochastic Self-Consistent Harmonic Approximation 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 advanced a lot and is close to be finished. 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.
In WP3 we are currently studying the possibility of high-temperature superconductivity in metastable states of PdH; the superconducting properties of the promising hydrogen boride monolayer recently synthesized; and the electronic ground state of polyacene chains, which are the building blocks of organic superconductors. Even if we have identified many metastable states in PdH, none of them seems to have an outstanding Tc despite the unconfirmed experimental evidence. The hydrogen boride monolayer also does not seem to be a good superconductor according to our results. Finally, we are identifying an important role of spin fluctuations in polyacenes.
The methodological work on WP1 related to the non-linear electron-phonon coupling will continue till the end of the project. The work we have done so far shows that the novel devised theoretical framework is possible to be translated into a useful program that can compute these effects in real materials through first-principles calculations. Moreover, confirming the initial hypothesis of the project, these non-linear effects seem to be extremely important in hydrogen-based superconductors according to our preliminary results. The new approach provides a completely new paradigm in the calculation of the electron-phonon coupling that considers the quantum nature of the ions and all the associated non-linearity in a non-perturbative way. This is a clear step forward in the state of the art of the field. Till the end of the project we will estimate the superconducting critical temperature (Tc) of many hydrogen-based compounds including these non-linear effects, providing the most accurate calculations of Tc performed in the literature. This new approach will allow us to perform calculations of Tc with an unprecedented accuracy. By the end of the project we will have a trustworthy and reliable code that will be shared with the community as an open source code that implements these ideas, and will be used in WP4.
WP2 and WP3 will be closed soon. WP2 has clearly advanced the state of the art in the field by providing the discovery of 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. Within this WP2 we will soon have a clear picture of the role of lattice quantum anharmonic effects on Tc, though our results already remark that these effects will make possible high-Tc superconductivity at low pressures as well. With respect to WP3, our results show that both in metastable states of PdH and hydrogen boride, no high-temperature superconductivity seems to be realistic, which was not clear before in the literature.
Having both the SSCHA method and the non-linear electron-phonon coupling code ready and benefiting from the knowledge acquired in WP2 and WP3, we will be able to conduct a careful and guided search of new high-Tc compounds at low pressures. We expect to provide new discoveries with the work that will be performed in WP4.
WP2 and WP3 will be closed soon. WP2 has clearly advanced the state of the art in the field by providing the discovery of 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. Within this WP2 we will soon have a clear picture of the role of lattice quantum anharmonic effects on Tc, though our results already remark that these effects will make possible high-Tc superconductivity at low pressures as well. With respect to WP3, our results show that both in metastable states of PdH and hydrogen boride, no high-temperature superconductivity seems to be realistic, which was not clear before in the literature.
Having both the SSCHA method and the non-linear electron-phonon coupling code ready and benefiting from the knowledge acquired in WP2 and WP3, we will be able to conduct a careful and guided search of new high-Tc compounds at low pressures. We expect to provide new discoveries with the work that will be performed in WP4.