Real-space understanding of electron-phonon coupling in superconducting hydrides

TL;DR

This paper introduces real-space functions to analyze electron-phonon coupling in superconducting hydrides, revealing the importance of higher-order derivatives and improving the understanding of factors influencing critical temperatures.

Contribution

The study develops new real-space descriptors to evaluate electron-phonon coupling, highlighting the role of higher-order derivatives and surpassing traditional electronic descriptors in predicting superconducting properties.

Findings

Derivatives of the potential are larger in regions of high electron localization in hydrides.

New descriptors distinguish structures with similar bonding but different critical temperatures.

Higher-order terms significantly impact electron-phonon coupling, especially in LaH10.

Abstract

Electron-phonon coupling is at the origin of conventional superconductivity, enabling the pairing of electrons into Cooper pairs. The electron-phonon matrix elements depend on the electronic eigenstates and, in the standard linear approximation, on the first derivative of the potential felt by the electrons with respect to ionic perturbations. Here, we focus on the derivatives of the potential with a twofold aim: to assess their contribution to the overall coupling and to analyze the limitations of neglecting higher-order derivatives. Several real-space functions are proposed to do the analysis, and are computed for some well-known superconductors. Our results show that, in hydrides, the derivatives of the potential tend to be larger in regions of high electron localization, explaining the success of electronic descriptors previously described to correlate with the critical temperature. The new functions introduced here are able to tell apart structures with similar types of bonding but very different critical temperatures, such as H3S and H3Se Im-3m phases, where electronic descriptors alone fail. Interestingly, our descriptors are capable of easily estimating the impact of higher-order terms in the electron-phonon coupling. In fact, we capture the limitations of the linear approximation expected for PdH, and predict an even more important non-linear behavior in LaH10.