Correlation-Enhanced Electron-Phonon Coupling: Applications of $GW$ and Screened Hybrid Functional to Bismuthates, Chloronitrides, and Other High-$T_c$ Superconductors

We show that the electron-phonon coupling (EPC) in many materials can be significantly underestimated by the standard density-functional theory (DFT) in the local-density approximation (LDA) due to large nonlocal correlation effects. We present a simple yet efficient methodology to evaluate the realistic EPC, going beyond the LDA by using more advanced and accurate $GW$ and screened-hybrid-functional DFT approaches. The corrections that we propose explain the extraordinarily high superconducting temperatures that are observed in two distinct classes of compounds—the bismuthates and the transition-metal chloronitrides—thus solving a 30-year-old puzzle. Our work calls for the critical reevaluation of the EPC of certain phonon modes in many other materials, such as cuprates and iron-based superconductors. The proposed methodology can be used to design new correlation-enhanced high-temperature superconductors and other functional materials that involve electron-phonon interaction.

Popular Summary

The interaction between electrons and crystal lattice vibrations (phonons) plays an important role in electronic transport, electronic-heat capacity, and superconductivity. In particular, it provides the “glue” for forming the so-called Cooper pairs between electrons in conventional superconductors. The task of establishing the strength of electron-phonon interaction in real materials is therefore of fundamental importance but faces two major difficulties: Experimental techniques are usually unable to evaluate the total electron-phonon interaction, and current standard theoretical tools are also unreliable for this purpose because of electronic correlation effects. It is, therefore, highly desirable to have a reliable first-principles method to accomplish this task.

In this paper, we propose just such a method. It is simple but computationally efficient. Conceptually, it goes beyond the standard density-functional theory in the local-density approximation (LDA). Through demonstrating and understanding that the LDA substantially underestimates the strength of the electron-phonon coupling in materials close to a metal-insulator transition, we propose important corrections. With these corrections, we can now explain the extraordinarily high superconducting temperatures that are observed in two distinct classes of compounds, the bismuthates and the transition-metal chloronitrides, thus solving a 30-year-old puzzle.

Our work calls for the critical reevaluation of the strength of the electron-phonon interaction in correlated materials in general, for example, the cuprate superconductors and the iron-based superconductors where the superconductivity is believed to be unconventional (not phonon mediated). In these materials, certain phonon modes have anomalously large linewidths relative to their frequencies, an experimental fact that has not yet received a proper first-principles explanation. The proposed methodology can also be used to design new high-temperature superconductors and other functional materials involving electron-phonon interaction.