Anomalously high value of Coulomb pseudopotential for the $\rm H_{5}S_{2}$ superconductor

TL;DR

This study reveals an unusually high Coulomb pseudopotential for H5S2, indicating that its superconducting properties can only be phenomenologically explained within the Eliashberg framework, unlike H2S which aligns with standard parameters.

Contribution

The paper identifies an anomalously high Coulomb pseudopotential in H5S2 and discusses its implications for understanding superconductivity in sulfur-hydrogen compounds.

Findings

Coulomb pseudopotential for H5S2 is significantly higher than typical values.

H2S can explain superconductivity with standard Coulomb pseudopotential.

Superconducting properties of H5S2 require phenomenological treatment.

Abstract

The ${\rm H_{5}S_{2}}$ and ${\rm H_{2}S}$ compounds are the two candidates for the low-temperature phase of compressed sulfur-hydrogen system. We have shown that the value of Coulomb pseudopotential ($\mu^{\star}$) for ${\rm H_{5}S_{2}}$ ($\left[T_{C}\right]_{\rm exp}=36$~K and $p=112$~GPa) is anomalously high. The numerical results give the limitation from below to $\mu^{\star}$ that is equal to $0.402$ ($\mu^{\star}=0.589$, if we consider the first order vertex corrections to the electron-phonon interaction). Presented data mean that the properties of superconducting phase in the ${\rm H_{5}S_{2}}$ compound can be understood within the classical framework of Eliashberg formalism only at the phenomenological level ($\mu^{\star}$ is the parameter of matching the theory to the experimental data). On the other hand, in the case of ${\rm H_{2}S}$ it is not necessary to take high value of Coulomb pseudopotential to reproduce the experimental critical temperature relatively well ($\mu^{\star}=0.15$). In our opinion, ${\rm H_{2}S}$ is mainly responsible for the observed superconductivity state in the sulfur-hydrogen system at low temperature.