Fermi-Liquid Nonadiabatic Highly Compressed Cesium Iodide Superconductor

Abstract

The experimental discovery that compressed sulfur hydride exhibits superconducting transition temperature of Tc=203 K by Drozdov et al. (Nature 2015, 525, 73–76) sparked studies of compressed hydrides. This discovery was not a straightforward experimental examination of a theoretically predicted phase, but instead it was a nearly five-decade-long experimental quest for superconductivity in highly compressed matters, varying from pure elements (hydrogen, oxygen, sulfur), hydrides (SiH4, AlH3) to semiconductors and ionic salts. One of these salts was cesium iodide, CsI, which exhibits the transition temperature of Tc≅1.5 K at P=206 GPa (Eremets et al., Science 1998, 281, 1333–1335). Detailed first principles calculations (Xu et al., Phys Rev B 2009, 79, 144110) showed that CsI should exhibit Tc~0.03 K (P=180 GPa). In an attempt to understand the nature of this discrepancy between the theory and the experiment, we analyzed the temperature-dependent resistance in compressed CsI and found that this compound is a perfect Fermi liquid metal which exhibits an extremely high ratio of Debye energy to Fermi energy, ℏωDkBTF≅17. This implies that direct use of the Migdal–Eliashberg theory of superconductivity to calculate the transition temperature in CsI is incorrect, because the theory is valid for ℏωDkBTF≪1. We also showed that CsI falls into the unconventional superconductors band in the Uemura plot.