On the isotope effect in compressed superconducting H3S and D3S

Abstract

A maximum superconductive transition temperature TC = 203.5 K has recently been reported for a sample of the binary compound tri-hydrogen sulfide (H3S) prepared at high pressure and with room temperature annealing. Measurements of TC for H3S and its deuterium counterpart D3S have suggested a mass isotope effect exponent α with anomalous enhancements for reduced applied pressures. While widely cited for evidence of phonon-based superconductivity, the measured TC is shown to exhibit important dependences on the quality and character of the H3S and D3S materials under study; examination of resistance versus temperature data shows that variations in TC and apparent α are strongly correlated with residual resistance ratio, indicative of sensitivity to metallic order. Correlations also extend to the fractional widths of the superconducting transitions. Using resistance data to quantify and compensate for the evident materials differences between H3S and D3S samples, a value of α = 0.043 ± 0.140 is obtained. Thus, when corrected for the varying levels of disorder, the experimental upper limit (≤0.183) lies well below α derived in phonon-based theories.