On the isotope effect in compressed superconducting H_{3}S and D_{3}S, D. R. Harshman and A. T. Fiory [arXiv]
A maximum superconductive transition temperature T_{C} = 203.5 K has recently been reported for a sample of the binary compound trihydrogen sulfide (H_{3}S) prepared at high pressure and with room temperature annealing. Measurements of T_{C} for H_{3}S and its deuterium counterpart D_{3}S have suggested a mass isotope effect exponent α with anomalous enhancements for reduced applied pressures. While widely cited for evidence of phononbased superconductivity, the measured T_{C} is shown to exhibit important dependences on the quality and character of the H_{3}S and D_{3}S materials under study; examination of resistance vs. temperature data shows that variations in T_{C} 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 H_{3}S and D_{3}S 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 phononbased theories.
Transition temperatures T_{C} for H_{3}S (green circles) and D_{3}S (blue circles) vs. applied pressure P with corresponding dashed lines fitted to joined linear trends using equation (1) of the article with parameters in table 1 of the article (left scale). Residual resistance ratios RRR are shown for H_{3}S (red triangles) and D_{3}S (red diamonds) vs. P (right scale). Data are for samples annealed at or above room temperature (see REfs. [1,2] of the article). 

Residual resistance ratio RRR vs. fractional transition width ΔT_{C}/T_{C} for samples of H_{3}S (green triangles) and D_{3}S (blue diamonds). A linearly fitted dashed line is drawn to indicate the trend.  Transition temperature T_{C} vs. residual resistance ratio RRR for H_{3}S (green triangles) and D_{3}S (blue diamonds). The eight symbols with dot fill correspond to data for P ≳ P_{m} with trend shown by the fitted dashed line. 
Dale R. Harshman and Anthony T. Fiory, Supercond. Sci. and Technol. 30, 045011 (2017).