Cooper-pair distribution function, Dcp(ω, Tc) , is a recent theoretical proposal that reveals information about the superconductor state through the determination of the spectral regions where Cooper pairs are formed. This is built from the well-established Eliashberg spectral function and phonon density of states, calculated by first-principles. From this function is possible to obtain the Ncp parameter, which is proportional to the total number of Cooper pairs formed at a critical temperature Tc. Herein, we reported Dcp(ω, Tc) function of the compressed D3S and H3S high-Tc conventional superconductors, including the effect of stable sulfur isotopes in H3S. Dcp(ω, Tc) suggests that the vibration energy range of 10–70 meV is where the Cooper pairs are possible for these superconductors, pointing out the possible importance of the low-energy region on the electron–phonon superconductivity. This has been confirmed by the fact that a simple variation in the low-frequency region induced for the substitution of S atoms in H3S by its stable isotopes can lead to important changes in Tc. The results also show proportionality between Ncp parameter and experimental or theoretical Tc values.
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