Comment on ‘High- Tc superconductivity in H₃S: pressure effects on the superconducting critical temperature and Cooper pair distribution function’

Abstract

The Monkhorst–Pack scheme is applied in a recent article in (2019 Supercond. Sci. Technol. 32 125013) implying exclusion of the umklapp phonons. But in a superconductor scattered electrons can land anywhere on the Fermi surface so that umklapp phonons must be included. The authors should take care to explain if and how the original Monkhorst–Pack scheme was modified to accommodate umklapp phonons for electron-phonon scattering.

On the use of Monkhorst–Pack scheme to evaluate superconductivity and the issue of umklapp electron–phonon interactions

“…In 2019, Camargo-Martı ´nez, Gonza ´lez-Pedreros and Baquero studied the pressure effect on high-T c superconductivity in H 3 S. 24 In response to our enquiry in ref. 50, they confirmed that ''the electron-phonon umklapp process has a vital relevance in explaining the behaviour of electrical resistance'' but claimed ''in the total absence of electrical resistance, . .…”

“…In 2019, Camargo-Martínez, González-Pedreros and Baquero studied the pressure effect on high- T c superconductivity in H 3 S. 24 In response to our enquiry in ref. 50, they confirmed that “the electron–phonon umklapp process has a vital relevance in explaining the behaviour of electrical resistance” but claimed “in the total absence of electrical resistance, …, perhaps an electron that undergoes the umklapp-process will contribute to electrical resistance rather than the superconducting phase”. 25 In short, they believe the umklapp process does not contribute much to superconductivity.…”

“…In a following publication, 26 González-Pedreros, Camargo-Martínez and Mesa continued to apply the MP scheme to evaluate superconductivity in D 3 H. They cited, 25,50 claiming that “the presence of umklapp process is mainly associated with the electrical resistivity, which is measured in the normal state and not in the superconductor one”. They announced “Our calculations were considered in the superconducting state, and Umklapp processes have not been included”.…”

Cooper-pair distribution function $$D_{cp}(\omega ,T_c)$$ for superconducting $$\hbox {D}_3\hbox {S}$$ and $$\hbox {H}_3\hbox {S}$$

“…In particular, the Umklapp phonons come from anharmonic terms. Some authors 23 – 25 suggest that Umklapp phonons must be included in theoretical studies that evaluating phonon interaction effects at high-temperature conditions (like room-temperature for superconductors). However, the presence of umklapp process is mainly associated with the electrical resistivity 26 – 29 , which is measured in the normal state and not in the superconductor one.…”

Reply to Comment on ‘Reply to comment on High-T c superconductivity in H3S : pressure effects on the superconducting critical temperature and Cooper-pair distribution function’

“…Possibly, we need to include three-and four-phonons anharmonic terms or perhaps propose a modified Eliashberg function. Also,'the umklapp phonons are not replicas of normal phonons' [8]. This is not trivial topic.…”