Ranga Dias and superconducting hydrides: a canary in the BCS gold mine
Jorge E. Hirsch, UC San Diego
In October 2020, Ranga Dias and coworkers announced the discovery of the first room temperature superconductor, a long-sought goal of condensed matter physics. The material was CSH, carbonaceous sulfur hydride, under 200 GPa pressure. It was acclaimed as a triumph of experimental physics but even more so of theory: the conventional BCS-electron-phonon theory of superconductivity had predicted high temperature superconductivity in pressurized hydrides in 2004, and motivated a large experimental effort to find it. In 2015, sulfur hydride had been announced as the first such superconductor by Eremets and coworkers [1], with 200K critical temperature, and several other such hydrides followed with Tc ~ 250K, culminating in the Dias discovery in 2020. By now, more than 15 pressurized hydrides are claimed to be high Tc superconductors, including another room temperature superconductor, LuHN, announced by the Dias group in March 2023 [2].
Yet the Dias 2020 results looked too good to be true. Transitions were unusually sharp, did not change with magnetic field the way they should, and had other peculiar features. A closer look at the published data and their underlying raw data revealed that they had been doctored in ways incompatible with accepted scientific practice. The CSH paper was retracted by the journal in September 2022 [3]. The recent Dias claim of room T superconductivity in LuHN has also been strongly called into question [4].
The Dias group undoubtedly saw signals in these materials that suggested superconductivity, similar to signals seen by Eremets and others in the other hydride materials claimed to be high Tc superconductors. I suggest that their and the community at large’s interpretation that they are due to superconductivity originates in confirmation bias, due to the expectation that they should be superconductors according to BCS theory. I will discuss our extensive analysis of experimental data for the hydrides that strongly suggests that no superconductivity exists.
If the latter is the case, it casts doubt on the applicability of BCS theory not only to hydrides but also to other materials [5]. I will outline the alternative theory of hole superconductivity, that predicts that high temperature superconductivity results from holes conducting through closely spaced negatively charged anions, as is the case in the “conventional” superconductor MgB2, and that the ionic mass is irrelevant. It also explains the Meissner effect, which I claim BCS theory does not and cannot.
[1] A. P. Drozdov et al, “Conventional superconductivity at 203 kelvin at high pressures in the sulfur hydride system”, Nature 525, 73–76 (2015).
[2] E. Snider et al, “Retraction Note: Room-temperature superconductivity in a carbonaceous sulfur hydride”, Nature 610, 804 (2022)
[3] N. Dasenbrock-Gammon et al, “Evidence of near-ambient superconductivity in a N-doped lutetium hydride”, Nature 615, 244–250 (2023).
[4] X. Ming et al, “Absence of near-ambient superconductivity in LuH2±xN”, Nature (2023). https://doi.org/10.1038/s41586-023-06162-w.
[5] J. E. Hirsch, “Superconducting materials: Judge and jury of BCS-electron–phonon theory”, Appl. Phys. Lett. 121, 080501 (2022).