The Meissner effect in high-temperature hydrogen-rich superconductors under high pressure

Abstract

In the last few years, the superconducting transition temperature, Tc, of hydrogen-rich compounds has increased dramatically, and is now approaching room temperature. However, the pressures at which these materials are stable exceed one million atmospheres and limit the number of available experimental probes - superconductivity has been primarily identified based on electrical transport measurements. Here, we report definitive evidence of the Meissner effect – a key feature of superconductivity – in H3S and LaH10. Furthermore, we have determined characteristic superconducting parameters: a lower critical field Hc1 of ∼1.9 and ∼1.0 T, and a London penetration depth λL of ∼13 and ∼21 nm in Im-3m-H3S and Fm-3m-LaH10, respectively. These compounds have low values of the Ginzburg-Landau parameter κ ∼7–14 and belong to the group of “moderate” type II superconductors.

Clear evidence against superconductivity in hydrides under high pressure

“…Associated with it, superconductors exclude applied magnetic fields. Recently Minkov et al presented experimental results reportedly showing "definitive evidence of the Meissner effect" in sulfur hydride and lanthanum hydride under high pressure [1]. Instead, we show here that the evidence presented in that paper does not support the case for superconductivity in these materials.…”

“…The experimental results on sulfur hydride reported by the Eremets group in 2015 [2] and 2021 [1] report critical temperatures of 203 K and 196 K respectively, i.e. they are very close to one another.…”

“…they are very close to one another. The size of the samples used is also reported to be similar, approximately 80 µm [2] and 85 µm [1] in diameter and "a few µm" in thickness in [2] and between 2.1 µm and 3.1 µm in thickness in [1]. However, the measured magnetic moment of the samples under an applied magnetic field of the same magnitude differ by a factor of 5, with the 2015 sample having the larger magnetic moment.…”

“…[12]. Recently, Eremets and coworkers provided new magnetic evidence for superconductivity in sulfur hydride as well as in lanthanum hydride [1], and argued that it provides definitive evidence for superconductivity. Instead, we argue in this paper that these new measurements together with the old measurements provide conclusive evidence against the existence of superconductivity in these hydrides.…”

“…However, the measured magnetic moment of the samples under an applied magnetic field of the same magnitude differ by a factor of 5, with the 2015 sample having the larger magnetic moment. No explanation for this large discrepancy is given in [1]. These magnetic moments are measured not by field cooling but by applying a magnetic field to an already cold sample, and for sufficiently small fields so that the field should not penetrate the sample, hence such large differences are not expected for samples of similar sizes.…”