Compressed H{sub 3}S, Superfluid Density and the Quest for Room-Temperature Superconductivity
The discovery of superconductivity at 203 K in highly compressed sulphur hydride validates the ideas put forward by Ashcroft 50 years ago and galvanises the quest for room-temperature superconductivity. But at such temperatures, thermal fluctuations might be expected to break up Cooper pairs. For example, in the cuprates, fluctuations reduce T{sub c} by 30% or more below the mean-field value. Similar effects are found in iron pnictides. Here, we ask: how does superconductivity survive in sulphur hydride at such high temperatures? To answer this, we examine the superfluid density which is the key parameter for quantifying fluctuations in both amplitude and phase. We show that dimensionality plays a key role in suppressing or enhancing thermal fluctuations to the benefit of hydrogen sulphide and the detriment of its more layered 2D competitors. We find that the temperature scale for phase fluctuations, T{sub φ}, in superconducting H{sub 3}S exceeds 1200 K, and therefore, these are irrelevant at 200 K. But the amplitude fluctuation temperature scale, T{sub amp}, at around 300 K is much lower and this has important implications for the ongoing quest for room temperature superconductivity. Appealing to the way in which superfluid density, T{sub φ}, T{sub amp} and T{sub c} scale with each other it seems that room temperature superconductivity is nearly ruled out, but perhaps not quite. It will require 3D systems with a large Fermi velocity to achieve this goal.