The characterization of superconductivity under high pressure

The discovery of superconductivity with a critical temperature (Tc) of 203 K in H3S (ref. 1) and soon after up to ~250 K in LaH10 (ref. 2 and references therein) has given rise to a new family of hydrogen-rich superconductors, termed superhydrides. Importantly, these materials exhibit conventional superconductivity, enabling the calculation of Tc and the prediction of new superconducting materials. We believe this Feature is particularly timely, given recent erroneous claims of room-temperature superconductivity in carbonaceous H3S (ref. 3), as well as claims of room-temperature superconductivity in the Lu–N–H ternary system at much lower pressures4. These claims have led to speculations about the validity of the entire field of high-pressure high-temperature superconductivity5. Superhydrides are stabilized by high pressures, necessitating the use of diamond anvil cells for their study. A high-pressure environment imposes limitations on the types of experiment that can be conducted. In particular, powerful techniques for probing electronic structure, such as angle-resolved photoemission spectroscopy and scanning tunnelling microscopy, are not feasible. The small (micrometre) size of samples is an additional constraint. Nevertheless, the hallmarks of superconductivity in hydrides are firmly established. Here, we provide a brief summary of experimental studies on hydrides at high pressures, emphasizing the evidence for, and independent confirmation of, superconductivity. Typically, superhydrides are synthesized by reacting pure metals or their stable hydrides with hydrogen at high pressures. Although light atoms are practically invisible to X-rays, the structure of the heavy-atom sublattice in the superconducting phase can be well determined. Compositions are estimated based on the difference in the determined molar volume of the product and the precursors, or by comparing the calculated lattice volumes. The existence of superconductivity can be demonstrated through unique properties such as zero resistance measured below Tc and the expulsion of an applied magnetic field from the sample, known as the Meissner effect. Remarkably, these and many other properties of a superconductor can be well characterized under high-pressure conditions using various methods. Tc is derived from a sharp change of the temperature dependence of electrical resistance (Fig. 1a), magnetic susceptibility (Fig. 1b) and the trapped magnetic flux (Fig. 1c). Four-probe electrical resistance measurements allow for the direct detection of the zero-resistance state2.