15 - Superconductivity at High Pressure

Summary

Room temperature superconductivity (RTSC) is one the most challenging and longstanding problems in solid-state physics. The Bardeen–Cooper–Schrieffer (BCS) theory (1956) explained superconductivity but could not predict high critical temperatures (T_c). Extension of the BCS theory allowed RTSC in principle; however, estimations for realistic materials gave low T_c, with the only exception being metallic hydrogen. Therefore, conventional superconductors were not considered potential RTSCs. This tendency strengthened after the experimental discovery of superconductivity in cuprates with very high T_c, up to 133 K. Later, other families of nonconventional superconductivity appeared, notably, iron-based superconductors with T_c reaching 100 K. However, the mechanism of superconductivity in these materials is still not understood, and there has been no progress for many years in increasing T_c. Unexpectedly, conventional superconductors recently showed a clear prospect to be the first RTSCs: T_c = 203 K was discovered in H₃S, and then T_c = 250 K in LaH₁₀. This breakthrough resulted from a combination of factors, including the general idea to consider hydrogen-dominant materials, the appearance of ab initio predictions of structures for searching new materials, and advances in synthesis and characterization of new superconductors at megabar pressures. There is a clear prospect to achieve higher T_c in other binary or ternary hydrides. At ambient pressures, there is also a distinct possibility for substantial superconductivity, likely in materials with strong covalent bonding.