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Abstract Designing materials with advanced functionalities is the main focus of contemporary solid-state physics and chemistry. Research efforts worldwide are funneled into a few high-end goals, one of the oldest, and most fascinating of which is the search for an ambient temperature superconductor (A-SC). The reason is clear: superconductivity at ambient conditions implies being able to handle, measure and access a single, coherent, macroscopic quantum mechanical state without the limitations associated with cryogenics and pressurization. This would not only open exciting avenues for fundamental research, but also pave the road for a wide range of technological applications, affecting strategic areas such as energy conservation and climate change. In this roadmap we have collected contributions from many of the main actors working on superconductivity, and asked them to share their personal viewpoint on the field. The hope is that this article will serve not only as an instantaneous picture of the status of research, but also as a true roadmap defining the main long-term theoretical and experimental challenges that lie ahead. Interestingly, although the current research in superconductor design is dominated by conventional (phonon-mediated) superconductors, there seems to be a widespread consensus that achieving A-SC may require different pairing mechanisms. In memoriam, to Neil Ashcroft, who inspired us all. 

Following the discovery of high-temperature superconductivity in the La–H system, we studied the formation of new chemical compounds in the barium-hydrogen system at pressures from 75 to 173 GPa. Using in situ generation of hydrogen from NH₃BH₃, we synthesized previously unknown superhydride BaH₁₂with a pseudocubic (fcc) Ba sublattice in four independent experiments. Density functional theory calculations indicate close agreement between the theoretical and experimental equations of state. In addition, we identified previously knownP6/mmm-BaH₂and possibly BaH₁₀and BaH₆as impurities in the samples. Ab initio calculations show that newly discovered semimetallic BaH₁₂contains H₂and H₃^–molecular units and detached H₁₂chains which are formed as a result of a Peierls-type distortion of the cubic cage structure. Barium dodecahydride is a unique molecular hydride with metallic conductivity that demonstrates the superconducting transition around 20 K at 140 GPa.

 

With the motivation of searching for new superconductors in the Mg–B system, we performed ab initio evolutionary searches for all the stable compounds in this binary system in the pressure range of 0–200 GPa. We found previously unknown, yet thermodynamically stable, compositions MgB 3 and Mg 3 B 10 . Experimentally known MgB 2 is stable in the entire pressure range 0–200 GPa, while MgB 7 and MgB 12 are stable at pressures below 90 GPa and 35 GPa, respectively. We predict a reentrant behavior for MgB 4 , which becomes unstable against decomposition into MgB 2 and MgB 7 at 4 GPa and then becomes stable above 61 GPa. We find ubiquity of phases with boron sandwich structures analogous to the AlB 2 -type structure. However, with the exception of MgB 2 , all other magnesium borides have low electron–phonon coupling constants λ of 0.32–0.39 and are predicted to have T c below 3 K.