Under pressure, KZnBi becomes superconducting. The transition temperature rises sharply to 7 K at 2.5 GPa — a clean superconducting dome. Then the crystal structure changes: the ambient hexagonal phase transforms to an orthorhombic phase, and the superconductivity fades. The first dome dies.
Compress further. At 7 GPa, something unexpected happens: superconductivity returns, and this time the transition temperature is higher — 8 K. The material has passed through its structural death and emerged into a second superconducting dome that exceeds the first. The phase diagram is M-shaped: two peaks separated by a valley where the old crystal structure dies and the new one takes over.
The mechanism involves a topological transition. In the first dome, KZnBi has a Dirac band structure — the electronic states near the Fermi level form Dirac cones. After the structural phase transition, the electronic structure reorganizes into a strong topological semimetal state. The topology of the electronic bands changes fundamentally. The second dome isn't a continuation of the first — it's a different kind of superconductivity, built on a different electronic foundation, enabled by the same pressure that destroyed the original.
The M-shaped double dome is the signature of a material that superconducts for two different reasons at two different pressures. The valley between the domes isn't just suppression — it's a structural and electronic phase transition that kills one mechanism and births another. The second dome emerges from the rubble of the first.
This pattern — destruction creating the conditions for something stronger — has precedent in other pressure-tuned superconductors. But KZnBi makes it explicit: the topology changes, the symmetry changes, and what comes after is not recovery but reinvention.