  High-pressure studies on elements play an essential role in superconductivity
research, with implications for both fundamental science and applications. Here
we report the experimental discovery of surprisingly low pressure driving a
novel germanium allotrope into a superconducting state in comparison to that
for alpha-Ge. Raman measurements revealed structural phase transitions and
possible electronic topological transitions under pressure up to 58 GPa. Based
on pressure-dependent resistivity measurements, superconductivity was induced
above 2 GPa and the maximum Tc of 6.8 K was observed under 4.6 GPa.
Interestingly, a superconductivity enhancement was discovered during
decompression, indicating the possibility of maintaining pressure-induced
superconductivity at ambient pressure with better superconducting performance.
Density functional theory analysis further suggested that the electronic
structure of Ge (oP32) is sensitive to its detailed geometry and revealed that
disorder in the beta-tin structure leads to a higher Tc in comparison to the
perfect beta-tin Ge.
