  Structural and superconducting transitions of layered van der Waals (vdW)
hydrogenated germanene (GeH) were observed under high-pressure compression and
decompression processes. GeH possesses a superconducting transition at critical
temperature (Tc) of 5.41 K at 8.39 GPa. A crystalline to amorphous transition
occurs at 16.80 GPa while superconductivity remains. An abnormally increased Tc
up to 6.1 K has been observed in the decompression process while the GeH
remained amorphous. Thorough in-situ high-pressure synchrotron X-ray
diffraction and in-situ high-pressure Raman spectroscopy with the density
functional theory simulations suggest that the superconductivity of GeH should
be attributed to the increased density of states at the Fermi level as well as
the enhanced electron-phonon coupling effect under high pressure. The
decompression-driven superconductivity enhancement arises from pressure-induced
phonon softening related to an in-plane Ge-Ge phonon mode. As an amorphous
metal hydride superconductor, GeH provides a platform to study amorphous
hydride superconductivity in layered vdW materials.
