  Recently, ZrTe5 and HfTe5 are theoretically studied to be the most promising
layered topological insulators since they are both interlayer weakly bonded
materials and also with a large bulk gap in the single layer. It paves a new
way for the study of novel topological quantum phenomenon tuned via external
parameters. Here, we report the discovery of superconductivity and properties
evolution in HfTe5 single crystal induced via pressures. Our experiments
indicated that anomaly resistance peak moves to low temperature first before
reverses to high temperature followed by disappearance which is opposite to the
low pressure effect on ZrTe5. HfTe5 became superconductive above ~5.5 GPa up to
at least 35 GPa in the measured range. The highest superconducting transition
temperature (Tc) around 5 K was achieved at 20 GPa. High pressure Raman
revealed that new modes appeared around pressure where superconductivity
occurs. Crystal structure studies shown that the superconductivity is related
to the phase transition from Cmcm structure to monoclinic C2/m structure. The
second phase transition from C2/m to P-1 structure occurs at 12 GPa. The
combination of transport, structure measurement and theoretical calculations
enable a completely phase diagram of HfTe5 at high pressures.
