Superconductivity in multiple phases of compressed GeSb$_{\mathrm{\mathbf{2}}}$\textbf{Te}$_{\mathrm{\mathbf{4}}}$*

Here we report the discovery of superconductivity in multiple phases of the compressed GeSb$_{\mathrm{2}}$Te$_{\mathrm{4\thinspace }}$(GST) phase change memory alloy, which has attracted considerable attention for the last decade due to its unusual physical properties with many potential applications. Superconductivity is observed through electrical transport measurements, both for the amorphous (a-GST) and for the crystalline (c-GST) phases. The superconducting critical temperature, T$_{\mathrm{c}}$, continuously increases with the applied pressure reaching a maximum T$_{\mathrm{c\thinspace }}=$6K at P$=$20 GPa for a-GST, whereas the critical temperature of the cubic phase reaches a maximum T$_{\mathrm{c\thinspace }}=$8 K at 30 GPa. This new material system, exhibiting a superconductor-insulator quantum phase transition (SIT) has an advantage over disordered metals since it has a continuous control of the crystal structure and the electronic properties using pressure as an external stimulus, which was lacking in SIT studies until today.