Pressure effect on the electronic transport properties of Fe$_{1+y}$Te$_{1-x}$Se$_{x}$

We present a systematic study of electronic transport as function of pressure up to 25 kbar of Fe$_{+y}$Te$_{1-x}$Se$_{x}$ single crystalline samples (with $y=0.02$, 0.05, and $x=0$, 0.2, and 0.3). Pressure is demonstrated to be a clean control parameter to drive the system with high Fe-excess through the metal-insulator (MIT) transition, in analogy with increasing the Se-doping or reducing the Fe-excess. The scaling of resistivity $\rho (T, p)$ below 50 K identified a critical pressure of $p_{c}=8$ kbar which separates non-metallic and metallic temperature dependences. At the $p_{c}$ the low-temperature sheet resistance is in the 6.5 k$\Omega$/square range. The Seebeck coefficient ($S$) at $p_{c}$ changes sign from negative to positive indicating a change in the electronic structure and in the balance between the electron and hole carriers. The $S$ at the highest pressure exhibits low positive values similar to the metallic, superconducting cuprates. The critical MIT behavior, related to a quantum phase transition, indicates a universality of the Fe- and Cu-based high-$T_{c}$ superconductors.