Evolution of superconducting and normal state properties of Fe_1.09Se_0.55Te_0.45 under pressure

The Fe_1+ySe_1-xTe_x family of iron-based superconductors are extensively investigated for their unconventional nature of superconductivity, which arises from a complex interplay of spin and orbital ordering. At ambient conditions, Fe_1.09Se_0.55Te_0.45 exhibits a superconducting transition below T_c∼14 K and a nematic ordering accompanied by a tetragonal to orthorhombic structural change at (T_s) which is marked by a sign change of Hall coefficient (R_H) from positive to negative. In addition, the normal state resistivity follows a -log(T) increase with decreasing temperature due to the presence of excess Fe impurity acting as Kondo scattering centre. In this work, we investigate the evolution of superconducting and normal state properties of Fe_1.09Se_0.55Te_0.45, a member of the Fe_1+ySe_1-xTe_x family, under hydrostatic pressure (P) using magneto-transport, dc magnetization and complementary first-principles band structure calculations. With applied P, the superconducting T_c reveals a dome-like shape, reaching a maximum T_c ∼19.9 K at critical pressure P_c ∼3.3 GPa. Simultaneously, with increasing pressure, both the -log(T) resistivity increase and T_s are gradually suppressed. Near P_c, T_s almost disappears, while the -log(T) resistivity increase persist beyond P_c up to 5 GPa and a Fermi liquid like behaviour emerges around 8 GPa. Furthermore, the band structure calculations suggest a pressure-induced structural change from orthorhombic to monoclinic symmetry near P_c. The nontrivial nature is evidenced by the effects of high pressure on the charge carrier balance, phase transition and superconductivity in Fe_1.09Se_0.55Te_0.45. This nontrivial superconductivity is strongly linked to the significant normal state that arises from the connection between Fermi surface reconstruction and structural phase transitions.