Evolution of superconducting and normal state properties of Fe1.09Se0.55Te0.45 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.