Pressure Dependence of Superconducting Properties, Pinning Mechanism, and Crystal Structure of the Fe_0.99Mn_0.01Se_0.5Te_0.5Superconductor

We have investigated the pressure (P) effect on structural (up to 10 GPa), transport [R(T): up to 10 GPa], and magnetic [(M(T): up to 1 GPa)] properties and analyzed the flux pinning mechanism of the Fe0.99Mn0.01Se0.5Te0.5 superconductor. The maximum superconducting transition temperature (Tc) of 22 K with the P coefficient of Tc dTc/dP = +2.6 K/GPa up to 3 GPa (dTc/dP =-3.6 K/GPa, 3 ≤ P ≥ 9 GPa) was evidenced from R(T) measurements. The high-pressure diffraction and density functional theory (DFT) calculations reveal structural phase transformation from tetragonal to hexagonal at 5.9 GPa, and a remarkable change in the unit cell volume is observed at ∼3 GPa where the Tc starts to decrease, which may be due to the reduction of charge carriers, as evidenced by a reduction in the density of states (DOS) close to the Fermi level. At higher pressures of 7.7 GPa ≤ P ≥ 10.2 GPa, a mixed phase (tetragonal + hexagonal phase) is observed, and the Tc completely vanishes at 9 GPa. A significant enhancement in the critical current density (JC) is observed due to the increase of pinning centers induced by external pressure. The field dependence of the critical current density under pressure shows a crossover from the δl pinning mechanism (at 0 GPa) to the δTc pinning mechanism (at 1.2 GPa). The field dependence of the pinning force at ambient condition and under pressure reveals the dense point pinning mechanism of Fe0.99Mn0.01Se0.5Te0.5. Moreover, both upper critical field (HC2) and JC are enhanced significantly by the application of an external P and change over to a high P phase (hexagonal ∼5.9 GPa) faster than a Fe0.99Ni0.01Se0.5Te0.5 (7.7 GPa) superconductor.