Superconductivity induced by external pressure in Eu_3-xSr _x Bi₂S₄F₄ (x = 1, 2) compounds

We have studied the temperature-pressure phase diagram of two materials Eu_3-xSr _x Bi₂S₄F₄ (x = 1 and x = 2) by electrical resistivity and magnetic measurements down to 2 K. Semiconducting resistive behavior observed in both the materials under ambient conditions transforms into metallic behavior as externally applied pressure gradually increases. Superconductivity is observed in both the materials at and above applied pressure P = 2.37 GPa. Under the highest pressure P ∼ 2.9 GPa applied in our measurements, T _c is ∼9.8 K in Eu₂SrBi₂S₄F₄ (x = 1) and 8.2 K in EuSr₂Bi₂S₄F₄ (x = 2). Upper critical field H _c2(0) ∼ 3.04 T (x = 1) and 1.17 T (x = 2) is estimated from magnetic field dependent resistivity measurements at 2.9 GPa. Using the Arrhenius equation, we estimate the thermally activated flux flow activation energy U ₀ as 116 K in Eu₂SrBi₂S₄F₄ and 39 K in EuSr₂Bi₂S₄F₄. At 2 K, DC magnetic susceptibility measurements indicate S-type paramagnetic behavior.