A new modulated structure in Sr2CuO3+δ superconductor synthesized under high pressure

A Sr2CuO3+δ superconductor with a new modulated structure has been synthesized using high pressure technique. Two superconducting transitions with Tc ∼ 75 K and ∼ 48 K respectively were found in Sr2CuO3+δ superconductor. The superconducting volume fraction is calculated to be 85% at 2 K, which is much higher than anyone else reported before. A new type of modulated phase with a periodicity of 2√2ap × 2√2ap × cp of Pmmm symmetry is found in the sample by using transmission electron microscopy. Our experimental results suggest that the new Pmmm modulated phase is responsible for superconductivity with Tc at 48 K, while C2/m modulated phase for that with Tc at 75 K found previously.