Phase separation in iron chalcogenide superconductor Rb0.8+xFe1.6+ySe2 as seen by Raman light scattering and band structure calculations

Анотація

We report Raman light scattering in the phase separated superconducting single crystal Rb_0.77Fe_1.61Se₂ with T_c = 32 K over a wide temperature region 3–500 K. The observed phonon lines from the majority vacancy ordered Rb₂Fe₄Se₅ (245) antiferromagnetic phase with T_N = 525 K demonstrate modest anomalies in the frequency, intensity and halfwidth at the superconductive phase transition. We identify phonon lines from the minority compressed Rb_δFe₂Se₂ (122) conductive phase. The superconducting gap with d_x²-y² symmetry has been detected in our spectra. In the range 0–600 cm^–1 we observe a weak but highly polarized B_1g-type backgroundwhich becomes well-structured upon cooling. A possible magnetic or multiorbital origin of this background is discussed. We argue that the phase separation in M_0.8+xFe_1.6+ySe₂ is of pure magnetic origin. It occurs below the Néel temperature when the magnetic moment of iron reaches a critical value. We state that there is a spacer between the majority 245 and minority 122 phases. Using ab initio spin-polarized band structure calculations we demonstrate that the compressed vacancy ordered Rb₂Fe₄Se₅ phase can be conductive and therefore may serve as a protective interface spacer between the purely metallic Rb_δFe₂Se₂ phase and the insulating Rb₂Fe₄Se₅ phase providing percolative Josephson-junction like superconductivity all throughout of Rb_0.8+xFe_1.6+ySe₂. Our lattice dynamics calculations show significant differences in the phonon spectra of the conductive and insulating Rb₂Fe₄Se₅ phases.