Superconducting phase in KxFe2−ySe2: Critical role of intact FeSe layers

Abstract

Alkali metal intercalated iron selenide superconductors AxFe2−ySe2 exhibit unique properties, such as the coexistence of superconductivity and antiferromagnetism, along with an iron-vacancy-order superstructure. However, due to microscopic phase separation, the relationship between these properties and superconductivity remains elusive. In this study, we have employed high-spatial-resolution magnetic force microscopy (MFM), Raman spectroscopy, and scanning electron microscopy–energy dispersive x-ray spectroscopy (SEM−EDX) to identify and characterize the superconducting phase. Below the superconducting critical temperature Tc, the striped regions exhibit a pronounced diamagnetic response, while the surrounding areas remain unaffected, providing direct evidence that the striped regions constitute the superconducting phase. SEM−EDX and high-spatial-resolution Raman scattering results reveal a stoichiometry of K0.57Fe1.89Se2 in the striped regions, with a single Raman peak at 173.3cm−1. Temperature- and polarization-dependent Raman scattering spectra further show that, at low temperatures, this peak shifts to 201cm−1 in the B1g channel, while a new peak emerges at 165cm−1 in the A1g channel. The Fe:Se ratio in the stripe phase (1.89:2), along with the number, symmetry, and frequencies of the phonon modes, is consistent with the FeSe layers. These results indicate that the superconducting phase in the striped regions retains an approximately intact FeSe layer. Our findings underscore the critical role of the intact FeSe layer in FeSe-based superconductors and provide new insights into the mechanism of high-Tc superconductivity.

Source-reported materials — not catalogue approval

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