Correlation between intercalated magnetic layers and superconductivity in pressurized EuFe₂(As_0.81P_0.19)₂

¹ International Center for Quantum Materials, School of Physics, Peking University - Beijing 100871, China
² Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences Beijing 100190, China
³ KYOKUGEN, Graduate School of Engineering Science, Osaka University Machikaneyama-cho 1-3, Toyonaka, Osaka 560-8531, Japan
⁴ Institute of High Energy Physics, Chinese Academy of Sciences - Beijing 100039, China
⁵ Shanghai Synchrotron Radiation Facilities, Shanghai Institute of Applied Physics, Chinese Academy of Sciences Shanghai 201204, China
⁶ Collaborative Innovation Center of Quantum Matter - Beijing, 100190 and 100871, China

^(a) llsun@iphy.ac.cn
^(b) zhxzhao@iphy.ac.cn

Received: 24 February 2015
Accepted: 26 August 2015

Abstract

We report comprehensive high-pressure studies on the correlation between intercalated magnetic layers and superconductivity in a EuFe₂(As_0.81P_0.19)₂ single crystal through in situ high-pressure resistance, specific-heat, X-ray diffraction and X-ray absorption measurements. We find that an unconfirmed magnetic order of the intercalated layers coexists with superconductivity in a narrow pressure range 0–0.5 GPa, and then it converts to a ferromagnetic (FM) order at pressure above 0.5 GPa, where its superconductivity is absent. The obtained temperature-pressure phase diagram clearly demonstrates that the unconfirmed magnetic order can emerge from the superconducting state. In stark contrast, the superconductivity cannot develop from the FM state that is evolved from the unconfirmed magnetic state. High-pressure X-ray absorption (XAS) measurements reveal that the pressure-induced enhancement of Eu's mean valence plays an important role in suppressing the superconductivity and tuning the transition from the unconfirmed magnetic state to a FM state. The unusual interplay among valence state of Eu ions, magnetism and superconductivity under pressure may shed new light on understanding the role of the intercalated magnetic layers in Fe-based superconductors.