[Paper Review] Connection between ambient-pressure and pressure-induced superconducting phases in alkaline iron selenide superconductors

Dachun Gu, Liling Sun+7 more|arXiv (Cornell University)|Jul 2, 2014

Iron-based superconductors research51 references3 citations

TL;DR

This study reveals that the ambient-pressure superconducting phase (SC-I) and pressure-induced superconducting phase (SC-II) in alkaline iron selenide superconductors are connected via the antiferromagnetic (AFM) order. By tuning Te doping, both SC-I and SC-II superconductivity, along with the AFM order, vanish simultaneously at x = 0.4, indicating that long-range AFM order stabilizes SC-I, while AFM fluctuations drive SC-II emergence under pressure. The findings establish a unified role for AFM states in enabling superconductivity across distinct phases.

ABSTRACT

A unique platform for investigating the correlation between the antiferromagnetic (AFM) and superconducting (SC) states in high temperature superconductors is created by the discovery of alkaline iron selenide superconductors which are composed of an AFM insulating phase and a SC phase separated spatially. Our previous studies showed that pressure can fully suppress the superconductivity of ambient-pressure superconducting phase (SC-I) and AFM order simultaneously, then induce another superconducting phase (SC-II) at higher pressure. Consequently, the connection between the two superconducting phases becomes an intriguing issue. In this study, on the basis of observing pressure-induced reemergence of superconductivity in Rb0.8Fe2-ySe2-xTex (x=0, 0.19 and 0.28) superconductors, we find that the superconductivity of the SC-I and SC-II phases as well as the AFM ordered state can be synchronously tuned by Te doping and disappear together at the doping level of x=0.4. We propose that the two superconducting phases are connected by the AFM phase, in other words, the state of long-ranged AFM order plays a role in giving rise to superconductivity of the SC-I phase, while the fluctuation state of the suppressed AFM phase drives the emergence of SC-II phase. These results comprehensively demonstrate the versatile roles of AFM states in stabilizing and developing superconductivity in the alkaline iron selenide superconductors.

Motivation & Objective

• To investigate the relationship between ambient-pressure superconducting phase (SC-I) and pressure-induced superconducting phase (SC-II) in alkaline iron selenide superconductors.
• To determine whether the antiferromagnetic (AFM) order mediates or influences the emergence of both superconducting phases.
• To explore how chemical doping (Te) affects the coexistence and suppression of SC-I, SC-II, and AFM order.
• To clarify the role of AFM states—both long-range and fluctuating—in stabilizing and driving superconductivity in these complex systems.

Proposed method

• Systematic pressure-dependent electrical transport measurements were performed on Rb0.8Fe2-ySe2-xTex solid solutions with varying Te content (x = 0, 0.19, 0.28).
• The evolution of superconducting transition temperatures (Tc) and antiferromagnetic order was monitored under hydrostatic pressure using a cubic anvil press.
• Te doping was used as a tuning parameter to control the strength of electron correlation and the stability of the AFM and superconducting phases.
• The suppression of superconductivity and AFM order was analyzed across different doping levels to identify a critical doping threshold (x = 0.4) where all three states vanish simultaneously.

Experimental results

Research questions

• RQ1 How are the ambient-pressure superconducting phase (SC-I) and pressure-induced superconducting phase (SC-II) in alkaline iron selenides related?
• RQ2 Does the antiferromagnetic (AFM) order play a direct role in stabilizing or inducing superconductivity in these materials?
• RQ3 Can Te doping simultaneously tune both the ambient-pressure and pressure-induced superconducting phases?
• RQ4 At what doping level do the SC-I, SC-II, and AFM phases coherently vanish, indicating a critical quantum phase boundary?

Key findings

• The ambient-pressure superconducting phase (SC-I) and pressure-induced superconducting phase (SC-II) are both suppressed and vanish simultaneously at Te doping level x = 0.4.
• The antiferromagnetic (AFM) order is fully suppressed at the same doping level (x = 0.4), indicating a common critical point for all three states.
• Long-range AFM order is essential for stabilizing the SC-I phase, as its suppression coincides with the disappearance of SC-I.
• AFM fluctuations in the suppressed state are proposed to drive the emergence of the pressure-induced SC-II phase, suggesting a dual role for AFM correlations.
• The simultaneous tuning and disappearance of SC-I, SC-II, and AFM order under Te doping confirms a deep connection between the two superconducting phases via the AFM state.
• The results support a unified scenario in which the AFM state—both long-range and fluctuating—plays a central role in enabling superconductivity across different phases in iron selenides.