Influence of interstitial Fe to the phase diagram of Fe$_{1+y}$Te$_{1-x}$Se$_x$ single crystals

TL;DR

This study investigates how interstitial Fe influences the phase diagram of Fe$_{1+y}$Te$_{1-x}$Se$_x$ single crystals, revealing that removing interstitial Fe clarifies the true relationship between antiferromagnetism and superconductivity.

Contribution

The paper provides the first detailed comparison of phase diagrams before and after interstitial Fe removal, clarifying its impact on magnetism and superconductivity in Fe$_{1+y}$Te$_{1-x}$Se$_x$.

Findings

Interstitial Fe affects the coexistence of AFM and SC.

Removing interstitial Fe suppresses AFM for x<0.05.

The phase diagram resembles the '1111' system, not the '122' system.

Abstract

Superconductivity (SC) with the suppression of long-range antiferromagnetic (AFM) order is observed in the parent compounds of both iron-based and cuprate superconductors. The AFM wave vectors are bicollinear ($\pi$, 0) in the parent compound FeTe different from the collinear AFM order ($\pi$, $\pi$) in most iron pnictides. Study of the phase diagram of Fe$_{1+y}$Te$_{1-x}$Se$_x$ is the most direct way to investigate the competition between bicollinear AFM and SC. However, presence of interstitial Fe affects both magnetism and SC of Fe$_{1+y}$Te$_{1-x}$Se$_x$, which hinders the establishment of the real phase diagram. Here, we report the comparison of doping-temperature ($x$-$T$) phase diagrams for Fe$_{1+y}$Te$_{1-x}$Se$_x$ (0 $\leq$ $x$ $\leq$ 0.43) single crystals before and after removing interstitial Fe. Without interstitial Fe, the AFM state survives only for $x$ $<$ 0.05, and bulk SC emerges from $x$ = 0.05, and does not coexist with the AFM state. The previously reported spin glass state, and the coexistence of AFM and SC may be originated from the effect of the interstitial Fe. The phase diagram of Fe$_{1+y}$Te$_{1-x}$Se$_x$ is found to be similar to the case of the "1111" system such as LaFeAsO$_{1-x}$F$_x$, and is different from that of the "122" system.