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arXiv · 1006.0073

Synthesis, structural and physical properties of $\delta'$-FeSe$_{1-x}$

Mariano de Souza·Amir-Abbas Haghighirad·Ulrich Tutsch·Wolf Assmus·Michael Lang
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Abstract

We report on synthesis, structural characterization, resistivity, magnetic and thermal expansion measurements on the as yet unexplored $\delta'$-phase of FeSe$_{1-x}$, here synthesized under ambient- (AP) and high-pressure (HP) conditions. We show that in contrast to $\beta$-FeSe$_{1-x}$, monophasic superconducting $\delta'$-FeSe$_{1-x}$ can be obtained in off-stoichiometric samples with excess Fe atoms preferentially residing in the van der Waals gap between the FeSe layers. The AP $\delta'$-FeSe$_{1-x}$ sample studied here ($T_c$ $\simeq$ 8.5\,K) possesses an unprecedented residual resistivity ratio RRR $\simeq$ 16. Thermal expansion data reveal a small feature around $\sim$90\,K, which resembles the anomaly observed at the structural and magnetic transitions for other Fe-based superconductors, suggesting that some kind of "magnetic state" is formed also in FeSe. %indicative of a fluctuating magnetic ordering. For HP samples (RRR $\simeq$ 3), the disorder within the FeSe layers is enhanced through the introduction of vacancies, the saturated magnetic moment of Fe is reduced and only spurious superconductivity is observed.

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Mariano de Souza, Amir-Abbas Haghighirad, Ulrich Tutsch, Wolf Assmus, Michael Lang. 2010-06-01. Synthesis, structural and physical properties of $\delta'$-FeSe$_{1-x}$. https://doi.org/10.1140/epjb/e2010-00254-7

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