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

Abstract

We report on synthesis, structural characterization, resistivity, magnetic and thermal expansion measurements on the as yet unexplored ${\delta }^{\prime }$-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 ${\delta }^{\prime }$-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 ${\delta }^{\prime }$-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.

7 pages, 8 figures, published version

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