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Pressure cycle of superconducting Cs0.8Fe2Se2 : A transport study

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Abstract

We report measurements of the temperature and pressure dependence of the electrical resistivity (ρ) of single-crystalline iron-based chalcogenide Cs0.8Fe2Se2. In this material, superconductivity with a transition temperature (Tc)30K develops from a normal state with extremely large resistivity. At ambient pressure, a large “hump” in the resistivity is observed around 200 K. Under pressure, the resistivity decreases by two orders of magnitude, concomitant with a sudden Tc suppression around pc8GPa. Even at 9 GPa a metallic resistivity state is not recovered, and the ρ(T) “hump” is still detected. A comparison of the data measured upon increasing and decreasing the external pressure leads us to suggest that the superconductivity is not related to this hump.

Highlights

► The resistivity of the Fe-based superconductor CsFe2Se2 has been measured under high pressure. ► The transition temperature is suppressed from 30 K (p=0) to zero at around 7.5 GPa. ► The resistivity hump at 200 K seems unrelated to the superconductivity.

Introduction

From a structural point of view, FeSe is the simplest material among the new Fe-based pnictide and chalcogenide “high-temperature” superconductors [1], [2]. It is a tetragonal compound in which layers formed by edge-sharing FeSe4 tetrahedra are stacked along the c-axis of the crystal. It possesses several outstanding characteristics, among them a large pressure effect on the superconducting temperature that increases from Tc(p=0)13K to Tc37K at 4 GPa [3]. This steep rise in Tc(p) is thought to be due in part to a reduction in the distance h between the Se anion and the Fe plane (known as the “anion height”), which approaches the optimum height for superconductivity, h1.38Å [4].

Recently, a new family of Fe-based superconductors with Tc30K and the general formula AxFe2−ySe2 has been identified. Element A is either the alkaline K [5], Rb [6] or Cs [7], or also Tl in the +1 valence state [8]. These compounds crystallize in the well-known ThCr2Si2-type tetragonal structure (space group I4/mmm), obtained by the intercalation of A in superconducting FeSe. Introducing the element A expands the tetragonal c-axis but reduces the anion height, which approaches the optimum value h [7]. Muon-spin spectroscopy, resistivity, magnetization, and differential scanning calorimetry investigations performed on the system A=Cs (hereafter Cs-122) have shown a microscopic coexistence between the superconductivity and a magnetic phase with very high TN=478K [9]. A similar behavior has subsequently been reported in the system A=K, based on a neutron study [10]. In addition, resistivity measurements on K0.8Fe1.7Se2 up to 11 GPa [11] seem to indicate that Tc cannot be further optimized by applying high pressures and suggest a relationship between the resistivity hump around 200 K and the occurrence of superconductivity. Our present results question this relation, in line with recent reports studying this interdependence in a large variety of samples at ambient pressure [12], [13], [14].

Section snippets

Experimental details

Single crystals of nominal composition Cs0.8(FeSe0.98)2 were grown using the Bridgman technique [7]. Detailed crystallographic analysis revealed the presence of only one single phase [15], and the magnetization data [9] are compatible with 100% superconducting volume fraction (on crystals of the same batch as the present one). Two samples were carefully cleaved from a larger crystal. High-pressure four-probe resistivity measurements along the basal (ab) plane were performed on sample S1, a 1020×

Results

In Fig. 1(a), we present the temperature dependence of the electrical resistivity of sample S1 at selected increasing pressures (p). At the lowest pressure, p1.2GPa, ρ(T) displays a similar T-dependence as sample S2 at p=0, which is typical of other AxFe2Se2 compounds reported in the literature [5], [14], [17], [6], [9], [18]. The resistivity first increases towards lower temperatures, it displays a “hump” at Tmax200K, and the superconducting transition occurs at Tc50%28K. Between the onset

Discussion

As already pointed out, superconductivity develops in AFe2Se2 compounds out of normal-state resistivities of 0.2Ωcm, which is about three orders of magnitude higher than say in FeSe [1]. For the Cs-122 samples studied here, we estimate ρ(295K)2.5Ωcm at p=0. We verified that this value does not change on exposing the sample to the air for around one hour, the time necessary to cleave it and close the high-pressure cell. To our knowledge, there are no other systems that exhibit such a large Tc

Conclusion

In conclusion, high-pressure measurements on single-crystalline Cs0.8Fe2Se2 show a suppression of Tc(p), which is almost constant up to 5 GPa and then decreases steeply, not being detected above 8 GPa. The resistivity hump of unknown origin is only very partially recovered with decreasing pressure, while Tc seems to be reversible. This questions the connection between Tc and the resistivity hump, in line with recent publications [12], [13], [14] that arrive at a similar conclusion. At the

Acknowledgements

This work was supported by the Swiss National Science Foundation through the NCCR “MaNEP”. P.P. is a member of CONICET.

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