Superconductivity in the PbO-type structure alpha-FeSe

Abstract

The recent discovery of superconductivity with relatively high transition temperature (Tc) in the layered iron-based quaternary oxypnictides La[O(1-x)F(x)] FeAs by Kamihara et al. [Kamihara Y, Watanabe T, Hirano M, Hosono H (2008) Iron-based layered superconductor La[O1-xFx] FeAs (x = 0.05-0.12) with Tc = 26 K. J Am Chem Soc 130:3296-3297.] was a real surprise and has generated tremendous interest. Although superconductivity exists in alloy that contains the element Fe, LaOMPn (with M = Fe, Ni; and Pn = P and As) is the first system where Fe plays the key role to the occurrence of superconductivity. LaOMPn has a layered crystal structure with an Fe-based plane. It is quite natural to search whether there exists other Fe based planar compounds that exhibit superconductivity. Here, we report the observation of superconductivity with zero-resistance transition temperature at 8 K in the PbO-type alpha-FeSe compound. A key observation is that the clean superconducting phase exists only in those samples prepared with intentional Se deficiency. FeSe, compared with LaOFeAs, is less toxic and much easier to handle. What is truly striking is that this compound has the same, perhaps simpler, planar crystal sublattice as the layered oxypnictides. Therefore, this result provides an opportunity to better understand the underlying mechanism of superconductivity in this class of unconventional superconductors.

Fig. 1.

Schematic crystal structure of α-FeSe. Four unit cells are shown to reveal the layered structure.

Fig. 2.

Powder x-ray diffraction patterns of FeSe_0.82 and FeSe_0.88. The patterns show that the resulting sample with starting composition of Fe (53%)/Se (47%) composes of primarily PbO-type tetragonal FeSe_1−x (P4/nmm), the α-phase, and partly of NiAs-type hexagonal FeSe (P63/mmc), the β-phase. The sample with higher initial iron content, Fe (55%)/Se (45%), shows no β-phase but trace amounts of possible impurity phases including elemental selenium, iron oxide, and iron silicide (marked with an asterisk). Question marks in the figure represent unknown phases.

Fig. 3.

Temperature dependence of electrical resistivity (ρ) of FeSe_0.88. The Left Inset shows the resistive measurement in magnetic fields (H) of 0, 1, 3, 5, 7, and 9T below 12 K. Tc decreases linearly with increasing magnetic field. The Right Inset displays the temperature dependence of upper critical field (Hc2), with the fit shown in blue.

Fig. 4.

Magnetic susceptibility and specific heat of FeSe_0.88. (A) Temperature dependence of magnetic susceptibility measured in a 30-G magnetic field. A small magnetic anomaly is observed at ≈105 K, which is more pronounced in the FC measurements. Inset shows the magnetic hysteresis of the sample measured at 2 K. It confirms the superconducting characteristic of the sample. (B) Low temperature-specific heat of FeSe_0.88. The red dotted line is the curve fitting of phonon and electronic contribution to the specific heat. The intercept at zero temperature gives γ = 9.17 mJ/mole-K². A specific jump appears at ≈8 K, which coincides with the zero-resistance temperature that confirms the superconducting transition. The Inset shows the semilogarithmic Ces/γTc vs. Tc/T in the superconducting state. The measured plot displays deviation from the linear curve of the fully gapped superconductor (solid blue straight line).

Fig. 5.

The moment transfer is M = 4πsin(θ)/λ. (A) Observed (open black circle) and calculated (red solid line) powder diffraction intensities of FeSe_0.88 at 300 K using space group P4/nmm. The Inset in A shows a single peak of the (2, 2, 0), (2, 0, 3), (2, 2, 1) reflection at room temperature. But double peaks show up for (2, 2, 0) and (2, 2, 1) at low temperature, as seen in B. The double-peak structure begins to show up at ≈105 K. Inset in B shows the temperature dependence of the γ angle fit with P-1 symmetry.