] 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 ␣-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.electronic properties ͉ Fe-oxypnictide A lthough superconductivity exists in alloy (1) that contains the element Fe, LaOMPn (2-9) (with M ϭ Fe, Ni; and Pn ϭ P and As) is the first system where Fe plays the key role in the occurrence of superconductivity. LaOMPn has a layered crystal structure with an Fe-based plane. It is quite natural to ask whether other Fe-based planar compounds exist that exhibit superconductivity. Here, we report the observation of superconductivity with zero resistance transition temperature at 8 K in the PbO-type ␣-FeSe compound. Although FeSe has been studied quite extensively (10, 11), a key observation is that the clean superconducting phase exists only in those samples prepared with intentional Se deficiency.FeSe comes in several phases: (i) a tetragonal phase ␣-FeSe with PbO-structure, (ii) a NiAs-type ␤-phase with a wide range of homogeneity showing a transformation from hexagonal to monoclinic symmetry, and (iii) an FeSe 2 phase that has the orthorhombic marcasite structure. The most studied of these compounds are the hexagonal Fe 7 Se 8 , which is a ferrimagnet with Curie temperature at Ϸ125 K, and monoclinic Fe 3 Se 4 .Unlike the high-temperature (high-Tc) superconductors (12) discovered Ͼ20 years ago that have a CuO 2 plane that is essential for the observed superconductivity, the tetragonal phase ␣-FeSe with PbO structure has an Fe-based planar sublattice equivalent to the layered iron-based quaternary oxypnictides, which have a layered crystal structure belonging to the P4/nmm space group (2). The crystal of ␣-FeSe is composed of a stack of edge-sharing FeSe 4 -tetrahedra layer by layer, as shown schematically in Fig. 1. Polycrystalline samples with nominal concentration FeSe 1Ϫx (x ϭ 0.03 and 0.18) were synthesized and studied. X-ray diffraction analysis of the samples in Fig. 2 shows that ␣-FeSe is dominant, and ␤-FeSe phases exist in trace amounts. This result is reasonable because in the Fe-Se binary alloy system, the...

We have carried out a systematic study of the PbO-type compound FeSe 1-x Te x (x = 0~1), where Te substitution effect on superconductivity is investigated. It is found that superconducting transition temperature reaches a maximum of Tc=15.2K at about 50% Te substitution. The pressure-enhanced Tc of FeSe 0.5 Te 0.5 is more than 10 times larger than that of FeSe. Interestingly, FeTe is no longer superconducting. A low temperature structural distortion changes FeTe from triclinic symmetry to orthorhombic symmetry. We believe that this structural change breaks the magnetic symmetry and suppresses superconductivity in FeTe.

We use bulk magnetic susceptibility, electronic specific heat, and neutron scattering to study structural and magnetic phase transitions in Fe1+ySexTe1−x. Fe1.068Te exhibits a first order phase transition near 67 K with a tetragonal to monoclinic structural transition and simultaneously develops a collinear antiferromagnetic (AF) order responsible for the entropy change across the transition. Systematic studies of FeSe1−xTex system reveal that the AF structure and lattice distortion in these materials are different from those of FeAs-based pnictides. These results call into question the conclusions of present density functional calculations, where FeSe1−xTex and FeAs-based pnictides are expected to have similar Fermi surfaces and therefore the same spin-density-wave AF order.

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