Superconductivity in the PbO-type structure α-FeSe

Abstract

] 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...

Validation of Interstitial Iron and Consequences of Nonstoichiometry in Mackinawite (Fe1+xS)

“…VASP calculations were carried out on models of stoichiometric tetragonal "FeS" and the following supercell structures designed to consider different amounts of interstitial Fe content: Fe 1 ) were not completed due to the size of the basis set for these large supercells. Instead, these structural models were constructed using the atomic positions determined by Berner and lattice parameters extrapolated from a linear fit of the optimized lattice parameters in iron-richer Fe 1+x S cases, i.e., for 0.125 ≤ x ≤ 0.50.…”

“…Exchange and correlation in LMTO were treated by the local density (LDA) and local spin density (LSDA) approximations. The hypothetical compositions of FeS, Fe 1 Additionally, the magnetic ordering of "FeS" was examined using two magnetically ordered models: (i) a checkerboard type arrangement; and (ii) a "striping" along the [110] direction. The checkerboard was calculated using P4m ̅ 2, in which the two Fe atoms in the unit cell of the square net were separated into two inequivalent sites, Fe1 at (0, 0, 0) and Fe2 at (1/2, 1/2, 0), allowing an antiferromagnetic checkerboard.…”

“…These [FeX 4/4 ] layers are believed to be the active conducting layer and, therefore, create a competition between superconductivity and magnetism. 1 The total composition of these structures, whether they are stoichiometric or metal-rich, has also been heavily debated. 7 McQueen et al reported a superconducting phase (T C = 8.5 K) with excess iron, Fe 1.01 Se, taking special care to prevent oxide impurities in these samples.…”

Temperature insensitivity of Fermi surface in electron-doped high-temperature FeSe films on SrTiO3

“…The reported superconducting [16][17][18][19] which is significantly higher than T c of ~8 K for bulk FeSe [20]. The importance of interfacial effects, such as the electron-phonon coupling across the interface between FeSe and SrTiO 3 , has been proposed as the origin of the observed drastic T c enhancement [15,21,22].…”

Bulk Superconductivity in Bismuth Oxysulfide Bi4O4S3

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