Iron-Based Superconductors
Since the discovery of superconductivity in \(\mathrm{LaFeAsO }_{1-x}\mathrm{F }_x\) , which generated a new route to solve the problem of high-\(T_c\) superconductivity, tremendous efforts have been devoted for identifying the pairing mechanism of this n
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Iron-Based Superconductors
Abstract Since the discovery of superconductivity in LaFeAsO1−x Fx , which generated a new route to solve the problem of high-Tc superconductivity, tremendous efforts have been devoted for identifying the pairing mechanism of this new class of superconductors. So far, various theories have been proposed for the pairing symmetry in this system, ranging from an s± -wave state with opposite signs between hole and electron pockets or s++ -wave state without sign change, to nodal s± -wave or d-wave state, and more exotic order parameter such as p-wave state, but a consensus for the pairing symmetry in this system is still lacking. Therefore, the first experimental task is to clarify the superconducting pairing symmetry in the iron-based superconductors. In this chapter, we will briefly review the experimental and theoretical studies of iron-based superconductors. Keywords Iron-based superconductors Superconducting gap structure
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Superconducting pairing symmetry
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3.1 Crystal Structure of Iron-Based Superconductors Since the discovery of superconductivity in LaFeAsO1−x Fx with Tc = 26 K [1], a series of R EFeAsO superconductors (the so-called ‘1111’ system, R E: rare earth such as La, Ce, Pr, Nd, and Sm) have been reported [2–10]. R EFeAsO has a tetragonal ZrCuSiAs-type crystal structure (space group P4/nmm) with alternating layers of R EO and FeAs, stacked sequentially along the c-axis, as shown in Fig. 3.1a. The chemical formula can be expressed as (R E 3+ O2− )+ (FeAs)− . The parent materials show an antiferromagnetic spin-density-wave (SDW) transition [11] as well as a tetragonal-to-orthorhombic structural phase transition, and exhibit no superconductivity. With electron doping by replacing oxygen with fluorine or creating oxygen deficit, or hole doping achieved by substituting, for example, La for Sr, superconductivity appears. In addition, it has been reported that superconductivity emerges by
K. Hashimoto, Non-Universal Superconducting Gap Structure in Iron-Pnictides Revealed by Magnetic Penetration Depth Measurements, Springer Theses, DOI: 10.1007/978-4-431-54294-0_3, © Springer Japan 2013
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3 Iron-Based Superconductors
(a)
O F
LaO layer La
eFe As
FeAs layer
(b) Ba Fe As
Fig. 3.1 Schematic crystal structures of a LaFeAsO1−x Fx and b BaFe2 As2 . FeAs tetrahedra form two-dimensional layers, surrounded by the layers of LaO or Ba. Fe ions inside tetrahedra form square lattice. The figures are taken from Refs. [2, 12]
3.1 Crystal Structure of Iron-Based Superconductors
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applying pressure [2]. The superconducting transition temperature in the ‘1111’ system was immediately raised to 56 K. Subsequently, oxygen-free iron-based superconductor Ba1−x Kx Fe2 As2 (‘122’ system) was discovered [13]. The parent material AFe2 As2 (A = Ba, Sr, Ca) has a tetragonal ThCr2 Si2 -type structure with space group I4/mmm, as shown in Fig. 3.1b. By replacing the alkaline earth element A of the parent compound AFe2 As2 with K, Cs, and Na, which corresponds to hole doping, superconductiv
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