Progress in Superconductivity, 13, 65-84 (2011) Brief review on iron-based superconductors: are there clues for unconventional superconductivity? Hyungju Oha, Jisoo Moona, Donghan Shina, Chang-Youn Moonb, Hyoung Joon Choi*,a a Department of Physics and IPAP, Yonsei University, Seoul 120-749, Korea b Department of Chemistry, Pohang University of Science and Technology, Pohang, 790-784 Korea December 30, 2011 Abstract Study of superconductivity in layered iron-based materials was initiated in 2006 by Hosono’s group, and boosted in 2008 by the superconducting transition temperature, Tc, of 26 K in LaFeAsO1-xFx. Since then, enormous researches have been done on the materials, with Tc reaching as high as 55 K. Here, we review briefly experimental and theoretical results on atomic and electronic structures and magnetic and superconducting properties of FeAs-based superconductors and related compounds. We seek for clues for unconventional superconductivity in the materials. Keywords : iron-based superconductor, FeAs, unconventional superconductivity I. Introduction 3d electrons is closely related with superconductivity. Since the discovery of superconductivity in mercury, it has been a key interest to find the origin II. FeAs-based and related materials of the superconductivity and materials with higher superconducting transition temperature (Tc). Up to Iron-based superconductors started with the now, Tc is the highest in copper-oxide perovskite discovery of superconductivity at 4 K in LaFePO in materials [1] with values higher than 150 K achieved 2006 [2], and great interests have been drawn since in 1980s and 1990s, but the origin of the high-Tc 2008 when Tc was raised to 26 K in LaFeAsO1-xFx by superconductivity in the copper oxides is still not replacing phosphorous with arsenic, and some of understood. After that, superconductivity in MgB2 oxygen with fluorine [3]. So far, iron-based (Tc = 39 K) found in 2001 renewed interest in superconductors have been extended to a large maximal Tc by conventional phonon-mediated variety of materials including four prototypical superconductivity, and more recently, discovery of families of iron-based superconductors, 1111, 122, superconductivity in iron pnictides [2, 3] opened a 111, and 11 types, as shown in Fig. 1, and further new family of unconventional superconductors variations such as 42622-type iron pnictides [4-7] having Tc relatively higher than conventional and 122-type iron chalcogenides [8-11]. superconductors. Iron-pnictide superconductors have 1111-type family.─The 1111-type family includes layer structures of FeAs or FeP, and magnetism from LaFePO and LaFeAsO1-xFx, which are mentioned above, and LnFeAsO with various lanthanide elements (Ln). The atomic structure of the 1111 *Corresponding author. Fax : +82 2 392 1592 e-mail : [email protected] 1 Fig. 2. (a) Electrical resistivity, ρ, vs. temperature, T, for pure LaFePO at various magnetic field, H. (b) ρ vs T and (c) magnetic susceptibility, χ, vs T for pure Fig. 1. Four families of iron-based superconductors, and F-doped LaFePO. Fig. reprinted from Ref. 2: Y. (a) 1111, (b) 122, (c) 111, (d) 11 type. Fig. (d) Kamihara et al., J. Am. Chem. Soc. 128, 10012 reprinted from Ref. 22: F.-C. Hsu et al., Proc. Natl. (2006). Copyright 2006 by the American Chemical Acad. Sci. U.S.A. 105, 14262 (2008). Copyright Society. 2008 by the National Academy of Sciences. Reported values of Tc in 1111-type materials include family consists of negatively charged FeP or FeAs 4 K in LaFePO [2], 26 K in LaFeAsO0.89F0.11 [3], 41 layers, where Fe atoms form a planar square lattice, K in CeFeAsO0.84F0.16 [14], 52 K in PrFeAsO0.89F0.11 and positively charged LnO layers, as shown in Fig. [15], 54.3 K in NdFeAsO1−y [16], 55 K in 1(a). With or without doping, electrons are SmFeAsO0.9F0.1 [17], and 54 K in GdFeAsO1−y [16]. conducting in FeP or FeAs layers. Figs. 2(a) and (b) 122- and 111-type families.─Right after discovery show that the electrical resistivity of pure LaFePO of 1111-type family, Ba1-xKxFe2As2 (122 type) with drops at 4 K and that of F-doped LaFePO drops at Tc of 38 K [18] were found, followed by LiFeAs (111 higher temperature. These superconducting type) with Tc of 18 K [19]. The 122- and 111-type transitions are confirmed by magnetic susceptibility families have simpler structures than the 1111-type [Fig. 2(c)]. It is noticeable that decrease of the family. While FeAs or FeP layers are present in resistivity starts at ~ 10 K in F-doped LaFePO. 1111-, 122-, and 111-type materials, the ‘blocking Unlike LaFePO, undoped LaFeAsO does not show layer’ which separates FeAs or FeP is different for superconductivity [Fig. 3(a)]. With doping of F each type: rare-earth oxide or fluoride (for example, LaO or SrF) for the 1111-type family, alkaline-earth replacing O in part, LaFeAsO0.89F0.11 becomes superconducting. When small pressure is applied to metals (for example, Ba) for the 122-type family, and alkali metals (for example, Li) for the 111-type LaFeAsO0.89F0.11, Tc increases, reaching a maximum family. Reported values of T in 122-type materials value of Tc = 43 K at 4 GPa, and then it decreases to c include 38 K in Ba K Fe As [18], 32 K in Tc = 9 K at 30 GPa [12]. At ambient pressure, Tc 0.6 0.4 2 2 Sr K Fe As [20], 26 K in Sr Na Fe As [21], higher than 40 K is achieved in SmFeAsO1-xFx [13]. 0.6 0.4 2 2 0.6 0.4 2 2 and 21 K in Ca0.6Na0.4Fe2As2 [20]. 2 Fig. 4. Antiferromagnetic ordering in LaFeAsO. The solid line is a simple fit to mean field theory that gives a Néel temperature TN = 137 K. The top-right inset shows the single-stripe-type antiferromagnetic ordering of Fe magnetic moments. Fig. reprinted Fig. 3. (a) Electrical resistivity, ρ, vs temperature in from Ref. 27: C. de la Cruz et al., Nature 453, 899 LaFeAsO F for x = 0.0, 0.04, 0.05, 0.11, and 0.12. 1-x x (2008). Copyright 2008 by Macmillan Publishers Fig. reprinted from Ref. 3: Y. Kamihara et al., J. Am. Ltd. Chem. Soc. 130, 3296 (2008). Copyright 2008 by the American Chemical Society. (b) Temperature dependence of electrical resistance above 3 GPa in III. Phase diagram LaFeAsO0.89F0.11. The maximum Tc is 43 K at 4 GPa. Fig. reprinted from Ref. 12: H. Takahashi et al., Different behaviors of undoped compounds.─The Nature 453, 376 (2008). Copyright 2008 by undoped parent compounds of iron-based Macmillan Publishers Ltd. superconductors are either superconducting or antiferromagnetic at low temperatures. LaFePO, LiFeAs, and FeSe, for example, are nonmagnetic and 11-type family.─The 11-type materials are iron superconducting even without doping. In contrast, chalcogenide, which started with FeSe having T of 8 c undoped LaFeAsO and BaFe As , for example, are K at ambient pressure [22] and 36.7 K with applied 2 2 non-superconducting antiferromagnetic metals, and pressure of 8.9 GPa [23]. This family also includes electron or hole doping suppresses the magnetic FeTe Se , and FeTe S . These materials have the 1-x x 1-x x order and induces superconductivity. simplest structure among iron-based superconductors, Structural and magnetic transitions in 1111-type in which iron-chalcogenide layers are simply stacked family.─The undoped 1111-type iron pnictides show together. Reported values of T in 11-type materials c structural and magnetic phase transitions at slightly include 8 K in FeSe at ambient pressure [22] and different temperatures. It is reported by neutron 36.7 K in FeSe under pressure of 8.9 GPa [23], as scattering experiments [27] that LaFeAsO undergoes mentioned above, and 14 K in FeTe Se [24], 2 K 0.5 0.5 an abrupt structural transition at 155 K, changing in Fe Te S [25], and 10 K in FeTe S [26]. 1.13 0.85 0.1 0.8 0.2 from high-temperature tetragonal structure (space group P4/nmm) to low-temperature monoclinic 3 Fig. 5. Phase diagram of CeFeAsO1-xFx. The red circles indicate the onset temperature of structural transition. Antiferromagnetic and superconducting phases are marked with AFM and SC, respectively. Fig. reprinted from Ref. 29: J. Zhao et al., Nature Mater. 7, 953 (2008). Copyright 2008 by Macmillan Publishers Ltd. structure (space group P112/n). At ~137 K, long-range antiferromagnetic order starts to develop Fig. 6. (a) Phase diagram of the BaFe2As2 system, with a small moment of 0.36 μB per atom and a shown for K [30], Co [31] and P [32] doping. The simple antiferromagnetic ordering of single-stripe dotted line indicates tetragonal (T) to orthorhombic type [27], as shown in Fig. 4. The direction of the Fe (O) structural phase transitions in Co doped samples magnetic moment was later measured to be [31]. Fig. reprinted from Ref. 33: J. Paglione and R. longitudinal in the sense that it is parallel or L. Greene, Nature Phys. 6, 645 (2010). Copyright anti-parallel to the antiferromagnetic ordering wave 2010 by Macmillan Publishers Ltd. (b) Doping vector [28]. With electron or hole doping, the dependent antiferromagnetic transition temperature, antiferromagnetic order in the parent compounds is TN, superconducting transition temperature, Tc, and suppressed and superconductivity emerges. Fig. 5 pseudogap crossover temperature, T*, in YBCO [35]. shows the typical phase diagram for the 1111-type Fig. reprinted from Ref. 35: N. Doiron-Leyraud et family which is determined by neutron-scattering al., Nature 447, 565 (2007). Copyright 2007 by measurements on CeFeAsO1-xFx [29].