Tuning the ground state of BaFe 2 As 2 : Phase diagrams and empirical trends
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Introduction LaFeAsO0:9F0:1 was reported in 2008 to be a superconductor with a Tc around 26 K at ambient pressure1 and around 43 K under applied pressure up to 4 GPa.2 Its parent compound, LaFeAsO, which has alternate La2O2 sheets made from edge-sharing OLa4 tetrahedra and Fe2As2 sheets made from edge-sharing FeAs4 tetrahedra, undergoes a tetragonal-toorthorhombic phase transition at a temperature Ts around 160 K and an antiferromagnetic ordering transition at Tm around 140 K. Since then, the iron pnictide superconductors have been under extensive investigation. In part, this is because several new families of superconductors were discovered based on the idea that superconductivity could arise from structurally and chemically similar compounds (see the article by Sefat and Singh in this issue). In addition, the interplay between superconductivity, magnetism, and crystal structure in these new Fe-based superconductors is so rich that it could significantly contribute to the understanding of the origin of high-Tc superconductivity (see References 3–5 and references therein). Among the Fe-based superconductors, doped BaFe2As2 compounds (members of the family called 122 because of their chemical formula) have been studied most systematically. They have become a model system for understanding high-temperature superconductivity in Fe-based superconductors because large, high-quality, homogeneous single
crystals can be easily grown and reproduced, especially transition-metal doped BaFe2As2.3 The parent compound, BaFe2As2, shows structural and physical properties similar to LaFeAsO: it consists of alternating Ba and Fe2As2 layers, the latter made from edge-sharing FeAs4 tetrahedra, as shown in Figure 1a. The compound also manifests simultaneous structural (tetragonal to orthorhombic) and magnetic (paramagnetic to antiferromagnetic) phase transitions at Ts = Tm = 134 K.6 When doped with K, BaFe2As2 is the first oxygen-free Fe-based superconductor to have a critical temperature Tc up to 38 K.7 With K doping, the structural and magnetic phase transitions are monotonically suppressed; superconductivity is stabilized in a dome-like region centered near the critical doping concentration where the antiferromagnetism (AFM) is completely suppressed; superconductivity and antiferromagnetism coexist in the underdoped region (Figure 1b).8 Although these are the universal features of the phase diagrams of all doped BaFe2As2 superconductors as well as BaFe2As2 under pressure, the suppression rates of Ts and Tm and the dome ranges differ with the dopant or other tuning parameter. In this article, we briefly summarize the current view of the phase diagrams of electron-, hole-, and isoelectronic-doped BaFe2As2, as well as BaFe2As2 under pressure, focusing on the empirical trends inferred from these phase diagrams and the universal behavior observed in these series.
Ni Ni, Department of Chemistry, Princeton University, Princeton, NJ 08544, USA; [email protected] Sergey L. Bud’ko, Ames Laboratory Iowa State University, Ames, IA 50011, USA; e-ma
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