A Model Platform for Unconventional ... - ACS Publications

Chem. Mater. 2010, 22, 715–723 715 DOI:10.1021/cm9027397

Materials Chemistry of BaFe2As2: A Model Platform for Unconventional Superconductivity† David Mandrus,* Athena S. Sefat, Michael A. McGuire, and Brian C. Sales Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received September 2, 2009. Revised Manuscript Received December 1, 2009

BaFe2As2 is the parent compound of a family of unconventional superconductors with critical temperatures approaching 40 K. BaFe2As2 is structurally simple, available as high-quality large crystals, can be both hole and electron doped, and is amenable to first-principles electronic structure calculations. BaFe2As2 has a rich and flexible materials chemistry that makes it an ideal model platform for the study of unconventional superconductivity. The key properties of this family of materials are briefly reviewed. 1. Introduction Superconductors conduct electricity with little or no dissipation and have a role to play as part of a multidimensional approach to energy, primarily involving electrical grid applications.1 Better materials are urgently needed in order to make superconductors practical and useful. Cuprates have high transition temperatures but their extreme two-dimensionality makes them subject to thermal depinning of vortices and limits their usefulness. A new generation of superconductors is needed with high transition temperatures but that are relatively isotropic. The recently discovered Fe-based superconductors are quite exciting in this regard. Although the maximum TC to date is only 55 K, the materials are fairly isotropic and tolerate disorder well, leading to high critical currents. The evidence points to an unconventional superconducting mechanism, so higher transition temperatures are likely achievable. What is most exciting about these materials, however, is that they both show great chemical flexibility and are amenable to first principles approaches. This means it may be possible to place the unconventional superconductivity in these materials on a predictive, quantitative footing. Such in-depth understanding will allow us for the first time to build new superconductors from the ground up, tailoring the properties of the materials as needed to suit the application. Materials based on the parent BaFe2As2 have emerged as the best model systems to date for Fe-based superconductors. The materials are structurally simple, and large crystals can be grown. The materials can be both hole- and electron-doped, and the crystals are cleavable, which is important for surface-sensitive probes such as angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy. In this article, we will review the crystal chemistry and physical properties of the BaFe2As2 † Accepted as part of the 2010 “Materials Chemistry of Energy Conversion Special Issue”. *Corresponding author. E-mail: [email protected].

r 2010 American Chemical Society

parent, and examine the effects of hole-, electron-, or isovalent-doping (chemical substitutions) on the structure and properties of BaFe2As2. 2. BaFe2As2 Parent At room temperature, BaFe2As2 is tetragonal, adopting the ThCr2Si2 structure type (I4/mmm, a = 3.963 A˚, c = 13.017 A˚, Ba at Wyckoff position 2a, Fe at 4d, As at 4e, zAs = 0.355).2,3 The crystal structure is illustrated in Figure 1. Covalently bonded layers of composition FeAs that lie in the ab-plane are separated along the c-axis by Ba ions. The Fe atoms form a square net, and each is coordinated by four As atoms, resulting in layers of edgesharing, slightly distorted tetrahedra. The Ba atoms are coordinated by eight As atoms in a distorted cubic environment. The distortions of both the Fe-centered tetrahedra and the Ba-centered cubes can be described by a contraction of the ideal coordination geometries along the c-axis. Rele√ vant interatomic distances are dFe-Fe = 2.80 A˚ (= a/ 2), dFe-As = 2.40 A˚, and dBa-As = 3.38 A˚. Unlike the prototype structures ThCr2Si2 and BaAl4, as well as many other members of this structure type, there is no direct chemical bonding between adjacent FeAs layers in BaFe2As2. BaFe2As2 is a semimetal with all five Fe-3d bands crossing the Fermi level. Substantial direct Fe-Fe interactions are expected due to the relatively short Fe-Fe distance. Electronic structure calculations show the states near the Fermi level are derived almost exclusively from these Fe orbitals.4 The Fermi surface of BaFe2As2 as calculated by Singh is shown in Figure 2.4 It strongly resembles the calculated Fermi surface of LaFeAsO,5 suggesting that the superconductivity in these materials results from the peculiarities of the electronic structure rather than from a conventional phonon mechanism. The (small) Fermi surface consists of 3 hole cylinders at the zone center, and 2 electron cylinders at the zone corners. Calculations indicate that BaFe2As2 is a low carrier density, high density of states N(Ef) metal,

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with a strongly increasing N(Ef) below the Fermi energy EF. The electronic structure near the Fermi energy is extremely sensitive to the internal coordinate of arsenic, as illustrated in the right panel of Figure 2, and there is a large (4%) discrepancy between the experimental and the relaxed local density approximation (LDA) coordinates. Such a discrepancy is indicative of strong spin fluctuations (not included in the LDA) and a large magnetoelastic effect that lead to a larger effective size for Fe. BaFe2As2 can by synthesized by various methods. Polycrystalline material can be obtained from direct reaction of stoichiometric mixtures of the elements,3 or from Ba and FeAs, at temperatures up to 900 °C placed in alumina crucibles, and then sealed in silica tubes under argon. High purity products require multiple grindings and annealings of pressed pellets. Single crystals can be grown in several ways. Growth from a Sn flux is convenient,6 but produces crystals with a few percent Sn

incorporated into the crystal structure.6-8 Higher quality crystals can be grown from an FeAs flux (Figure 3).9 It has been suggested that BaFe2As2 melts congruently above 1170 °C, and the Bridgman technique has been used to grow BaFe2As2.10 Upon cooling below about 140 K, BaFe2As2 undergoes a structural phase transition (To).3 The resulting distortion can be visualized as a stretching of the tetragonal structure along the diagonal of the square face (along the [110] direction). The square nets of Fe become rectangular with Fe-Fe distances of 2.79 and 2.81 A˚, and the resulting symmetry is orthorhombic (Fmmm). Chemical

Figure 1. Structure of BaFe2As2 consisting of layers of edge-sharing FeAs4 tetrahedra separated by layers of Ba.

Figure 3. Typical flux-grown single crystals of BaFe2As2. The scale is in millimeters.

Figure 2. Fermi surface of BaFe2As2 (left) and LDA density of states (right) calculated using the experimental As coordinate (top) and relaxed LDA coordinate (bottom), illustrating that the details of the electronic structure near EF are strongly dependent on the As position. Reprinted with permission from ref 4. Copyright 2008 American Physical Society.

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substitutions can be used to suppress the structural phase transition and induce superconductivity. The similarity of the physical properties of BaFe2As2 to LaFeAsO was first pointed out by the Johrendt group,3 who discovered and characterized the coupled structural and magnetic transition in this material at approximately the same temperature as LaFeAsO. In BaFe2As2, the structural transition is accompanied by an antiferro-

Figure 4. Left: Heat capacity results for two parents of Fe-based superconductors in the temperature region of 125-165 K. The anomalies in Cp illustrate the antiferromagnetic (TN) and structural (To) transitions. For BaFe2As2, TN and To are the same, whereas for LaFeAsO, these transitions are separated in temperature. Right: Heat capacity results for BaFe2As2 from 1.8 to 200 K. The inset shows the C/T vs T2 dependence and a linear fit below ∼6 K. Adapted from refs 11 and 12. Copyright 2008 and 2009 American Physical Society.

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magnetic, spin-density-wave (SDW) transition (TN). In contrast to LaFeAsO, the heat capacity anomaly in BaFe2As2 is large and well-defined (see Figure 4).11,12 There is some debate as to whether the transition is continuous, first-order, or weakly first order. This topic was discussed at length by Wilson, et al.13 They conclude that in strain-free crystals, the transition is essentially continuous, but they cannot rule out a very weak first-order transition. Analysis of the heat capacity yields a Debye temperature of θD = 260 K, and a Sommerfeld coefficient, γ, of 6.1 mJ/(K2 mol) [or 3.0 mJ/(K2 mol Fe)].9,11 The antiferromagnetic transition has been established through temperature-dependent neutron diffraction results14 and noted in features of bulk property results.15-17 Figure 5 illustrates the results of temperature-dependent magnetic susceptibility (χ).11,17 At room temperature, χc = χab ≈ 6
10-4 cm3 mol-1. The polycrystalline average of the susceptibility data are presented as the Fisher’s d(χT)/dT in the inset of Figure 5. In the parents of Fe-based superconductors, the magnetic susceptibility above TN increases with temperature (the opposite of Curie-Weiss) and resembles that of chromium (a model SDW system) as shown in right panel of Figure 5. The non-Curie-Weiss behavior suggests the absence of local magnetic moments above TN.18

Figure 5. Left: Anisotropic magnetic susceptibility of BaFe2As2 showing linear dependence above TN. Right: Magnetic susceptibility obtained on a number of parents of Fe-based superconductors and also on Cr metal. Figures follow refs 11, 17, and 18.

Figure 6. Anisotropic resistivity of several BaFe2As2 crystals. The sample-to-sample variation is mainly due to delamination of the crystals. Reprinted with permission from ref 16. Copyright 2009 American Physical Society.

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Figure 6 illustrates the temperature-dependent resistivity (F) results.16 The room temperature in-plane Fab is ∼ 300 μΩ cm, but there is considerable variation in the literature with reported room temperature values ranging from 200 μΩ cm to 1000 μΩ cm. The reported c-axis resistivities have even a larger scatter, but given the tendency of the crystals to delaminate the lower reported values are probably more accurate.16 A careful study by Tanatar, et al. finds that Fc/Fab is in the range of 4-5 as shown in Figure 6.16 The residual-resistivity ratios (RRR) of the crystals are about 4, but it was reported by Chu, et al.19 that annealing the crystals can increase the RRR to 10. Interestingly, the resistivity drops sharply at To in the undoped crystals, presumably because of a strong reduction in scattering from magnetic fluctuations. In doped crystals, conversely, the resistivity increases at To. These effects are not well understood, but point to a fragile electronic structure in BaFe2As2 that is perhaps related to the sensitivity of the electronic structure to the As height.

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Figure 7. Phase diagram of Ba1-xKxFe2As2. Reprinted with permission from ref 23. Copyright 2009 European Physical Society.

3. Hole-Doping of BaFe2As2 Superconductivity was first reported in BaFe2As2based materials with TC as high as 38 K by hole doping with K partially replacing Ba.20 The most detailed studies of these materials have been carried out on polycrystalline samples, synthesized as described above for BaFe2As2, or in sealed niobium or tantalum tubes to contain the reactive, volatile potassium.21,22 High-quality single crystals are more difficult to obtain. Growth of Ba1-xKxFe2As2 from an FeAs flux is complicated in part by the volatility of K at the high temperatures required by the high melting point of FeAs (1030 °C). The K content of the resulting crystals is typically inhomogeneous because of volatilization and the changing concentration of K in the flux as the crystals grow. Some of these problems can be avoided by using a lower melting Sn flux; however, Sn incorporation must then be tolerated. Chemistry of the Ba1-xKxFe2As2 system has been recently reviewed by Johrendt and P€ ottgen.21 Replacing 2þ 1þ Ba with the larger K results in the expected increase in c, but a decrease in a. The decrease in a must be attributed to electronic effects in the covalent FeAs layer. Because it has been shown that states near the Fermi level are primarily of Fe character,4 Fe-Fe interactions are likely the ones most strongly affected by doping. The shortening of the Fe-Fe contacts by hole doping suggest that the Fe-Fe interactions near the Fermi level in BaFe2As2 may be primarily antibonding. As K replaces Ba the SDW transition is suppressed and a superconducting dome forms, as illustrated in Figure 7.23 Rb doping has also been found to produce superconductivity.24 At low doping there is a region of coexistence between magnetism and superconductivity that has attracted a great deal of attention. The superposition of magnetic and nonmagnetic components in the M€ ossbauer spectra over a temperature range of ∼10 K has been reported in underdoped polycrystalline Ba1-xKxFe2As2.26 This was interpreted as the result of a narrow distribution

Figure 8. Illustration of electronic phase separation in the underdoped region of Ba1-xKxFe2As2 system. This shows the coexistence of magnetic order and superconductivity. Reprinted with permission from ref 25. Copyright 2009 American Physical Society.

Figure 9. Temperature dependence of specific heat for BaFe2-xCrxAs2, for 0 e x e 0.75. Adapted from ref 27. Copyright 2009 American Physical Society.

of K concentrations resulting in a continuous distribution of magnetic transition temperatures, and not electronic phase separation. Other evidence indicates that the coexistence involves electronic phase separation with characteristic length scale of about 65 nm as illustrated in Figure 8.25 It should be noted, however, that some skepticism regarding crystal quality has appeared in the literature. In ref 21, Johrendt and P€ ottgen argue that “...crystals can never be homogeneous as a consequence of the phase diagram, because the potassium concentration in the flux is varying

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Figure 10. Phase diagram of Ba(Fe1-xCox)2As2. Note that the structural and magnetic transitions separate upon doping with Co. Reprinted with permission from ref 36. Copyright 2009 American Physical Society.

during the crystal growth” and that the “growth of highquality homogeneous single crystals of Ba1-xKxFe2As2 is an unresolved problem.” Hole-doping on the Fe crystallographic site in BaFe2As2 was first reported by Sefat et al.,27 by use of chromium. This form of hole-doping leads to suppression of the magnetic and structural phase transitions in BaFe2-xCrxAs2 crystals (Figure 9), without inducing superconductivity. This suggests that superconductivity does not derive simply from the suppression of the phase transitions. The materials show signatures of approaching a ferromagnetic state for x as little as 0.36 by an enhanced Wilson ratio. 4. Electron-Doping of BaFe2As2 We know of no careful structural studies on electron doped systems in which the chemical substitution occurs on the Ba site in BaFe2As2. One report on a polycrystalline sample of nominal composition Ba0.85La0.15Fe2As2 suggests a very small decrease in both a and c when the smaller La3þ is substituted for Ba2þ.28 The contraction of both lattice constants in this case suggests that if any competing electronic effects are present, they are overwhelmed by the size difference between La and Ba. The first report of electron doping on the Fe site was reported by Sefat, et al., by use of cobalt.9 Superconductivity occurs for low cobalt concentration, with a maximum TC of 22 K for optimal doping (x ≈ 0.06-0.08). Substitution by cobalt, which is similar in size to iron, leads to a decrease in both a and c. This is expected based on the difference in unit cell sizes for the ternary end members BaFe2As2 (a = 3.963 A˚, c = 13.017 A˚) and BaCo2As2 (a = 3.954 A˚, c = 12.659 A˚).29 It has been found that Fe can also be partially or completely replaced by other transition metals Ni and Cu,30,31 Rh and Pd.32 Of these systems, Ba(Fe1-xCox)2As2 has received the most attention. Several phase diagrams for Ba(Fe1-xCox)2As2 system have appeared, and all are in broad agreement.33-36 A recent phase diagram is shown in Figure 10.36 This phase

Figure 11. Phase diagrams of Ba(Fe1-xCox)2As2 and Ba(Fe1-xRhx)2As2 (a) and Ba(Fe1-xNix)2As2 and Ba(Fe1-xPdx)2As2 (b). The transition temperatures were determined similar to that described in the bottom inset. Reprinted with permission from ref 32. Copyright 2009 American Physical Society.

diagram is remarkable on several levels. In the first place, it shows that superconductivity is surprisingly robust to in-plane disorder, in striking contrast to the cuprates.37 Second, maximum TC is achieved by replacing only 6-8% of the Fe as compared with nearly 50% of the Ba in the hole-doped case. This suggests that the electrondoped systems are likely to be chemically more homogeneous than the hole-doped systems. Lastly, doping with Co splits the structural and the magnetic transitions, so that as the sample is cooled it first undergoes a structural phase transition and at a lower temperature it orders magnetically. Such a split has also been observed in the LaFeAsO system,38 and has been attributed to formation of a nematic phase39 or magnetoelastic coupling.40 The use of other electron-dopants30-32 has been found to produce a “universal phase diagram” as shown in Figure 11, in which dopant elements from the same column of the periodic table produce an equal effect.30 It is difficult to understand how a conventional phonon mechanism could account for this observation. Considerable progress has been made in understanding the electrical transport in electron doped BaFe2As2. The evolution of the resistivity and Hall number with temperature and doping for Ba(Fe1-xCox)2As2 is shown in Figure 12.41 The behavior is very regular and the analysis by Rullier-Albenque, et al. indicates that each cobalt atom adds about 0.9 electrons for a large range of cobalt doping (0 < x < 0.2).41 They also point out that since the Fermi energy is only 20-40 meV above the bottom of the electron bands, the usual assumption that kBT , EF is no longer valid and the temperature dependence of the Hall number is due to a change in the carrier concentration.

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Figure 12. Left: Resistivity vs temperature for a series of Ba(Fe1-xCox)2As2 samples. Right: Hall number vs temperature for the same series. Reprinted with permission from ref 41. Copyright 2009 American Physical Society.

Figure 13. Left: Two bands used to model the transport and magnetic susceptibility of Ba(Fe1-xCox)2As2 as described in the text. The position of the Fermi energy is shown for x = 0. As x is increased, the Fermi energy moves up and is above Eh for x ≈ 0.1. Right: Model calculation of the Seebeck coefficient vs temperature, for x values as indicated.

These values for EF are also consistent with photoemission results.42 Similar Hall data is also reported by L. Fang, et al.43 This line of analysis was taken a step further by B. C. Sales, et al., who considered a model two-band system as shown in Figure 13.44 The only parameters in this model are the x = 0 electron concentration, N0, the effective electron and hole masses me* and mh*, and the valence band maximum, Eh. It was found that for values of the parameters N0 = 1
1020, me* = 25 m0, mh*= 50 m0, and EH = 150 K, the Hall number, Seebeck coefficient, and magnetic susceptibility could all be semiquantitatively modeled. The effective mass parameters incorporate multiple bands, multivalley degeneracy, and Fermi surface averages and hence are much larger than those obtained from ARPES data. The results of the Seebeck coefficient modeling are also shown in Figure 13, and are in fair agreement with the data reported by Mun, et al.45 5. Isovalent-Doping of BaFe2As2 Some indication of size effects within the transition metal layer in BaFe2As2 can be revealed by isoelectronic doping in the system BaFe2-xMxAs2, e.g. M = Ru. Superconductivity has been reported with a maximum TC of 21 K for the nominal composition Ba(Fe0.62Ru0.38)2As2.46 Substitution with Ru increases a, likely

because of steric effects between Fe/Ru sites. Interestingly, c decreases as Ru replaces Fe. Comparison of the crystal structure of the end members BaFe2As2 and BaRu2As247 reveals that as the M = Fe/Ru square nets are stretched, the MAs4 tetrahedra become more compressed along the c-axis. Although the Ru-As bonds (2.43 A˚) are longer than the Fe-As bonds, the thickness of the MAs layer is significantly smaller in BaRu2As2 (2.52 A˚) than in BaFe2As2 (2.72 A˚). This accounts for about half of the observed decrease in c. The rest is due to a decrease in the separation of the MAs layers upon Ru substitution, which occurs despite an increase in Ba-As interatomic distances. This suggests that the larger size of Ru is responsible for both the contraction of the MAs layer through M-M interactions, and the interlayer spacing due the resulting expansion of the As layers. Replacing As with P results in superconductivity with TC near 30 K at the optimal composition BaFe2(As0.68P0.32)2.48 This isoelectronic substitution formally does not dope charge carriers into the FeAs layer, and the suppression of magnetism and emergence of superconductivity may be due to disruption of the spin density wave and the resulting changes in the Fe-Fe magnetic interactions, which involve the pnictogen atoms. Polycrystalline samples of BaFe2(As1-xPx)2 have been synthesized by direct reaction of the

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Figure 14. Phase diagram of BaFe2(As1-xPx)2. The inset shows the value of the exponent n as a function of doping obtained by fitting the low temperature resistivity to the equation F(T ) = F0 + RT n. Reprinted with permission from ref 48. Copyright 2009 Institute of Physics Publishing.

elements,48 and very small single crystals have been prepared by melting and slowly cooling a nearly stoichiometric polycrystalline precursor.49 Phosphorus doping results in a shorter Fe-pnictogen distance, which is primarily realized through a decrease in the height of the As/P site above the Fe plane. This increases the distortion of the FeAs4 tetrahedra. Contraction of the FeAs layer in the ab-plane is likely limited sterically by the direct Fe-Fe interactions. For comparison, the unit cell parameters of BaFe2P2 are a = 3.840 A˚ and c = 12.442 A˚.50 BaFe2Pn2 for the heavier pnictides Pn = Sb and Bi have not been reported. This suggests that the solubility of Sb or Bi may be low in BaFe2As2. We know of no studies of these systems to date. In addition, no aleovalent substitutions (e.g., Ge or Se) on the As site have been reported. There have been several reports of pressure-induced superconductivity in BaFe2As2,51-53 but other studies have failed to reproduce these results.34,54 A possible explanation for the disagreement involves the role of uniaxial stress along the c-axis, which affects the As height above the Fe plane, zAs. We have previously seen in Figure 2 that the electronic structure near EF is highly sensitive to zAs. Supporting these ideas is the appearance of superconductivity in isovalently doped materials such as P for As48,49,55 and Ru for Fe.46 The phase diagram obtained on single crystals of BaFe2(As1-xPx)2 is shown in Figure 14. As no carriers are being introduced into the system, the primary effect of doping is to suppress the SDW, presumably by tuning the structural parameters. 6. Magnetic Structure and Spin Dynamics As the superconductive pairing is believed to be mediated by spin fluctuations, a detailed understanding of the magnetism in BaFe2As2 is essential. The magnetic structure (often termed “stripe order”) of undoped BaFe2As2 is illustrated in Figure 15.8 Note that the moments form ferromagnetic chains along b (longer Fe-Fe distance in rectangular Fe nets), but are antiferromagnetic along a and c. The magnetic moment was found to be 0.87 μB per Fe atom.8,14 There has been considerable debate as to whether the antiferromagnetism is due to Fermi surface nesting or to second neighbor superexchange (see Mazin and Schmalian56 for a review), but recent thinking finds both of these views inadequate.57 According to Johannes and Mazin,57

Figure 15. Magnetic structure of BaFe2As2 below the ∼140 K SDW transition. Reprinted with permission from ref 8. Copyright 2009 American Physical Society.

the ground state is selected by itinerant, one-electron interactions, but Fermi surface nesting only plays a small part in the overall energy balance. Neutron scattering was used to study the spin waves in BaFe2As2 by Matan, et al.,58 and they found a spin gap of Δ = 9.8(4) meV, an in-plane spin wave velocity of vab = 280(150) meV A˚, and an out of plane velocity of vc = 57(7) meV A˚. Above TN, spin-wave scattering is replaced by broad quasi-inelastic scattering yielding in-plane correlation lengths of ξ = 13(2) A˚ at pω = 10 meV and ξ = 15(2) A˚ at pω = 12 meV.58 Out of plane scattering above TN indicated uncorrelated interlayer spins, illustrating the 2D nature of the spin fluctuations in this system. One of the most striking features of unconventional superconductivity is the appearance of a magnetic excitation (commonly called a “resonance”) that grows as the sample is cooled below TC. Resonances have been observed in a wide variety of cuprate and heavy fermion superconductors, and follow a universal scaling with TC and maximum value of the gap.59-61 Resonances are predicted to occur when the superconducting gap function has opposite signs on different parts of the Fermi surface.59-64 The spin resonance was first observed in iron-based superconductors by Christianson, et al. on polycrystalline samples of Ba1-xKxFe2As2.65 This was followed by measurements on single crystals of Co-doped BaFe2As2 by Lumsden, et al.66 and on Ni-doped BaFe2As2 by Chi, et al.67 The resonance occurs in reciprocal space at the same place in-plane antiferromagnetic order is observed in the parent compound, namely (1/2, 1/2) (see Figure 16). Another important probe of spin fluctuations is NMR. This technique was recently applied to a series of Ba(Fe1-xCox)2As2 samples to observe the evolution of spin fluctuations as superconductivity was suppressed through overdoping. ARPES measurements have shown that when x ≈ 0.15 in Ba(Fe1-xCox)2As2 the hole pocket is filled.68 When the hole pocket is full, the interband spin excitations should disappear leaving only the intraband excitations.69

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Figure 16. Left: Sharp resonance magnetic excitation at wavevector (1/2 1/2) observed as a sample of Ba(Fe0.92Co0.08)2As2 is cooled below TC. Right: Top figure shows the transfer of spectral weight into the resonance, and the bottom figure shows the onset of scattering at TC. Adapted from ref 66. Copyright 2009 American Physical Society.

Figure 17. 75As NMR nuclear spin-lattice relaxation rate 1/T1 divided by T (1/T1T ) vs temperature for Ba(Fe1-xCox)2As2 spanning the superconducting dome. This set of data is for B//c-axis. Reprinted with permission from ref 69. Copyright 2009 American Physical Society.

These ideas have been recently tested with NMR experiments and it has been found that filling the hole pocket results in the complete suppression of antiferromagnetic spin fluctuations. Plotted in Figure 17 is 1/T1T data for Ba(Fe1-xCox)2As2 showing that for optimally doped samples the spin fluctuations grow as the sample is cooled below 100 K.34,69,70 As x is increased and the samples become overdoped, the enhancement of the spin fluctuations is reduced and by x = 0.26 there is no enhancement. This is precisely the behavior one would expect if interband spin excitations were responsible for the pairing in Ba(Fe1-xCox)2As2. 7. Conclusions One of the grand challenges of materials chemistry and physics is to understand complex, emergent phenomena such as unconventional superconductivity on a predictive level. To achieve this goal, we must identify and characterize good model systems in detail. In this short review, we have argued that the family of superconductors based on BaFe2As2 are excellent model systems for the study of unconventional superconductivity, and we have briefly discussed their key properties.

Acknowledgment. This work was supported by the Division of Materials Sciences and Engineering (D.M., B.C.S., M.A.M.), Office of Basic Energy Sciences, U.S. Department of Energy. This research is sponsored in part by the Eugene P. Wigner Fellowship Program (A.S., M.A.M.).

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