Abstract
Hightemperature superconductivity in iron pnictides occurs when electrons and holes are doped into their antiferromagnetic parent compounds. Since spin excitations may be responsible for electron pairing and superconductivity, it is important to determine their electron/holedoping evolution and connection with superconductivity. Here we use inelastic neutron scattering to show that while electron doping to the antiferromagnetic BaFe_{2}As_{2} parent compound modifies the lowenergy spin excitations and their correlation with superconductivity (<50 meV) without affecting the highenergy spin excitations (>100 meV), holedoping suppresses the highenergy spin excitations and shifts the magnetic spectral weight to lowenergies. In addition, our absolute spin susceptibility measurements for the optimally holedoped iron pnictide reveal that the change in magnetic exchange energy below and above T_{c} can account for the superconducting condensation energy. These results suggest that highT_{c} superconductivity in iron pnictides is associated with both the presence of highenergy spin excitations and a coupling between lowenergy spin excitations and itinerant electrons.
Introduction
In conventional BardeenCooperSchrieffer (BCS) superconductors^{1}, superconductivity occurs when electrons form coherent Cooper pairs below the superconducting transition temperature T_{c}. Although the kinetic energy of paired electrons increases in the superconducting state relative to the normal state, the reduction in the ion lattice energy is sufficient to give the superconducting condensation energy (E_{c}=−N(0)Δ^{2}/2 and Δ≈2ħω_{D}, where N(0) is the electron density of states at zero temperature, ħω_{D} is the Debye energy, and V_{0} is the strength electronlattice coupling)^{1,2,3}. For iron pnictide superconductors derived from electron or holedoping of their antiferromagnetic (AF) parent compounds^{4,5,6,7,8,9}, the microscopic origin for superconductivity is unclear. Although spin excitations arising from quasiparticle excitations between the hole pockets near Γ and electron pockets at M in reciprocal space have been suggested as the microscopic origin for superconductivity^{10,11}, orbital fluctuations may also induce superconductivity in these materials^{12}. Here we use inelastic neutron scattering (INS) to systematically map out energy and wave vector dependence of the spin excitations in electron and holedoped iron pnictides with different superconducting transition temperatures. By comparing the outcome with previous spin wave measurements on the undoped parent compound BaFe_{2}As_{2} (ref. 13), we find that highT_{c} superconductivity only occurs for iron pnictides with lowenergy (≤25 meV or ~6.5 k_{B}T_{c}) itinerant electronspin excitation coupling and highenergy (>100 meV) spin excitations. Since our absolute spin susceptibility measurements for optimally holedoped iron pnictide reveal that the change in magnetic exchange energy below and above T_{c}^{14,15} can account for the superconducting condensation energy, we suggest that the presence of both highenergy spin excitations giving rise to a large magnetic exchange coupling J and lowenergy spin excitations coupled to the itinerant electrons are important for highT_{c} superconductivity in iron pnictides.
For BCS superconductors, the superconducting condensation energy E_{c} and T_{c} are controlled by the strength of the Debye energy ħω_{D} and electronlattice coupling V_{0} (refs 1, 2, 3). A material with large ħω_{D} and lattice exchange coupling is a necessary but not a sufficient condition to have highT_{c} superconductivity. On the other hand, a soft metal with small ħω_{D} (such as lead and mercury) will also not exhibit superconductivity with highT_{c}. For unconventional superconductors such as iron pnictides, the superconducting phase is derived from hole and electron doping from their AF parent compounds^{4,5,6,7,8,9}. Although the static longrange AF order is gradually suppressed when electrons or holes are doped into the iron pnictide parent compound such as BaFe_{2}As_{2} (refs 5, 6, 7, 8, 9), shortrange spin excitations remain throughout the superconducting phase and are coupled directly with the occurrence of superconductivity^{16,17,18,19,20,21,22,23,24,25}. For spin excitationsmediated superconductors, the superconducting condensation energy should be accounted for by the change in magnetic exchange energy between the normal (N) and superconducting (S) phases at zero temperature. For an isotropic tJ model^{26}, ΔE_{ex}(T)=2J[‹S_{i+x}·S_{i}›_{N}−‹S_{i+x}·S_{i}›_{S}], where J is the nearest neighbour magnetic exchange coupling and ‹S_{i+x}·S_{i}› is the dynamic spin susceptibility in absolute units at temperature T^{14,15}. Since the dominant magnetic exchange couplings are isotropic nearest neighbour exchanges for copper oxide superconductors^{27,28}, the magnetic exchange energy ΔE_{ex}(T) can be directly estimated using the formula through carefully measuring of J and the dynamic spin susceptibility in absolute units between the normal and superconducting states^{29,30,31}. For heavy Fermion superconductor such as CeCu_{2}Si_{2}, one has to modify the formula to include both the nearest neighbour and next nearest neighbour magnetic exchange couplings appropriate for the tetragonal unit cell of CeCu_{2}Si_{2} to determine ΔE_{ex}(T) (ref. 32). In the case of iron pnictide superconductors^{5,6,7,8,9}, the effective magnetic exchange couplings in their parent compounds are strongly anisotropic along the nearest neighbour a_{o} and b_{o} axis directions of the orthorhombic structure (see inset in Fig. 1a)^{13,33,34}. Although the electron doping induced lattice distortions in iron pnictides^{35} may affect the effective magnetic exchange couplings^{36}, our INS experiments on optimally electrondoped BaFe_{1.9}Ni_{0.1}As_{2} indicate that the highenergy spin excitations, which determines the effective magnetic exchange couplings^{13,33,34}, are weakly electron dopingdependent^{24}. Therefore, we can rewrite the relation between the magnetic exchange coupling and the magnetic exchange energy as^{15}
Here the scattering function S(Q,E=ħω) is related to the imaginary part of the dynamic susceptibility χ^{″}(Q,ω) via S(Q,ω)=[1+n(ω,T)]χ^{″}(Q,ω), where [1+n(ω,T)] is the Bose population factor, Q the wave vector, and E=ħω the excitation energy. J_{1a} is the effective magnetic coupling strength between two nearest sites along the a_{o} direction, while J_{1b} is that along the b_{o} direction and J_{2} is the coupling between the next nearest neighbour sites (see inset in Fig. 1a)^{13}.
To determine how highT_{c} superconductivity in iron pnictides is associated with spin excitations, we consider the phase diagram of electron and holedoped iron pnictide BaFe_{2}As_{2} (Fig. 1a)^{9}. In the undoped state, BaFe_{2}As_{2} forms a metallic lowtemperature orthorhombic phase with collinear AF structure as shown in the inset of Fig. 1a. INS measurements have mapped out spin waves throughout the Brillouin zone and determined the effective magnetic exchange couplings^{13}. Upon doping electrons to BaFe_{2}As_{2} by partially replacing Fe with Ni to induce superconductivity in BaFe_{2−x}Ni_{x}As_{2} with maximum T_{c}≈20 K at x_{e}=0.1 (ref. 37), the lowenergy (<80 meV) spin waves in the parent compounds are broadened and form a neutron spin resonance coupled to superconductivity^{20,21,22,23}, while highenergy spin excitations are weakly affected^{24}. With further electron doping to x_{e}≥0.25, superconductivity is suppressed and the system becomes a paramagnetic metal (Fig. 1a)^{37}. For holedoped Ba_{1−x}K_{x}Fe_{2}As_{2} (ref. 38), superconductivity with maximum T_{c}=38.5 K appears at x_{h}≈0.33 (ref. 5) and pure KFe_{2}As_{2} at x_{h}=1 is a T_{c}=3.1 K superconductor^{6}. In order to determine how spin excitations throughout the Brillouin zone are correlated with superconductivity in iron pnictides, we study optimally holedoped Ba_{0.67}K_{0.33}Fe_{2}As_{2} (T_{c}=38.5 K, Fig. 1c), pure KFe_{2}As_{2} (T_{c}=3 K, Fig. 1b), and nonsuperconducting electronoverdoped BaFe_{1.7}Ni_{0.3}As_{2} (Fig. 1d). If spin excitations are responsible for mediating electron pairing and superconductivity, the change in magnetic exchange energy between the normal and superconducting state should be large enough to account for the superconducting condensation energy^{15}.
From density functional theory (DFT) calculations, one finds that Fermi surfaces for the undoped parent compound BaFe_{2}As_{2} consist of holelike pockets near the Brillouin zone centre and electron pockets near the zone corner (Fig. 2a)^{10}. Figure 2a–d shows the evolution of Fermi surfaces as a function of electron and holedoping obtained from the tightbinding model of Graser et al.^{39} When electrons are doped into the parent compounds, the hole Fermi surfaces decrease in size while the electron pocket sizes increase (Fig. 2b)^{40}. As a consequence, quasiparticle excitations between the hole and electron Fermi surfaces form transversely elongated spin excitations that increase with increasing electron doping^{23,24,25}. For electronoverdoped iron pnictides, the hole pockets sunk below the Fermi surface (Fig. 2c)^{40}. The absence of interband transition between the hole and electron Fermi surfaces is expected to result in a complete suppression of the lowenergy spin excitations at the AF ordering wave vector. This is indeed confirmed by nuclear magnetic resonance experiments on electron overdoped Ba(Fe_{1−x}Co_{x})_{2}As_{2} (ref. 41). For optimally holedoped iron pnictide Ba_{0.67}K_{0.33}Fe_{2}As_{2} (inset in Fig. 1a), DFT theory based on sign reversed quasiparticle excitations between hole and electron pockets (Fig. 2d) predicts correctly the longitudinally elongated spin excitations from Q_{AF}=(1,0)^{23,18}. In addition, INS work on powder^{16,17} and single crystals^{18} of holedoped Ba_{1−x}K_{x}Fe_{2}As_{2} reveal that the lowenergy spin excitations are dominated by a resonance coupled to superconductivity. For pure KFe_{2}As_{2} (x_{h}=1), lowenergy (<14 meV) spin excitations become longitudinally incommensurate from Q_{AF}=(1,0) (inset in Fig. 1a)^{19}. These results, as well as the work on electrondoped iron pnictides^{23,24,25}, have shown that the lowenergy spin excitations in ironbased superconductors can be accounted for by itinerant electrons on the hole and electronnested Fermi surfaces^{9}.
Here we use INS to show that the effect of holedoping to BaFe_{2}As_{2} is to suppress highenergy spin excitations and transfer the spectral weight to lowenergies that couple to the appearance of superconductivity (Fig. 1h). The overall spin excitations spectrum in optimally holedoped superconducting Ba_{0.67}K_{0.33}Fe_{2}As_{2} is qualitatively consistent with theoretical methods based on DFT and dynamic mean filed theory (DMFT)^{42}. By using the INS measured magnetic exchange couplings and spin susceptibility in absolute units, we calculate the superconductivityinduced lowering of magnetic exchange energy and find it to be about seven times larger than the superconducting condensation energy determined from specific heat measurements for Ba_{0.67}K_{0.33}Fe_{2}As_{2} (ref. 43). These results are consistent with spin excitationsmediated electron pairing mechanism^{15}. For the nonsuperconducting electronoverdoped BaFe_{1.7}Ni_{0.3}As_{2}, we find that while the effective magnetic exchange couplings are similar to that of optimally electrondoped BaFe_{1.9}Ni_{0.1}As_{2} (Fig. 1g)^{24}, the lowenergy spin excitations (<50 meV) associated with the hole and electron pocket Fermi surface nesting disappear (Fig. 2c), thus revealing the importance of Fermi surface nesting and itinerant electronspin excitation coupling to the occurrence of superconductivity (Fig. 1h). Finally, for heavily holedoped KFe_{2}As_{2} with lowT_{c} superconductivity (Fig. 1b), there are only incommensurate spin excitations below ~25 meV possibly due to the mismatched electronhole Fermi surfaces^{19,44} and the correlated highenergy spin excitations prevalent in electrondoped and optimally holedoped iron pnictides are strongly suppressed (Fig. 1e), indicating a dramatic softening of effective magnetic exchange coupling (inset Fig. 1h). Therefore, highT_{c} superconductivity is likely associated with two ingredients: a large effective magnetic exchange coupling^{15}, much like the large Debye energy for highT_{c} BCS superconductors, and a strong itinerant electronsspin excitations coupling from the Fermi surface nesting^{10}, similar to electronphonon coupling in BCS superconductors.
Results
Electron and holedoping evolution of spin excitations
To substantiate the key conclusions of Fig. 1, we present the twodimensional (2D) constantenergy images of spin excitations in the (H,K) plane at different energies for KFe_{2}As_{2} (Fig. 3a–c), Ba_{0.67}K_{0.33}Fe_{2}As_{2} (Fig. 3d–f), and BaFe_{1.7}Ni_{0.3}As_{2} (Fig. 3g–i) above T_{c}. In previous INS work on KFe_{2}As_{2}, longitudinal incommensurate spin excitations were found by triple axis spectrometer measurements for energies from 3 to 14 meV in the normal state^{19}. While we confirmed the earlier work using timeofflight INS for energies below E=15±1 meV (Fig. 3a,b), our new data collected at higher excitation energies reveal that incommensurate spin excitations converge into a broad spin excitation near E=20 meV and disappear for energies above 25 meV (Fig. 3c; Supplementary Fig. S1). For Ba_{0.67}K_{0.33}Fe_{2}As_{2}, spin excitations at E=5±1 meV are longitudinally elongated from Q_{AF} as expected from the DFT calculations (Fig. 3d)^{18,23}. At the resonance energy (E=15±1 meV)^{16}, spin excitations are isotropic above T_{c} (Fig. 3e). On increasing energy further to E=50±10 meV, spin excitations change to transversely elongated from Q_{AF} similar to spin excitations in optimally electrondoped superconductor BaFe_{1.9}Ni_{0.1}As (Fig. 3f)^{24}. Figure 3g–i summarizes similar 2D constantenergy images of spin excitations for nonsuperconducting BaFe_{1.7}Ni_{0.3}As. At E=9±3 (Fig. 3g) and 30±10 meV (Fig. 3h), there are no correlated spin excitations near the Q_{AF}. Upon increasing energy to E=59±10 meV (Fig. 3i), we see clear spin excitations transversely elongated from Q_{AF} (Fig. 3i; Supplementary Fig. S2). To further illustrate the presence of a large spin gap in BaFe_{1.7}Ni_{0.3}As, we compare spin waves in BaFe_{2}As_{2}^{13} and paramagnetic spin excitations in BaFe_{1.7}Ni_{0.3}As. Figure 4a,b shows the background subtracted spin wave scattering of BaFe_{2}As_{2} for the E_{i}=250, 450 meV data, respectively, projected in the wave vector (Q=[1,K]) and energy space at 7 K^{13}. Sharp spin waves are seen to stem from the AF ordering wave vector Q_{AF}=(1,0) above the ~15 meV spin gap^{45}. Figure 4c,d shows identical projections for spin excitations of BaFe_{1.7}Ni_{0.3}As at 5 K. A large spin gap of ~50 meV is clearly seen in the data near Q_{AF}=(1,0). A detailed comparison of spin excitations in BaFe_{2−x}Ni_{x}As_{2} with x_{e}=0, 0.1, 0.3 is made in Supplementary Figs S3 and S4.
Figure 5a–d shows 2D images of spin excitations in BaFe_{1.7}Ni_{0.3}As_{2} at E=70±10, 112±10, 157±10, and 214±10 meV, respectively. Figure 5e–h shows wave vector dependence of spin excitations at energies E=70±10, 115±10, 155±10 and 195±10 meV, respectively, for Ba_{0.67}K_{0.33}Fe_{2}As_{2}. Similar to spin waves in BaFe_{2}As_{2} (ref. 13), spin excitations in BaFe_{1.7}Ni_{0.3}As_{2} and Ba_{0.67}K_{0.33}Fe_{2}As_{2} split along the Kdirection for energies above 80 meV and form rings around Q=(±1,±1) positions near the zone boundary, albeit at slightly different energies. Comparing spin excitations in Fig. 5a–d for BaFe_{1.7}Ni_{0.3}As_{2} with those in Fig. 5e–h for Ba_{0.67}K_{0.33}Fe_{2}As_{2} in absolute units, we see that spin excitations in BaFe_{1.7}Ni_{0.3}As_{2} extend to slightly higher energies and have larger intensity above 100 meV.
As discussed in the spin wave measurements of BaFe_{2}As_{2} (ref. 13), the magnon band top energy at Q=(1,1) governs the effective magnetic exchange couplings J (J_{1a}, J_{1b}, and J_{2}). To estimate the change of J for holedoped Ba_{0.67}K_{0.33}Fe_{2}As_{2}, we calculate the energy cut at Q=(1,1) by exploring the Heisenberg Hamiltonian of the parent compound. It turns out that J_{1a}, J_{1b}, and J_{2} have comparable effect on the band top. On the basis of the dispersion of Ba_{0.67}K_{0.33}Fe_{2}As_{2}, the effective magnetic exchange J is found to be about 10% smaller for Ba_{0.67}K_{0.33}Fe_{2}As_{2} compared with that of BaFe_{2}As_{2} (Fig. 6). For comparison, if we assume the band top for KFe_{2}As_{2} is around E=25 meV, the effective magnetic exchange should be about 90% smaller for KFe_{2}As_{2}. Of course, we know this is not an accurate estimation since spin excitations in KFe_{2}As_{2} are incommensurate and have an inverse dispersion. In any case, given the zone boundary energy of E≈25 meV (Supplementary Fig. S1), the effective magnetic exchange couplings in KFe_{2}As_{2} must be much smaller than that of BaFe_{2}As_{2}.
Theoretical calculations of spin excitations
To understand the wave vector dependence of spin excitations in holedoped Ba_{0.67}K_{0.33}Fe_{2}As_{2}, we have carried out the random phase approximation (RPA) calculation of the dynamic susceptibility in a pure itinerant electron picture using the method described before^{25}. Figure 5i,j shows RPA calculations of spin excitations at E=70 and 155 meV, respectively, for Ba_{0.67}K_{0.33}Fe_{2}As_{2} assuming that hole doping induces a rigid band shift^{25}. While a pure RPA type itinerant model can explain longitudinally elongated spin excitations at lowenergies^{18}, it clearly fails to describe the transversely elongated spin excitations in holedoped iron pinctides at high energies (Fig. 5e,g). For comparison with the RPA calculation, we also used a combined DFT and DMFT approach^{24,42} to calculate the imaginary part of the dynamic susceptibility χ^{″}(Q,ω) in the paramagnetic state. Figure 5k,i shows calculated spin excitations at E=70 and 155 meV, respectively. Although the model still does not agree in detail with the data in Fig. 5e,g, it captures the trend of spectral weight transfer away from Q_{AF}=(1,0) on increasing energy and forming a pocket centred at Q=(1,1).
Dispersions of spin excitations and local dynamic susceptibility
By carrying out cuts through the 2D images similar to Figs 3d–f and 5e–h along the [1,K] and [H,0] directions (Supplementary Figs S5 and S6), we establish the spin excitation dispersion along the two high symmetry directions for Ba_{0.67}K_{0.33}Fe_{2}As_{2} and compare with the dispersion of BaFe_{2}As_{2} (Fig. 1f)^{13}. In contrast to the dispersion of electrondoped BaFe_{1.9}Ni_{0.1}As_{2} (ref. 24), we find clear softening of the zone boundary spin excitations in holedoped Ba_{0.67}K_{0.33}Fe_{2}As_{2} from spin waves in BaFe_{2}As_{2} (ref. 13). We estimate that the effective magnetic exchange coupling in Ba_{0.67}K_{0.33}Fe_{2}As_{2} is reduced by about 10% from that of BaFe_{2}As_{2} (Fig. 6). Similarly, Fig. 1g shows the dispersion curve of BaFe_{1.7}Ni_{0.3}As_{2} along the [1,K] direction plotted together with that of BaFe_{2}As_{2} (ref. 13). For energies below ~50 meV, spin excitations are completely gapped marked in the dashed area probably due to the missing holeelectron Fermi pocket quasiparticle excitations^{40}. On the basis of the 2D spin excitation images similar to Fig. 3a–c; we plot in Fig. 1e the dispersion of incommensurate spin excitations in KFe_{2}As_{2}. The incommensurability of spin excitations is weakly energy dependent below E=12 meV but becomes smaller with increasing energy above 12 meV. Correlated spin excitations for energies above 25 meV are suppressed as shown in the shaded area in Fig. 1e.
To quantitatively determine the effect of electron and hole doping on the overall spin excitations spectra, we calculate the local dynamic susceptibility per formula unit (f.u.) in absolute units, defined as χ^{″}(ω)=∫χ^{″}(q,ω)dq/∫dq (ref. 24), where , at different energies for Ba_{0.67}K_{0.33}Fe_{2}As_{2}, BaFe_{1.7}Ni_{0.3}As_{2}, and KFe_{2}As_{2}. Figure 1h shows the outcome together with previous data on optimally electrondoped superconductor BaFe_{1.9}Ni_{0.1}As_{2} (ref. 24). While electron doping up to BaFe_{1.7}Ni_{0.3}As_{2} does not change much the spectral weight of highenergy spin excitations from that of BaFe_{1.9}Ni_{0.1}As_{2}, holedoping dramatically suppresses the highenergy spin excitations and shift the spectral weight to lower energies (Fig. 1h). For heavily holedoped KFe_{2}As_{2}, spin excitations are mostly confined to energies below about E=25 meV (inset in Fig. 1h).
The solid green, red, black, and blue lines in Fig. 7 show calculated local susceptibility in absolute units based on a combined DFT and DMFT approach for KFe_{2}As_{2}, Ba_{0.67}K_{0.33}Fe_{2}As_{2}, BaFe_{2}As_{2}, and BaFe_{1.9}Ni_{0.1}As_{2}^{24}, respectively. This theoretical method predicts that electron doping to BaFe_{2}As_{2} does not affect the spin susceptibility at high energy (E>150 meV), while spin excitations in the holedoped compound beyond 100 meV are suppressed by shifting the spectral weight to lower energies. This is in qualitative agreement with our absolute intensity measurements (Fig. 1h). The reduction of the highenergy spin spectral weight and its transfer to low energy with hole doping, but not with electron doping, is not naturally explained by the band theory (Fig. 7) and requires models that incorporate both the itinerant quasiparticles and the local moment physics^{9,42}. The hole doping makes the electronic state more correlated, as local moment formation is strongest in the halffilled d^{5} shell, and mass enhancement larger thereby reducing the electronic energy scale in the problem. The dashed lines in Fig. 7 show the results of calculated local susceptibility using RPA, which clearly fails to describe the electron and holedoping dependence of the local susceptibility.
Estimation of the superconductivityinduced magnetic exchange energy
Finally, to determine how lowenergy spin excitations are coupled to superconductivity in Ba_{0.67}K_{0.33}Fe_{2}As_{2}, we carried out a detailed temperaturedependent study of spin excitation at E=15±1 meV. Comparing with strongly c axis modulated lowenergy (E<7 meV) spin excitations^{18}, spin excitations at the resonance energy are essentially 2D without much c axis modulations. In previous work^{18}, we have shown that spin excitations near the neutron spin resonance are longitudinally elongated and change dramatically in intensity across T_{c}. However, these measurements are obtained in arbitrary units and therefore cannot be used to determine the magnetic exchange energy. Figure 8a–d shows the 2D mapping of the resonance at T=25, 38, 40, and 45 K, respectively. While the resonance reveals a clear oval shape at temperatures below T_{c} consistent with earlier work (Fig. 8a,b)^{18}, it changes into an isotropic circular shape abruptly at T_{c} (Fig. 8c,d) as shown by the dashed lines representing fullwidthathalfmaximum of the excitations (Supplementary Fig. S7). Temperature dependence of the resonance width along the [H,0] and [1,K] directions in Fig. 8e reveals that the isotropic to anisotropic transition in momentum space occurs at T_{c}. Figure 8f shows temperature dependence of the resonance from 9–40 K, which vanishes at T_{c}. Figure 8g plots temperature dependence of the mode energy together with the sum of the superconducting gaps from the hole and electron pockets^{40}. Figure 8h compares temperature dependence of the superconducting condensation energy^{43} with superconductivityinduced intensity gain of the resonance. By calculating spin excitations induced changes in magnetic exchange energy using equation (1) (see Methods and Supplementary Fig. S8)^{15}, we find that the difference of magnetic exchange interaction energy between the superconducting and normal state is approximately seven times larger than the superconducting condensation energy^{43}, thus indicating that AF spin excitations can be the major driving force for superconductivity in Ba_{0.67}K_{0.33}Fe_{2}As_{2}.
Discussion
One way to quantitatively estimate the impact of hole/electron doping and superconductivity to spin waves of BaFe_{2}As_{2} is to determine the energy dependence of the local moment and total fluctuating moments ‹m^{2}› (ref. 24). From Fig. 1h, we see that holedoping suppresses highenergy spin waves of BaFe_{2}As_{2} and pushes the spectral weight to resonance at lower energies. The total fluctuating moment of Ba_{0.67}K_{0.33}Fe_{2}As_{2} below 300 meV is ‹m^{2}›=1.7±0.3 per Fe, somewhat smaller than per Fe for BaFe_{2}As_{2} and BaFe_{1.9}Ni_{0.1}As_{2} (ref. 24). For comparison, BaFe_{1.7}Ni_{0.3}As_{2} and KFe_{2}As_{2} have ‹m^{2}›=2.74±0.11 and per Fe, respectively. Therefore, the total magnetic spectral weights for different iron pnictides have no direct correlation with their superconducting T_{c}s. Table 1 summarizes the comparison of effective magnetic exchange couplings, total fluctuating moments, and spin excitation band widths for BaFe_{2−x}Ni_{x}As_{2} with x_{e}=0,0.1,0.3 and Ba_{1−x}K_{x}Fe_{2}As_{2} with x_{h}=0.33, 1.
From Fig. 1h, we also see that the spectral weight of the resonance and lowenergy (<100 meV) magnetic scattering in Ba_{0.67}K_{0.33}Fe_{2}As_{2} is much larger than that of electrondoped BaFe_{1.9}Ni_{0.1}As_{2}. This is consistent with a large superconducting condensation energy in Ba_{0.67}K_{0.33}Fe_{2}As_{2} since its effective magnetic exchange coupling J is only ~10% smaller than that of BaFe_{1.9}Ni_{0.1}As_{2} (Fig. 1h)^{43,46}. For electronoverdoped nonsuperconducting BaFe_{1.7}Ni_{0.3}As_{2}, the lack of superconductivity is correlated with the absence of lowenergy spin excitations coupled to the hole and electron Fermi surface nesting even though the effective magnetic exchange couplings remain large^{40,41}. This means that by eliminating [‹S_{i+x}·S_{i}›_{N}−‹S_{i+x}·S_{i}›_{S}], there is no magnetic driven superconducting condensation energy, and thus no superconductivity. On the other hand, although the suppression of correlated highenergy spin excitations in KFe_{2}As_{2} can dramatically reduce the effective magnetic exchange coupling in KFe_{2}As_{2} (Figs 1e and 6), one can still have superconductivity with reduced T_{c}. If spin excitations are a common thread of the electron pairing interactions for unconventional superconductors^{15}, our results reveal that the large effective magnetic exchange couplings and itinerant electronspin excitation interactions may both be important ingredients to achieve highT_{c} superconductivity, much like the large Debye energy and the strength of electronlattice coupling are necessary for highT_{c} BCS superconductors. Therefore, our data indicate a possible correlation between the overall magnetic excitation band width, the presence of lowenergy spin excitations, and the scale of T_{c}. This suggests that both highenergy spin excitations and lowenergy spin excitation itinerant electron coupling are important for highT_{c} superconductivity.
Methods
Sample preparation
Single crystals of Ba_{0.67}K_{0.33}Fe_{2}As_{2}, KFe_{2}As_{2}, and BaFe_{1.7}Ni_{0.3}As_{2} are grown using the flux method^{18,25}. The actual crystal compositions were determined using the inductively coupled plasma analysis. We coaligned 19 g of single crystals of Ba_{0.67}K_{0.33}Fe_{2}As_{2} (with inplane and outofplane mosaic of 4°), 3 g of KFe_{2}As_{2} (with inplane and outofplane mosaic of ~7.5°), and 40 g of BaFe_{1.7}Ni_{0.3}As_{2} (with inplane and outofplane mosaic of ~3°).
Neutron scattering experiments
Our INS experiments were carried out on the MERLIN and MAPS timeofflight chopper spectrometers at the RutherfordAppleton Laboratory, UK^{13,24}. Various incident beam energies were used as specified, and mostly with E_{i} parallel to the c axis. To facilitate easy comparison with spin waves in BaFe_{2}As_{2} (ref. 13), we defined the wave vector Q at (q_{x}, q_{y}, q_{z}) as (H,K,L)=(q_{x}a_{o}/2π, q_{y}b_{o}/2π, q_{z}c/2π) reciprocal lattice units (rlu) using the orthorhombic unit cell, where a_{o}≈b_{o}=5.57 Å, and c=13.135 Å for Ba_{0.67}K_{0.33}Fe_{2}As_{2}, a_{o}≈b_{o}=5.43 Å, and c=13.8 Å for KFe_{2}As_{2}, and a_{o}=b_{o}=5.6 Å, and c=12.96 Å for BaFe_{1.7}Ni_{0.3}As_{2}. The data are normalized to absolute units using a vanadium standard with 20% errors^{24} and confirmed by acoustic phonon normalization (see Supplementary Note 1). Supplementary Discussion provides additional data analysis on electrondoped iron pnictides, focusing on the comparison of electron overdoped nonsuperconducting BaFe_{1.3}Ni_{0.3}As_{2} with optimally electrondoped superconductor BaFe_{1.9}Ni_{0.1}As_{2} and AF BaFe_{2}As_{2}.
DFT+DMFT calculations
Our theoretical DFT+DMFT method for computing the magnetic excitation spectrum employs the ab initio full potential implementation of the method, as detailed in ref. 47. The DFT part is based on the code of Wien2k^{48}. The DMFT method requires solution of the generalized quantum impurity problem, which is here solved by the numerically exact continuoustime quantum Monte Carlo method^{49,50}. The Coulomb interaction matrix for electrons on iron atom was determined by the selfconsistent GW method in Kutepov et al.^{51}, giving U=5 eV and J=0.8 eV for the local basis functions within the all electron approach employed in our DFT+DMFT method. The dynamical magnetic susceptibility χ^{″}(Q,E) is computed from the ab initio perspective by solving the BetheSalpeter equation, which involves the fully interacting one particle Greens function computed by DFT+DMFT, and the twoparticle vertex, also computed within the same method (for details see Park et al.^{42}). We computed the twoparticle irreducible vertex functions of the DMFT impurity model, which coincides with the local twoparticle irreducible vertex within the DFT+DMFT method. The latter is assumed to be local in the same basis in which the DMFT selfenergy is local, here implemented by projection to the muffintin sphere.
Calculation of magnetic exchange energy and superconducting condensation energy for Ba_{0.67}K_{0.33}Fe_{2}As_{2}
In a neutron scattering experiment, we measure scattering function S(Q,E=ħω) which is related to the imaginary part of the dynamic susceptibility via S(Q,ω)=[1+n(ω,T)]χ^{″}(Q,ω), where n(ω,T) is the Bose population factor. The magnetic exchange coupling and the imaginary part of spin susceptibility are related via the formula^{15}:
where g=2 is the Landé gfactor. Hence, we are able to obtain the change in magnetic exchange energy between the superconducting and normal states by the experimental data of χ^{″}(Q,ω) in both states using equation (1). Strictly speaking, we want to estimate the zero temperature difference of the magnetic exchange energy between the normal and the superconducting states, and use the outcome to compare with the superconducting condensation energy^{15}. Unfortunately, we do not have direct information on the normal state χ^{″}(Q,ω) at zero temperature. Nevertheless, since our neutron scattering measurements at lowenergies showed that the χ^{″}(Q,ω) are very similar below and above T_{c} near the AF wave vector Q_{AF}=(1,0,1) and only a very shallow spin gap at Q=(1,0,0) (see Fig. 1f,h in Zhang et al.^{18}), we assume that there are negligible changes in χ^{″}(Q,ω) above and below T_{c} at zero temperature for energies below 5 meV. For spin excitation energies above 6 meV, the Bose population factors between 7 and 45 K are negligibly small. In previous work on optimally doped YBa_{2}Cu_{3}O_{6.95} superconductor, we have assumed that spin excitations in the normal state at zero temperature are negligibly small and thus do not contribute to the exchange energy^{30}.
The directly measured quantity is the scattering differential cross section
where k_{i} and k_{f} are the magnitudes of initial and final neutron momentum and F(Q) is the Fe^{2+} magnetic form factor, and (γr_{e})^{2}=0.2905, barn·sr^{−1}.
The quantity χ^{″}(Q,E) in both superconducting and normal states can be fitted by a Gaussian for resonance wave vector (1,0) and by cutting the raw data. The outcome is summarized in the Supplementary Table S1, where the unit of E is meV and that of A_{s(n)} is mbarn˙meV^{−1}·sr^{−1}·Fe^{−1}. For the case below 5 meV, we assume that A_{n} decreases to zero linearly with energy and A_{s}=A_{n} (see Fig. 1h in Zhang et al.^{18}), while the σ keeps the value at 5 meV. The assumption is shown in Supplementary Figure S8, where the resonance is seen at E=15 meV.
Because the condensation energy is only defined at zero temperature, we take T=0 in the formula equation (3) and the integral gives:
The magnetic exchange coupling constants in an anisotropic model are estimated to be
which are 10% smaller than that of BaFe_{2}As_{2} (ref. 13) and we estimate S to be close to ½ (ref. 24). Hence the exchange energy change is
The condensation energy U_{c} for optimally doped Ba_{0.67}K_{0.33}Fe_{2}As_{2} can be calculated to be
from the specific heat data of Popovich et al.^{43} Therefore, we have the ratio ΔE_{ex}/U_{c}≈7.4, meaning that the change in the magnetic exchange energy is sufficient to account for the superconducting condensation energy in Ba_{0.67}K_{0.33}Fe_{2}As_{2}.
Additional information
How to cite this article: Wang, M. et al. Doping dependence of spin excitations and its correlations with hightemperature superconductivity in iron pnictides. Nat. Commun. 4:2874 doi: 10.1038/ncomms3874 (2013).
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Acknowledgements
The single crystal growth and neutron scattering work at Rice/UTK is supported by the US DOE BES under Grant No. DEFG0205ER46202. Work at IOP is supported by the MOST of China 973 programs (2012CB821400, 2011CBA00110) and NSFC. The LDA+DMFT computations were made possible by an Oak Ridge leadership computing facility director discretion allocation to Rutgers. The work at Rutgers is supported by DOE BES DEFG0299ER45761 (G.K.) and NSFDMR 0746395 (K.H.). T.A.M. acknowledges the Center for Nanophase Materials Sciences, which is sponsored at ORNL by the Scientific User Facilities Division, BES, US DOE.
Author information
Author notes
 Meng Wang
 , Chenglin Zhang
 & Xingye Lu
These authors contributed equally to this work
Affiliations
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
 Meng Wang
 , Xingye Lu
 , Huiqian Luo
 , Xiaotian Zhang
 & Pengcheng Dai
Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA
 Chenglin Zhang
 , Yu Song
 & Pengcheng Dai
Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 379961200, USA
 Chenglin Zhang
 , Xingye Lu
 , Guotai Tan
 , Yu Song
 , Miaoyin Wang
 & Pengcheng Dai
ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK
 E.A. Goremychkin
 & T.G. Perring
Center for Nanophase Materials Sciences and Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 378316494, USA
 T.A. Maier
Department of Physics, Rutgers University, Piscataway, New Jersey 08854, USA
 Zhiping Yin
 , Kristjan Haule
 & Gabriel Kotliar
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Contributions
This paper contains data from three different neutron scattering experiments in the group of P.D. lead by M.W. (Ba_{0.67}K_{0.33}Fe_{2}As_{2}), C.Z. (KFe_{2}As_{2}), and X.L. (BaFe_{1.7}Ni_{0.3}As_{2}). These authors made equal contributions to the results reported in the paper. For Ba_{0.67}K_{0.33}Fe_{2}As_{2}, M.W., H.L., E.A.G. and P.D. carried out neutron scattering experiments. Data analysis was done by M.W. with help from H.L. and E.A.G. The samples were grown by C.Z., M.W., Y.S., X.L. and coaligned by M.W. and H.L. RPA calculation is carried out by T.A.M. The DFT and DMFT calculations were done by Z.Y., K.H. and G.K. Superconducting condensation energy was estimated by X.Z. For KFe_{2}As_{2}, the samples were grown by C.Z and G.T. Neutron scattering experiments were carried out by C.Z., E.A.G. and P.D. For BaFe_{1.7}Ni_{0.3}As_{2}, the samples were grown by X.L., H.L., and coaligned by. X.Y.L. and M.Y.W. Neutron scattering experiments were carried out by X.L., T.G.P. and P.D. The data are analysed by X.L. The paper was written by P.D., M.W., X.L. and C.L.Z. with input from T.M., K.H. and G.K. All coauthors provided comments on the paper.
Competing interests
The authors declare no competing financial interests.
Corresponding author
Correspondence to Pengcheng Dai.
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Supplementary Figures S1S9, Supplementary Tables S1S2, Supplementary Note 1 and Supplementary Discussion
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Magnetic moment evolution and spin freezing in doped BaFe2As2
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