Correlation-induced self-doping in intercalated iron-pnictide superconductor Ba2Ti2Fe2As4O
Junzhang Ma, Ambroise van Roekeghem, Pierre Richard, Zhonghao Liu, Hu Miao, Lingkun Zeng, Nan Xu, Ming Shi, Chao Cao, Junbao He, Gengfu Chen, Yunlei Sun, Guanghan Cao, Shancai Wang, Silke Biermann, Tian Qian, Hong Ding
CCorrelation-induced self-doping in intercalated iron-pnictide superconductor Ba Ti Fe As O J.-Z. Ma , ∗ A. van Roekeghem , , ∗ P. Richard , , Z.-H. Liu , H. Miao , L.-K. Zeng , N. Xu , M. Shi , C. Cao ,J.-B. He , G.-F. Chen , , , Y.-L. Sun , G.-H. Cao , S.-C. Wang , S. Biermann , , , T. Qian , † and H. Ding , ‡ Beijing National Laboratory for Condensed Matter Physics,and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Centre de Physique Th´eorique, Ecole Polytechnique, CNRS-UMR7644, 91128 Palaiseau, France Collaborative Innovation Center of Quantum Matter, Beijing, China Department of Physics, Renmin University, Beijing, 100872, China Paul Scherrer Institute, Swiss Light Source, CH-5232 Villigen PSI, Switzerland Condensed Matter Physics Group, Department of Physics,Hangzhou Normal University, Hangzhou 310036, China Department of Physics, Zhejiang University, Hangzhou 310027, China Coll ` ege de France, 11 place Marcelin Berthelot, 75005 Paris, France and European Theoretical Synchrotron Facility (ETSF), Europe
The electronic structure of the intercalated iron-based superconductor Ba Ti Fe As O ( T c ∼ d and Ti 3 d orbitals, indicating that the spacing layers separating di ff erent FeAs layers are also metallic.By counting the enclosed volumes of the Fermi surface sheets, we observe a large self-doping e ff ect, i . e . As O layer, leaving the FeAs layer in ahole-doped state. This exotic behavior is successfully reproduced by our dynamical mean field calculations, inwhich the self-doping e ff ect is attributed to the electronic correlations in the Fe 3 d shell. Our work provides analternative route of e ff ective doping without element substitution for iron-based superconductors. PACS numbers: 71.27. + a, 73.20.-r, 74.70.Xa, 79.60.-i In iron-based superconductors (IBSCs), the most commonways to suppress long-range antiferromagnetic order and ob-tain high- T c superconductivity is to introduce carriers [1–3]and / or internal strain [4–6] by element substitution. How-ever, an inevitable problem is that element substitution alsointroduces disorder, and impurity scattering is believed to bedetrimental to superconductivity [7], though not as seriouslyas in cuprate superconductors. It has been revealed that theimpurity scattering e ff ects are site dependent and the scat-tering strength is gradually reduced when the dopants moveaway from the Fe plane [8, 9]. This may partially explainwhy the maximum T c is much higher and the superconduct-ing dome is much wider in (Ba,K)Fe As as compared withBa(Fe,Co) As [8]. Therefore, finding an alternative way todope carriers but without introducing disorder would be apromising path for reaching higher T c superconductivity.A remarkable feature in the IBSCs is that there is an inti-mate relationship between the electronic correlations and the d -shell occupancy. For hole doping, extremely low coherencetemperatures are expected, while electron doping reinforcesFermi-liquid properties [10, 11]. Moreover, the orbital polar-ization can be tuned by the magnitudes of the Coulomb inter-action U and the Hund’s rule coupling J , leading to a redistri-bution of electrons among five Fe 3 d orbitals [12]. Electroniccorrelations weaken the hybridization between Fe and ligandatoms, reducing the e ff ective occupancy of the Fe 3 d orbitals[13]. However, as the total electron count on the Fe and lig-and atoms in crystals, such as BaFe As , is conserved, suchcharge redistribution between them does not produce any dop-ing e ff ect on the Fermi surfaces (FSs). In this work, we prove that doping can be induced by elec-tronic correlations in the IBSC Ba Ti Fe As O (Ba22241, T c ∼ As O lay-ers. Ba22241 can be regarded as a superlattice consisting ofalternating stacking of BaFe As and BaTi As O layers [Fig.1(a)] [14]. Compared with other IBSCs, the most distinctivecharacteristic of Ba22241 is the metallic nature of the inter-calated layers, which contributes distinctly to the density ofstates (DOS) at the Fermi level ( E F ) [15]. Our ARPES mea-surements suggest that the low-energy band dispersions can beregarded as a superposition of the band structures of the FeAsand Ti As O layers. By counting the volumes of the FSs, wefind that about 0.25 electrons per unit cell are transferred fromthe FeAs layer to the Ti As O layer. This exotic behavior issuccessfully reproduced by dynamical mean field-based elec-tronic structure calculations, which allow us to identify elec-tronic Coulomb correlations in the 3 d shells as the main causefor this self-doping e ff ect.High-quality single crystals of Ba22241 were synthesizedby the flux method [16]. ARPES measurements were per-formed at beamlines PGM and APPLE-PGM of the Syn-chrotron Radiation Center (Wisconsin) with Scienta R4000and SES 200 analyzers, respectively, as well as at beamlineSIS of the Swiss Light Source (PSI) with Scienta R4000. Theenergy and angular resolutions were set at 15-30 meV and0.2 ◦ , respectively. The samples were cleaved in situ and mea-sured in the temperature range between 25 and 150 K in avacuum better than 3 × − Torr. The ARPES data weretaken with vertical exit slits under horizontal (HP) or vertical(VP) polarized lights at SRC and with horizontal exit slit at a r X i v : . [ c ond - m a t . s up r- c on ] S e p PSI.
M X (b)(a) -2.0-1.5-1.0-0.50.0 E - E F ( e V ) M Z X M (d) -2.0-1.5-1.0-0.50.0 E - E F ( e V ) M MX (c) (cid:75)(cid:75)(cid:75)
BaFe As BaTi As O Ba Ti Fe As O ZZ FIG. 1. (Color online) (a) Crystal structures of BaFe As ,BaTi As O and Ba Ti Fe As O. (b) Schematic Brillouin zones forone and two Fe (Ti) per unit cell. Red lines indicate the measurementlocations in panel (c). (c) Two-dimensional (2D) curvature intensityplot [17] of the ARPES data along MZX Γ M recorded at 150 K andphoton energy h ν =
55 eV with HP. The LDA bands are also plottedwithout renormalization for comparison. (d) Momentum-resolvedspectral function calculated within LDA + DMFT at T =
145 K. Redcurves represent the extracted experimental band dispersions.
Figure 1(c) shows the experimental band dispersions alongthe high-symmetry lines MZX Γ M in an energy range within2.2 eV below E F . The band dispersions are much more com-plex than those of other IBSCs due to the contribution of themetallic Ti As O layers. To understand the multiband elec-tronic structure, we superimpose the Kohn-Sham band struc-ture of density functional theory within the local density ap-proximation (LDA) as calculated within Wien2k [18] on topof the experimental data in Fig. 1(c). We use the experimentalcrystal structure of Ref. [14] as input. The experimental banddispersions below –1 eV match well the LDA bands, whichare mainly derived from As 4 p of the Ti As O layers [Fig.S3 in the Supplement Materials]. Obvious discrepancies be-tween the experimental results and the LDA calculations areobserved within 1 eV below E F , where the DOS is mainly ofFe 3 d and Ti 3 d characters. The deviation can be qualitativelyattributed to non-negligible correlation e ff ects between the3 d electrons, for which the self-energy leads to strong bandrenormalizations near E F , as observed in other IBSCs [19].To analyze the e ff ects of Coulomb correlations on the elec-tronic structure, we have performed LDA + dynamical meanfield theory (DMFT) calculations of the momentum-resolvedspectral function, starting from the above Kohn-Sham bandstructure and the implementation of Ref. [20] using projectedatomic orbitals. Since our target compound contains two dif-ferent atomic species with partially filled narrow d -shells, wehave generalized the usual LDA + DMFT scheme to includee ff ective local Coulomb interactions on both the Fe-3 d andTi-3 d shells. We use U = J = and Hund’s coupling on Fe [Ti]), and a multi-orbital around mean field double counting based on the LDA elec-tron count [21]. The resulting 10-orbital many-body problemwas solved within a continuous-time Quantum Monte Carloscheme, as implemented in the TRIQS toolbox [22]. The cor-responding results are presented in Fig. 1(d), which reproducewell the experimental band dispersions. The Fe 3 d derivedbands are strongly renormalized compared with the LDA re-sults and their spectral intensities are very di ff use due to alarge quasiparticle broadening encoded in the imaginary partof the DMFT self energy. -0.6-0.4-0.20.0 E - E F ( e V ) -0.2 0.0 0.2 k y ( (cid:47) / a) (cid:75) -0.3-0.2-0.10.0 E - E F ( e V ) -0.75 -0.50 -0.25 k y ( (cid:47) / a) X -0.2 0.0 0.2 k y ( (cid:47) / a) M M X (cid:75)(cid:75) ' HPVP -0.4 -0.2 0.0 0.2 k y ( (cid:47) / a) M (a) VP cut1(c) HP cut2 (e) HP cut3(d) VP cut3 MaxMin (f) -0.4 -0.2 0.0 k y ( (cid:47) / a) (cid:75) (b) HP cut1 FIG. 2. (Color online) (a-e) 2D curvature intensity plots of the near- E F ARPES data recorded at h ν =
55 eV. The momentum locationsare indicated as red vertical lines in panel (f). To illustrate the Fe-3 d related band dispersions more clearly, the data taken at 30 K areshown in panels (a), (b) and (d). As the Ti-3 d related band disper-sions are dramatically changed across the transition at 125 K [Fig.S2 in the Supplement Materials], the data taken at 150 K are shownin panels (c) and (e). A prominent feature in the calculations is that the interca-lated Ti As O layers contribute significantly to the DOS at E F , demonstrating the metallic nature of the intercalated lay-ers in this system. To clarify the e ff ects on the low-energyelectronic states of the FeAs layers, we show the band disper-sions near E F in Fig. 2. All the cuts are parallel to the Γ Mdirection, as schematically plotted in Fig. 2(f). We identifythree hole-like bands near Γ [Figs. 2(a) and 2(b)], one hole-like band near X [Fig. 2(c)] and two electron-like bands nearM [Figs. 2(d) and 2(e)], which cross E F . The band disper-sions can be regarded as a superposition of the band structuresof the FeAs and Ti As O layers. We assign the three hole-likebands near Γ [Figs. 2(a) and 2(b)] and the shallow electron-like bands near M [Fig. 2(d)] to the Fe 3 d orbitals, while thehole-like band near X [Fig. 2(c)] and the deep electron-likeband near M [Fig. 2(e)] are attributed to the Ti 3 d orbitals.The Fe-3 d related band structure resembles those of other IB-SCs. In the IBSCs, there are generally two electron-like bandsfrom Fe 3 d near M. Only one electron-like band from Fe 3 d is observed in our experiments [Fig. 2(d)], while another onefrom Fe 3 d is not identified, most likely due to its extremelylow spectral weight as it is folded from the adjacent M due toinequivalent As sites around Fe. The Ti-3 d related band struc-ture is similar to that of BaTi As O [23]. In BaTi As O, thereare one electron-like band near Γ , one hole-like band near Xand one electron-like band near M, which cross E F . We doobserve one electron-like band with a bottom of –0.5 eV at Γ [Fig. 2(b)] and our photon energy dependence measure-ments suggest it to originate mainly from Ti 3 d states [Fig. S1in the Supplement Materials], in agreement with the observa-tion in BaTi As O [23]. The spectral intensity of this band issmeared out as dispersing towards E F , and its wave vector isestimated to be close to 0.4 π / a along Γ M by extrapolating theband dispersion to E F . (cid:75) M (b) VP k y ( (cid:47) / a ) -1.0 -0.5 0.0 0.5 k x ( (cid:47) / a) k y ( (cid:47) / a ) -1.0 -0.5 0.0 0.5 k x ( (cid:47) / a) M (cid:75) (d) Ba K Fe As -1.5-1.0-0.50.0 k y ( (cid:47) / a ) -1.0 -0.5 0.0 0.5 k x ( (cid:47) / a) (cid:75)(cid:77) M MX (c) -1.5-1.0-0.50.0 k y ( (cid:47) / a ) -1.0 -0.5 0.0 0.5 k x ( (cid:47) / a) M (cid:75) X (cid:77) M (a) HP FIG. 3. (Color online) (a) FS intensity plot of Ba22241 recorded at150 K and h ν =
55 eV with HP. The intensity is obtained by integrat-ing the spectra within ±
10 meV with respect to E F . a represents thenearest neighbor Fe(Ti)-Fe(Ti) distance. (b) Same as (a) but takenwith VP. (c) Extracted FSs of Ba22241. Red and blue symbols repre-sent the extracted k F points related to the FeAs and Ti As O layers,respectively. Solid and hollow symbols are extracted from the datataken at 150 and 30 K, respectively. Solid curves are guides for eyes.(d) FS intensity plot of Ba . K . Fe As recorded with He I α reso-nance line ( h ν = With the identification of the near- E F bands, we extract thecorresponding FSs and summarize them in Fig. 3(c). Theextracted FSs related to the Fe 3 d orbitals resemble those ofother IBSCs, indicating that the FS topology of the FeAs lay-ers is not changed by the intercalated Ti As O layers. Oneprominent feature of the Fe-related FSs is that the total en-closed area of the hole pockets near Γ is much larger thanthat of the electron pockets near M. Our k z dependent mea-surements suggest that the electronic structure of Ba22241 isquasi-2D [Fig. S1 in the Supplement Materials]. By countingthe Luttinger volume of 2D FS sheets and assuming a purely2D FS, we obtain a hole doping of ∼ . K . Fe As with a hole doping of 0.2 per Fe site, asshown in Fig. 3(d). Note that the inner hole pockets are almostdoubly-degenerate at k z ∼ . K . Fe As . By countingthe volumes of the Ti-related FS sheets, we obtain an electrondoping of ∼ k z and temperature de-pendent ARPES results reflect bulk features [Figs. S1 and S2in the Supplement Materials], thus excluding the possibilityof charge polarization on the surface. Therefore, the most sig-nificant e ff ect of the intercalation of metallic Ti As O layersis that the electrons are transferred from the FeAs layer to theTi As O layer. The “washed out” nature of the Fe-3 d bandsis then a direct consequence of the interlayer charge transfer,since the e ff ective hole-doping places this compound in theregime of strongly doping- and temperature-dependent coher-ence properties induced by Hund’s rule coupling [10]. An-alyzing further the many-body self-energies obtained withinour DMFT calculations we find qualitatively similar incoher-ent behavior for the Fe-3 d states in Ba22241 and optimallyhole-doped BaFe As .We analyze the electron transfer from Fe to Ti by compar-ing LDA and LDA + DMFT calculations for Ba22241 withexperimental crystal structure, BaFe As , Ba . K . Fe As and the compounds derived by splitting Ba22241 intoBaTi As O and BaFe As while keeping the same distanceand angle between atoms within one layer. We construct local-ized Wannier-like orbitals within the same window [–8.16 eV,8.16 eV] for all the compounds and extract the number of elec-trons, and the orbital-resolved electron counts for Fe- d andTi- d , which are displayed on Table I. For Ba . K . Fe As we have taken the band structure of BaFe As with the ex-perimental crystal structure of optimally doped BaFe As andcalculated the chemical potential such as to obtain the correcttotal number of electrons. The number of electrons in the dif-ferent orbitals of Ba22241 obtained from LDA is independentof the particular stacking structure of this material, as we cansee by comparing the calculation on Ba22241 to calculationson BaFe As and BaTi As O crystals. Two e ff ects are con-tributing to the global hole-doping of the Fe- d orbitals whengoing from BaFe As to Ba22241. The e ff ect of correlationsin BaFe As is to reduce the number of electrons in the cor-related shell. If the Fe-As hybridization is weaker, electronsare more localized and this e ff ect is enhanced. In Ba22241,the Fe-As distance is about 1% larger than in BaFe As , thuslowering the hybridization with As. The consequence canbe seen by comparing the number of electrons in the Fe- d Wannier orbitals in BaFe As in the structure of Ba22241 toBaFe As in the experimental crystal structure: There is al-ready a loss of 0.05 electrons per Fe in Ba22241. The seconde ff ect is a transfer of electrons from Fe to Ti due to correla-tions. In BaTi As O, correlations tend to increase the numberof electrons in the Ti- d orbitals. This e ff ect is enhanced whenBaTi As O and BaFe As are put together, leading to a trans-fer of about 0.1 electrons from Fe to Ti. That transfer happensnotably (but not exclusively) through the Fe- d z – Ti- d z hy-bridization, as Ti and Fe are on top of each other. We stress TABLE I. Orbital-resolved electron count in Wannier functions calculated by LDA + DMFT (LDA) for Ba22241 (1st column), BaFe As withthe same structure than in Ba22241 (2nd column), BaTi As O with the same structure than in Ba22241 (3rd column), BaFe As (4th column)and optimally doped BaFe As (5th column).Orbital Ba22241 BaFe As @Ba22241 BaTi As O@Ba22241 BaFe As Ba . K . Fe As Fe d z d x − y d xy d xz + d yz d d z d x − y d xy d xz d yz d that the band with bottom around -0.5 eV at the Γ point isof mixed Ti d z and Fe d z characters, and wrongly predictedby LDA while correctly captured by our two-correlated shellsLDA + DMFT.In conclusion, we studied the electronic structure ofBa22241 and revealed a large charge transfer between theFeAs and intercalated Ti As O layers, which is identified asa consequence of electronic correlations in the 3 d shells bythe LDA + DMFT calculations. This provides an alternativeroute of e ff ective doping without element substitution for theIBSCs and thus without introducing disorder. Furthermore,our results prove the presence of strong interlayer couplingin Ba22241 arising from the Fe- d z – Ti- d z hybridization andthe metallic nature of the intercalated Ti As O layer. It hasbeen argued that, as in the high- T c cuprate superconductors,the FeAs interlayer coupling can play a crucial role in enhanc-ing T c in the IBSCs [24]. Further studies for the IBSCs withmetallic intercalated layers may be a fruitful path for reachinghigher T c in the IBSC family.This work was supported by grants from CAS(2010Y1JB6 and XDB07000000), MOST (2010CB923000,2013CB921700, and 2011CBA001000), NSFC (11004232,11234014, and 11274362), the Cai Yuanpei program, theFrench ANR via project PNICTIDES, IDRIS / GENCI underproject 091393, the European Research Council under project617196, and the Sino-Swiss Science and Technology Coop-eration (IZLCZ2138954). This work is based in part uponresearch conducted at the Synchrotron Radiation Center,which is primarily funded by the University of Wisconsin-Madison with supplemental support from facility Users andthe University of Wisconsin-Milwaukee. ∗ These authors contribute equally to this work. † [email protected] ‡ [email protected][1] Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am.Chem. Soc. , 3296 (2008). [2] M. Rotter, M. Tegel, and D. Johrendt, Phys. Rev. Lett. ,107006 (2008).[3] A. S. Sefat, R. Y. Jin, M. A. McGuire, B. C. Sales, D. J. Singh,and D. Mandrus, Phys. Rev. Lett. , 117004 (2008).[4] S. Jiang, H. Xing, G. F. Xuan, C. Wang, Z. Ren, C. M. Feng, J.H. Dai, Z. A. Xu, and G. H. Cao, J. Phys.: Condens. Matter ,382203 (2009).[5] S. Sharma, A. Bharathi, S. Chandra, V. R. Reddy, S. Paulraj, A.T. Satya, V. S. Sastry, A. Gupta, and C. S. Sundar, Phys. Rev. B , 174512 (2010).[6] W. Schnelle, A. Leithe-Jasper, R. Gumeniuk, U. Burkhardt, D.Kasinathan, and H. Rosner, Phys. Rev. B , 214516 (2009).[7] K. Kirshenbaum, S. R. Saha, S. Ziemak, T. Drye and J. Paglione,Phys. Rev. B , 140505 (2012).[8] Z. R. Ye, Y. Zhang, F. Chen, M. Xu, J. Jiang, X. H. Niu, C. H.P. Wen, L. Y. Xing, X. C. Wang, C. Q. Jin, B. P. Xie, and D. L.Feng, arXiv:1404.6716.[9] M. Nakajima, S. Ishida, T. Tanaka, K. Kihou, Y. Tomioka, T.Saito, C. H. Lee, H. Fukazawa, Y. Kohori, T. Kakeshita, A. Iyo,T. Ito, H. Eisaki, S. Uchida, Sci. Rep. , 5873 (2014)[10] P. Werner, M. Casula, T. Miyake, F. Aryasetiawan, A. J. Millis,and S. Biermann, Nature Phys. , 331 (2012).[11] N. Xu, P. Richard, A. van Roekeghem, P. Zhang, H. Miao, W.-L. Zhang, T. Qian, M. Ferrero, A. S. Sefat, S. Biermann, and H.Ding, Phys. Rev. X , 011006 (2013).[12] G. T. Wang, Y. M. Qian, G. Xu, X. Dai, and Z. Fang, Phys. Rev.Lett. , 047002 (2010).[13] M. Aichhorn, S. Biermann, T. Miyake, A. Georges, and M.Imada, Phys. Rev. B , 064504 (2010).[14] Y. L. Sun, H. Jiang, H. F. Zhai, J. K. Bao, W. H. Jiao, Q. Tao,C. Y. Shen, Y. W. Zeng, Z. A. Xu, and G. H. Cao, J. Am. Chem.Soc , 12893 (2012).[15] H. Jiang, Y. L. Sun, J. Dai, G. H. Cao, and C. Cao,arXiv:1207.6705.[16] Y. L. Sun, A. Ablimit, J. K. Bao, H. Jiang, J. Zhou, and G. H.Cao, Sci. Technol. Adv. Mat. , 055008 (2013).[17] P. Zhang, P. Richard, T. Qian, Y. M. Xu, X. Dai and H. Ding.Rev. Sci. Instrum. // , 124512 (2011).[20] M. Aichhorn, L. Pourovskii, V. Vildosola, M. Ferrero, O. Par-collet, T. Miyake, A. Georges, and S. Biermann, Phys. Rev. B , 085101 (2009). [21] A. van Roekeghem, T. Ayral, J. M. Tomczak, M. Casula, N. Xu,H. Ding, M. Ferrero, O. Parcollet, H. Jiang, and S. Biermann,arXiv:1408.3136.[22] M. Ferrero and O. Parcollet. TRIQS: A Toolbox for Researchon Interacting Quantum Systems (2011) http: // ipht.cea.fr / triqs.[23] H. C. Xu, M. Xu, R. Peng, Y. Zhang, Q. Q. Ge, F. Qin, M. Xia, J. J. Ying, X. H. Chen, X. L. Yu, L. J. Zou, M. Arita, K. Shimada,M. Taniguchi, D. H. Lu, B. P. Xie, and D. L. Feng, Phys. Rev. B , 155108 (2014).[24] N. Ni, J. M. Allred, B. C. Chan, and R. J. Cava, Proc. Natl.Acad. Sci. , 1019 (2011). Correlation-induced self-doping in intercalated iron-pnictide superconductor Ba Ti Fe As O -Supplementary material
As seen in Fig. S1 (a)-(c), the Fermi surface near the Bril-louin zone centre exhibits an obvious warping with a periodof 2 π/ c (cid:48) , suggesting that the ARPES data reflect the bulk elec-tronic structure. This Fermi surface is assigned to the hole-likeFe 3 d xz band. This indicates that the interlayer coupling be-tween the adjacent FeAs layers is not negligible, which can beattributed to the metallic nature of the intercalated BaTi As Olayer.In Fig. S1 (d), the spectrum at the Brillouin zone centrehas two structures, which correspond to the shallow flat bandand the bottom of the deep band, respectively, in Fig.S1 (e)and (f). The spectral intensities of the shallow and deep onesare strongly suppressed at h ν =
53 and 34 eV, respectively,corresponding to the Fe 3 p -3 d and Ti 3 p -3 d antiresonances,respectively [S1]. k z ( π / c ' ) Γ ZMA .. .. I n t e n s it y ( a . u ) -0.4 -0.2 0.0 0.2 0.4 k y ( π / a) .. ΓΖ -0.8 -0.6 -0.4 -0.2 0.0 E-E F (eV) -0.4 0.0 0.4 k y ( π / a)-0.6-0.4-0.20.0 E - E F ( e V ) -0.4 0.0 0.4 k y ( π / a)98 k z ( π / c ' ) -1.0 -0.5 0.0 0.5 k y ( π / a) Γ ZMA .. .. (a)(b)(c) (d)(e)34eV (f)53eV
FIG. S1. (a) ARPES intensity plot in the k y - k z plane recorded at 25K with di ff erent photon energies from 35 to 60 eV with horizon-tal polarization. The intensity is obtained by integrating the spectrawithin ±
10 meV with respect to E F . An inner potential of 14 eV isused to obtain k z . c (cid:48) = c /
2, where c represents the lattice parame-ter perpendicular to the FeAs plane. (b) Curvature intensity plot ofthe ARPES intensity in panel (a). (c) Corresponding momentum dis-tribution curves (MDCs) at E F with di ff erent k z . (d) Valence bandspectra at the Brillouin zone center recorded with di ff erent photonenergies from 26 to 62 eV. (e) and (f) ARPES intensity plots at k x = The band dispersion around M derived from Ti 3 d ex-hibits a dramatic change around 120 K, which is consistentwith the anomaly in resistivity and magnetic susceptibility ofBa Ti Fe As O [S2]. Since both the k z and temperature de-pendent ARPES results reflect bulk features, the possibility ofcharge polarization on the surface can be excluded. -0.2-0.10.0
30K 110K-0.2-0.10.0 -0.3 0.0 0.3 k y ( π / a) E - E F ( e V ) -0.3 0.0 0.3 k y ( π / a) I n t e n s it y ( a . u ) (b) 30K H u m p B E ( m e V ) I n t e n s it y ( a . u ) -0.2 -0.1 0.0 E-E F (eV) M (c) 150K M (e) -0.2-0.10.0 E - E F ( e V ) k y ( π / a) edc30k mdc30kmdc150k edc150k M Γ ' 30K (d) FIG. S2. (a) Two-dimension curvature intensity plots of the cuts at M( k x = π/ a ) at various temperatures between 30 and 150 K. (b) and (c)Corresponding energy distribution curves (EDCs) at 30 and 150 K,respectively. (d) Band dispersions taken from EDCs and MDCs at 30and 150 K. (e) Binding energy of the top of the hump as a functionof temperature. XZ MM Γ -2.5-2.0-1.5-1.0-0.50.00.51.0 E - E F ( e V ) M MZ X Γ -2.5-2.0-1.5-1.0-0.50.00.51.0 E - E F ( e V ) k // ( π / a) 2.01.51.00.50.0-0.5-1.0 k // ( π / a) (a) Fe d (c) As1 p (b) Ti d (d) As2 p MaxMin
FIG. S3. (a)-(d) Contributions of the Fe 3 d , Ti 3 d , As1 4 p in theFeAs layers, and As2 4 p in the Ti As O layers to the LDA calculatedband structure of Ba Ti Fe As O, respectively. The contribution isrepresented by both the symbol size and the color scale. ∗ These authors contribute equally to this work. † [email protected] ‡ [email protected][S1] H. Ding, K. Nakayama, P. Richard, S. Souma, T. Sato, T. Taka-hashi, M. Neupane, Y.-M. Xu, Z.-H. Pan, A. V. Fedorov, Z.Wang, X. Dai, Z. Fang, G. F. Chen, J. L. Luo, and N. Wang, J.Phys.: Condens. Matter , 135701 (2011).[S2] Y.-L. Sun, H. Jiang, H.-F. Zhai, J.-K. Bao, W.-H. Jiao, Q. Tao,C.-Y. Shen, Y.-W. Zeng, Z.-A. Xu, and G.-H. Cao, J. Am.Chem. Soc.134