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Fermi Surface Dichotomy of the Superconducting Gap and Pseudogap in Underdoped Pnictides

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Fermi Surface Dichotomy of the Superconducting Gap and Pseudogap in Underdoped Pnictides ARTICLE Received 3 Jun 2010 | Accepted 16 Jun 2011 | Published 12 Jul 2011 DOI: 10.1038/ncomms1394 Fermi surface dichotomy of the superconducting gap and pseudogap in underdoped pnictides Y.-M. Xu1,2, P. Richard3,4, K. Nakayama5, T. Kawahara5, Y. Sekiba5, T. Qian3,5, M. Neupane1, S. Souma4, T. Sato5,6, T. Takahashi4,5, H.-Q. Luo3, H.-H. Wen3, G.-F. Chen3,7, N.-L. Wang3, Z. Wang1, Z. Fang3, X. Dai3 & H. Ding3 High-temperature superconductivity in iron-arsenic materials (pnictides) near an anti- ferromagnetic phase raises the possibility of spin-fluctuation-mediated pairing. However, the interplay between antiferromagnetic fluctuations and superconductivity remains unclear in the underdoped regime, which is closer to the antiferromagnetic phase. Here we report that the superconducting gap of underdoped pnictides scales linearly with the transition temperature, and that a distinct pseudogap coexisting with the superconducting gap develops on under- doping. This pseudogap occurs on Fermi surface sheets connected by the antiferromagnetic wavevector, where the superconducting pairing is stronger as well, suggesting that anti- ferromagnetic fluctuations drive both the pseudogap and superconductivity. Interestingly, we found that the pseudogap and the spectral lineshape vary with the Fermi surface quasi-nesting conditions in a fashion that shares similarities with the nodal–antinodal dichotomous behaviour observed in underdoped copper oxide superconductors. 1 Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, USA. 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkley, California 94720, USA. 3 Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. 4 WPI Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. 5 Department of Physics, Tohoku University, Sendai 980-8578, Japan. 6 TRiP, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan. 7 Department of Physics, Renmin University of China, Beijing 100872, China. Correspondence and requests for materials should be addressed to H.D. (email: [email protected]). NATURE COMMUNICATIONS | 2:392 | DOI: 10.1038/ncomms1394 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1394 he newly discovered iron arsenic (pnictide) superconduc- a b c tors1–4 have joined the copper oxide (cuprate) superconduc- T = 45 K T = 45 K T = 50 K 40 K 35 K 40 K Ttors in the category of high-temperature superconductors 35 K 30 K 30 K 30 K (HTCSs). Superconductors are characterized by the size and the ) ) ) 25 K 25 K symmetry of the superconducting (SC) gap that forms below a criti- 25 K 20 K 20 K cal temperature Tc. In optimally hole-doped cuprates, the SC gap 15 K 20 K 15 K 5 10 K function has a d-wave symmetry in momentum (k) space . Its size 15 K 10 K evolves smoothly along the Fermi surface (FS) from zero at locations Intensity (a.u. 10 K Intensity (a.u. Intensity (a.u. 45 K 10 K 5 K referred to as nodes to a large maximum ∆ at the ‘antinodes’ that 45 K (Smoothed) 6,7 puts the value of 2∆/kBTc in the strong coupling regime . In con- α β γ,δ trast, angle-resolved photoemission spectroscopy (ARPES) studies 20 0 –20 20 0 –20 20 0 –20 E (meV) E (meV) E (meV) on optimally hole-doped pnictide Ba0.6K0.4Fe2As2 (Tc = 37 K; labelled B B B as OPD37K), have revealed isotropic gaps that have different values 8–11 d e BCS on different FSs , with strong pairing occurring on the quasi- (or OPD37K α band band nearly) nested FSs. The properties associated with the quasi-nested UD26K 25 β FSs have been described in a previous theoretical work12. γ,δ band A remarkable resemblance between these two HTCS systems is 1 X M γ 20 the emergence of their SC domes from an antiferromagnetic (AF) 13,14 (meV) ) l phase when adding extra carriers (dopants), suggesting that a / δ ∆ l π 15 ( 2 AF fluctuations may have a role in the SC pairing mechanism. For y k this reason, much attention has been devoted in the past to under- β α doped (UD) cuprates. In addition to the SC gap, evidence for a large 10 0 Γ X doping-dependent pseudogap (PG) above Tc at the antinodes of UD 15,16 cuprates has existed for some time . Nevertheless, debates on the 5 interplay between superconductivity, antiferromagnetism and PG in cuprates are ongoing. Using the recently discovered pnictides, it is now possible to look into this fundamental issue from a dif- 0 1 0 10 20 30 40 ferent perspective. Unfortunately, there is no ARPES report on the kx (π/a) Tc (K) UD region of hole-doped pnictides investigating the relationship Figure 1 | SC gaps and FS of UD pnictides. (a–c) show the T dependence between AF fluctuations and superconductivity. Thus, it is particu- of the symmetrized energy distribution curves (EDCs) of Ba0.75K0.25Fe2As2 larly important to conduct a comparison study of optimally doped (T = 26 K) divided by a smoothed spectrum recorded above T (45–50 K) on (OPD) and UD pnictides to determine whether high-temperature c c the α, β and γ,δ FSs, respectively. The corresponding momentum locations superconductivity in cuprates and pnictides can possibly share the are given in the inset of (c) (red, blue and green dots for the α, β and γ,δ same mechanism. FSs, respectively). Dashed lines indicate the QP peak position at low T, Here we focus mainly on underdoped Ba K Fe As (T = 26 K; 0.75 0.25 2 2 c which can be regarded as the SC gap value. We find∆ (0) = 8.5 meV, labelled as UD26K). We observe that the SC gaps scale with T in α c ∆ (0) = 4.0 meV and ∆ (0) = 7.8 meV. The raw curves obtained at the UD regime, suggesting that the gap size is controlled by the pair- β γ/δ 10 K (blue) and 45 K (red) for the α FS, as well as the smoothed 45 K ing strength. In addition, we reveal the existence of a 18 meV pseu- (black dashed), are shown at the bottom of panel (a) as examples. (d) dogap on a quasi-nested FS pocket that coexists in momentum with Comparison of FS contours between the OPD37K and UD26K samples. the SC gap at low temperature but survives up to T* = 115 − 125 K, The dots indicate Fermi wave vectors extracted from ARPES measurements temperature at which it completely fills in. Its band-selectivity above T by tracing the peak position of momentum distribution curves strongly suggests an origin related to AF fluctuations. Moreover, c (MDCs). (e) Full SC gap values (2∆) versus T . The dashed red, blue and the evolution of the spectral lineshape on underdoping indicates a c green lines are linear fits of the SC gaps on the α, β and γ,δ FSs passing dichotomy behaviour between unnested and quasi-nested FSs that through the origin. The dashed black line is the BCS line with a slope of shows striking similarities with the nodal–antinodal dichotomy 3.52 kB. We defined the vertical error bars as twice the derivation that found in cuprates, and that is naturally explained in pnictides in we get by fitting the SC coherent peaks with Lorentzien functions. The terms of short-range AF scattering. horizontal error bars represent the transition width measured by transport experiments. Results Superconducting gaps in underdoped pnictides. As illustrated in Figure 1d, the FS of UD26K differs from that of the OPD material the effect of the Fermi function and then normalized by the previously reported8,9,11,17 only by the size of the holelike and corresponding normal state spectra. We clearly observe the opening electronlike FS pockets, which become, respectively, smaller and of SC gaps on all the FS sheets at low T and their disappearance larger in UD26K and satisfy the Luttinger theorem (Supplementary above Tc. The SC gap value ∆( ) on each FS sheet of the UD26K Discussions for the electron counting). Similarly to the OPD samples is reduced by approximately 30% as compared with the sample, the quasi-nesting condition between the inner hole-FS OPD37K samples8,11, which is almost the same as the reduction of (α) and the electron-FSs is maintained, albeit slightly weakened the transition temperature (from 37 to 26 K). Interestingly, the 2∆ in the UD samples. We performed high-resolution measurements values vary linearly with Tc from moderate underdoping to optimal on all the FS pockets observed. Below Tc, we observe the opening doping, as illustrated in Figure 1e. Thus, Tc is determined by the of SC gaps at their corresponding Fermi vectors (kF). As with the energy scale of the pairing gap within this range. The FS-dependent 8,9,11 OPD compound , these gaps are isotropic within experimental but doping-independent 2∆/kBTc ratio varies from typical BCS uncertainties (Supplementary Discussion for the superconducting values (for the β FS) to strong coupling values on the quasi-nested gap symmetry). To characterize these SC gaps further, temperature FS sheets. A good agreement between the Tc (doping) dependence (T)-dependent ARPES measurements have been performed on of Hc2 deduced from ARPES and transport measurements supports different FS sheets (α, β and γ,δ). We show in Figure 1a–c spectra the bulk nature of the linear scaling of ∆ and Tc (Supplementary at various kF positions, which have been symmetrized to remove Figure S2). It reinforces the notion that Tc in UD pnictides is NATURE COMMUNICATIONS | 2:392 | DOI: 10.1038/ncomms1394 | www.nature.com/naturecommunications © 2011 Macmillan Publishers Limited.
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