Unconventional pairing mechanism remains an important unsolved problem in condensed matter physics. The newly discovered iron-arsenic (pnictide) superconductors have joined the copper oxide (cuprate) superconductors into the category of high-temperature superconductors. Their differences and similarities in the superconducting phase are essential problems to be tackled to understand the high-temperature superconductivity.　 
　  A collaboration organized by Professor Hong Ding of the Institute of Physics (IOP), Chinese Academy of Sciences (CAS), performed a comprehensive angle-resolved photoemission spectroscopy (ARPES) investigation of p-doped iron pnictide materials, Ba_1-xK_xFe₂As₂ (barium potassium iron arsenic). For the first time, they observed that the superconducting gap scales linearly with the transition temperature, and that a distinct pseudogap coexisting with the superconducting gap develops upon underdoping. This pseudogap occurs on Fermi surface sheets connected by the antiferromagnetic (AF) wave vector, where the superconducting pairing is stronger as well. Earlier, a ubiquitous gap function obtained by decoupling three-dimensional next-nearest-neighbor AF exchange coupling within the pairing channel has been introduced to describe all superconducting gaps with both intralayer and interlayer pairing energy scales only. Their results are the strongest evidence yet that short-range AF fluctuations drive both the pseudogap and superconductivity in iron-based superconductors, raising the possibility for a unifying picture of high-temperature superconductors involving AF fluctuations. 
　  “This is a significant milestone in the way we search for the solution of the pairing mechanism in high-temperature superconductivity,” says Yiming Xu, leading author of the paper in the July 12 issue of the online journal Nature Communications that describes the team’s findings. “A possible unifying picture in both pnictides and cuprates might direct an effort to achieve unconventional superconductivity in these and other materials at even higher temperatures.” When the research was done, Yiming was a member of Boston College’s Department of Physics and he is now a postdoctoral fellow of the Materials Sciences Division (MSD) of Lawrence Berkeley National Laboratory (LBNL).

　  Why underdoped pnictides?

　  A remarkable similarity between pnictides and cuprates is that both of them have a superconducting dome emerging from an AF phase when adding extra carriers (dopants), suggesting that AF fluctuations may play a role in the superconducting pairing mechanism. In underdoped cuprates, in addition to the superconducting gap, a large doping-dependent pseudogap above superconducting transition temperature (T_c) exists at the antinodes and its origin is still undergoing an intensive debate. “We can investigate the relationship between AF fluctuations and superconductivity in the underdoped pnictides,” says Yiming Xu, “to determine whether high-temperature superconductivity in cuprates and pnictides can possibly share the same mechanism.” 

　  What controls the pairing strength?

　  Superconductors are characterized by the size and the symmetry of the superconducting gap. Upon hole-underdoping, all Γ-centered hole-like Fermi surfaces (FSs) shrink, while M-centered electron-like FSs expand. High-resolution temperature-dependent ARPES measurements reveal that the superconducting gap on each FS sheet of underdoped Ba_0.75K_0.25Fe₂As₂ (T_c = 26 K) remains isotropic, indicating a s-wave pairing symmetry. The gap value (Δ) is reduced by approximately 30% as compared with the optimally doped Ba_0.6K_0.4Fe₂As₂ (T_c = 37 K) samples, which is almost the same as the reduction of the transition temperature (from 37 to 26 K). Furthermore, the values of 2Δ scale linearly as function of T_c from moderate underdoping to optimal doping (as shown in the inset). Thus, T_c is determined by the energy scale of the pairing gap within this range. The ratio 2Δ/k_BT_c varies from typical Bardeen-Cooper-Schrieffer (BCS) values (for the β FS) to strong coupling values (~7) on the FS sheets connected by the AF wave vector (α and electron-like FSs), behaving FS-dependently but doping-independently.
　  AF fluctuations inducing pairing has been also reported by the same group in their previous paper published in the March issue of the journal Nature Physics by using kz-tunable ARPES to study the optimally doped samples. Superconducting gaps on the hole-like FSs disperse significantly as a function of the photon energy (kz) but not to zero, while the ones on the electron-like FSs remain at a nearly constant energy (as shown in the right panel). But all gap values on different FS sheets can be fit with a simple three-dimensional gap function with only two variables, the in-plane pairing and the out-of-plane pairing. This gap function is derived from the next-nearest-neighbor AF exchange couplings, taking into account the s-wave symmetry. “Our findings strongly demonstrate that there is no node along the kz direction in this material”, says Yiming, the leading author of this paper as well, “and suggest that the pairing in pnictides is mediated by short-range AF fluctuations.”

　  What is the origin of the Pseudogap?

　  In the superconducting phase, other than the superconducting gap, a second energy gap (~ 18 meV) emerges in the ARPES spectra on the α FS. And this gap survives in the normal state. It gradually fills up as temperature increases and eventually closes around T* = 115 ~ 125 K. “Motivated by the similar behavior observed in the underdoped cuprates”, says Yiming, “we refer to this gap as a pseudogap.” Interestingly, there is no sign of such pseudogap feature on the β FS. Such momentum dependence of the pesudogap can be naturally associated with the (π, π) interband scattering that exists in the underdoped region in iron-pnictides. Within this scenario, the quasiparticle (QP) spectral weight should exhibit a similar FS dependence, which has been observed in high-resolution ARPES results. As known, the underdoped cuprates also exhibit a momentum dependent QP weight suppression. The nodal QP remains robust, while the antinodal QP suppresses quickly with underdoping, which is so-called nodal-antinodal dichotomy. This is because the QPs can be scattered strongly by spin fluctuations via the (π, π) AF wave vector.

　  What do underdoped pnictides hint us?

　  Unlike cuprates, pnictides have more than one FS. It is easy to distinguish all observed FS sheets of two different types according to the results on underdoped pnictides: (i) quasi-nested FSs are accompanied by large superconducting gaps, the formation of a pseudogap, and a strong QP spectral weight suppression upon underdoping; (ii) FSs that are not quasi-nested are accompanied by a smaller superconducting gap, the absence of pseudogap, and a quite robust QP upon underdoping. Also, FS in cuprates can be similarly attributed to two sections: the antinodal and the nodal section of the Fermi surface. Such dichotomous behaviors in underdoped pnictides and underdoped cuprates leave the possibility for a unifying picture where AF fluctuations mediate the superconducting pairing and contribute to the origin of the pseudogap in high-temperature superconductors. 
　  ARPES measurements were performed at Professor Takashi Takahashi’s group at Tohoku University, Japan and at beamline SIS of Swiss Light Source (SLS), Switzerland supported by Dr. Ming Shi. The project was collaborated with Professor Nanlin Wang, the director of EX Division at IOP, CAS, Professor Haihu Wen, a former member of the National Lab for Superconductivity (SC) at IOP, CAS, Professor Pengcheng Dai, a faculty member of Department of Physics at the University of Tennessee and the a member of SC Division at IOP, CAS, Professor Genfu Chen, a member of SC Division at IOP, CAS, Professor Zhong Fang and Professor Xi Dai, members of Laboratory of Condensed Matter Theory and Materials Computation (T) at IOP, CAS, Professor Jiangping Hu, a faculty member of Department of Physics at Purdue University, USA and a member of T Division at IOP, CAS, and Professor Ziqiang Wang, a faculty member of Department of Physics at Boston College, USA. “This is a team work,” says Professor Hong Ding, “We could not make it without internal support from IOP and international collaborations, which all belong to one of the best groups in the field.”
　  “Observation of a ubiquitous three-dimensional superconducting gap function in optimally doped Ba_0.6K_0.4Fe₂As₂”, by Yiming Xu, Yaobo Huang, Xiaoyu Cui, Elia Razzoli, Milan Radovic, Ming Shi, Genfu Chen, Ping Zheng, Nanling Wang, Chenglin Zhang, Pengcheng Dai, Jiangping Hu, Ziqiang Wang, and Hong Ding, appears in Nature Physics 7, 198 (2011). This work was supported by grants from the Chinese Academy of Sciences, NSF, the Ministry of Science and Technology of China, NSF, DOE of US, and the Sino-Swiss Science and Technology Cooperation.
　  “Fermi surface dichotomy of the superconducting gap and pseudogap in underdoped pnictides”, by Yiming Xu, Pierre Richard, Kosuke Nakayama, Yoichi Sekiba, Takuma Kawahara, Tian Qian, Madhab Neupane, Seigo Souma, Takafumi Sato, Takashi, Takahashi, Huiqian Luo, Haihu Wen, Genfu Chen, Nanlin Wang, Ziqiang Wang, Zhong Fang, Xi Dai, and Hong Ding, appears in Nature Communications 2, 392 (2011). This work was supported by grants from the Chinese Academy of Sciences, NSF, Ministry of Science and Technology of China, TRiP-JST, CREST-JST, JSPS and MEXT of Japan, and NSF, DOE of US.

Left panel: a schematic phase diagram for Ba1-xKxFe2As2, where PG is the pseudogap region observed in the underdoped region. The scaling behavior between the superconducting gaps and the transition temperatureTcin the underdoped regime is shown in the inset. Right panel: Superconducting gaps of Ba0.6K0.4Fe2As2on three-dimensional Fermi surface sheets.