We performed a high-resolution angle-resolved photoemission spectroscopy study of the Tl0.63K0.37Fe1.78Se2 superconductor (Tc = 29 K). We show the existence of two electronlike bands at the M(π, 0) point which cross the Fermi level at similar Fermi wave vectors to form nearly circular electronlike Fermi surface pockets. We observe a nearly isotropic ∼ 8.5 meV superconducting gap (∆/kBTc ∼ 7) on these Fermi surfaces. Our analysis of the band structure around the Brillouin zone centre reveals two additional electronlike Fermi surfaces: a very small one and a larger one with kF comparable to the FS pockets at M. Interestingly, a SC gap with a magnitude of ∼ 8 meV also develops along the latter FS. Our observations are consistent with the s-wave strong coupling scenario.PACS numbers: 74.25.Jb, 74.70.Xa, The amplitude and symmetry of the superconducting (SC) gap of a material are determined by its band structure, its Fermi surface (FS) topology and the pairing mechanism itself. The experimental observation of enhanced gap amplitude on holelike and electronlike FS pockets quasi-nested by the antiferromagnetic (AF) wave vector in iron-based superconductors [1][2][3][4][5] has been widely considered as suggestive of the importance of AF interband scattering in these materials. In particular, the quasi-nesting model is consistent with the strong suppression of superconductivity in heavily hole-doped [6] and heavily electron-doped [7] BaFe 2 As 2 compounds, for which the FS quasi-nesting conditions vanish. Recently, this model faced a serious challenge with the discovery of superconductivity above 30 K in heavily electron-doped K 0.8 Fe 2−x Se 2 and (Tl,K)Fe 2−x Se 2 [8,9]. Indeed, previous angle-resolved photoemission spectroscopy (ARPES) measurements revealed only electronlike FS pockets [10,11].In this letter, we report high-energy resolution ARPES measurements on the Tl 0.63 K 0.37 Fe 1.78 Se 2 superconductor (T c = 29 K). We observed two electronlike M(π, 0)centred FS pockets that develop a nearly isotropic SC gap below T c with a magnitude of ∼ 8.5 meV, leading to a 2∆/k B T c of ∼ 7. In addition, a weak electronlike FS pocket with a similar size and a tiny electronlike pocket are also observed at the Γ(0, 0) point. The former one also exhibits a SC gap size of about 8 meV. In addition, a high-energy (∼ 0.8 eV) incoherent peak undergoes a significant energy shift of ∼ 100 meV through the metalnonmetal crossover around 70K, while the low-energy valence band shows little change. We discuss the possible implications of the SC gap symmetry and the FS topology for the SC pairing mechanism in this unusual iron- * Electronic address: dingh@iphy.ac.cn based superconductor .Single crystals of Tl 0.63 K 0.37 Fe 1.78 Se 2 (T onset c = 29.1 K; T mid c = 28.6 K; T zero c = 27.5 K) were grown by the Bridgeman method [9]. The precise composition was determined using an Energy Dispersive X-ray Spectrometer (EDXS). The lattice parameters a = 3.85Å and c = 14.05Å were obtained by fitting XRD data. We performed ARPES measurements at th...

The momentum distribution of the energy gap opening at the Fermi level of superconductors is a direct fingerprint of the pairing mechanism. While the phase diagram of the iron-based superconductors promotes antiferromagnetic fluctuations as a natural candidate for electron pairing, the precise origin of the interaction is highly debated. We used angle-resolved photoemission spectroscopy to reveal directly the momentum distribution of the superconducting gap in FeTe 1-x Se x , which has the simplest structure of all iron-based superconductors. We found isotropic superconducting gaps on all Fermi surfaces whose sizes can be fitted by a single gap function derived from a strong coupling approach, strongly suggesting local antiferromagnetic exchange interactions as the pairing origin.

We report an investigation with angle-resolved photoemission spectroscopy of the Fermi surface and electronic band structure of BaCo 2 As 2 . Although its quasinesting-free Fermi surface differs drastically from that of its Fe-pnictide cousins, we show that the BaCo 2 As 2 system can be used as an approximation to the bare unoccupied band structure of the related BaFe 2Àx Co x As 2 and Ba 1Àx K x Fe 2 As 2 compounds. However, our experimental results, in agreement with dynamical-mean-field-theory calculations, indicate that electronic correlations are much less important in BaCo 2 As 2 than in the ferropnictides. Our findings suggest that this effect is due to the increased filling of the electronic 3d shell in the presence of significant Hund's exchange coupling. DOI: 10.1103/PhysRevX.3.011006 Subject Areas: Condensed Matter Physics, Strongly Correlated Materials, SuperconductivityAlthough the role they play in ferropnictide superconductivity is still not settled, there is sufficient evidence indicating that Fe-based superconductor systems exhibit non-negligible electronic correlations. Indeed, previous angle-resolved-photoemission-spectroscopy (ARPES) studies reported typical overall bandwidth renormalization of a factor of about 2-5 [1]. Interestingly, Hund's-rule coupling was identified as an efficient tuning parameter for electronic correlations [2][3][4][5], indicating the importance of local moments in the physics of these materials. Very recently [6], a study of hole-doped BaFe 2 As 2 evidenced an unusual non-Fermi-liquid behavior with frozen moments in the paramagnetic phase. The doping-temperature phase diagram of these materials exhibits exotic fractional power-law behavior of the many-body self-energies consistent with the phase diagram of the ''spin-freezing'' scenario [7], where fractional behaviors occur in some temperature range at fillings close to half-filling. (For a review, see Ref.[8].) Following this logic, one would expect the strength of the electronic correlations to vary significantly when the electronic structure is tuned away from the d 6Àx filling of hole-doped Ba 1Àx K x Fe 2 As 2 , with reduced correlation effects when the d 7 filling is reached.An experimental characterization of the d 7 filling state is thus called for.In this work, we report ARPES results on BaCo 2 As 2 with a d 7 filling. We determine its Fermi surface (FS) and electronic band structure by using polarized photons over a wide energy range. To quantify the strength of the electronic correlations, we compare the overall bandwidth found experimentally with the one predicted by our local-densityapproximation (LDA) calculations, which ignore the electronic correlations. We find that the overall band structure is only 1.4 times narrower than predicted by our LDA calculations, indicating much weaker correlation effects than in its cousin BaFe 2 As 2 , for which a renormalization factor of 3 was reported [9]. We have performed dynamical-mean-fieldtheory (DMFT) calculations, which include the electronic correlations, a...

No abstract

We report the observation by angle-resolved photoemission spectroscopy of an impurity state located inside the superconducting gap of Ba 0.6 K 0.4 Fe 2 As 2 and vanishing above the superconducting critical temperature, for which the spectral weight is confined in momentum space near the Fermi wave-vector positions. We demonstrate, supported by theoretical simulations, that this in-gap state originates from weak scattering between bands with opposite sign of the superconducting-gap phase. This weak scattering, likely due to off-plane nonmagnetic (Ba, K) disorder, occurs mostly among neighboring Fermi surfaces, suggesting that the superconducting-gap phase changes sign within holelike (and electronlike) bands. Our results impose severe restrictions on the models promoted to explain high-temperature superconductivity in these materials.

No abstract

We report a systematic angle-resolved photoemission spectroscopy study on Ba(Fe1−xRux)2As2 for a wide range of Ru concentrations (0.15 ≤ x ≤ 0.74). We observed a crossover from twodimension to three-dimension for some of the hole-like Fermi surfaces with Ru substitution and a large reduction in the mass renormalization close to optimal doping. These results suggest that isovalent Ru substitution has remarkable effects on the low-energy electron excitations, which are important for the evolution of superconductivity and antiferromagnetism in this system. PACS numbers: 74.70.Xa, 71.18.+y, 74.25.Jb, Superconductivity in the iron-based materials usually emerges from a magnetic state by several kinds of routes leading to very similar phase diagrams of magnetism and superconductivity. In Ba 1−x K x Fe 2 As 2 [1] and Ba(Fe 1−x Co x ) 2 As 2 [2], the introduction of extra hole or electron carriers shifts the chemical potential so that the sizes of the hole and electron Fermi surface (FS) pockets evolve oppositely [3], which eventually suppresses the nesting between the hole and electron FS pockets that play a role in the formation of spin-density-wave (SDW) with exotic Dirac cone dispersion [4] in the parent compound. While it is generally believed that external pressure also changes the FS topology by modifying the chemical bonds [5], the role of isovalent element substitution is still debated. Various scenarios, for example changes of the FS topology by chemical pressure [6][7][8], the reduction of electron correlations [8,9], magnetic dilution [10], and even the addition of extra hole carriers [11], have been suggested to explain the suppression of the SDW order with isovalent element substitution in the BaFe 2 (As 1−x P x ) 2 and Ba(Fe 1−x Ru x ) 2 As 2 systems. Surprisingly, only little attention has been devoted to answer the reversed but somehow similarly important question: how does superconductivity is suppressed by increasing the substitution further than the optimal concentration?Since single-crystals can be grown for the entire phase diagram, the Ba(Fe 1−x Ru x ) 2 As 2 system is ideal to investigate the suppression of the SDW order, the emergence of superconductivity and its disappearance with isovalent-substitution. We expect that the electronic structure near the Fermi level (E F ) be substantially modified by the Ru substitution. Indeed, the isovalent Ru substitution at the Fe site leads to an anisotropic lattice distortion, resulting in a strong increase of the As-Fe(Ru)-As bond angle and a decrease of the As height from the Fe(Ru) plane [6,12,13]. The Hall coefficient, which is always negative and decreases with decreasing temperature in the parent compound BaFe 2 As 2 , increases with decreasing temperature in the Ru-substituted samples and even changes sign for large Ru concentrations [13]. With its capacity to resolve dispersive electronic states in the vicinity of E F , angleresolved photoemission spectroscopy (ARPES) is a powerful tool to determine which parameters drive the system from a SDW orde...

Abstract. In this study, we report on the development of a lamp-based vacuum ultraviolet photoionization mass spectrometer (VUV-PIMS) in our laboratory; it is composed of a radio-frequency-powered VUV lamp, a VUV photoionizer, an ion-migration lens assembly, and a reflection timeof-flight mass spectrometer. By utilizing the novel photoionizer consisting of a photoionization cavity and a VUV light baffle, the baselines of the mass spectra decreased from 263.6 ± 15.7 counts to 4.1 ± 1.8 counts. A detection limit (2σ ) of 3 pptv was achieved for benzene after an acquisition time of 10 s. To examine its potential for real-time monitoring applications of samples, the developed VUV-PIMS was employed for the continuous measurement of urban air for 6 days in Beijing, China. Strong signals of trace-level volatile organic compounds, such as benzene and its alkylated derivatives, were observed in the mass spectra. These initial experimental results reveal that the instrument can be used for the online monitoring of trace-level species in the atmosphere.