Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-T(c)) copper oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at approximately 137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-T(c) copper oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.

We have performed high-resolution angle-resolved photoemission spectroscopy on heavily overdoped KFe2As2 (transition temperature Tc = 3 K). We observed several renormalized bands near the Fermi level with a renormalization factor of 2-4. While the Fermi surface (FS) around the Brillouin-zone center is qualitatively similar to that of optimally-doped Ba1−xKxFe2As2 (x = 0.4; Tc = 37 K), the FS topology around the zone corner (M point) is markedly different: the two electron FS pockets are completely absent due to excess of hole doping. This result indicates that the electronic states around the M point play an important role in the high-Tc superconductivity of Ba1−xKxFe2As2 and suggests that the interband scattering via the antiferromagnetic wave vector essentially controls the Tc value in the overdoped region.PACS numbers: 74.70. Dd, 71.18.+y, 74.25.Jb, 79.60.Bm The discovery of superconductivity at 26 K [1] (43 K under high pressure [2]) in LaFeAsO 1−x F x , has triggered intensive researches on the high-temperature (T c ) superconductivity of iron (Fe) pnictides. The T c value has already exceeded 55 K by replacing La atom with other rare-earth atoms or by introducing oxygen vacancies [3,4], opening a new avenue for high-T c material research beside cuprates. Remarkable aspects of the FeAs-based superconductors are (i) electrons in the Fe orbitals, generally believed to be the foe, indeed play an essential role in superconductivity [1,5,6], (ii) non-doped parent compounds commonly exhibit a collinear antiferomagnetic (AF) spin density wave (SDW) [7,8], and (iii) the superconductivity emerges either by the hole or electron doping into the parent compounds [1,9]. To elucidate the mechanism of high-T c superconductivity in terms of the electronic structure, angle-resolved photoemission spectroscopy (ARPES) has been performed on both holeand electron-doped compounds in the optimally-and non(under)-doped region [6,10,11,12,13,14,15,16,17] and it clarified key features on the band structure, the FS topology, and the superconducting gap. On the other hand, little is known about the electronic states in the overdoped region. As demonstrated by electrical resistivity measurements, the T c value of the holedoped Ba 1−x K x Fe 2 As 2 monotonically decreases from the optimally-doped region (T c = 37 K) upon hole doping but does not completely disappear even at the highest doping level (x = 1.0; T c ∼3 K) [18,19], unlike the overdoped cuprates. The resistivity does not show SDW-related anomalies in the overdoped region [19]. Clarifying the microscopic origin of this T c reduction would be a key to find an essential ingredient to achieve high-T c values in the iron-based superconductors. It is thus of particular importance to gain insight into the band structure and the FS by performing ARPES measurements on overdoped samples and directly compare the electronic states with the optimally-doped ones for a comprehensive understanding of the high-T c mechanism.In this Letter, we report high-resolution ARPES results on KFe 2 A...

We performed optical spectroscopy measurement on single crystals of BaFe2As2 and SrFe2As2, the parent compounds of FeAs-based superconductors. Both are found to be quite metallic with fairly large plasma frequencies at high temperature. Upon entering the spin-density-wave state, the formation of partial energy gaps was clearly observed with the surprising presence of two different energy scales. A large part of the Drude component was removed by the gapping of Fermi surfaces. Meanwhile, the carrier scattering rate was even more dramatically reduced. We elaborate that the spin-density-wave instability is more likely to be driven by the Fermi surface nesting of itinerant electrons than a local-exchange mechanism.

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We report resistivity measurements performed on KFe 2 As 2 single crystals down to T = 0.3 K and in magnetic fields up to 17.5 T. The in-plane resistivity vs. T curve has a convex shape down to ∼50 K and shows a T 2 dependence below ∼45 K. The ratio of the c-axis to in-plane resistivities is ∼10 at room temperature and ∼40 at 4.2 K. The superconducting

We present a systematic angle-resolved photoemission spectroscopic study of the high-Tc superconductor class ͑Sr/ Ba͒ 1−x K x Fe 2 As 2 . By utilizing a photon-energy-modulation contrast and scattering geometry we report the Fermi surface and the momentum dependence of the superconducting gap, ⌬͑k ជ ͒. A prominent quasiparticle dispersion kink reflecting strong scattering processes is observed in a binding-energy range of 25-55 meV in the superconducting state, and the coherence length or the extent of the Cooper pair wave function is found to be about 20 Å, which is uncharacteristic of a superconducting phase realized by the BCS-phonon-retardation mechanism. The observed 40Ϯ 15 meV kink likely reflects contributions from the frustrated spin excitations in a J 1 -J 2 magnetic background and scattering from the soft phonons. Results taken collectively provide direct clues to the nature of the pairing potential including an internal phase-shift factor in the superconducting order parameter which leads to a Brillouin zone node in a strong-coupling setting. DOI: 10.1103/PhysRevB.78.184508 PACS number͑s͒: 74.20.Mn, 74.25.Jb, 74.70.Ϫb, 74.72.Ϫh The recent discovery of superconductivity ͑T c up to 55 K͒ in iron-based layered compounds promises a new route to high-temperature superconductivity. 1-3 This is quite remarkable in the view that T c in the pnictides is already larger than that observed in the single-layer cuprates. Preliminary studies suggest that the superconducting state in these materials competes with a magnetically ordered state, and the proper description of the ordered state lies somewhere in between a strong correlation-mediated local-moment magnetism and quasi-itineracy with stripelike frustration. [3][4][5][6][7][8][9][10][11][12][13][14][15] This calls for a microscopic investigation of pair formation and related electron dynamics in these superconductors. Angle-resolved photoemission spectroscopy ͑ARPES͒ is a powerful tool for investigating the microscopic electronic behavior of layered superconductors. 16 In this work we report electronic structure results focusing on the details of the low-lying quasiparticle dynamics on very high-quality ͑␦T c Շ 1 K and surface flatness root mean square ϳ1 Å͒ single domain single-crystal samples, which allow us to gain insight into connections between the superconductivity and magnetism. We observe that the electrons are strongly scattered by collective processes around the 15-50 meV binding-energy range depending on the Fermi-surface ͑FS͒ sheet while a magnitude-oscillating gap structure persists nearly along the spin density wave ͑SDW͒ vector of the parent compound. We also show that a Cooper pair in this superconductor is strongly bound ͑ Շ4a o ͒. Our overall results can be self-consistently interpreted in a phase-shifting order-parameter scenario.ARPES measurements were performed using 18-60 eV photons with better than 8-15 meV energy resolution, respectively, and overall angular resolution better than 1% of the Brillouin zone ͑BZ͒. Most of the data wer...

We use neutron scattering to study the structural distortion and antiferromagnetic ͑AFM͒ order in LaFeAsO 1−x F x as the system is doped with fluorine ͑F͒ to induce superconductivity. In the undoped state, LaFeAsO exhibits a structural distortion, changing the symmetry from tetragonal ͑space group P4 / nmm͒ to orthorhombic ͑space group Cmma͒ at 155 K, and then followed by an AFM order at 137 K. Doping the system with F gradually decreases the structural distortion temperature, but suppresses the long range AFM order before the emergence of superconductivity. Therefore, while superconductivity in these Fe oxypnictides can survive in either the tetragonal or the orthorhombic crystal structure, it competes directly with static AFM order.

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