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...

A superconductor has the unique properties of zero resistance and Meissner effect. Lenz's law is a fundamental law of physics. People have occasionally brought up the question that if a superconductor abides by Lenz's law. There has been lack of an explicit answer to this question so far. Recently, we carried out experiments with superconductor coils and a magnet in search of an answer to this question. We find out that the interacting behavior between a superconducting coil and a magnet does not comply with one of the primary interpretations of Lenz's law: the current induced in a circuit due to a change or a motion in a magnetic field is so directed as to exert a mechanical force opposing the motion. Our experimental results show that the induced current in the superconducting coil do not always oppose the motion of magnet during their interaction. Instead, in a certain portion of the interaction the induced current aids the motion of the magnet. This finding may require the aforementioned interpretation of Lenz's law to be revised as superconductors are involved.

No abstract

Differently from most of the other 1111-type iron-based superconductors, ThFeAsN itself shows superconductivity at 30 K without antiferromagnetism, even in the absence of chemical doping and other treating. In order to understand its peculiar behavior better, it is necessary to investigate the evolution of the superconducting phase through electron doping. Chemically, Co doping is a more effective way to introduce electrons, as carriers are doped directly into the FeAs planes. It also could provide information on how well the ThFeAsN tolerates in-plane disorder. Here we have substituted Co for Fe to synthesize ThFe1−xCoxAsN. It is found that the superconductivity of ThFeAsN parent compound is quickly suppressed upon Co doping. With a doping amount of 5% (x = 0.05), the superconductivity of ThFe1−xCoxAsN vanishes. ThCoAsN has been synthesized and characterized. It shows no superconductivity at 1.8 K. As both its crystal structure and transport behaviour are similar to those of itinerant ferromagnets LaCoAsO and LaCoPO, it is expected that ThCoAsN would be a kind of itinerant ferromagnet. However, until 1.8 K, the expected itinerant ferromagnetism could not be confirmed. The experimental data support that ThCoAsN is a kind of metallic paramagnet above 1.8 K.

No abstract