The constructive interference of surface plasmon polaritons (SPP) launched by nanometric holes allows us to focus SPP into a spot of high near-field intensity having subwavelength width. Near-field scanning optical microscopy is used to map the local SPP intensity. The resulting SPP patterns and their polarization dependence are accurately described in model calculations based on a dipolar model for the SPP emission at each hole. Furthermore, we show that the high SPP intensity in the focal spot can be launched and propagated on a Ag strip guide with a 250 x 50 nm2 cross section, thus overcoming the diffraction limit of conventional optics. The combination of focusing arrays and nano-waveguides may serve as a basic element in planar nano-photonic circuits.
Iron selenide (FeSe x ) crystals with lateral dimensions up to millimeters were grown via a vapor self-transport method. The crystals consist of the dominant α -phase with trace amounts of β -phase as identified by powder x-ray diffraction. With four-probe resistance measurements we obtained a zero-resistance critical temperature of 7.5 K and a superconducting onset transition temperature of up to 11.8 K in zero magnetic field as well as an anisotropy of 1.5 ± 0.1 for the critical field. Magnetization measurements on individual crystals reveal the co-existence of superconductivity and ferromagnetism.
Irradiation with Pb ions was used to study the effect of disorder on the in-plane London penetration depth, λ(T ), in single crystals of Ba(Fe1−xTx)2As2 (T=Co, Ni). An increase of the irradiation dose results in a monotonic decrease of the superconducting transition temperature, Tc, without affecting much the transition width. In both Co and Ni doped systems we find a power-law behavior, ∆λ(T ) ∝ T n , with the exponent n systematically decreasing with the increase of disorder. This observation, supported by the theoretical analysis, conclusively points to a nodeless s ± state with pairbreaking impurity scattering (interband) with strength being intermediate between Born and unitary limits.
We investigate the origin of the matching effect observed in superconducting Nb films containing regular arrays of holes near the zero-field critical temperature. We find "dips" in the resistance vs magnetic field curves at matching fields where the magnitude of the magnetic flux threading each unit cell is an integer number of the flux quantum. By comparing the magnetic field dependences of the resistance and critical temperature in perpendicular and parallel magnetic field directions, we find that the matching effect in Nb films containing triangular hole arrays originates from hole-induced suppression of the critical temperature rather than the widely assumed flux pinning enhancement.
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