Zn and Ni doping effects on the low-energy spin excitations in La1.85_{1.85}Sr0.15_{0.15}CuO4_{4}

Impurity effects of Zn and Ni on the low-energy spin excitations were systematically studied in optimally doped La1.85Sr0.15Cu1-yAyO4 (A=Zn, Ni) by neutron scattering. Impurity-free La1.85Sr0.15CuO4 shows a spin gap of 4meV below Tc in the antiferromagnetic(AF) incommensurate spin excitation. In Zn:y=0.004, the spin excitation shows a spin gap of 3meV below Tc. In Zn:y=0.008 and Zn:y=0.011, however, the magnetic signals at 3meV decrease below Tc and increase again at lower temperature, indicating an in-gap state. In Zn:y=0.017, the low-energy spin state remains unchanged with decreasing temperature, and elastic magnetic peaks appear below 20K then exponentially increase. As for Ni:y=0.009 and Ni:y=0.018, the low-energy excitations below 3meV and 2meV disappear below Tc. The temperature dependence at 3meV, however, shows no upturn in constrast with Zn:y=0.008 and Zn:y=0.011, indicating the absence of in-gap state. In Ni:y=0.029, the magnetic signals were observed also at 0meV. Thus the spin gap closes with increasing Ni. Furthermore, as omega increases, the magnetic peak width broadens and the peak position, i.e. incommensurability, shifts toward the magnetic zone center (pi pi). We interpret the impurity effects as follows: Zn locally makes a non-superconducting island exhibiting the in-gap state in the superconducting sea with the spin gap. Zn reduces the superconducting volume fraction, thus suppressing Tc. On the other hand, Ni primarily affects the superconducting sea, and the spin excitations become more dispersive and broaden with increasing energy, which is recognized as a consequence of the reduction of energy scale of spin excitations. We believe that the reduction of energy scale is relevant to the suppression of Tc.Comment: 13pages, 14figures; submitted to Phys. Rev.

Overview of the North Ecliptic Pole multi-wavelength survey (NEP-DEEP)

An overview of the North Ecliptic Pole (NEP) deep multi-wavelength survey covering from X-ray to radio wavelengths is presented. The main science objective of this multi-wavelength project is to unveil the star-formation and AGN activities obscured by dust in the violent epoch of the Universe (z=0.5-2), when star-formation and black hole activities were much stronger than at present. The NEP deep survey with AKARI/IRC consists of two survey projects: shallow wide (8.2 sq. deg, NEP-Wide) and the deep one (0.6 sq. deg, NEP-Deep). The NEP-Deep provides us with a 15 μm or 18 μm selected sample of several thousands of galaxies, the largest sample ever made at these wavelengths. A continuous filter coverage at mid-IR wavelengths (7, 9, 11, 15, 18, and 24 μm) is unique and vital to diagnose the contribution from starbursts and AGNs in the galaxies at the violent epoch. The recent updates of the ancillary data are also provided: optical/near-IR magnitudes (Subaru, CFHT), X-ray (Chandra), FUV/NUV (GALEX), radio (WSRT, GMRT), optical spectra (Keck/DEIMOS etc.), Subaru/FMOS, Herschel/SPIRE, and JCMT/SCUBA-2