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Magnetic neutron scattering studies on the Fe-based superconductor system Fe$_{1+y}$Te$_{1-x}$Se$_x$

I present a brief overview on the interplay between magnetism and superconductivity in one of the Fe-based superconductor systems, \fts, where the research of our group has centered. The parent compound Fe$_{1+y}$Te is an antiferromagnet; with Se doping, antiferromagnetic order is suppressed, followed by the appearance of superconductivity; optimal superconductivity is achieved when $x\sim50\%$, with a superconducting temperature \tc of $\sim$15~K. The parent compound has an in-plane magnetic ordering wave vector around (0.5,\,0) (using the tetragonal notation with two Fe atoms per cell). When Se concentration increases, the spectral weight appears to be shifted to the wave vector around (0.5,\,0.5), accompanying the optimization of superconductivity. A neutron-spin resonance has been observed around (0.5,\,0.5) below \tc, and is suppressed, along with superconductivity, by an external magnetic field. Taking these evidences into account, it is concluded that magnetism and superconductivity in this system couple to each other closely---while the static magnetic order around (0.5,\,0) competes with superconductivity, the spin excitations around (0.5,\,0.5) may be an important ingredient for it. I will also discuss the nature of magnetism and substitution effects of 3$d$ transition metals.